Introduction

Chronic bone marrow lesions (BML) in the weight-bearing portions of the knee are often associated with symptomatic degenerative arthritis resulting in pain and dysfunction [1,2,3,4]. BML may not be apparent on radiographs and are diagnosed on magnetic resonance imaging (MRI) using fat-saturated T2-weighted, proton density-weighted or short tau inversion recovery (STIR) image sequences [5, 6]. They appear as poorly defined areas of high signal intensity in the subchondral region [5,6,7,8]. These lesions represent sites of increased mechanical stress [9] with subchondral hyperintense signal suggestive of an insufficiency fracture [5, 6]. They may also be correlated with common degenerative conditions of the knee such as meniscal tears [2], cartilage deterioration [5, 6, 9], subchondral bone attrition and cyst formation [3], mechanical malalignment [4] and ultimately, progression of osteoarthritis of the knee [2].

Management of symptomatic BML in patients with degenerate change of the knee using percutaneous injection of bone substitute material like calcium phosphate has been described in the literature [10,11,12]. The proprietary term Subchondroplasty® (SCP) (Zimmer Knee Creations; West Chester, PA, USA) refers to a novel technique in which a BML is injected with a bone substitute material composed of calcium phosphate, using fluoroscopic guidance [13]. The perceived advantages of Subchondroplasty® in managing these patients include the minimally invasive percutaneous technique [14], the potential for improved structural integrity and biomechanical strength of the subchondral bone from the injected calcium phosphate, which may additionally function as a porous osteoconductive scaffold allowing formation of new bone capable of sustaining load [1, 12, 13]. Since the description of the initial procedure in 2008 for chronic BML around the knee, the indications for this technique has been extended by some authors to include osteochondritis dessicans of the lateral tibial plateau in skeletally immature patients [15], subchondral bone marrow oedema of the talar dome due to ankle instability [16], osteochondral lesion of the talus [16] and BML of the shoulder and hip [2]. Initial studies of this procedure as reported in the literature have had encouraging results [1, 11, 13]. However, it is noted that these studies involve small cohorts of patients with limited follow up. Furthermore, some authors have reported unsatisfactory clinical results questioning the efficacy of this procedure in advanced osteoarthritis of the knee [17], femoral BML and lesions with mild bone marrow oedema [18]. It has been reported that there is considerable variation in the biomechanical properties of the commercially available calcium phosphate used as injectable bone substitute material during this procedure [19].

The primary objective of this review was to collate the available clinical studies in the published literature and review the results of percutaneous injection of bone substitute material like calcium phosphate including Subchondroplasty®. The secondary objective was to answer the following questions encountered in the clinical decision-making process of managing patients with BML associated with osteoarthritis of the knee:

  1. 1.

    Does calcium phosphate injection or Subchondroplasty® provide effective long-lasting pain relief to avoid further surgical intervention?

  2. 2.

    Which factors (patient/surgical) significantly influence the outcome following this type of procedure?

  3. 3.

    Does calcium phosphate injection or Subchondroplasty® adversely affect the outcomes of subsequent arthroplasty?

Materials and methods

Literature search and databases

A literature search of all the available evidence was undertaken (June 2019) using the healthcare database website (https://hdas.nice.org.uk). The databases searched were Medline, CINAHL, Embase and the Cochrane library.

Search criteria included boolean statements and the wildcard symbol (femur* OR femoral* OR tibia* OR tibial* OR knee*) AND (bone* OR marrow*) AND (lesion* OR edema* OR oedema*) AND (subchondroplasty*)”. The Cochrane database was reviewed for relevant articles.

An adjunctive bibliography search was undertaken to identify additional relevant articles. Subsequently duplicate articles were excluded and the clinical results of calcium phosphate injection including Subchondroplasty® were reviewed. Animal model and in vitro studies were excluded.

Results

The aforementioned database search and adjunctive bibliography search returned 53 relevant articles of which 7 articles were noted to be duplicates (Fig. 1). Thus, a total of 46 relevant articles were identified in the current literature: amongst these, 8 articles described the clinical results of calcium phosphate injection/subchondroplasty and were selected for further review. Details of the eight articles that describe the clinical outcomes of subchondroplasty are provided in Tables 1 and 2. The salient points are discussed below.

Fig. 1
figure 1

Results of literature search

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Table 1 Details of clinical studies
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Table 2 Reported clinical outcomes of calcium phosphate injection including Subchondroplasty®
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Patient demographics and physiology

Wide variation was noted in the patient population with the youngest patient reported to be 35 years of age at the time of the procedure [11, 13] whereas the oldest patient was 82 years [22].

Body mass index (BMI) is also reported inconsistently in the literature. Some studies [1, 7, 11, 13, 17] provide the details of BMI for their patient cohort whereas other studies [12, 18, 21, 22] do not provide any information on this vital parameter.

