Subchondral cystic lesions of the medial femoral condyle in the stifle cause a lot of problems for owners of various breeds of performance horses—if not for the pain they can cause, for how radiographic evidence can create concern for potential buyers. This is especially the case in young Thoroughbreds intended for public auction. Equine surgeons have developed variety of ways to handle these lesions but continue to explore new approaches. One research group recently studied how a novel absorbable implant might aid in the effort.

“This is a condition most likely all of us have dealt with in our equine patients,” said Cole Sandow, DVM, MS, DACVS-LA, associate surgeon at Hagyard Equine Medical Institute, in Lexington, who presented their results as part of the 2020 American Association of Equine Practitioners convention, held online. “They occur in numerous locations and can be different shapes and sizes. Usually, they can be readily diagnosed on radiographs, and the relevance to clinical practice is they can be a source of lameness and poor performance or decrease sale value if seen on prepurchase radiographs.”

Sandow explained that veterinarians used to believe subchondral cystic lesions (SCLs) were caused by osteochondrosis. But SCLs usually occur in areas of maximal weight-bearing in the joint, while osteochondrosis affects both non-weight-bearing and weight-bearing surfaces. Infection might play a role in these cases—usually in foals—but is unlikely the cause.

“Trauma is probably the most currently accepted theory, which causes a defect in the articular cartilage and a fissure of the subchondral bone, creating somewhat of a valve that allows for synovial fluid to be propulsed in the subchondral bone, creating a subchondral cystic lesion,” he said. “This is also known as the hydraulic theory.”

Inflammation can also cause SCLs, he explained. No matter the cause, the cavitylike lesions usually progress—getting bigger with time—but can also stay the same size or shrink.

“Ultimately, treatment options are dependent on the location and size of the subchondral cystic lesion, as well as taking into consideration the intended use of the horse,” said Sandow. “And that can also affect the goals of treatment, whether it be to improve lameness or the radiographic appearance or both, which is ideally how treatment options should be evaluated.”

Sandow and his co-authors conducted a retrospective multicenter study of 57 horses (36 fillies, 20 colts, one gelding), 10-26 months of age, with subchondral cystic lesions of the medial femoral condyle that veterinarians had identified on radiographs prior to public auction or during lameness exams. Thoroughbreds were overrepresented with 41 cases, he said, and there were 16 Standardbreds.

Surgeons used a procedure developed by Paolo Ravanetti, DVM, of Equitecnica Equine Hospital, in Parma, Italy, involving curettage of (tissue removal by scraping or scooping) each subchondral cystic lesion and placement of an absorbable implant under radiographic guidance. The implant they used is a 9-mm-wide, 30-mm-long cannulated interference screw that surgeons use commonly in human surgery as a graft anchor for ACL surgery or to fill bone voids. “It is composed of hydroxyapatite (HA), poly-L-lactic acid (PLLA), and carbonate ions,” said Sandow. “The hydroxyapatite is a fine 3- to 5-micrometer particles, which is beneficial to trigger phagocytosis and incorporation into the PLLA matrix for improved absorption.”

“The implant is not readily visible, depending on radiographic quality, but it does extend 8 millimeters past the tip of the metallic screwdriver,” said Sandow.

Horses received antibiotics and non-steroidal anti-inflammatory drugs for three days after the procedure, their sutures were removed at 12 days, and they were kept on stall rest for four weeks. Next steps depended on reevaluation, and then they transitioned back to work.

Veterinarians performed follow-up lameness and radiographic examinations one, two, three, and four months after the procedure. Observations included:

• Two independent observers objectively evaluating the horses saw 70% reduction of the cysts in 86% of the cases.
• In the 48 horses with available lameness exam follow-ups, 47 (97.9%) were sound up to four months postoperatively.
• Two of the horses needed repeat surgeries because their implants migrated (one into the joint, the other into the subcutaneous tissue).

For those of racing age at the time of writing, 76% raced, which is similar to what is seen in horses without lesions.

Sandow said some cases might not have improved because of incomplete healing or recurrence of the bone fissure.

“(The procedure) appears to be an effective technique in younger horses, and the mean age at the time of surgery in the study was 14 months,” he said. “The metabolic activity of the bone is more active with an open physis in these younger horses may help with remodeling and absorption of the screw. Further investigation is needed for the use of this technique in older horses.”

In comparison, Sandow said that in research using another technique to correct these lesions—the  transcondylar screw—the mean age of horses treated in research was 2.4 years old, and the procedure was less successful in horses older than 3 (Santschi, 2015). And in a report using IGF-1 and a chondrocyte graft (Ortved, 2011), the median age among horses was 3.3 years and the approach was more successful in horses older than 3, compared to other studies.

“Since the implant is absorbable, it does not need to be removed, which is beneficial in young horses intended for sale,” he said. “And even though it costs more than a cortical screw at $500, it can be less expensive than a second surgery.”

Sandow acknowledged that the multicenter nature of this study and some of the subjective assessments present inherent limitations.

“In conclusion, the technique developed by Dr. Ravanetti is an effective surgical treatment for MFC SCLs in young horses,” said Sandow. “There was an improvement in lameness and radiographic appearance at four months post-surgery, and this technique had comparable long-term results to other accepted methods.