Anatomy of the Orbit: Overall Skeletal and Topographical Configuration

Cornelius, Carl-Peter; Gooris, Peter J. J.

doi:10.1007/978-3-031-40697-3_2

Carl-Peter Cornelius⁴ &
Peter J. J. Gooris^5,6,7

2143 Accesses

Abstract

Objective: The intent of this chapter is to provide a systematic reappraisal of the bony anatomy of the orbit. The studies and the available literature on this topic are ever-expanding. The present knowledge shall be put into perspective.

Material and Methods: Textbook and atlas descriptions served as a starting point for a comprehensive outline of the osseous anatomy of the orbits. To deepen the insight and understanding, a small selection of recent reports on specific structures has been reviewed. For visual purposes, photographs of PMHS (post mortem human subject) specimens and illustrations were used.

Review/Results: The most distinctive feature of each orbit is the quadrangular pyramidal shape with a tetrahedron spire making up the orbital apex. The bony elements of the orbits are delineated with the sphenoid as backward foundation and starting base for the overall architectural composition. A closer look goes into the openings within the orbital precincts and reveals the subtleties and variations of the fissures, canals, grooves, foramina, notches, and fossae monitoring recent publications. The research modalities of these studies have shifted from dry skull investigations and PMHS dissections to modern imaging techniques (CT, MRI, CBCT) within defined living populations facilitating the analysis of hidden spaces or hard-to-reach structures in a non-destructive manner. Additional advantages of imaging include quantification and a detailed morphometric evaluation in the spirit of computational anatomy.

Conclusion: In essence, the combination of traditional anatomic knowledge and the understanding of the tremendous complexity of variations characterized in new publications demands for a patient-specific diagnostic workup as exemplified in this book. As a matter of fact, however it is more likely to recognize and grasp features that one knows or knows about from previous intensive learning.

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Keywords

FormalPara Learning Objectives

To acquire a/an

Comprehensive knowledge of the bony composition within and around the precincts of the orbits.
Understanding of the topographical relationships of the orbit to the middle cranial fossa, cavernous sinus, infratemporal and pterygopalatine fossa, the paranasal air sinuses, and the lateral nasal wall.
Impression of the individual variability of passageways in and out the orbit, which becomes specifically apparent in large-scale data pools of morphometric imaging (CT, MRI, CBCT) studies.
Habitude for a thorough diagnostic workup to anticipate anatomic normal-variants for increased finesse in surgical planning and postoperative analysis.

Introduction

The orbits are paired mirror symmetric cavities of bone on either side of an intermediary compartment that is made up by the external and inner nose as well as the ethmoid and sphenoid sinuses. The orbits contain the eyeballs, i.e., the optical and light/image processing units, that represent the entry of the visual organ. Besides the eyeballs or globes with their refractive apparatus and retinal receptors, the visual organ consists of auxiliary adnexa, such as the lacrimal system, the extraocular muscles, and the adipose body including a multitude of neural connections and vascular supply. The posterior end of each orbital cavity (orbital apex) centers over the bony surfaces in the close vicinity of the superior orbital fissure and the posterior sinkhole of the inferior orbital fissure. These apex areas of the orbit are identical with the regions in juxtaposition of the lateral surfaces of the cubic body of the sphenoid bone. This coincidence fostered the idea of this fundamental anatomy chapter to describe the osseous construction of the orbits systematically building up from scratch at the orbital apex in the manner of a technical blueprint and exploded view drawings of the individual components. This process will result in a depiction of the overall assembly of both orbits and an outline of their spatial and topographical relationships at the transition between the maxillae, central and lateral midface components, and the cranial base and vault. Finally, the position, shape, dimensions, and important variations of the gates and passageways within this framework of bones and paranasal sinuses are addressed.

Method

Classic textbook and atlas descriptions were used to model the basic bony anatomy of the orbit and serve as introduction to outline the details of the passageways as well as topographical aspects. The recent relevant literature on the bony openings was identified and a small selection assigned to review the details of the bony openings. Photographs of post mortem human subject dissections, illustrations and digital drawings were synthesized to present a comprehensive survey of topics spanning the breadth of current knowledge on the bony constituents inside and around the orbit.

I. Osteology—Bony Building Blocks

Overall Skeletal and Topographical Configuration

The orbits are formed by facial and cranial bones. Each orbit is assembled of seven bones: sphenoid, frontal ethmoid, lacrimal, palatine, maxilla, zygoma [1]. These skeletal building blocks outline a cone- or pear-shaped cavity with a thick marginal rim framing the aperture at its base in contrast to the thin-walled cone construction. In terms of a geometric concept, the orbit can be translated into a pyramid with a quadrangular base, giving rise to four concentric walls which carry a three-sided spire or apex on the top end (Fig. 2.1a–f). The open base or aditus orbitae is projecting fronto-laterally and the apex posteromedially toward the optic foramen. The aditus is framed by thick and prominent marginal rims. These are well defined except for the medial side, which is discontinuous due to the interposition of the fossa for the lacrimal sac between its lower and upper part.¹ The medial rim or nasal orbital margin consists of the frontonasal process of the maxilla, the lacrimal bone, and the maxillary process of the frontal bone. The frontonasal process extends upward into the anterior lacrimal crest and forms the medial orbital margin in the lower part (MOMLP). The supraorbital rim continues into the posterior lacrimal crest in a more backward plane and creates a second parallel bone ridge, the upper part of the medial orbital margin (MOMUP) with the lacrimal fossa in between at a fluted bevel. The superior and inferior orbital margins curve distinctly posterior, so that the lateral rim is least projecting in the whole orbital circumference. The four walls of the human orbit or the internal orbit, respectively, are formed by the seven bones named above. The roof of the orbit is composed largely of the orbital plate of the frontal bone anteriorly and of the lesser wing of the sphenoid (LWS) with a minor part in the posterior part. The triangular shape of the roof narrows toward the orbital apex. The anterior portion of the frontal bone is containing the frontal sinuses which can extend far up into the squamous part of the frontal bone and far back over the orbital roof when extremely pneumatized. The floor of the anterior cranial fossa forms the endocranial side of both orbital roofs. The fossa for the lacrimal gland is a shallow depression in the anterolateral aspect of the roof next to the zygomaticofrontal suture (ZFS). A small depression in the anteromedial portion of the roof, the trochlear fovea, is the site of attachment for the fibrocartilaginous ring (pulley) girdling the tendon of the superior oblique muscle.

5 anatomical photos. A and B depict the skull with orbits in an anterior view with the regions labeled C O F, F O F N, S O M N, S O F N, and I O F. C to E. A peeled banana is placed on the orbital walls of the skull. F. A geometrical illustration of a quadrangular pyramid with a triangular apex. — **Fig. 2.1**

The junctions of the walls in the superomedial, superolateral, inferolateral, and inferomedial quadrants or pyramidal corners are rounded in reality. The overall configuration of the internal orbit (i.e., the inside) comes closer to the pyramid model than the external orbit due to its integration into frames, pillars, support, and pneumatized paranasal structures. With regard to their anatomical subunits, the entire orbital margin is occasionally divided into three sections, a supraorbital rim, an inferomedial or maxillary rim, and an inferolateral or zygomatic rim.

Sphenoid Bone—Constituent of the Upper, Medial, and Lateral Orbital Wall

The sphenoid bone with its cuboidal body at the center and three pairs of outstretched wings or leg-like processes, respectively, can be appreciated as the principial constructive element of the skull base. It has often been considered as the most complex polymorphous bone of the human skeleton (Figs. 2.2a–d and 2.3).

4 anatomical photos exhibit the sphenoid bone in anterior, superoanterior, superior, and posterior views. An inset photo in A and C depicts the sphenoid bones of a bee and a bat. The labeled regions include the temporal and orbital surfaces, L W S, A C P, P C P, L P P, M S, F R, F M N, and L I O F M. 4 anatomical photos of the sphenoid bone from different views exhibit labels l i o f m, g w s, f m n, f o, s q m, a c p, c a g, h f, a c p, orbital surface, f m n, and s z s, cerebral surface, s s p, l p p, f r, h f, p c s, o f c, i s, a c p, etcetera in parts c and d. — **Fig. 2.2**

An anatomical photo of the sphenoid bone within the cranial fossa in the endocranial surface view exhibits a cup-shaped structure with a depression at the center. The labeled regions include the squamous part, orbital part, F B, F C, A C F, O F C, O C C B, G W S, C O F, S B P T, I A O, and M F C. — **Fig. 2.3**

Its front view silhouette has the appearance of a wasp (Fig. 2.2a). Because of this appearance, a common term for it was “sphecoid” bone, according to Ancient Greek σφήξ (sphḗx ≈ “wasp” or “hornet”) which was used every now and then from the beginnings of medical science in Greco-Roman antiquity until the nineteenth century. The tale (historically unconfirmed) is still bandied that the c in sphenoid was erroneously confused with an n resulting in sphenoid which has the meaning of σφην (sphēnoeid ≈ “wedge”) in the publishing process of an anatomy textbook and never set right again since then [3, 4]. The triangular lesser wings (alae minores) of the sphenoid (LWS) extend from the superolateral aspect of the body to form the upper border of the superior orbital fissure (SOF), the most posterior part of the orbital roof, and the posterior ridge of the floor of the anterior cranial fossa on either side (Fig. 2.2a). The greater wing (ala major) of the sphenoid (GWS) separates the orbit from the middle cranial fossa and is part of the vertical pterygomaxillary buttress. Each GWS is attached on the lateral aspect of the body by a radix (root). The GWS can be conceived as assembled of three divisions (anterolateral/posterolateral/inferior) which are arranged along and below a vertical compact pillar with a triangular horizontal cross section, nowadays called the sphenoid door jamb (SDJ see later). It ensues a complicated 3-dimensional configuration of two adjacent corners exhibiting several external and cerebral (internal/endocranial) surfaces. The external medial surface of the anterolateral division corresponds to the plane posterior portion of the lateral orbital wall. The external lateral surface of the same division is docked close to its anterior border at a T-junction by the posterolateral division. The external surfaces of both the anterolateral and posterolateral divisions blend to form parts of the concave medial boundary of the temporal fossa and the external lateral orbital wall. The inferior GWS division spreads underneath the other two divisions starting posteriorly from their vertical intersection (Fig. 2.2c, d). The upper surface of the inferior division conforms to the floor of the medial cranial fossa which is bordered anteriorly and laterally by the cerebral surfaces of the vertical GWS divisions moreover. The bottom surface of the inferior GWS division circumscribes the infratemporal fossa. Axial cross sections unveil the posterior GWS as a central trigone with a spongious bony space between the orbital, temporal, and cranial cortical surfaces (Fig. 2.15b). This potential space for surgical decompression is termed the sphenoid door jamb (SDJ). The superior orbital fissure (SOF) is a gap between the LWS and the GWS. The lower end of the SOF is bounded by the maxillary strut; this is a bony bridge across the foramen rotundum and integral part of the upper GWS radix. The paired pterygoid processes project bilaterally from the inferior aspect of the sphenoid body and its connection to the GWS downwards. It is the base of the lateral pterygoid plate (lamina) that fuses with the root of the GWS in a longitudinal direction. The medial and lateral pterygoid plates band together along the upper portion of their anterior delineations to form the posterior concavity of the pterygopalatine fossa (PPF). The anterior bottom portions of the pterygoid plates remain set asunder forming the pterygoid notch. The notch gap is interposed by the solid inferoposterior end (pyramidal process) of the perpendicular plate of the palatine bone. The suture line arrangements differ and are collectively addressed as the pterygopalatine fissure.

The pterygoid fossa is the vertically oriented open recess between the pterygoid process plates viewed from the posterior aspect. The sphenoid connects to all of the other cranial base bones (Fig. 2.3) and to all bones of the facial skeleton except for the lacrimal and nasal bones.

Bony Orbital Apex—A Three-Walled/Tetrahedron Spire

The orbital apex within the sphenoid bone needs an elaborated description.

