Horse Stifle Joint Anatomy

The horse’s stifle, like the human knee, is an intricate structure that plays a major role in his hindquarter movement. It is formed by the femur bone, the tibia, and a patella which sits towards the front of the joint. Within the stifle joint are two crescent shaped discs of fibrocartilage called menisci that help provide support and a locking mechanism for the hindquarter. The stifle also has collateral ligaments that help keep the joint in place.

The stifle has between one and four sesamoid bones which assist with the smooth movement of tendon/muscle over the stifle joint. The most well-known is the patella (knee cap) which helps guide the quadriceps femoris muscle over the stifle joint to its point of insertion on the tibia. The horse does not have the popliteal or gastrocnemius sesamoid bones found in other species but does have a medial and lateral trochlear ridge which are separated by a deep groove that is akin to the trochlear groove of a human knee.

During flexion, the patella floats over this trochlear ridge and is controlled by the collateral ligaments. This is part of the stifle’s passive stay apparatus and allows for weight bearing with minimal muscular effort. Inappropriate hooking or catching of the cartilage extension of the patella over this medial trochlear ridge is what causes a locked stifle.

In addition to these structures, the stifle is also held in place by the cruciate ligaments which are responsible for limiting rotation of the stifle and keeping it firmly in flexion. The cruciate ligaments are not as prone to injury as the collateral ligaments but they can be damaged by quick changes in direction, rapid deceleration and when jumping. Horses that routinely jump are at a higher risk for injury to the medial cruciate ligament.

Radiographic evaluation is important for evaluating stifle joint pathology. A craniocaudal view of the femoropatellar joint will show evidence of osteoarthritis characterized by flattening of the articular surfaces, sclerosis or lucent zones on the subchondral bone and narrowing of the tibial trochlear groove.

Ultrasound of the stifle can be useful for evaluation of the collateral and cruciate ligaments. Ultrasound of the stifle is often more difficult than other joints as it requires the limb to be in slight flexion to obtain adequate images. It is important to have a trained ultrasound technician available for this evaluation as it is not a common procedure in practice.

The stifle is an extremely complex joint with numerous bony structures, muscles, and ligaments all working together. Damage to any of these can have profound impact on the function of the stifle and result in lameness. It is therefore crucial for the equine practitioner to be aware of all potential stifle injuries and seek out the advice of a board certified veterinary orthopedic surgeon if there are any concerns.