Upward Patellar Fixation Intermittent upward patellar fixation is a condition whereby the horse’s pelvic limb temporarily "locks" in extension. As a result, there is a delay in flexion of the limb. The delay in flexion can range from milli- seconds to over several minutes. A short delay in flexion may manifest only as a subtle pelvic limb asymmetry or lameness; severely affected horses (with a long delay in flexion) may be unable to flex the affected limb without assistance. See what this looks like: What is the "Patella"? The horse’s stifle joint is analogous to the human knee. Just like humans, horses have a patella, or "knee cap", which slides along the distal aspect of the femur (thigh bone) during flexion of the joint. The patella slides within a groove (called the trochlear groove) and serves as a fulcrum for the extensor muscles and their tendons as they course over the front of the stifle (or knee) joint. The patella is attached proximally to the quadriceps and biceps femoris muscles and distally to the tibia. In humans, the patella is attached to the tibia by one distal patellar ligament. Horses have 3 distal patellar ligaments: the medial patellar ligament, the middle patellar ligament, and the lateral patellar ligament. How does the horse ‘lock’ the pelvic limb? Horses have the ability to lock (or fixate) the pelvic limb in extension. This is possible due to the unique anatomy associated with the horse’s stifle joint. The proximal aspect of the medial femoral trochlea is shaped similar to a hook or ski jump. By placing the space between the medial and middle patellar ligaments over this hook, horses can "lock" their pelvic limbs in extension. Once locked, minimal effort is required to maintain limb extension. A similar locking apparatus in the thoracic limbs allows horses to sleep while standing. Therefore, patellar fixation while standing is a normal process in the horse. What is ‘intermittent upward patellar fixation’? Although patellar fixation is normal in the standing horse, it can produce pelvic limb dysfunction if it occurs during exercise. Inadvertent locking of the patella over the medial femoral trochlea prevents normal flexion of the affected limb(s). Consequently, pelvic limb asymmetry and lameness frequently become evident. What causes upward patellar fixation? There are 3 primary causes of upward patellar fixation in the horse: Lack of fitness: Lack of quadriceps and/or biceps femoris muscle tone results in an inability to quickly pull the patella up and off of the medial femoral trochlea. Straight or upright pelvic limb conformation: This places the medial femoral trochlea further distad in closer proximity with the patella, facilitating patellar fixation. Negative plantar angles: Derangement of foot angle can increase the degree of pelvic limb extension, thereby increasing the likelihood for patellar fixation to occur. Excessive distal patellar ligament length: This places the patella proximad in closer proximity with the medial femoral trochlea, where it can inadvertently "catch" or "lock" It should be noted that the factors which cause upward patellar fixation are often interrelated. For example, an unfit horse will generally have increased laxity (and therefore increased length) of the distal patellar ligaments. Furthermore, if unfitness is secondary to another disease process (such as neurologic disease), intermittent upward fixation may also occur secondarily. Therefore, it is important to assess the horse as a whole prior to determining the cause for upward patellar fixation. What are the clinical signs? Horses with intermittent upward patellar fixation will exhibit clinical signs during their attempt to flex the pelvic limb from an extended position. In acute severe cases, the pelvic limb may stay locked in extension. The horse may not be able to flex the stifle and tarsus without assistance. In some instances, the condition may temporarily resolve only to recur after taking a few steps. These signs are quite obvious and diagnosis is relatively simple if the condition is severe. Most of the time, however, there is only a "catching" of the patella as it slides up and over the hook and the limb does not truly lock in extension. In this situation, there may only be a mild pelvic limb asymmetry or lameness. This type of lameness can be easily confused with other problems and therefore may present a dilemma in regard to accurate diagnosis. Following are common clinical signs associated with mild to moderate forms of intermittent upward patellar fixation: Non-weightbearing pelvic limb lameness This may be distinguished from tarsal (hock) soreness which is usually weightbearing in nature The horse will frequently drag the toe of t
Posted on: Fri, 20 Sep 2013 23:38:41 +0000
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