So far there is no unanimously consented definition of the sagittal extent of the orbital apex [5]. For the purposes here, it is regarded as the most posterior projection of the orbital cavity that starts in front of the level of the maxillary strut or foramen rotundum with a narrow triangular base in the frontal cross section (Fig. 2.4a, b) and ends with a pointed tip within the openings of the optic foramen or SOF, respectively. The roof side of the orbital apex is formed by the LWS, the medial side bordered by the lateral surface of the sphenoid body, and the lateral side adjoined by the GWS. The orbital floor with the orbital surface of the palatine bone at its posterior portion comes to a halt at the posterior (rear) IOF sinkhole and does not reach into the orbital apex (Fig. 2.4a, b). In general, two openings can be distinguished within the orbital apex, the optic foramen (OF) representing the intraorbital end of the optic canal and the SOF. The optic strut (OS) constitutes the inferolateral wall of the OC and separates the latter from the SOF (Fig. 2.5a, b).

2 anatomical photos of the left orbital apex. A. The marked region in the orbital apex indicates the gaps between the three-walled spire. B. An orbit in the anterior view exhibits the regions of S O F N, F Z P, S Z S, M S, F R, and L O M. An inset photo highlights the triangular orbital apex. — **Fig. 2.4**

2 anatomical photos. A. An orbital fissure system that extends into the orbital apex exhibits the regions of Z S S, G W S, S O F, I O F, I O F R S, A L I O F. B. The junction of the orbital apex and lateral sellar region is labeled with P C P, H F, A C P, O F C, I F C, M S, F R, I O F, and O P P B. — **Fig.2.5**

Frontal Bone—Orbital Roof Constituents

The unpaired frontal bone features external and cerebral surfaces on the sides of a large squamous vertical forehead part (squama frontalis), a nasal part, and two horizontally oriented orbital parts. Each orbital part passes backward behind the supraorbital rim and configures the superior wall (roof) of the orbit in unison with the floor of the anterior cranial fossa to a major extent (Fig. 2.6). The posterior margin of the orbital part articulates with the LWS, which forms a minor most posterior roof portion congruous with the roof of the apex. The posterolateral orbital part margin connects with the upper edge of the GWS along the sphenofrontal suture line (Fig. 2.7). The anterolateral orbital part brings up the zygomatic process and joins with the orbital plate and the temporal process of the zygoma along the frontozygomatic suture line. The orbital roof has the outline of an isoceles triangle that bends up into a concavity. The lacrimal fossa is a shallow depression in the anterolateral roof for the lacrimal gland. The trochlear fovea conforms to the anteromedial adherence zone of trochlear fiber condensations.

An anatomical photo of the frontal bone in the anterior view exhibits regions of G L A B, N P F, N M F, and N S P located medially. It denotes the depressions on the O P F B region at two edges. The labeled regions include Z M F, F Z P, S O F N, F Z P, S M F, T R F, and L F. — **Fig. 2.6**

An anatomical photo of the articulation of the frontal bone and sphenoid in an anterior view exhibits the regions of F E F, C O F, S R, L W S, G W S, S M F, and I F F. — **Fig. 2.7**

The ethmoidal notch is a cove-like slot in between the two orbital plates with a quadrilateral outline, from an endocranial view it is located in the central region of the anterior cranial fossa. Anteriorly the frontal notch turns into the nasal process of the frontal bone (frontal beak) with a serrated margin on either side of the superior nasal spine, a sharp downward process in the midline. The nasal margins articulate with the nasal bones and the frontonasal maxillary processes. The anterior and superior borders of the lacrimal bone connect to the frontonasal maxillary process and to a small strip of the notch margin just behind its anterior corner. In the intact, articulated cranium the interorbital slot is filled by the cribriform plate and the crista galli of the ethmoid bone. The margins of the notch contain several partially opened sinus cells, which have their counterparts in the upper surface of the ethmoid. The sandglass- or two-funnel-shaped frontonasal communication and drainage tract presents with an isthmus (frontal ostium) caudal to the bottom of the medial floor of each sinus. The cranial funnel of the tract is accommodated within the confines of the anterolateral notch margin and proceeds further downwards in the niches between the nasal margins and the superior nasal spine to reach into the frontal recess or the ethmoidal infundibulum [5,6,8]. The extensions of the frontal sinus over the orbital roofs and behind the squamous part of the frontal bone can be intensively pneumatized. The two tables of bone can be extremely thin and may contain dehiscences, so that the periorbita in the roof is in direct contact with the dura mater. The aeration patterns of the frontal sinus show great gender and interindividual variations in number, dimensions, outline, symmetry, septation, and laterality.

The superior orbital wall (roof) to its greatest extent consists of the orbital part of the frontal bone (Fig. 2.8). The most posterior minor portion at the apex is formed by the lesser wing of the sphenoid. The orbital roof takes a triangle shape bent up into a dome.

An anatomical photo of the right orbital roof in the inferior view exhibits the regions of S O F N, T R F, L F, O P F B, S O F, F P Z B, S D J, T R, S O F, M C F, L W S, O F C, O C R, S B, E B, and A C P. — **Fig. 2.8**

Ethmoid, Lacrimal Bone and Frontonasal Maxillary Process—Constituents of the Medial Orbital Wall

The medial wall of the orbit is part of the centro-facial or naso-orbito-ethmoido-nasal unit. It is built up by the sphenoid, ethmoid, lacrimal bone, and the maxilla in a posterior to anterior order (Fig. 2.9).

An anatomical photo of the bony constituents of the right medial orbital in the inferolateral view highlights the Ethmoid bone in colors at the center. Some of its labeled regions include F Z P, O P F B, S O F N, T R F, N M F, G L A B, N L C, Z M S, I O G, O S M, I M M, N F P, and L B. — **Fig. 2.9**

The lateral wall of the sphenoid body is lined up in front with the lamina papyracea, the paper-thin orbital plate of the ethmoid, and the lateral surfaces of the lacrimal bone and the naso-frontal maxillary process (NFP). The unpaired ethmoid (ethmoidal bone) is located between the two orbits. It contains the ethmoid sinus cells and is forming parts of the nasal and the orbital cavity. The ethmoid bone is built up of the perpendicular plate along the sagittal plane with the crista galli at the upper end (Fig. 2.10a–e). The perpendicular plate corresponds to the bony nasal septum. The cribriform plate extends on each side of the upper perpendicular plate margin in kind of the bar of a T-beam profile. The ethmoidal labyrinths refer to honeycomb arrays of small air cells hanging downwards from the lateral bounds of the cribriform plates. The ethmoidal air cells vary in number and volume on each side and extend in the orbital (i.e., edges of the ethmoidal notch) and squamous part of the frontal bone (Fig. 2.11), occasionally into the sphenoid and into frontonasal process of the maxilla, too. The paramedian portions of the cribriform plate form the roof of the nasal cavity. The rudimentary supreme, the superior, and the middle nasal turbinates (conchae) go down from the medial side of each labyrinth. The boundaries of an ethmoidal labyrinth are often reviewed for didactical illustration in analogy to the sides of a matchbox cover standing on its small longitudinal side (Fig. 2.10a). The box cover is closed posteriorly with the frontal surface of the sphenoid body. In addition to the already open anterior entrance the bottom side of the box is absent in accordance to the open sides of the middle nasal meatus. The lateral wall of the box represents the orbital plate, which is identical with the large rectangular center piece of the medial orbital wall. The orbital plate is paper-thin (lamina papyracea) and transparent, so that the contours of ethmoidal air cells become visible through it (Fig. 2.12). The lower lateral wall of the box or in other terms the inferior ethmoidal cells are adjacent to the medial roof side of the maxillary sinus. The medial wall of the labyrinthine box corresponds to the upper and middle turbinates in alignment at the lateral wall of the inner nose (turbinal wall). The curved basal (ground) lamellae of the superior and middle turbinates fuse with each other and intermingle with the air cell septa network buttressing the paper plate medial orbital wall. The basal lamina of the middle turbinate also reinforces the maxilloethmoidal suture resulting in a firm bony thickening, which is referred to as the inferomedial orbital strut (IOS) (Fig. 2.24a, b).

5-part images. A. A graphical illustration of the ethmoid bone in the left anterolateral view exhibits the cribriform plate, orbital plate, perpendicular plate, and medial concha. B to E. Four anatomical photos of the ethmoid bone are labeled with C G A, P P E, M N T, L P, and L C. — **Fig. 2.10**

An anatomical photo of the squamous part of the frontal and ethmoid bones exhibits the cribriform plate that forms the roof of the nasal cavity and its notched edges. — **Fig. 2.11**

An anatomical photo of the medial orbital wall from lateral to medial view exhibits a slight depression at the center. The labeled regions include O F C, E B A, P E F, F E S, A E F, F L S, O S M, M E S, O P P B, I F C, and L P P. An inset with arrows denotes the supraorbital rim at the center. — **Fig. 2.12**

Clinical Implications

Significance of morphologic properties of the medial orbital wall:

Few and large-volume ethmoidal air cells extending over larger areas of the lamina papyracea are supposed to predispose more fracture susceptibility of the medial orbital wall, in contrast to a high-density reinforcing honeycomb structure [9].

The small trapezoidal lacrimal bones have a nasal and an orbital surface. They are located right in front of the orbital plate of the ethmoid and close to the medial orbital wall across the nasofrontal maxillary process (NFP). Concurrently, they complete the lateral wall of the anterior ethmoidal cells and are part of the lateral wall of the nose where this is overlying the tip of the middle turbinate. A vertical ridge, the posterior lacrimal crest, protrudes along the orbital surface and divides it into a smooth plane posteriorly and into a carved out longitudinal groove (lacrimal sulcus). The anterior margin of this sulcus joins to the likewise grooved posterior margin of the NFP and completes the entrance into the nasolacrimal canal. The orifice at the initial canal section is widened and creating a fossa for the lacrimal sac. The upper lateral slope of the NFP carries the anterior lacrimal crest in parallel to the anterior circumference of the canal inlet zone.

The posterior lacrimal crest may be elongated together with a small portion of the lower smooth plane into a hooked process. This lacrimal hamulus interdigitates with a corresponding notch of the NFP at the caudal periphery of the canal entrance.

Palatine Bone and Maxilla—Major Constituents of the Orbital Floor

The palatine bone is interposed between the back of the maxilla and the base as well as the medial and lateral plates of the pterygoid process (Fig. 2.13a). It contributes to the posterior lateral wall of the nasal cavity, the hard palate, the PPF, and the orbital floor in an intriguing configuration. Basically, the palatine bone consists of a vertical part (perpendicular plate) with a medial oblique orientation and a horizontal part or plate providing the posterior hemi-hard palate portion. The upper end of the perpendicular plate is overtopped by the orbital and sphenoidal processes. The orbital process of the palatine bone (OPPB) is situated at a higher level than the sphenoid process and projecting anterosuperiorly. It is based on a neck narrowed by the half-oval sphenopalatine notch (SPNPB) at the posterior aspect which also sets it apart from the sphenoidal process that is attached at the inferior margin of the notch and diverges medially and upwards. The orbital process presents with five surfaces in total. The anterior surface connects with upper medial end of the posterior surface of the maxilla. It is common to refer to the superior surface of the orbital process of the palatine bone, as to the orbital plate of the palatine bone (OPPB) more briefly. The OPPB is triangular in shape and sloping slightly upwards in combination with a downward tilting laterally (Fig. 2.13b–d). Since the OPPB conforms the rear end of the orbital floor, that is often preserved in defect fractures and may serve as support in surgical reconstruction therefore, it is simply addressed as the “posterior ledge” in surgical jargon. The remaining three surfaces are directed either posteriorly to abut the anterior sphenoid body, or medially to join with the ethmoid and laterally toward the pterygopalatine fossa (PPF). The orbital process may enclose an air cell, which is open either to the sphenoid sinus, the posterior ethmoidal cells or with both at once [9,10,12]. The sphenoidal process attaches to the medial base of the pterygoid process. The anterior border of the sphenoidal process bounds the posterior SPNPB margin, while the anterior notch margin is molded by the posterior neck of the orbital process (OPPB).