Indications and severity of osteoarthritis

There is good agreement in the literature for treating chronic painful bone marrow lesions noted in knee arthritis with injection of calcium phosphate [1, 7, 10, 12, 13, 22]. However, there is a lack of consensus on the degree of radiographic severity of knee arthritis for which this procedure is effective. Whilst some authors [11] have included patients with Kellgren-Lawrence (K-L) grade 4 arthritis, other authors consider it a contraindication [12, 17].

Mechanical parameters (range of motion/lower limb alignment)

Inconsistency is noted in the reported literature on mechanical parameters such as range of motion and mechanical axis in patients undergoing subchondroplasty. Several studies evaluated mechanical axis preoperatively and excluded patients with > 8° of varus/valgus mal-alignment [1, 11, 12, 17]; however, some authors have not stated the mechanical parameters in their series [1, 13, 18].

Surgical technique and postoperative regimen

There is considerable variation in the reported surgical technique used to perform the injection of bone substitute material like calcium phosphate to treat BML associated with knee arthritis. These includes (1) use of different types of commercially available injectable calcium phosphate such as Graftys HBS® (Graftys, Aix en Provence, France) [12], AccuFill® (Zimmer Knee Creations; West Chester, PA, USA) [7, 11, 13, 17]; (2) injection of calcium phosphate into BML under fluoroscopic guidance alone [7, 12] or combining it with arthroscopic assessment of chondral surfaces; (3) adjunctive arthroscopic procedures [1, 10, 11, 13, 17] and viscosupplementation [17, 22] and (4) injection of calcium phosphate with [10, 17] and without [12, 13] the use of a targeting jig/device.

Additionally, variable perioperative and postoperative regimens have been described for this procedure. These include (1) weight-bearing regimens ranging from full weight-bearing [12], weight-bearing as tolerated with crutches for 1 week [11] or for 2 weeks postoperatively [10, 13] to partial weight-bearing for 2 weeks postoperatively [17] and (2) type of analgesia used is variable ranging from none [17], oral/parenteral dipyrone with tramadol [12], acetaminophen with anti-inflammatory medications viz. indomethacin/ibuprofen [17], opiates such as. oxycodone/hydrocodone [17] and oral opiates alone [10, 13] to not reported [1, 11].

Outcome measures and follow-up duration

Different outcome measures have been used to describe the results of this procedure (Tables 1, 2 and 3). These include visual analog scale (VAS) score for pain [1, 11, 12, 22], Knee injury and Osteoarthritis Outcome Score (KOOS) [12, 17], Lysholm and Tegner score [17], International Knee Documentation Committee (IKDC) score [11, 13] and the Short Form Survey (SF-12) score [13]. The duration of follow up ranged from 6 months [12] to a maximum of 43 months [21].

Table 3 Reported clinical outcomes of calcium phosphate injection including Subchondroplasty®
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Complications

Extravasation of calcium phosphate into soft tissues or the joint has been reported [10, 12, 13]. Other reported complications include significant postoperative pain [10, 13, 20], which may last up to 12 months [20], persistent drainage at the injection site [11] and deep vein thrombosis [11].

Discussion

Bone marrow lesions

Bone marrow lesions associated with osteoarthritis of the knee have been well-described in the current literature [1,2,3, 6, 8, 9, 23]. It has been suggested that altered biomechanics due to abnormal joint loading and the corresponding subchondral BML contribute to the aetiopathogenesis of osteoarthritis [3, 23]. Chronic BML around the weight-bearing regions of the knee are often associated with symptomatic degenerative arthritis resulting in pain [1,2,3,4]. Management of these lesions has been broadly described using non-operative and operative methods alongside osteoarthritis of the knee. Non-operative methods including oral nonsteroidal anti-inflammatory medications have been demonstrated to provide symptomatic relief in patients with osteoarthritis and BML [24]. Use of other medications like bisphosphonates [25] and prostacyclins [26, 27] have been described in the management of BML; however, the long-term efficacy of these measures is limited [2]. Operative methods such as core decompression [28, 29] and high tibial osteotomy [30, 31] have been proposed as treatment options for BML associated with osteoarthritis of the knee.

Surgical technique - paucity of literature

Despite only limited studies being available in the decade since this procedure was first described, the indications have been widely extended to include osteochondritis dessicans of the lateral tibial plateau in skeletally immature patients [15], subchondral bone marrow oedema of the talar dome due to ankle instability [16], osteochondral lesion of the talus [16] and BML of the shoulder and hip [2].

