Could something even smaller than a pea end a horse's athletic career? If that something is a cystic lesion in the stifle (or femoral condylar cyst), it's entirely possible. Fortunately, specific management approaches can help some horses return to their jobs in the arena or on the track with few, if any, lasting effects.
Wayne McIlwraith, BVSc, PhD, Dipl. ACVS, ACVSMR of the Colorado State University Equine Orthopaedic Research Center, in Ft. Collins, described stifle bone cysts at the 2013 American Association of Equine Practitioners' Convention, held Dec. 7-11 in Nashville, Tenn., in the “Lessons Learned ” session. The stifle consists of several different joints between the patella (knee cap), the femur (the long upper bone of the hind leg), and the tibia (the inner of the two bones that extends down from the knee to the hock).
McIlwraith emphasized that there are five different types of cystic lesions that arise in the stifle joint ranging from dome-shaped deficits to circular ones. Historically, clinical signs tend to recur following treatment when the horse returns to its previous athletic level.
In general, McIlwraith noted that intra-articular (in the joint) corticosteroid injections tend to be unrewarding. Surgeons performing arthrotomy (opening up the joint) with debridement of the cystic lesion have experienced a relatively high success rate, but the procedure is quite invasive. Instead, they often debride the lesion arthroscopically and inject corticosteroids directly into the cystic lesion with success; sometimes they augment this approach by adding a bone graft over the lesion. Another treatment option that shows promise is the placement of a stainless steel lag screw across the condyle through the cystic lesion.
A flow chart McIlwraith showed could help veterinarians sort through possible therapeutic approaches: “If the margin of the cyst is stable without collapse, then arthroscopy with intralesional steroids is a good approach,” said McIlwraith. If the cystic edges are unstable or arthroscopic surgery fails to resolve lameness, then he recommends treating with surgical debridement. Success rates for return to soundness following arthroscopic debridement are 64-72%, depending on the size of lesion and horse’s age. The best results are seen in horses less than 3 years old and in lesions smaller than 15 millimeters.
McIlwraith explained that lack of resolution could be related to tears in the stifle's meniscus or meniscal ligaments; about 9% of horses with stifle cysts have concurrent meniscal lesions. He also said it is possible for a meniscal lesion to develop after surgical debridement—the debrided edges of the cyst might act like a cookie cutter and cause inadvertent meniscus injury, he said. Administering certain types of intralesional steroids (such as methylprednisolone) might be counterproductive and increase bone resorption (in other words, the bone would lose tissue or break down); other corticosteroids, such as triamcinolone, provide better outcomes. In horses with cysts in only one stifle, 90% improved in their comfort with triamcinolone injected under arthroscopic guidance, whereas a 67% of horses with cysts in both stifles improved.
The screw technique might help relieve strain on a subchondral bone cyst (located just under the cartilage surface within a joint) and promote formation of trabecular bone (the precursor to hard bone tissue) within the cyst and bone remodeling. McIlwraith said anecdotal reports regarding the use of the screw technique has been shown to decrease lameness grades and to improve density of the cystic lesion with new bone growth. The screw can be left in place permanently with no untoward effects on horses in active training.
In summary, McIlwraith said success rates of horses treated with unilateral cystic lesions treated arthroscopically with intralesional injection of triamcinolone acetonide can reach 90%, and success rates of horses treated with debridement range from 30% to 70% depending on the size of the cyst and the age of the horse. He also said the lag screw technique is an excellent option to use in horses when previous treatment approaches have failed.
Disclaimer: Seek the advice of a qualified veterinarian before proceeding with any diagnosis, treatment, or therapy.