5 anatomical photos exhibit a skull with colored regions in part a, bones of maxillae exhibiting o p p b, s p f g, p p p b, i t, p y r p b, h p p b, and c c p b in part b, right palatine bone from front, above, and antero-posterior views in c, d, and e, respectively with labels. — **Fig. 2.13**

The sphenopalatine foramen (SPF) has a posteromedial angulation to the sagittal plane. The SPF results from the union of the inferior surface of the sphenoid body with the superomedial marginal SPNPB circumference. The communication between the PPF and the posterior nasal cavity is via the SPF. In topographical relations, the bony SPF surroundings form the medial wall of the posterior (rear) IOF sinkhole and the vertically oriented surface descending from the posterior OPPB border.

Clinical Implications

The position of the posterior ledge in terms of linear distances between the infraorbital margin above the infra-orbital foramen and landmark points at the OPPB, the anterior SOF and ventrolateral entrance of the OFC has been assessed in a CT-based cohort study as a guideline for the safe surgical approach in the repair of inferomedial orbital wall fractures [13].

The potential of an OPPB resection for expansion of the surgical corridor in the endoscopic approach to the orbital apex has been checked in a morphometric analysis of the area dimensions in macerated skulls [14].

The orbital surface of the maxilla (OSM) makes up the largest part of the orbital floor [15]. It is the upper of a total of four surfaces of the body of the maxilla (corpus maxillae) which contains the maxillary sinus or the antrum (of Highmore), respectively. Hence, the OSM is also the ceiling of the antrum. The outline markings of the orbital surface fit a smooth triangular plate, which has an inclination angle of 45° to the horizontal plane. The posterior OSM border converts into the rounded anteromedial IOF margin and its consecutive vertical wall, which is interchangeable with the infratemporal surface of the maxillary body. At its front, the anterior OSM blends into the infraorbital margin (IMM). The IMM extends into the lower part of the medial orbital margin (MOMLP) and further on into the nasofrontal maxillary process (NFP). Laterally, the IMM terminates with the zygomatic process. The lower part of the medial orbital plate of the zygoma (OPZLP) provides a small anterolateral flange to the orbital floor just behind the lower lateral corner of the rim. A small bony depression behind the lower medial corner of the rim and immediately lateral alongside the nasolacrimal canal is the origin to the inferior oblique muscle.

The NFP has a plate-like shape and is forming part of the sidewall of the nose along the upper lateral border of the piriform aperture. At the cranio-medial end, the NFP articulates with the frontal bone and the nasal bone (Fig. 2.14). The posterior margin raises the anterior lacrimal crest (ALC) before it transforms into the lacrimal fossa and the entrance into the nasolacrimal groove and canal (NLC), which emanate in combination with the contours and borders of the lacrimal bone.

An anatomical photo of the skull highlights the ethmoid bone at the center. Some of its labeled regions include F B, G L A B, S O F, S M F, L I O F M, G W S, O S M, N B, Z P M, I O F N, A N S, N A, I M M, and N F P. — **Fig. 2.14**

The zygomatic or malar process is passing laterally for articulation with the zygomatic bone. This process rests on the zygomaticomaxillary crest, a porch jutting out with a slightly arched border from the junction of the anterior and infratemporal surfaces of the maxillary body that extends upward at the level of the first upper molar. The upper surface of this process is a rough and serrated triangular platform that has a downward angulation to the lateral side. The borders of this platform bring up the zygomaticomaxillary suture lines together with the edges of the medial undersurface of the zygoma (Figs. 2.15a, b, 2.16b, and 2.17a, b). The orbital floor is shorter in anteroposterior extent than the three other orbitals walls and thus is missing in the orbital apex (Fig. 2.15a, b).

3 anatomical photos. A and B. The orbital floor in the anterocranial and posterior views exhibits the regions of S O F, S D J, G W S, F P Z B, O P Z, I O F, O M S. C. An assembly of the maxilla, zygoma, and sphenoid bone in the inferolateral aspect denotes the regions of Z T S, Z F F, and I O F N. — **Fig. 2.15**

2 anatomical photos of the right zygoma in the lateral and medial aspects with curved margins on the lateral ends. The labeled regions include M T Z, F P Z, Z F F, I P Z, T P Z, and M M Z in A and F P Z, O P Z, Z O F, O P Z L P, M M Z, and temporal surface in B. — **Fig. 2.16**

2 anatomical photos. A. The zygomatic bones in the closed-lock position exhibit the frontal, temporal, and maxillary regions with depressions at the edges. B. The midface in the anterior view denotes the regions of the frontal eminence of the frontal bone and greater wings of the sphenoid bone. — **Fig. 2.17**

The posterior or infratemporal surface of the maxillary body covers the curving vertical area between the posterior aspect of the nasal surface to the lateral margin of the zygomaticomaxillary crest (Fig. 2.15c). The maxillary tuberosity is bowing out backward from the lower lateral portion of this surface. A small crescentic rough site at the medial posteroinferior aspect at the junction of the posterior and medial surface is attached to the maxillary process at the lateral surface of the perpendicular plate of the palatine bone. The greater palatine canal is formed by the apposition of two obliquely oriented grooves descending on each side of the bony interface.

The melding seamline along the upper border of posterior surface is the link to the orbital plate and corresponds to the anteromedial IOF margin. This rounded hillslope at the back of the seamline into the IOF rear sinkhole (IOFRS) is engraved by the infraorbital groove (sulcus) [16]. The infraorbital groove (IOG) often begins with a deep U-shaped hollowing in the middle of the posterior maxillary floor. It passes forward over varying distances before it transforms into a canal structure [17]. The infraorbital canal (IOC) moves downwards and gets suspended on a bony beam resembling a mesentery protruding from underneath the orbital floor [18]. This conduit ends in the anterior antral wall usually with a single infraorbital foramen (IOFN). In adults, the foramen is located 7–10 mm below the infraorbital margin and above the canine fossa purportedly on a plumb line passing through the mid-pupil.

Clinical Implications

The position and relative length of the infraorbital groove and canal are supposed to be predisposing factors to the occurrence and pattern of fractures within the orbital floor [19].

The medial or nasal surface is dominated by the maxillary hiatus, a large irregular aperture to the maxillary sinus, which is covered and downsized by the perpendicular plate of the palatine bone posteriorly, the inferior turbinate inferiorly, and the ethmoidal labyrinth—in particular by the uncinate process—together with the lacrimal bone superoanteriorly. The medial OSM border is assembled alongside the maxilla-ethmoidal suture line as part of the inferomedial orbital strut (IOS) and comes up against the lower borders of the lamina papyracea and the lacrimal bone [20]. The two paired maxillae are the largest bones in the midface. Uniting below the nasal aperture at the level of the anterior nasal spine and along the midline of the hard palate, they aggregate to the upper jaw (Fig. 2.14).

Apart from the body including the frontonasal and zygomatic process, there are two other components contributing to the overall architecture of a maxilla, the horizontal palatine process, and the alveolar process curvature. Altogether, these components are involved in the structural organization of the orbital, nasal/paranasal and oral cavities.

Zygoma—Constituent of the Lateral Orbital Wall

The zygoma, malar, or cheek bone is the prominent cornerstone of the upper lateral midface (Figs. 2.4b and 2.16a, b). The orbital plate (facies orbitalis) of the zygoma (OPZ) is directed posteromedially at an angle of 45° toward the sagittal plane. In conjunction with the vertical frontal process (FPZ), the OPZ completes the lateral orbital wall and makes up the lateral orbital margin (LOM) including the rim around the lower lateral quadrant. Thereto the OPZ joins up with the anterior GWS border along the sphenozygomatic suture line (SZS) (Fig. 2.17a), while the zygomatic-frontal-suture line (ZFS) is the anterosuperior contact zone between the two according frontal processes from above and below (Fig. 2.17b). Anterior to the massive central trigone of the GWS, the lateral orbital wall turns into a monocortical layer with the SZS line located in the thinnest portion. The indented bay between the margins of the anterior loop of the IOF partitions a small horizontally oriented lower part (OPZLP) from the overall orbital plate. The OPZLP conforms the anterolateral orbital floor (Figs. 2.15a, b and 2.17a, b).

The orbital tubercle or Whitnall’s tubercle [21] is a small roundly protuberance of 2 mm or 3 mm diameter on the inner OPZ immediately (2–4 mm) in the marginal territory behind the orbital rim and about 10–11 mm beneath the ZFS. The tubercle gives attachment to the lateral retinacular suspension complex. The marginal tubercle (MTZ) corresponds to a spine at the posterior upper edge of a FPZ widening that is occasionally present somewhat below the ZFS.

The solid zygomatic body has a rhombus shape and exhibits three further extensions in a medial, caudal, and posterior direction. The infraorbital process (IPZ) courses medially, while the adjacent maxillary margin is beveled diagonally downwards and laterally to merge with the tapering inferior tip of the zygoma (malar tuberosity), at last the temporal process reaches out backward to convey into the anterior zygomatic arch. The outer surface of the zygoma is commonly addressed as the malar or oftentimes inappropriately as lateral surface. The inner surface has been summed up as the temporal or infratemporal surface without drawing any clear distinction between the rear sides neither of the orbital plate or frontal process, the zygomatic body, or the temporal process.

A set of foramina with a network of interconnecting zygomatic channels perforates the OPZ as well as the malar and infratemporal surfaces of the zygoma. These foramina show a lot of variations from complete absence to multiplicity [22, 23]. The single or plural orifice(s) of the zygomatic-orbital foramen (ZOF) open(s) at the inner surface of the anterior inferolateral orbital quadrant next to the IOF loop (Fig. 2.15b). The zygomatic-facial foramen (ZFF) is located within the central range of the outer zygoma surface (Fig. 2.16a). The zygomatic-temporal foramen (ZTF) occupies an ascended position behind the frontal process or the zygomatic body.

Clinical Implications

The sphenozygomatic suture line (SZS) is an essential reference and reliable guide in the reduction of the fractured zygoma and consecutively for reestablishing of the outer facial frame. For this purpose, the lateral orbital wall is under intraoperative control from inside the orbit for accurate restoration into a flat continuous plane [24, 25].

Anterior Orbit—Midorbit—Posterior Orbit (Apex)

It is still often used practice to divide the length of the orbit into three-thirds according to the anteroposterior extension. This division is intuitive and not based on acknowledged anatomical references or appropriate metric distances between consented measuring points or planes. The orbital floor which is shorter than the other three walls is often the only zone taken into consideration and divided into three parts. Within the perspectives of the entire orbit, this must lead to confusion, since the apex with its triangular cross section is usually regarded as the posterior part, whereas the orbital floor extends over the remaining two thirds of the orbital depth. A more pragmatic approach is the partitioning into an anterior orbit, a midorbit, and a posterior orbit (apex orbitae) based on reproducible anatomic landmarks. The IOF appears most suitable for the provision of reproducible topographic indicators (Fig. 2.18a, b). A frontal plane at the level of a tangent to the tip of the anterior IOF loop provides the boundary between anterior orbit and the beginning of the midorbit. A second frontal plane placed at the transition of the IOF into the SOF anterior to the maxillary strut separates the midorbit from the posterior orbit or the apex orbitae. In other words, the anterior orbit lies in front and the posterior orbit in the rear of the IOF while the midorbit corresponds to the anteroposterior extent of the IOF. Obviously, this way of division does not result in an even metric tripartition but puts up the midorbit as the major component. The IOF landmarks need extrapolation around the entire orbital circumference, so that a series of concentric circles for a topographical allocation inside the orbit ensues (Fig. 2.18b).

2 anatomical illustrations. A. The orbital floor exhibits the borderlines between anterior, mid, and posterior orbits. B. A left orbit in the frontal view highlights the transition between I O F and S O F, the orbital plate of the palatine bone, and the anterior loop of I O F. — **Fig. 2.18**

II. Craniomaxillofacial Transition and Passageways

Fissures, Canals, Grooves, Foramina, Notches, Fossae—Bony Openings within the Precincts of the Orbit

Bony openings (canals, grooves, fissures, foramina, notches) lend pathways for neural and vascular linkages from the internal orbit to the cranial cavity, infratemporal fossa, paranasal sinuses, inner nose, and the face (Details see Chap. 3).