Currently there is a lack of consensus in the literature on the surgical technique for this procedure. Some authors [1, 10, 11, 13, 17] performed adjunctive arthroscopic procedures to address meniscal and chondral lesions in addition to the injection of calcium phosphate. The influence of these adjunctive procedures on the functional improvement in clinical outcomes is difficult to quantify given the small patient cohorts and limited follow up. Careful interpretation of the results from these studies is essential. Intraoperative localisation of BML has been aided using fluoroscopy and targeting devices [17]; however, the use of targeting devices is not consistently reported in the literature [12].

Bone substitute material - nanocrystalline calcium phosphate

As highlighted above, different types of calcium phosphate have been described for injection of painful BML. It has been demonstrated that different types of commercially available calcium phosphate used as bone substitute material have considerable variation in their in vitro injectability and biomechanical properties [19]. Using synthetic (polyurethane foam sheets) and cadaveric cancellous bone blocks, Colon et al. [19] noted that only AccuFill® (Zimmer, Inc.) and StrucSure™ CP (Smith and Nephew) were able to flow into a closed structure such as cancellous bone. Furthermore, they observed that amongst the eight bone substitute materials tested, AccuFill® had the lowest injection force and achieved desired fill of the void. It is interesting to note that both AccuFill® and StrucSure™ CP are composed of nanocrystalline calcium phosphate [19]. Hence the type of calcium phosphate used to inject BML and the pressure used may have a critical influence on the outcome of Subchondroplasty®. The surgical experience of the surgeon performing Subchondroplasty® may be an additional factor.

Imaging features after subchondroplasty

Subsequent to injection of BML with calcium phosphate imaging modalities like magnetic resonance imaging (MRI) have been used to evaluate the subchondral bone. The injected calcium phosphate has been described to demonstrate a low signal surrounded by hyperintense rim on T2-weighted images [5, 7].

Lack of consensus on outcome measures

There is considerable variation in the literature on the outcome measures used to describe the clinical results of calcium phosphate injection into BML (Tables 2 and 3). Analysis of the different outcome measures is presented in Table 3. The VAS score for pain has been demonstrated to significantly decrease following Subchondroplasty® [1, 11, 12, 22]. Some authors have reported an improvement in the KOOS [12, 17] and IKDC score [11, 13], whereas the Tegner Lysholm score [17] was noted to deteriorate in a different study. This lack of consensus is a major limitation in the interpretation of results. Hence it is essential to establish a set of robust clinical and radiological outcome measures that will be consistently used in future studies to report the results of this procedure.

Limited follow up and natural history of osteoarthritis

The maximum reported follow up after this procedure is 43 months [21]. Given the long-standing natural history of osteoarthritis, the limited follow up reported in the current literature is a further limitation in the interpretation of results.

Morbidity and implications for future procedures after subchondroplasty

Overall, the injection of calcium phosphate into BML around the knee appears to be associated with limited morbidity. Extravasation of the material into the soft tissues or into the joint have been reported [10, 12, 13]. However, calcium phosphate is a hydrophilic material and it has been suggested that the residual material can be removed arthroscopically using standard instruments [13].

The perceived advantage of injectable calcium phosphate is that it may be able to function as a scaffold following the setting process involving an endothermic reaction [13, 14]. This enables bone growth providing structural support [11]. Nonetheless there has been concern that this alteration in bony architecture may limit future procedures like arthroplasty. This has been addressed in a recent study by Yoo et al. [32]. They reported no increase in surgical complexity among patients undergoing arthroplasty following Subchondroplasty®. Furthermore, they noted that these patients had a similar postoperative Oxford score to their control group.

Lack of comparative studies for novel techniques

From a basic science perspective, calcium phosphate injection of BML has been suggested to play a mechanical role providing structural support [1, 11, 13]. BML are sites of altered bone remodelling activity in which the mesenchymal stem/stromal cells (MSCs) play a crucial role [33]. It has been demonstrated that as the frequency of symptomatic BML increases with progression of knee osteoarthritis, necessitating arthroplasty [34]. Hence novel techniques using biological agents such as platelet-rich plasma (PRP) or dual-action antiresorptive agents such as strontium ranelate delivered directly into BML has been proposed to improve bone homeostasis and joint preservation [34]. However, there is no study in the current literature comparing these novel techniques with calcium phosphate injection of BML.

Conclusions

The limited data from the current studies would suggest that injection of calcium phosphate in BML including Subchondroplasty® may improve pain and function. The procedure is associated with limited morbidity and it does not appear to negatively influence the outcome of subsequent arthroplasty. However, the type of injectable calcium phosphate and adjunctive procedures undertaken may have an influence on the clinical outcome. Hence pragmatic, prospective studies with stratified patient cohorts, longer follow-up duration and robust outcome measures are essential to improve the current understanding of the indications and the clinical effectiveness of this procedure.