Intermittent upward fixation of the patella occurs when the medial patellar ligament remains hooked over the medial trochlear ridge of the femur and locks the reciprocal apparatus with the limb in extension. A horse with upward fixation of the patella stands with the hindlimb fixed in extension with the fetlock flexed. The leg will usually release with a sudden snap or jerking movement.
Some horses demonstrate a milder form of this condition. In these horses, there is delayed release of the patella during limb protraction, most commonly evident as the horse moves off or in downward transitions. This appears as a jerky movement of the patella. Horses with recurrent upward fixation or delayed release of the patella may develop chronic, low-grade lameness due to stifle soreness and may be reluctant to work on soft, deep surfaces or up or down hills.
Upward fixation or delayed release of the patella is most commonly seen in young horses and ponies, particularly if they are in poor body condition and poorly muscled. Straight hindlimb conformation may predispose to this condition. It may also be seen in older horses that have had trauma to the stifle region, particularly if the horses are stabled or have been inactive.
Diagnosis is based on recognition of typical clinical signs. In some horses, upward fixation of the patella may be induced by pushing the horse backward or manually pushing the patella proximally. Radiographs of the stifle should be taken in horses with femoropatellar joint effusion and lameness to establish concurrent or secondary pathology.
To release an upward fixated patella, the horse should be pushed backward while simultaneously pushing the patella medially and distally. Alternatively, pulling the limb forward with a rope around the pastern may unlock the patella. If upward fixation of the patella is intermittent and not causing lameness, a conditioning program should be instituted. This involves daily lungeing or riding of the horse, appropriate to its age and type, as well as ensuring an adequate plane of nutrition, good dentistry, and anthelmintic administration. Stable rest is contraindicated, and the horse should be turned out to pasture as much as possible. Remedial foot trimming to ensure the foot is well balanced and shoeing with a bevel-edged shoe with or without a lateral heel wedge may be beneficial. A significant proportion of horses will improve with maturity and conservative treatment, although signs may recur if the horse undergoes prolonged stall rest.
Medial patellar ligament desmotomy is indicated in horses that do not respond to conservative treatment or in horses with lameness caused by upward fixation of the patella. Medial patellar ligament desmotomy is most often performed in the sedated horse under local anesthesia, although some surgeons prefer general anesthesia. After surgery, the horse should be restricted to stable or small stall rest for 2 mo to reduce the risk of complications. Fragmentation of the apex of the patella is the most common complication after surgery (see below); if it results in lameness, it may be treated arthroscopically with good results. Other complications include lameness, local swelling, or even patella fracture. The prognosis after medial patellar ligament desmotomy is generally considered to be good, with recurrence of the condition rare. Medial patella ligament splitting, which involves making numerous small incisions in the ligament, has also been used successfully to manage these horses.