Optic Canal (OC)

The optic foramen/canal (OC) opens into the superomedial corner of the orbital apex, where the posterior medial wall extension meets with the roof. The canal has an elliptical cross section (approx. Diameters 4–9.5 mm) over a length between 5.5 mm and 11.5 mm. The optic foramen may lie posterior to the anterior face of sphenoid sinus, though this position can differ on the sides [26].

The posterior canal end is located medial to the anterior clinoid process in the middle cranial fossa. The conventional OC route (frequency 90%) courses along the lateral wall of the sphenoid and through the medial wall of the orbital apex. The mean angle of entry is about 60° to the coronal plane then. As a variant (frequency 10%), the OC enters the orbit through the roof of the apex at a more acute angle of 32° to the coronal plane [27]. Asymmetry of the optic canals is not uncommon, however [28]. The canal is formed by the sphenoid body inferomedially, by the anterior root of the lesser wing of the sphenoid (LWS) superiorly and the optic strut inferolaterally (Figs. 2.5a, b). The optic strut (OS) is a bony abutment linking the sphenoid body and the anterior clinoid process, which on account of its original designation as “sphenoid strut” is considered as a posterior [29] or lateral root [30] of the LWS. The anteroposterior OS location in terms of attachment to the sphenoid body relative to the prechiasmatic sulcus varies and was classified as presulcal, sulcal, and postsulcal [31, 32]. The OS angulation varies over a wide range between 30 degrees to almost 60 degrees. The OS dimensions measure about up to 8 mm in length, up to 5 mm in width and between 3 mm to 4 mm in thickness. Optic canal variations include a duplication and a keyhole anomaly at the inferolateral OS wall [33]. The occurrence of a sphenoethmoidal cell [34, 35] can further affect the OC disposition. Onodi cells are particular posterior ethmoid air cells that pneumatize the sphenoid body superior and lateral to the sphenoid sinus (Fig. 2.2a, b) directly related to the OC and optic nerve (CN II), respectively. The medial OC wall can protrude the lateral portion of Onodi cell or the OC can even pass through the middle of the cell as a bony tube. The prevalence of Onodi cells is reported as high as 60% in clinico-anatomical studies.

Excessive pneumatization can also involve the OS and the anterior clinoid process by lateral extension from the opticocarotid recess [36]. As the name implies, the opticocarotid recess [37] refers to a small space between the prominences of the OC and the carotid canal in the clinoid segment (C5) of the internal carotid artery [38].

The OS separates the OC from the superior orbital fissure (SOF). The inferior OS border corresponds to the superomedial SOF border. The minute infraoptic tubercle corresponds to a thickening located just beneath the anterior base of the OS [39]. It is the origin of the common annular tendon (Zinn’s ring).

Superior Orbital Fissure (SOF)

The SOF extends inferolaterally to the OC and OS. It is frequently described as a club- or L-shaped gap interposed between the LWS and the GWS or the orbital roof and the lateral wall, respectively (Figs. 2.5a, b, 2.7, and 2.8). It slopes alongside the lateral apex wall inferomedially, where it levels along the sphenoid body to reach the top of the maxillary strut (MS). The MS is a transverse bony bridge above the foramen rotundum. At the lower end, the SOF margins blend into the posterior outlines of the inferior orbital fissure (IOF) (Fig. 2.19). The posterior (rear) sinkhole of the IOF (IOFRS) separates the lower SOF end from the orbital process of the palatine bone anteriorly, however.

The inferomedial portion of the SOF has a widened configuration next to the sphenoid body that becomes narrower toward the superolateral end between the LWS and GWS [29, 38,39,42]. The lateral SOF margin is projecting somewhat more anteriorly than the medial margin, so that the SOF outlines do not lie in a strictly coronal plane.

Midway between the narrow and broad SOF partition the lateral (GWS) margin of the SOF may exhibit the lateral rectus spine (spina rectus lateralis—SRL). If present (frequency 60%), the form of this bony spur can vary from pointed as a tubercle over a tongue-shaped, rectangular, or irregular projection to two spines [43]. The spur serves as attachment site of the lateral part of the common annular tendon (Zinn’s ring), more specifically the tendon of the lateral rectus muscle (LRM) and/or an additional tendinous LRM slip. An irregular projection or a double spine may be due to a bipartite tendon insertion.

The medial SOF margin is formed by the OS in its upper part, the lateral surface of the sphenoid body borders the lower part. The cavernous segment (C4) of the internal carotid artery (ICA) is indicated by the carotid groove (sulcus) running an S-shaped course (i.e., “carotid siphon”) along the lateral surface of the sphenoid body. This part of the ICA is implemented in the cavernous sinus and covered by its lining membrane.

Coming from a bend at the posterior clinoid process, the C4 ICA segment passes forward to curve upward posterior to the medial SOF edge and the OS on to the medial side of the anterior clinoid process.

The crescentiform SOF shape has been classified in up to nine different basic types [44, 45] with different prevalence and metrics. The morphological types appear to have a bearing on the topography of soft tissue structures within the SOF opening [46]. The SOF is the major communication between the internal orbit and the medial cranial fossa. The dura mater lining of the middle cranial fossa and the cavernous sinus are continuous with the periorbita via the SOF. The periorbita within the orbital apex and the fibrous components enfolding the densely packed neurovascular structures passing through the OC and SOF fuse to Zinn’s ring or more precisely to the common circular connective tissue funnel, which is the origin of the four extraocular rectus muscles. Zinn’s ring subdivides the SOF into three individual hubs according to a superolateral, central, and inferior section (Chap. 3).

Cranio-Orbital Foramen (COF)

The COF is laying either in the GWS or the orbital surface of the frontal bone close to the superolateral extremity of the SOF near or within the sphenofrontal suture line. The COF may also merge with the tip of the SOF ending in kind of an incompletely separate foramen. Synonyms in use for the COF are orbito-meningeal foramen, lacrimal foramen, sphenofrontal foramen, and anastomotic foramen [47]. The foramina may be single or multiple, whereby multiple refers to 1 or 2 accessory foramina. Numerous possible permutations result from unilateral or bilateral occurrence and from differences of the number between the sides [47,48,50]. COF are not consistently present; reported incidences range from 30% to 85% with an average of 50% [51]. The shape of a COF is usually circular, rarely oval. The COF diameters usually vary between 0.3 and 2.5 mm. If accessory foramina are present, the diameter of the main COF is comparatively larger.

Larger COF communicate with the cranial fossa through a short conduit and join the orbit either to the anterior (A-subtype) or middle (M-subtype) cranial fossa [52, 53]. Smaller foramina are mostly blind-ending. The orbital branch of the middle meningeal artery (MMA), in general with a comitant vein, is transmitted through the COF (Figs. 2.2a, d, 2.3, and 2.7) or as an alternative through the superolateral SOF. This vessel is inconstant (frequency 40–50%) and anastomoses with the lacrimal artery (LA) branch of the ophthalmic artery (OA) as a common branching pattern (see Chap. 3).

An anatomical photo of the lateral orbital wall in medial view exhibits the regions of F S, F Z P, F P Z B, S Z S, O P Z, G W S, S O F, L W S, M C F, Z T S, Z A, M F, S T P, C O C H, I O C, and M A S. — **Fig. 2.19**

The vascular pattern of arteries entering or leaving the orbit through the COF or the SOF demonstrates great diversity [52, 54]. Variants apart from the above-mentioned meningo-lacrimal connection (MMA–LA) are a meningo-ophthalmic anastomosis (MMA–OA) for the collateral vascular supply of the orbit or there are recurrent vessel courses back from the orbit to the middle cranial fossa originating from the LA or OA to the MMA or through a network of small branches for the vascularization of the meninges and tentorium. Meningo-lacrimal and meningo-ophthalmic arteries may run in parallel with discrepancies in diameter.

An anomaly paramount to note is the origin of the OA from the MMA as the sole vascular source of the orbit (estimated frequency 1%), when the usual OA exit from the ICA is either absent or obliterated [53,54,55,56,58]. An unusually large-sized COF (diameter 3–4 mm) should be considered as a warning sign that a vessel of the same size might be the major or the sole blood supply to the orbit [59]. Without any doubt, the exact COF location is of relevance to prevent complications (hemorrhage/erroneous ligation of a terminal vessel connection/amaurosis) during subperiorbital dissection of the deep lateral orbital wall.

A plenty of morphometric studies have addressed the COF distances to the lateral or medial SOF end, the FZS, the superior orbital notch, and Whitnall’s orbital tubercle [53, 60]. The FZS appears as the most suitable landmark for surgical purposes and its distances to COF have been recorded in almost every of the existing studies showing values between 22 and 35 mm depending on the ethnicity of the sample.

Clinical Implications

Caveat:

If the provenance of a larger vessel passing across the COF or the narrow superolateral SOF has not been unequivocally clarified, one should refrain from performing a ligation.

The presence of a groove on the lateral wall of the orbit (Fig. 2.3) running between the SOF or COF and the IOF (frequency 30–70%) has been associated with the course of an anastomosis between the MMA and infraorbital blood vessels [2, 61]. This could not be confirmed later and the groove has been considered as an abrupt thinning of the bone [47].

Inferior Orbital Fissure (IOF)

The IOF prototype outline corresponds to a silhouette reminiscent of a cat-tongue chocolate or a double-ended spoon with a short intermediate handle inside the orbit. Though overall, it is a complex 3D opening of varying shapes [62] (Fig. 2.15a–c). The IOF separates the floor portion of the midorbit from the lateral orbital wall and provides passageways and portals for vessels, nerves, or fat pads to the pterygopalatine, infratemporal, and temporal fossa [63, 64]. The long IOF axis runs a posteromedial to anterolateral route starting at the maxillary strut and extending to the tip of a loop in between the upper lateral and lower anteromedial orbital surfaces of the zygoma. Occasionally, this part of the zygomatic-orbital flange is absent, so that the loop ends after the IOF margins formed by the maxilla and GWS have joined at an anterior junction. Posteromedially, the body of the sphenoid and the palatine bone contribute to the rear IOF sinkhole (IOFRS). The foramen rotundum opens into this sinkhole, which is continuous with the lower SOF along a floored level above the maxillary strut.

The narrowing in the center of the IOF originates from a crescent-shaped overhang (“isthmus promontory”) (IOFIP) of the orbital floor next to the surface of the palatine bone.

Morphometric data of the IOF are rather scarce. An often-cited study performed on dry skulls went for average IOF dimensions [62]. The IOF covered a mean area of 60 ± 40 mm² with a range of 2mm² to 232 mm². The longest distance from the posterior sinkhole to the anterior loop (= diagonal length) had a mean of 18.2 ± 4.9 mm (range 2.8–32.4 mm). The span between the isthmus promontory to the GWS margin (= narrowest borders) amounted to a mean of 1.9 mm ± 1.3 mm (range 0.2–6.6 mm). The width of the anterior IOF loop (= widest distance) was 5.7 ± 2.6 mm (range 1.5–17.4 mm). The range of values allowed a distinction into eight IOF types, which had different frequencies.

However, these findings need to be treated with caution because the measurements derived from photographs taken from an anterior view at an angle of 78° degrees to the horizontal. This angulation obstructs the view in particular into the rear IOF sinkhole due to the isthmus promontory sticking out in the foreground.

The obvious consequence is that the values do not reflect the true size and shape variants. A more realistic outcome might have been yielded from a perpendicular view onto the IOF from a basal sight onto to the IOF outline in the temporal fossa (Fig. 2.15c) or a look from inside the orbit either with an 90 °angulated endoscope or an axial cut of 3D reformatted CT scans.

It is of note that the term posterior or rear IOF sinkhole in this text replaces what has been previously labelled posterior basin of the IOF in related papers of our group [65, 66]. Originally, the designation basin of the inferior orbital fissure had been referred to a conceptual area of thick bone in the lateral orbit amenable to removal in decompression surgery [67]. Pertaining to this original meaning the IOF basin consists of a bony area corresponding to the lower part of the orbital flange of the zygoma including the posterior portion of lateral orbital rim at the same vertical level and reaching medially all the way to the zygomaticomaxillary suture or even over it into the lateral portion of the maxillary sinus.

