Locking stifle (upward fixation of the patellar)
Fixation of the patellar is a normal occurrence in horses and allows the animal to rest in a weight-bearing, standing position. When fixed the patellar hooks into place to keep the stifle extended. To move out of this position the horse uses muscle contractions to free the patellar and allow normal movement.
Horses with straight hindlimb conformation, poor body condition or poor muscle condition can have problems in releasing the patellar from this fixed position.
Signs
After a period of rest or at the start of movement the horse is unable to flex the hock or stifle and the affected leg is dragged. This can last for only a few strides or may remain for days to weeks. Other animals show intermittent fixation where as the animal walks, again only ever after rest, the limb is held in extension and then suddenly jerks into the flexed position.
Diagnosis
A diagnosis can only be made by seeing the characteristic gait. This can be infuriating as the condition may only occur sporadically and not be seen at the time the vet examines the horse.
Treatment
If the condition is occurring intermittently the condition or fitness can be improved to alleviate the condition. Many young horses will grow out of the condition with time. The heels can be raised and the toes shortened by farriery to help the animal. If the patellar is permanently fixed then surgical release is required by cutting one of the patellar ligaments. This is often performed under sedation.