Upward Fixation of the Patella
This video sequence was originally prepared for a friend who needed information about a stifle problem known as upward fixation of the patella. The problem is also referred to as accidental lock of the patella. This video shows an accidental lock of the patella. I was recording the horse in turn out and the incident occurred in front of the camera.
James Rooney explains that "In normal standing position, with the stifle angle at about 135º, the patella is not locked, but when the angle approaches 143-145º it must hook. Patellar fixation often occurs in horses and ponies with straight hind legs. In such animals the joint is closer to 140º rather than 135º in the normal standing position." (James R. Rooney, Biomechanics of lameness in horses, 1969)
Due to a system of tendons and ligaments known as reciprocal mechanism, when the stifle is locked, the hock and the fetlock are locked as well. When patellar fixation occur, the whole limb is held rigid, which is demonstrated by the horse on the video. The horse unlocks the stifle contracting the quadriceps which lifts the patella. The biceps contracts, pulling the patella laterally off the ridge of the femoral trochlea and unlock the stifle.
Since most upward fixation of the patella occurs when the stifle extends, we resolve this type of problem increasing the dorso-ventral rotation of the pelvis. Greater pelvis rotation reduces the angle of the stifle at impact minimizing the risk of accidental lock. It is not possible to change the horse's morphology, but it is possible to teach the horse to use his body more efficiently.
The still picture shows the horse as he was energetically driven onto the bit. This type of riding condemned this horse as well as every straight legged horse to stifle problem. The horse becomes sound once proper functioning of his vertebral column mechanism allowed greater dorso-ventral rotations of the pelvis. The rider on the video clip is the horse's owner.
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