Foramen Rotundum (FR)—Maxillary Strut (MS)

Unlike suggested by its name, the foramen rotundum is not a foramen but a short canal structure that penetrates the common base of the lateral pterygoid plate and the GWS (Figs. 2.2a, d, 2.3, 2.4b, 2.5b, and 2.7) in the upper portion of the PPF and near to the transitional region between the orbit and the cavernous sinus via the SOF. The medial canal border is formed by the lateral wall of the sphenoid sinus. The FR is the path of communication for the maxillary nerve (CN V2) between the middle cranial and the pterygopalatine fossa (PPF). The FR/canal is not easily accessible in intact dry skulls, so that quantitative measurements are all CT based on axial and coronal sections on patient series. The length of the canal has an average of 6.3 mm (range 2.1 mm–10.8 mm) with a mean diameter of 2 mm (0.8–4.4 mm) and an intercanal distance to the pterygoid canal (PC) of 2.6 mm (range 0–8.8 mm). These indicative measurements from a retrospective study on 50 patients [68] show disparities to similar studies [68,69,71], which are explained by difficulties to determine the anterior and posterior limits of the canal accurately due to angulated planes of the in- and outlets. The canal usually runs a course from behind in an anterolateral downward direction and can assume various types. These FR/canal types differ with regard to their placement in the sphenoid bone, at or within the sinus wall or as a conduit inside the sinus [72]. The vertical and transverse position of the FR/canal can be located above the base of the lateral pterygoid plate coinciding with the base of the lateral pterygoid plate (frequency 50%) or with a medial (47%) or lateral offset (3%) from the midline [72]. A rare but not negligible variant is a branching of a canal from the lateral wall of the foramen rotundum that opens into the orbit (length 5 mm, diameter 0.2–0.5 -1.0 mm). This canal is straight and directed slightly superolaterally and likely transmits the zygomatic nerve and/or a portion of the infraorbital nerve [73].

The maxillary strut (MS) relates to the bony bridge across the foramen rotundum (FR). An accessory small foramen going through the MS is rarely present and not to be confused with the FR [74].

Pterygopalatine Fossa (PPF)—Sphenopalatine Foramen (SPF)—Pterygoid Canal (Vidian) (VC)

By convention, the PPF is the larger space anteriorly bounded by the curving backside of the maxilla, posteriorly by the fused front of the pterygoid process plates together with the base of the sphenoid bone and anteromedially by the perpendicular plate of the palatine bone [12, 75].

The PPF (Fig. 2.5, 2.12, and 2.15) represents a central transit hub connecting within the facial skeleton [76] via the:

Pterygomaxillary fissure—to the infratemporal fossa (maxillary artery),
Foramen rotundum (FR)—to the MCF (CN V2),
Inferior orbital fissure (IOF)—to the posterior midorbit (CN V2),
Sphenopalatine foramen—to the posterior nasal cavity (sphenopalatine artery/posterior superior nasal nerve branches of pterygopalatine ganglion—PPG),
Pterygoid (Vidian) canal—to the MCF (greater and deep petrosal nerves),
Greater (“pterygopalatine”) (GPC) and lesser palatine canal—to the oral cavity (major and minor palatine neurovascular bundles).

From the lateral view, the PPF looks like a narrow space tapering inferiorly to form the greater palatine canal.

The lateral opening of the PPF, named the pterygomaxillary fissure (PMF), has a sharp-edged posterior border. This is formed by the lateral margin of the anterior surface of the base of the pterygoid process and inferiorly by the fused pterygoid plates. The anterior PMF border is more evasive owing to the convex contour of the posterior wall of the maxillary sinus [12]. A variety of PMF types and sizes has been reported [77].

The sphenopalatine foramen (SPF) connects the superior part of the PPF with the posterior nasal cavity. The SPF and its thin-walled constituents, the sphenopalatine notch (SPN), and the inferior margin of the sphenoid body are angulated in an anterolateral plane, so that the SPF is located anterior to the opening of the Vidian canal into the PPF [12]. The Vidian Canal (VC) traverses the base of the medial pterygoid plate from the anterior border of the foramen lacerum to the exit in the posterior PPF (Figs. 2.2a, b, 2.4b, 2.5b, 2.7, and 2.17b).

Analogous to the FR, three VC types can be distinguished as completely embedded in the sphenoid bone under the floor, through the floor or protruding into the sphenoid sinus and within the sinus [72, 78]. These conditions are undoubtedly correlated to the type and degree of SS pneumatization [79]. A myriad of studies is dealing with the morphometry of the VC, FR, and their surroundings (e.g., [71, 72, 77,78,79,80,81,82,83,87]) due to their utmost importance as a landmark in endoscopic skull base surgery. A metanalysis of this immense data pool is beyond the scope of this article.

Ethmoidal Foramina (EF)

The EF are the funnel-shaped openings of the ethmoid canals (EC). The EF are laid out in an anterior–posterior row alongside the frontoethmoidal suture (FES) line (Figs. 2.5a, 2.11, and 2.12). As a common finding, there are two foramina [88]. They may be supernummary up to a maximum of six that can appear unilaterally, bilaterally, or in side asymmetry [86,87,91]. The most anterior and the most posterior EF are addressed as the anterior EF (AEF) and the posterior EF (PEF). The topography of the holes not only varies in the sagittal direction but vertically, too, with an intra-sutural (frequency 85%) or extrasutural position, below or above the FES [92, 93].

Clinical Implications

Awareness of the proximity of the posterior most foramen (PEF) to the optic canal (OFC) is regarded as critically important in the prevention of accidental optic nerve lesions and amaurosis during dissection of the periorbital lining along the medial orbital wall.

Injuries of the ethmoid arteries may result in massive hemorrhage, retroorbital hematomas, and an orbital compartment syndrome.

Predictably deroofing ethmoidal canals (EC) at and in particular above the FES level carries increased risk for bleeding and accidental entry into the anterior cranial fossa and cribriform plate.

A guideline to prevent interference with the OFC and CN II during subperiorbital dissection of the medial orbital wall been summarized in “the rule of halves,” which indicates the relationships of the ALC to the AEF, the AEF to the PEF, and the PEF to the OFC in the brief formula: 24 mm–12 mm–6 mm [94] (see Chap. 3 - Periorbital Dissection, Fig. 3.43a, b). A vast number of morphometric studies have scrutinized the reliability of this mnemonic rule, not all of them fully consolidating it (e.g., [60, 92,93,97]). Differences in the morphometric parameters arose from gender and age [98]. Ethnicity was deemed as a major variable for the presence of supernumerary EF [89]. Orbits of Asian or African descent had greater EF numbers and a shorter length of the orbit with the consequence of an increased density of the EF in comparison with Caucasians [99].

Insofar it has been repeatedly emphasized that the ratio between the EF distances from anterior landmarks and the length of the medial orbital wall must be accounted for in a meaningful evaluation [88, 90]. The preference should be given to case-based individual measurements, because exceptions from the rule are to be expected [48, 49]. The ethmoidal canals (EC) or sulci (incomplete canals) provide connections between the orbit, the ethmoid nasal roof and the anterior cranial fossa.

They commence with funnel-shaped openings to pass the ethmoidal labyrinth and enter the olfactory groove on the top surface of the cribriform plate. The frontoethmoidal suture line (FES) marks the level of the ethmoidal roof. The cribriform plate, however, may lie up to 10-mm caudal to the FES. The EC run a diagonal course in anterior direction containing the homonymous neurovascular structures [100]. Their 3D topography and dimensions (diameter, length) leave ample room for variations. A mean of 8.2 mm (range 4–12 mm) was reported for the average length of the anterior ethmoid canal (AEC) and a mean of 7.6 mm (range 2 mm −13 mm) (mean 7.6 mm) for the posterior ethmoid canal (PEC) [101].

Infraorbital Foramen (IOFN)

The infraorbital canal opens with a same named foramen at the anterior facial wall (Figs. 2.1a, b, 2.14, 2.15c, 2.17a, b, and 2.20). The foramen is typically single and located 7–10 mm below the infraorbital margin. The canal follows coursing underneath the anterior orbital floor and turns into the infraorbital groove (sulcus) further backward, which runs below the floor surface to its end at the IOF rear sink. This transformation from a true groove into a canal occurs halfway along the course within the orbital floor. Otherwise, the IOC can be closed over the whole length, so that an IOG is absent or the groove is covered with a very thin, transparent osseous layer pretending a “pseudocanal” [102, 103]. An abundance of normative data on the morphological characteristics and dimensions of the IOG/IOC as well as the occurrence, number and location of accessory infraorbital foramina, and side channels has been produced over the last decade [100,101,102,103,104,105,106,107,108,109,114].

An anatomical photo of the foramina in the inferolateral orbit exhibits the spherical-shaped depression at the center with regions of F S S, C O F, G W S, and Z O F. The regions at the bottom portion denote Z F F and I O F N. — **Fig. 2.20**

Supraorbital and Frontal Foramina/Notches (SOFN/FOFN)

The supraorbital margin can embody supraorbital and frontal foramina and/or passages formed as incisurae (notches) (Figs. 2.1a, 2.4b, 2.6, 2.7, 2.8, 2.14, and 2.20) or it may have plane and even contours without breaks or perforations [115]. Foramina or notches either occur bilaterally, but also alternate between the two sides of an individual [116, 117]; they appear single or in clusters of multiple openings with overlapping assignment as SOFN or FOFN. SOFN have an oval shape, the horizontal diameter being larger than the vertical. Supraorbital notches are usually wider than supraorbital foramina.

The mean SOFN distances from to the facial midline, frontozygomatic suture, or temporal crest of frontal bone display a wide variation with regard to gender, age, and ethnicities [116, 114,115,120]. The frontal SOFN plane can be angulated at all three spatial levels. Supraorbital notches are usually completed by fascial bands encircling their floor. The fascial bands exhibit variation patterns—osteofibrotic partially containing bone and/or either divided by additional horizontal or vertical septations [121]. The vertical height position of SOFN differs—notches are of course contiguous with the margin; foramina are located superior to the rim, sometimes in high positions up to 19 mm above [115].

Clinical Implications

The course and depth of the bony canal subsequent to such high positioned supraorbital foramina is of interest in surgical approaches to the superior orbital rim and orbital roof requiring osteotomies to release and mobilize the supraorbital nerve. The course of the canal in terms of a horizontal or steep inclination toward the end in the orbital roof in conjunction with the thickness of the rim determines the anteroposterior length (depth). The depth was reported accordingly to range between 2 mm and 12 mm [122].

While SOFN are a constant finding FOFN are facultative at varying low frequencies [123]. The average distance between the medial FOFN edge from the lateral SOFN edge ranges between 2 and 15 mm [117]. Either foramina or notches may co-occur bilaterally or differ at the two sides. The FOFN number, size, and shapes of apertures are most diverse.

Zygomaticoorbital (ZOF), Zygomaticofacial (ZFF), and Zygomaticotemporal (ZTF) Foramina

The openings and subsequent intrabony courses of the ZOF, ZFF, and ZTF (Fig. 2.15a–c) follow various patterns [22]. As the name indicates, ZOF refers to single or multiple (≤ 6 in skulls of African American descent, [124]) openings in the inner surface of the anterior inferolateral orbital quadrant of the orbit (Figs. 2.16b, and 2.20). Each ZOF represents the entrance to a separate or an interconnected canal which exits at the facial (ZFF) and/or temporal (ZTF) zygomatic surface. Hence, there can be an array of independent canals (ZOF– ZFF and ZOF–ZTF) only as well as a principal interconnected canal system with Y-kind divisions/subdivisions standing either alone or with additional independent connections [22]. The ZFF was mapped in an investigation of 429 (i.e., 858 zygomas) adult skulls [125] from nine ethnic groups from geographies around the world and both genders. An across-line laser module was used to generate two reproducible reference lines superimposed onto each zygoma and measure the distances to the ZFF. The ZFF occurrence per zygoma varied from no identifiable foramen (frequency 16.3%) to one foramen (49.8%) over two (29%) and three (3.4%) to a maximum of four (1.4%) foramina. The incidence and ZFF sites differed considerably among the ethnicities but were consistent between female and male subjects. The distance of the ZFF from the infraorbital margin, however, was larger in male (mean 7.1 mm) than in female (6.2 mm) subjects.

