Stifle lameness: Update on subchondral bone cyst management

Subchondral bone cysts (SBCs), and more specifically medial femoral condyle (MFC) cysts, are associated with lameness and poor performance in sport and racehorses and research has focused on them for years. Unfortunately, their etiology remains unclear although most studies suggest a traumatic origin. In fact, SBCs have been experimentally created after performing a small cartilage lesion with or without an additional subchondral lesion in the MFC of horses and ponies.

SBC scan cause lameness due to the inflammatory and morphological changes produced in the subchondral bone plate. With time, SBCs have been suggested to be also associated with mechanical abrasion of the meniscus and tibial plateau and with changes in the biomechanical environment of the joint. Ex-vivo and finite element studies performed have found that the medial meniscus and the tibial plateau uncovered by meniscus are subject to additional stresses and prone to damage when a defect in the MFC was present.

Several techniques are used to treat MFC SBCs, from conservative treatment to intracystic injection of steroids to surgical procedures. While conservative treatments were used in the past, they are currently not recommended unless the cyst is only a subtle concavity. Intracystic injection of triamcinolone (10-18 mg in several spots within the cyst) is used to minimize the inflammatory environment and bone resorption that has been histologically identified in naturally occurring cysts. This technique can be used ultrasonographically or radiographically-guided but best results are obtained when performed arthroscopically-guided. Additionally, higher success rates are obtained when attempted in younger animals, non-arthritic joints or smaller cysts.

Cyst debridement with or without cyst filling and resurfacing is another surgical possibility. However, only performing cyst debridement (traditional surgical technique) have been suggested to potentially cause meniscal and tibial damage and rarely promotes radiographic healing. Thus, it could be considered out of favor by many. Different materials can be used for cyst filling +/- resurfacing such as cancellous bone, bone marrow concentrate , tricalcium phosphate granules or chondrocytes. Debridement in conjunction with cyst filling led to a successful outcome in up to 74% of cases.

In the last decade, a novel surgical technique consisting of placing a transcondylar cortical screw through the cyst via an extra-articular approach has obtained encouraging results with success rates of 75% and higher. The benefits of this technique are thought to be associated with the preservation of the cartilage overlying the SBC and with the high rate of radiographic cyst healing (up to 75%). Additionally, debridement of the inflammatory cyst lining is achieved during drilling for screw placement. Some special drills can be used to maximize this debridement. Transcondylar screw placement is currently the gold standard for many surgeons as it achieves radiographic healing and lameness resolution in most cases. However, prognosis still depends on the health of the joint compartment (i.e: lack of additional damage to the medial meniscus, tibial plateau or femoral condyle surface), especially when older horses or chronic cases are considered.

Thus, pre-operative radiographic and ultrasonographic screening in addition to arthroscopic assessment of the joint during screw placement are key for patient selection and the success of the surgical technique. In recent years, the use of a reabsorbable interference screw has been tested for SBC in the stifle and other locations. The use of these screws rather than cortical screws was introduced due to concerns regarding horse selling value when stainless steel implants were radiographically visualized later in life, especially in young Thoroughbreds going to auctions. Despite the fact that these implants produce radiographic healing and lameness resolutions in short term rates similar to the cortical screw, their long term effects are still to be proven. Bone healing seems to be produced due to the osteoinductive and osteoconductive properties of the screw materials rather than to biomechanics as is the case for the cortical screw. Different healing patterns can be seen at the subchondral bone plate level with both techniques.

Newer techniques such as subchondroplasty are being extrapolated from human medicine and results will be available soon. Similar to screw placement, the cyst is accessed extra-articularly with a cannulated drill and then a biomaterial +/- PRP or BMAC are injected to achieve cyst filling. This material is replaced overtime by bone.