Surgery STAT: Options for treating subchondral bone cysts

A 3-month old Warmblood filly was referred to our hospital for evaluation for a left front-limb lameness of one week duration On presentation, the filly was grade 2/5 lame on the left front limb. The left front fetlock had moderate joint effusion and the filly was sore on flexion of the left front fetlock.

Radiographs of the left front fetlock showed a 10 mm by 9 mm subchondral bone cyst present on the medial metacarpal condyle, just abaxial to the saggital ridge The cyst did not appear to have direct communication with the joint. The following day, the filly was placed under general anesthesia in right lateral recumbency. The left front fetlock was clipped and prepped in routine fashion. Using radiographic guidance and with the fetlock in a flexed position, a 20-gauge, 1- and ½-inch needle was placed through the articular cartilage and thin layer of subchondral bone into the cyst using a dorsal approach and 10 mg of triamcinilone was injected into the cyst. The joint was injected with 10 mg of sodium hyaluronate and 125 mg of amikacin. The filly was recovered from general anesthesia and discharged the following day.

The left front fetlock was injected with 10 mg of sodium hyaluronate again at two and four weeks post-operatively. The left front lameness had resolved completely at two weeks postoperatively. Radiographs of the left front fetlock at four weeks indicated the cyst had decreased in size by approximately half Radiographs made at 8 weeks postoperatively showed complete resolution of the cyst

Subchondral bone cysts occur in all diarthrodal joints of the equine appendicular skeleton, with the most common location involving the media femoral condyle. They occur on weight-bearing regions of the joint, underlying the articular cartilage. Subchondral bone cysts reportedly communicate with the joint approximately 30 percent of the time, and often this communication can be confirmed only with arthroscopy. The pathophysi-ology of subchondral bone cysts has not been entirely elucidated. They were initially thought to be associated with the osteochondrosis complex; however, they may have an inflammatory origin. The prognosis for athletic soundness once a lesion is identified is variable, depending on the location of the cyst, the presence of concurrent osteoarthritis and the treatment chosen.

There are several therapeutic options described for treatment of subchondral bone cysts, including conservative therapy, surgical debridement of the cyst arthroscopically or trans-osseously, direct injection of the cyst with corticosteroids with arthro-scopic, ultrasonographic or radiographic control, surgical debridement of the cyst in combination with filling of the cyst with bone or bone replacements, joint resurfacing techniques and surgical debridement of the cyst followed by injection of the cyst with corticosteroids.

Conservative therapy is not recommended because progression of osteoarthritis and lameness is a likely sequel. Surgical debridement of the cyst often is the treatment of choice; however, surgical access to the cyst can be difficult. Recently, direct injection of the cyst with corticosteroids has gained popularity, and success rates are comparable to surgical debridement.

This case illustrates that direct injection of a cyst with corticosteroids is a viable alternative to surgical debridement of subchondral bone cysts and can result in resolution of the cyst without more aggressive surgical intervention. This is particularly important when surgical access to the cyst is difficult due to its location within a joint. Corticosteroids are thought to reduce local bone inflammation, arrest cyst progression and promote cyst healing. Direct injection of the cyst with corticosteroids was elected in this case because of the comparable success rates vs. surgical debridement, and to minimize the potential for surgical trauma to the fetlock joint in a young, growing filly.

Dr. Shawn Mattson is an ACVS board-certified surgeon who practices at Moore and Co. Veterinary Services, a full-service equine hospital in Calgary, Alberta, Canada. Dr. Mattson, previously at the Ontario Veterinary College in Guelph, Ontario, has published scientific articles in the American Journal of Veterinary Research and Veterinary Surgery related to research on orthopedic infections in horses.