The ZFF sites were related to the intersection of a horizontal line going through the deepest point of the infraorbital rim in parallel with the Frankfurt plane and a perpendicular vertical line going through the posterior end of the zygomaticofrontal suture (ZFS). The majority of ZFF were concentrated to a field in the upper and lower quadrants of the intersection with outliers in the posterior (upper > lower) quadrants (total approximate size 30 mm x 25 mm). If the ZFF sites were reevaluated within the limits of a circle of 25 mm in diameter and its center 5 mm anterior to the intersection of the reference lines, a percentage of 93% ZFF on the right zygoma and 94% ZFF on the left were enclosed in this risk spot area irrespective of their ethnicity. A surgical safe zone was then defined beyond the boundaries of this circle and delineated clinically using surface landmarks.

The position and size of the intraorbital (“entry”) opening (= ZFFin – equal to the ZFO) and the malar exit of the ZFF (= ZFFout) were the focus of a morphometric study in n = 10 fresh-frozen PMHS Caucasian heads [126]. The ZFFin number (total n = 23) per side side ranged from 0 to 3: none on 2 sides (frequency 10%), 1 on 14 sides (70%), 2 on 3 sides (15%), and 3 on 1 side (5%). The mean ZFFin diameter was 1.1 ± 0.5 mm. The ZFFin were located 5.1 ± 2.0 mm superior to the inferior margin of the orbit and 4.3 ± 1.6 mm medial to the lateral margin of the orbit. The ZFFout number (total n = 22) per side ranged from 0 to 2: none on 2 sides (10%), 1 on 12 sides (70%), and 2 on 4 sides (20%). The ZFFout were located 1.2 ± 2.9 mm inferior to the inferior margin of the orbit and 1.1 ± 3.0 mm lateral to the lateral margin of the orbit. With 1.4 ± 0.6 mm diameter, the ZFFout was larger than ZFFin. Surprisingly, the length as well as the continuity or reticulum of the canal(s) in between the foramina was not dealt with in detail as opposed to what the study title suggests.

Clinical Implications

A clear understanding of the zygomatic foramina, their prevalence, and variations as well as their nerve crossings is essential for a number of surgical procedures [127]:

Localization and guidance to the inferior orbital fissure (IOF)—[128].
Dissection and manipulation along the lateral orbital wall.
Osteosynthesis/plate and screw placement in orbital/zygoma/midface trauma.
Orbital decompression.
Design and bone cuts of orbito-zygomatic osteotomies [129, 130].
Advancement or on-lay augmentation of the inferolateral orbital rim.
Transmaxillary approaches to the orbit.
Subperiosteal face lift.
Retrobulbar anesthesia [131].

Nasolacrimal Fossa (NLF)/ Groove (NLG)/ Canal (NLC)

The proper nasolacrimal fossa (NLF) is strictly circumscribed and demarcated by the anterior (ALC) and the posterior lacrimal crest (PLC) (Fig. 2.21a). The lateral lower edge of the fossa pointing outward into the anteromedial orbital floor is termed the lacrimal notch (LN). The lacrimo-maxillary suture (LMS) runs in parallel in between the ALC and the PLC and corresponds to the union between the nasofrontal process of the maxilla (NFP) and the lacrimal bone [132]. The LMS corresponds to the so-called “maxillary line” (ML—[133]), a curvilinear (mucosal) eminence along the inside of the lateral nasal wall that runs from the anterior attachment of the middle turbinate to the anterior end (lacrimal process) of the inferior turbinate (Fig. 2.21b). One of the more detailed NLF descriptions based on endoscopic findings and CT scans [134] is too elaborate to be reproduced here in all detail. A clear distinction between a nasolacrimal fossa and a nasolacrimal groove (NLG) has never been thoroughly defined and the designations sometimes appear to be used synonymously. One description in terms of a vertically oriented groove addresses the anterior edge of the lacrimal bone depression as the lacrimal margin (LM). This LM is suggested to extend inferiorly to an end point at the conchal crest—a horizontal ridge which is projecting from the lateral sidewall to give an attachment for the inferior turbinate. The bony walls below the notch down to the level of this crista are then referred to as the lacrimal groove [135], what obviously does not fall short of the whole length of the nasolacrimal canal (NLC).

2 anatomical photos of the nasofrontal maxillary process with the lacrimal bone in the superolateral and medial views. Some of its labeled regions include L M S, A L C, L N, O M S, I O G, Z P M, hiatus maxillaris, O S M, and N P P M. Arrows point to the lacrimal margin and N L F in photo B. — **Fig. 2.21**

So, the NLC apparently encompasses the entity of NLF, NLG, and the bony tract underneath the inferior turbinate down to the terminal aperture (“lacrimal ostium”) in the lateral wall of the inferior nasal meatus. In fact, there is no consented definition on the NLC vertical extent concerning its exact upper and lower ends. The morphometric measurements as well as the morphologic features vary at all NLC levels.

The NLF from ALC to PLC in a Caucasian population [132] consisting of n = 47 orbits from n = 24 PMHS (postmortem human subjects) had a mean width of 8.8 mm (range 6–12 mm). The average distance of the LMS was 4.3 mm from the ALC and 4.5 mm from the PLC in 25% of the orbits under investigation, what means they were almost equidistant from the mid-vertical line. In 42.5% of the orbits, the LMS deviated anteriorly to the ALC and in 32.5% the LMC was located posteriorly toward the PLC. These deviations exceeded 1 standard deviation in 34%. An LMS located closer to the PLC implies that the proportions within the NLF are shifted to the thicker frontonasal maxillary bone, which is an unfavorable condition for dacryocystorhinostomies. In populations with East Asian descent, the NLF is formed predominantly by a thick maxillary bone layer [136]. As an extremely rare finding case, the entire NLF can be formed by the maxillary bone [137]. On the other hand, the anterior ethmoid air cells may extend beyond the PLC producing unusual thinness.

The anteroposterior diameters of the nasolacrimal canal entrance (i.e., NLF) ranked between 5.7 and 7.2 mm and the corresponding transverse diameters between 4.7 and 6.1 mm [138, 139]. The NLG in the configuration outlined above [135] had an average vertical length of 9.6 ± 2.1 mm. It displayed various morphological patterns (S-, boat-, hourglass-, cylinder-, barrel-, or funnel-shapes) ensuing in a different width according to the vertical height level with an average of 5.88 ± 1.53 mm at the upper one-third, 8.04 ± 2.05 mm at the middle one-third, and 5.94 ± 1.28 mm at lower one-third.

The morphometric parameters of the overall NLC have recently been reevaluated in a CBCT investigation (patients n = 100, age range 18–83 years with a mean of 42 years) including a summary of several preexisting studies [140].

The NLC diameters were assessed at the level of the infraorbital margin. Not surprisingly both the anteroposterior diameter with a mean of 6.6 ± 1.53 mm as well as the transverse diameter with a mean of 4.3 ± 1.0 mm had a similar magnitude as the former NLF measurements. The corresponding NLC sectional area averaged to 7.39 ± 3.29 mm² and the angle between lines through the length of the NLC and in parallel to the nasal floor was 73.5 ± 6.8°. This angulation was influenced by gender (female > male) and age. All the other parameters did not show these dependencies. The length of the NLC remained unconsidered.

The narrowest part along the length of the NLC was identified at the entrance to the canal in a small series of PMSH dissections [139].

A multiplanar CT study is particularly noteworthy in so far as it provides accurate 3 D information on the NLC at six height levels along the vertical axis [141]. The major and minor diameters of the elliptical canals, the cross-sectional areas, and volume were assessed and correlated with gender, a younger (mean age 25 years) versus an elderly (mean age 60 years) age group and Black/White American cohorts (total n = 72 individuals). The NLC length was longer in men (12.3 mm) when compared with women (10.8 mm), just as the NLC volume was greater in men (327 mm³) than in women (244 mm³). Neither minimum canal diameters (mean 3.5 ± 0.8 mm) nor minimum cross-sectional areas (mean 11.7 ± 2.5 mm²) demonstrated significant differences depending on gender, ethnicity, or age group. In the elderly age group, a trend was noticeable for greater canal cross-sectional diameters, possibly indicating an expansion of the canal aperture with aging. The diameters measured at two specific positions of the six height locations of the canal showed differences in elderly in comparison with younger patients. In the elders, the major axis diameters were increased at the superior NLC (apex) end with no alteration of the minor axis. Next to the inferior NLC (base) end, the minor axis diameters were larger, while the major axis kept constant. From a geometric aspect, the cross section of the upper canal became progressively more elongated and elliptical with aging, whereas the lower canal was getting circular. Moreover, the cross-sectional area at the canal base was greater in the people of color cohort than in Caucasians. No gender differences in the diameter or of the cross-sectional area along the canal length were noted.

Another recent CT study on the NLC using semi-automated segmentation techniques [142] refined the metric results above and added a classification into 5 morphological variants which is reminiscent to the NLG morphology patterns: A—cylindrical type with consistent diameter from top to bottom; B—lower-thicker type, wider at bottom than top; C—upper-thicker type, wider at top than bottom; D—spindle type, with extended middle portion; and E—hourglass type, with a particularly reduced middle portion.

The NLC lumen, basically modeled as a cylindrical bony pipe, is lined by the mucosal tissue layers of the nasolacrimal duct (NLD) which additionally modify the internal constrictions and openings of the lacrimal drainage system.

Clinical Implications

Average (normal) morphometric data of the NLF, NLG, and NLC are a prerequisite to identify mechanical risk factors in the etiology of primary acquired nasolacrimal duct obstruction (PANDO).

Paranasal Sinuses

This brief account has the intent to give a cursory summary about the juxtaposition of paranasal sinuses and the osseous structures of the orbit. The paranasal air sinuses (PAS) are paired mucosa-lined, aerated bony cavities, adjoining the orbits at four sides—the roof, the medial wall, the floor, and the apex.

The frontal sinus is located above the orbital roof, the ethmoid and sphenoid sinuses are adjacent to the medial wall and the apex, while the roof of the maxillary sinus is identical with the floor of the orbit. The paranasal sinuses communicate with the nasal cavity through small apertures or orifices. The anatomy of the paranasal sinuses is most complex with endless numbers of variations in size, shape, and symmetry as individual as fingerprints, so that exceptions are more common than rules [143]. The frontal sinus (FPS) may overlie large areas over the orbital roof and extend vertically far up into the squamous part frontal bone. The frontal sinus opens through a superior funnel medially into an inferior funnel and the nasofrontal duct that passes the anterior ethmoid and drains into the frontal recess of the middle nasal meatus.

