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. When the patella is inappropriately locked, the horse stands with the hindlimb fixed in extension and the fetlock flexed. The leg usually releases with a sudden snap or jerking movement.
Some horses demonstrate a milder form of upward patellar fixation. In these horses, release of the patella during limb protraction is delayed, most commonly evident as the horse initiates movement or in downward transitions. The release appears as a jerky movement of the patella. Horses with recurrent upward fixation or delayed release of the patella can develop chronic, low-grade lameness as a result of stifle soreness and might be reluctant to work on soft, deep surfaces or up or down hills.
Upward fixation or delayed release of the patella most commonly occurs in young horses and ponies, particularly if they are in poor body condition and poorly muscled. Straight hindlimb conformation might predispose the horse to this condition. The condition might also occur in older horses that have had trauma to the stifle region, have recently changed disciplines, or have been out of work for a period of time.
Diagnosis of upward fixation of the patella is based on recognition of typical clinical signs. In some affected horses, the patella can be locked manually by pushing the horse backward or by pushing the patella proximally. Radiographs of the stifle should be taken in horses with femoropatellar joint effusion and lameness to investigate for any concurrent or secondary pathology.
To release an upward fixated patella, the veterinarian should push the horse backward while simultaneously pushing the patella medially and distally. Alternatively, pulling the limb forward with a rope around the pastern might unlock the patella.
If upward fixation of the patella is intermittent and not causing lameness, as is most common, a conditioning program should be instituted. The program involves daily lunging or riding the horse and exercise on hills, appropriate to the horse's age and type, as well as ensuring an adequate plane of nutrition to improve muscling and strength, particularly of the quadriceps.
Stall rest is contraindicated in horses with upward fixation of the patella, and the horse should be turned out to pasture as much as possible. Remedial foot trimming to ensure that the foot is well balanced, as well as shoeing with a bevel-edged shoe with or without a lateral heel wedge, might be beneficial. Many horses improve with maturity and conservative treatment; however, signs might recur if the horse undergoes prolonged stall rest.
Patellar ligament counterirritation can be applied if exercise and corrective farriery do not resolve intermittent upward fixation of the patella. In this procedure, which is performed under standing sedation, strong iodine in an oily base is injected into the medial and middle patellar ligaments and quadriceps musculature. The resulting inflammation and some degree of fibrosis within the ligaments often resolves the condition.
Medial patellar ligament desmoplasty (or "splitting"), in which numerous small incisions are made on the ligament, can be performed in horses that do not respond to conservative treatment or in horses with lameness caused by upward fixation of the patella. Desmoplasty is most often performed in sedated horses under local anesthesia; however, some surgeons prefer general anesthesia. After surgery, the horse should be restricted to stable or small stall rest for 2 months to decrease the risk of complications. The prognosis after medial patellar ligament desmoplasty is generally considered to be good.
Complete medial patellar desmotomy was historically performed as a treatment for upward fixation of the patella, but fragmentation of the apex of the patella is a possible complication after this procedure, and it has been largely abandoned as a treatment option.
For More Information
Also see pet owner content regarding disorders of the stifle in horses.
