Upward Fixation of the Patella 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-weight-bearing pelvic limb lameness This may be distinguished from tarsal (hock) soreness which is usually weight-bearing in nature The horse will frequently drag the toe of the affected limb(s) during exercise, causing visible wearing of the dorsal aspect of the toe/shoe may be apparent. The foot of the affected limb(s) will have a low-arc flight pattern The horse will usually exhibit a shortened cranial phase to the stride • Resistance in the canter The horse will resist the canter, particularly if circled toward the more affected limb Resistance may be most noticeable during the transition between the trot and canter, when the horse is forced to extend the pelvic limb for a prolonged period Many horses will toss their head, rear, or stop when asked to canter. This may be due to their anticipation of impending upward patellar fixation. The horse would rather trot than canter (which is harder for the normal horse) • Consistent lead changes or cantering on the wrong lead The horse avoids prolonged pelvic limb extension with the affected limb. This is particularly apparent when cantering in a circle towards the affected limb. • The canter is very rough or bouncy This occurs as a result of consistent delay in pelvic limb flexion from the extended position • Swelling, heat, and/or pain may be associated with one or both stifle joints Upward patellar fixation causes patellar instability which in turn may result in femoro-patellar synovitis • The horse drags his hind toes during exercise • Resistance and/or difficulty when walking up and down hills, or when backing up These situations force the horse to extend the pelvic limb for a prolonged period Rather then fully extend the pelvic limb(s), the horse may crouch while walking Rather than flex the pelvic limb(s) normally, horses will often swing their limbs to the outside This may cause the lameness to be confused with neurologic disease (such as EPM or stringhalt) • Lameness is most severe when the horse is first taken out of the stall Many horses will improve as the workout progresses • Lameness becomes more obvious following an extended period of stall rest Loss of muscle and patellar ligament tone exacerbate the upward patellar fixation The horse does not improve (and may worsen) as a result of taking time off • The horse does not respond to anti-inflammatory (e.g. Phenylbutazone) therapy Intermittent upward patellar fixation is a mechanical problem and is not inflammatory-mediated As with many cases of pelvic limb lameness, secondary abnormalities such as thoracolumbar epaxial (back) and proximal thoracic suspensory ligament soreness are also present. These are generally detected during the passive lameness evaluation and are suggestive of chronic pelvic limb asymmetry/ lameness. How is upward patellar fixation diagnosed? Clinical signs are characteristic and, if the limb is locked in extension (i.e. the case is severe), diagnosis is simple. As previously mentioned, however, most cases are mild and diagnosis may be more difficult. A detailed history and careful clinical evaluation are essential parts of a proper workup. One helpful diagnostic aid involves placing the horse in one or more situations where prolonged pelvic limb extension is normally required. Such situations include walking up and down hills, the trot-to-canter transition, and backing up. When confronted with these situations, the affected horse will either 1) demonstrate upward patellar fixation by temporarily locking the pelvic limb, or 2) cheat by switching leads, swinging the limbs to the outside, avoiding pelvic limb extension, etc. Many times, a slight hitch or catch is visible as the pelvic limb begins to flex from an extended position. This catch is most easily detected by visualizing the point of the hock as the horse picks the limb up to advance it cranially. Infrequently, an audible snap or popping sound is also evident during exercise (particularly walking). Upward patellar fixation can sometimes be produced in affected horses by manually forcing the patella upward and outward. The examiner may actually be able to keep the pelvic limb locked in extension using minimal effort. Since the problem is usually secondary to conformation and/or level of fitness, it is almost always bilateral; however, affected horses historically exhibit clinical signs in one pelvic limb. It is not until the more affected limb is successfully treated that a problem in the opposite limb is manifested. Fox Run Equine Center foxrunequine
Posted on: Tue, 23 Sep 2014 16:00:00 +0000
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