The thin lateral wall of the ethmoid sinus (EPS) or the lamina papyracea (LP) is the major constituent of the medial orbital wall. The orbital roof connects with the ethmoid roof along the fronto-ethmoid suture line (FES) at the upper LP (Fig. 2.12). Alongside the maxillary-ethmoidal suture line, the lower LP border including the inferior labyrinthine cells joins with the transverse bone plate built by the orbital floor in coincidence with the maxillary sinus roof. A Haller cell, an additional basal ethmoid cell, may be interposed between the maxillary sinus and the lamina papyracea at their transition. The Haller cell may extend into the superomedial maxillary recess in medial direction allowing for a transmaxillary access to the ethmoid without the risk of entry into the orbit. Sometimes, the anterior ethmoid sinus cell group continues into the lacrimal bone or even underneath the frontonasal process of the maxilla. The anterior ethmoid cell group drains into the middle nasal meatus by way of the infundibulum. The posterior cell group drains into the superior meatus. The sphenoid sinus (SPS) commonly aerates the sphenoid body [144]. The two-sided sinuses are usually separated and do not communicate with each other. The pneumatization can go far beyond the confines of the body into OS, ACP, GWS, and clivus. Large-size SPS in particular are characterized by bony imprinting and embossments. The lateral SPS wall, which is the partition to the orbital apex, is bulged inwards by the bony prominences of the ICA and CN II with the opticocarotid recess in between. Posteriorly, the lateral SPS wall relates to the cavernous sinus. The SPS roof is part of the anterior cranial fossa and provides the outlines of the sella turcica with a protruding pituitary gland fossa. The SPS floor may be indented by the passage belonging to the foramen rotundum (FR) and to the maxillary nerve (CN V2) as well as by the pterygoid/Vidian canal (VC). Each SPS drains through an ostium of its own on its anterior wall into the sphenoethmoidal recess. The maxillary sinus (MPS) is the largest paranasal sinus (Fig. 2.22). It occupies the body of the maxilla. It is uncommon that the zygoma is pneumatized by the MPS though the MPS floor often reaches down to the level of the alveolar process base and may invade into toothless rim portions. Canine, premolar, and molar tooth roots may protrude into the MPS floor. The MPS roof or orbital floor is engraved by the IOG and/or tubuled by the IOC that forms a crease and is suspended in a mesentery-like bony plication on its way to the infraorbital foramen. The MPS medial wall forms the lateral wall of the nose. The MPS is draining over the medial wall into the semilunar hiatus of the middle nasal meatus.

An anatomical photo of the axial section of the maxillary sinus exhibits the tube inserted below the periorbital region. The perforators connecting intraperiorbital vessels on the right are indicated with arrowheads. — **Fig. 2.22**

Clinical Implications

Antrum Implosion or Silent sinus syndrome is a spontaneous, progressive disorder due to asymptomatic chronic maxillary sinusitis resulting in atelectasis of the maxillary sinus and in bony demineralization with long-term displacement of the orbital floor (see Chap. 8.)

Typical signs are facial asymmetry, enophthalmos, hypoglobus, and recession of the upper orbito-palpebral eyelid junction with unilateral manifestation in middle aged-patients between 30 and 40 years. The diagnosis is suspected clinically. CT imaging is pathognomonic and will confirm the underlying obstruction of the osteomeatal outlet and opacification/chronic mucosal inflammation of the maxillary sinus, leading to hypoventilation and negative sinus pressure. Antrostomy and reconstruction of the orbital floor using titanium implants are established treatment options [146].

The signs of a silent sinus syndrome may occur months after non-displaced orbital floor fractures [147], or even after repair of such fractures [148]. Posttraumatic is indistinguishable from spontaneous silent sinus and is presumably attributable to the same pathophysiology—subatmospheric pressure and antrum implosion. Secondary enophthalmos after repair of orbital floor fractures with resorbable implants should consider maxillary atelectasis and its etiology as a potential confounding factor besides the potential inadequacy of the implant material.

Lateral Wall of the Nose

The lateral nasal wall can be broken down into an arrangement of three or four turbinates in association with three meatus and the PAS outlets (Fig. 2.23a–c). The turbinates and meatus all run anteroposteriorly and direct the air flow. A turbinate or concha corresponds to a scroll of bone projecting from the lateral nasal wall (Fig. 2.23a, b). The turbinates are named in ascending sequence as inferior, middle, superior, and supreme turbinate. The meatus are the passages beneath and lateral to the turbinates and are named as the turbinates passing along their superior aspect.

3 anatomical photos. A. A skull split along the midsagittal plane exhibits the regions of S N T, M N T, F S, and A G. B. The nasal lateral wall with labeled regions includes S S, S P F, H M S, U C P, I N T, H P P B, and ANS. C. It indicates the regions of N A, I O F N, B E, and U C P. 3 anatomical photo of the anterolateral view with labels f s, s n t, a g, m n t, u c p, i n t, and p p m, in part A. S s, s p f, m n t, m p p, p n s, h p p b, i n t, p p m, a n s, in b, and i o f n, n a, b e, and u c p, in part C, — **Fig. 2.23**

The inferior meatus is the space above the nasal floor and below the inferior turbinate. The inferior nasal turbinate (INT) is an independent spongy bone converging to a pointed posterior end. Anteriorly, the INT joins with the conchal crest, an oblique ridge inside the frontonasal process of the maxilla. The medial convex INT surface is numerously perforated and finely grooved for the harboring of a pronounced vascular network.

The concave lateral INT surface features three processes along its upper border. The lacrimal process projects anterosuperiorly, articulates with the lacrimal bone and assists in the formation of the lower portion of the nasal lacrimal canal (NLC). The NLC exit is found in the anterior portion of the lateral wall of the inferior meatus. The ethmoidal process ascends from the midportion of the superior INT border to join the uncinate process of the ethmoid. The thin lamina of the maxillary process turns inferolaterally and forms part of the medial wall of the maxillary sinus.

The middle, superior, and supreme nasal turbinates are components of the ethmoid.

The middle nasal turbinate (MNT) attaches to the lateral edge of the cribriform plate as superior part of its supporting overall basal (ground) lamella. The sphenopalatine foramen is located within the reach of the posterior end of the MNT.

The superior nasal turbinate (SNT) is approximately half as long as the MNT; it also extends into the ethmoid roof and attaches to the posterior part of the cribriform plate.

The supreme nasal turbinate is an uppermost rudimentary concha which occurs unilaterally or bilaterally in about 60% of individuals. The PAS openings into the meatus have been described in the previous paragraph. If a supreme turbinate and meatus are present, the posterior ethmoid cell group may open there. The secondary features of the lateral nasal wall are revealed at best if the INT and MNT are removed. These include the agger nasi, the uncinate process, the ethmoid bulla, and the ethmoid infundibulum including the hiatus semilunaris. The agger nasi (AN) is a mound like prominence directly in front of the conchal crest and the MNT. The AN represents a vestige of the middle naso-turbinal. It is frequently aerated with an opening into the anterior middle meatus and into the ethmoid infundibulum. On the posterolateral side, the agger nasi may be fusing with the lacrimal bone and/or the medial orbital wall.

The hooked uncinate process (UCP) arises from the anterior ethmoid lateral to the anterior MNT attachment (Fig. 2.23b, c). The UCP is directed posteroinferiorly and projects at variable length across the wide medial ostium of the maxillary sinus. The MNT usually conceals the UCP. The ethmoid bulla refers to a single or more large ethmoidal cells that protrude from the anterolateral wall of the MNT. The space configured between the anterior inferior convex border of the bulla and the superior free edge of the uncinate process is the hiatus semilunaris. This hiatus is a two-dimensional opening leading into the ethmoidal infundibulum. The ethmoid infundibulum is a three-dimensional space extending downward and posteriorly between the lateral nasal wall and the uncinate process medially. The posterosuperior boundary is the ethmoid bulla. The lateral side is consisting of the lamina papyracea and the frontonasal process, rarely of the lacrimal bone, too. Depending on the superior attachment of the uncinate process either laterally to the lamina papyracea or medially to the ethmoid roof, the infundibulum is closed as a terminal recess or continuous with the frontal recess.

Internal Orbital Buttresses

A set of three buttresses running in parallel stabilize the orbital floor in sagittal direction (Fig. 2.24a, b), the inferomedial orbital strut (IOS) along the maxilloethmoidal suture line. [20, 149, 150], the intermediary bony underpinning of the infraorbital groove/canal, and the reinforcement along the medial IOF margin laterally on par with the lateral floor strut (LFS). The involvement and fragmentation of these buttresses are an indicator for the severity of the trauma. The sagittal buttresses integrate deliberately into the overall framework of facial buttresses (Fig. 2.24c).

Orbital Dimensions, Volume, Surface Contours, 3D Globe Position—Interrelationship

Numerous factors determine the 3D position of the ocular globe inside the orbit [151, 152], its anterior projection, and radial relationships to the orbital rims; these are the overall dimension and volume as well as the geometry (width and angulation of the medial and lateral walls) and surface contours of the internal orbit, such as slopes, angles, planes, concavities, and convexities—in particular the so-called posterior medial bulge within the inferomedial wall transition [146,147,155].

The overall dimensions and volumes of the orbit show great variations owing to age, gender, and ethnicity [149,150,224,228,160]. The adult orbital aditus may show some typical average values approximating to 4 cm horizontal × 3.5 cm vertical in size with wide ranging deviations (Fig. 2.25). The anteroposterior depth is 4.5–5 cm approximately and the overall volume of the orbital cavity measures up to 30 cm³ including the ocular globe, a sphere of 22–27 mm in diameter, up to 7.5 cm³ in volume [161], and 69–85 mm in equatorial circumference.

5 anatomical photos. A. 3 photos of the orbital aditus exhibit the frontal and nasal bones and greater wings of the sphenoid bone with variations in shape and size. B. The dimensions of the orbital cavity denote posteromedial bulges on the wall surface with openings at the center. — **Fig. 2.25**

The sagittal projection of the globe (corneal apex plane and/or equator) is referenced relative to the sagittal projection of the orbital rims, traditionally to the retruded lateral orbital rim only (exophthalmometry), more recently using 360° polar plots (CT morphometry) to measure distances to preassigned points all over the outer rim periphery [162]. Another CT method is the 3D characterization of the globe position in the orbit in relation to the center of the globe [163].

The globe projection in the coronal plane or the vertical/horizontal globe position is correlated to the delineation, shape, and curvatures of the orbital openings and the interorbital distance. The shape variations of the orbital aditus (Fig. 2.25a) have been classified into manifold categories, e.g., oval, rectangular, rhomboid, trapezoid, and subdivisions with a greater percentage of symmetry types in females (29.1%) than in males (23.81%) in a dry skull (n = 184) study [164].

For all aditus types, consistently the inclination of the medial part of the infraorbital rim (MOMLP) is parallel to the angle of the orbital floor while the lateral part (IPZ) and the adjacent floor both run in a horizontal plane.

The objective of linear morphometric measurements studies of the orbital skeleton is to provide guidelines for safe distances in periorbital dissection along each orbital wall [165] before the danger zone, viz. the orbital apex, is entered. Often quoted but just as often discredited mean maximum (standard) distances for adults (n = 24 dry skulls from India) on the orbital floor were 25 mm from the infraorbital foramen, in the orbital roof 30 mm from the supraorbital notch, in the lateral wall 25 mm from the frontozygomatic suture and in the medial wall (to the PEF) 30 mm from the anterior lacrimal crest [94]. Over time by the investigation of small series of dry skulls [166, 167] or PMHS specimens [168] and ultimately with the application of modern imaging techniques (CT—[169, 170], CBCT–[165]; MRI—[171, 172]) an extensive set of the reference points/distance lines/orbital cavity length/floor length and width, etc., was accumulated to delineate the spatial arrangement of the internal orbit and systematically assess its metrics.

Since orbital wall fractures typically involve the inferior (floor) and medial orbital wall their natural surface profile was thoroughly scrutinized and on the basis of CT scans translated into a data matrix to recognize the subtle slopes, depressions, and curvatures [173] enabling a true to original bony reconstruction [174]. In descriptive topographical terms, the lowest lateral point of the orbital floor is located next to the anterior IOF loop. The lowest portion of the orbital floor coincides with a short-pathed, circumferentially running concavity just behind the orbital rims. Originating from the periorbital dissection sequence this widening is named as “postentry zone” (Fig. 2.12 Detail). The orbital floor steadily ascends from the bottom of the postentry zone until it reaches the convex top site of the orbital plate of the palatine bone (OPPB—posterior ledge, Fig. 2.13a–e).

This curved process drops down abruptly backward along the infratemporal surface of the maxilla into the posterior IOF sinkhole (Fig .2.12 and 2.26a). In a paramedian sagittal plane, the orbital floor from the deepest point to the OPPB assumes an undulating shape, which has been addressed as the “Lazy-S configuration”.

An anatomical and 2 intra-operative photos of the orbit exhibit the regions of the posterior medial bulge, indicated by arrowheads in C. D. An illustration of the orbital floors is labeled with the inferior orbital fissure, posterior medial bulge, I O F isthmus promontory, and infraorbital groove. An anatomical photo in part A, is labeled with f o f n, o f c, a e f, e b a, p m b, o s, z t f, etcetera on the right orbit. Photos in b and c reveal the inner triangular flesh of the dissected area. Illustration in d, reveals inferior orbital fissure, palatine bone, infraorbital groove, and so on. — **Fig. 2.26**

In the posterior transition of the orbital floor into the medial orbital wall, the bony surface profile is protruded by a focal convexity, i.e. the posterior medial bulge (PMB) (Figs. 2.25b and 2.26a–d). The PMB has reached the status of an undeletable engram (Fig. 2.1f). The PMB is considered the most essential support area to maintain the vertical globe height and anterior globe projection. It is referred to as “Beat Hammer’s key area,” because this author [152] has emphasized the tremendous importance for the appropriate surgical reconstruction of the internal orbit time and again. The PMB has become some sort of trademark characteristic which is perpetuated in the design of commercially available anatomically preformed orbital titanium meshes/plates [155] and 3D models or molds [175]. The design information for 3D modeling of the inferomedial orbital circumference and the subsequent design of preformed orbital floor/medial orbital wall implants is gathered from segmentation or meanwhile from statistical shape analysis of CT data in unaffected orbits [155, 173, [165,166,186,167,168,169,170,171,184].

The graphic visualization of the variance models in all their details and beauty of their “heat maps” is fascinating but it should be borne in mind that the shape is reductionistic in pursuit of the target to come up with a few plate types which fit to a maximum of individual surface profiles. To that end, the shape requires technical modifications; for instance, undercuts are filled, openings, and depressions sealed and outliers eliminated. Therefore, it seems nothing more than a logical consequence for estimating the versatility and in particular the range of application of a preformed orbital mesh/plate that the manipulation specifics of the underlying statistical shape model are known to the end user in addition to the size and stratification of the population and the datasets according to laterality, gender, age, and ethnic group. At best, the statistical shape model as well as the STL files of the preformed meshes should be handy during preoperative virtual planning of fracture repair. Gross incongruency between the virtual (statistical shape) model and the patient’s intact orbital wall anatomy would obviate the implant positioning from the outset.

The PMB, the sagittal “Lazy-S,” the width of the orbital cavity between the lateral and medial wall and the inferior orbital rim are consistently reflected in the CT analysis studies though the difficulties to procure accurate and reproducible anatomic boundaries of the PMB are noteworthy. The PBM is still regarded a “fuzzy” characteristic of the orbital surface profile, since the distinct contour outlines are difficult to capture in the automated shape analysis [155].

The volume of the orbital cavity is a superordinate parameter having direct impact on the orbital contents and on changes in ocular globe position. Thus, quantitative orbital volume measurements (i.e., volume orbitometry) are crucial in the diagnostic workup and date back more than 3 decades [185]. The methods for orbital volume measurements are still striving to reach perfection, nowadays enhanced by computer-assisted programs and automatization [27, 157, 171, 174,175,176,177,178,191]. The orbital volume and morphology is correlated to anthropometric measurements (facial height and width, interorbital distance, etc.) [192]. On the other hand, the size of the adult orbital volume is associated with eyeball volume and with the grey matter volume of the visual cortex in a linear relationship [193].

The orbital volume has been shown to range between 16 cm³ and 30 cm³.

In a study characterizing the orbital volume in a normal population [194] from Chile (n = 398 orbits), the mean total orbital volume was 24.5 ± 3.1 cm³ (range 16.9–35.0 cm³). There were no differences in laterality but gender differences (male > female) and a steady orbital volume increase beyond the age of 30 years. Interestingly enough, if the orbits were divided into three zones in nearly the same way as in our proposition (Fig. 2.18a, b), the anterior zone had a mean volume of 17.30 ± 2.6 cm³ (range 9.4–25.6 cm³), which represented 70.7% of the total orbital volume.

The volume of the central zone (comparable to the midorbit) was 5.4 ± 1.8 cm³ (range 2.2–12.3 cm³) accounting for 22.1% of the total orbital volume. The posterior zone volume was 5.4 ± 1.8 cm³ (range 2.2–12.3 cm³) corresponding to a percentage of 8.2% of the total orbital volume. In line with previous correlations between fracture location, viz. volume increase in different regions of the orbital cavity [195], it was hypothesized that a volume increase in the anterior orbital zone has a negligible effect on the globe position. However, a volume increase in the central zone, which includes Beat Hammer’s key area, was held responsible for the occurrence of hypoglobus. A volume increase in the posterior area was implicated to generate enophthalmos, eventually.

Intraindividual volume differences in the orbit to a percentage of 7–8% were considered to represent a normal range [196] for a long period.

A recent CT study (n = 93 subjects) has shown that the intraindividual difference of orbital volumes amounts to approximately 2% [154]. A dry skull study evaluating 242 orbits using alginate impressions and their water displacement demonstrated an average orbital volume of 26.75 ml for the right side and 26.65 ml for the left side [197]. The mean volume difference was indicated with 0.8 ml (range 0.02 to 3.64 ml) corresponding to around 3%. Despite numerous investigations into average values [160, 173, 177, 198], age-related changes of the surface profile of the internal orbit (orbital floor and the medial orbital wall) have eluded quantitative assessment so far. From a qualitative perspective, the inferior orbital rim contours are resorbed enlarging the height of the aditus orbitae [187,188,201], while the attaching orbital floor flattens toward its posterior end, so that its lowermost portion tends to move backward [153] (Fig. 2.27).

A 3-D framework of the midface exhibits the orbital floor, which flattens toward the posterior end, and the dissection of its skeletal regions is highlighted in colors. — **Fig. 2.27**

Summary/Conclusion

To acquire macroscopic anatomical knowledge by traditional PMHS dissections is often purported as no longer appropriate in today’s medical student education, because of competition with unprecedented modern learning opportunities and information overflow (conventional textbooks and atlases, videos, interactive audiovisual programs, digital apps, e-mail newsletters, journals, Wikipedia, online or presence academic lectures, plastinated specimens, etc.). Trainees as well as professionals from all surgical specialties return into anatomical dissection rooms, however, with the aim to enhance their practice and to directly experience the visual and haptic properties of soft embalmed or even better frozen-thawed tissues [203]. CMF specialists involved in orbital surgery make no exception to optimize their expertise and skills or to perform preclinical tests of novel techniques for access, navigation, repair, reconstruction, and even allotransplantation (e.g., [53, 64, 102, 191,192,193,194,208]).

This article is intended to go ahead and support any kind of learning ambitions with a reappraisal of common textbook knowledge of the bony orbital precincts, systematic, and topographical connotations [see Basic Literature] and an introduction into the enormous morphologic and morphometric variability of the orbital openings and passageways as described in recent publications.

Textbooks and atlases as well as digital apps often draw a picture representing the average or abstraction of limited samples.

Enormously large collections of dry skulls have been evaluated to get detailed insights into the entire spectrum of normal variations of the bony architecture depending on ethnicity, age, gender, body height and somatotype. Such demographic criteria may remain inhomogeneous or incomplete leading to controversial results opposite to the needs of a health care with regard to a rapidly increasing globalization. The advent of modern imaging techniques with a limitless morphologic data production (CT, CBCT, MRI, ultrasound) as well as internal views of the body by endoscopes necessitated to acquire a more refined interpretation and understanding of the anatomy with paralleling computer and/or artificial intelligence assisted post-processing (segmentation- [209]) into multiplanar views (Fig. 2.27), internal surface, 3D formats (Fig. 2.25b), and volume calculations). Indeed, the contemporary literature is replete with publications on morphological and morphometric findings of the bony orbit, going into previously inconceivable subdivisions (e.g., [173, 210]) and sub-millimeter differentiation. As outlined above, the quality of any such studies differs substantially and it is not clear what will remain actively referenced in the future. For sure, there will be no standstill until a “big data/evidence-based anatomy” [211, 212] with consecutive translation into clinical settings allowing for interactive man/machine reading of findings and images will be established.

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Acknowledgments

Great thanks go to Gerhard Poetzel and Rudolf Herzig, the photographers at the OMFS Departments, LMU Munich, for their excellent support.

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The authors declare no conflict of interest in relation to the content of this article. Dissections of anatomic specimen were conducted in compliance with state and local laws.

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Department of Oral and Maxillofacial Surgery, Facial Plastic Surgery, Ludwig Maximilians University, Munich, Bavaria, Germany
Carl-Peter Cornelius
Department of Oral and Maxillofacial Surgery, Amphia Hospital Breda, Breda, The Netherlands
Peter J. J. Gooris
Amsterdam University Medical Centers, Amsterdam, The Netherlands
Peter J. J. Gooris
Department of Oral and Maxillofacial Surgery, University of Washington, Seattle, WA, USA
Peter J. J. Gooris

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Carl-Peter Cornelius
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Peter J. J. Gooris
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Correspondence to Carl-Peter Cornelius .

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Affiliate Professor, Department of Oral and Maxillofacial Surgery, Amphia Hospital Breda, Breda, The Netherlands
Peter J.J. Gooris
Professor Emeritus, Department of Ophthalmology, Amsterdam University Medical Centers, Location AMC, Amsterdam, The Netherlands
Maarten P. Mourits
Professor, Department of Oral and Maxillofacial Surgery, Amphia Hospital Breda, Breda, The Netherlands
J.Eelco Bergsma

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Cornelius, CP., Gooris, P.J.J. (2023). Anatomy of the Orbit: Overall Skeletal and Topographical Configuration. In: Gooris, P.J., Mourits, M.P., Bergsma, J. (eds) Surgery in and around the Orbit. Springer, Cham. https://doi.org/10.1007/978-3-031-40697-3_2

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DOI: https://doi.org/10.1007/978-3-031-40697-3_2
Published: 29 December 2023
Publisher Name: Springer, Cham
Print ISBN: 978-3-031-40696-6
Online ISBN: 978-3-031-40697-3
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Anatomy of the Orbit: Overall Skeletal and Topographical Configuration

Abstract

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Keywords

Introduction

Method

I. Osteology—Bony Building Blocks

Overall Skeletal and Topographical Configuration

Sphenoid Bone—Constituent of the Upper, Medial, and Lateral Orbital Wall

Bony Orbital Apex—A Three-Walled/Tetrahedron Spire

Frontal Bone—Orbital Roof Constituents

Ethmoid, Lacrimal Bone and Frontonasal Maxillary Process—Constituents of the Medial Orbital Wall

Clinical Implications

Palatine Bone and Maxilla—Major Constituents of the Orbital Floor

Clinical Implications

Clinical Implications

Zygoma—Constituent of the Lateral Orbital Wall

Clinical Implications

Anterior Orbit—Midorbit—Posterior Orbit (Apex)

II. Craniomaxillofacial Transition and Passageways

Fissures, Canals, Grooves, Foramina, Notches, Fossae—Bony Openings within the Precincts of the Orbit

Optic Canal (OC)

Superior Orbital Fissure (SOF)

Cranio-Orbital Foramen (COF)

Clinical Implications

Inferior Orbital Fissure (IOF)

Foramen Rotundum (FR)—Maxillary Strut (MS)

Pterygopalatine Fossa (PPF)—Sphenopalatine Foramen (SPF)—Pterygoid Canal (Vidian) (VC)

Ethmoidal Foramina (EF)

Clinical Implications

Infraorbital Foramen (IOFN)

Supraorbital and Frontal Foramina/Notches (SOFN/FOFN)

Clinical Implications

Zygomaticoorbital (ZOF), Zygomaticofacial (ZFF), and Zygomaticotemporal (ZTF) Foramina

Clinical Implications

Nasolacrimal Fossa (NLF)/ Groove (NLG)/ Canal (NLC)

Clinical Implications

Paranasal Sinuses

Clinical Implications

Lateral Wall of the Nose

Internal Orbital Buttresses

Orbital Dimensions, Volume, Surface Contours, 3D Globe Position—Interrelationship

Summary/Conclusion

References

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