Distal Patellar Hyperextension
Distal patellar hyperextension occurs as the horse tends to flex a locked pelvic limb from its extended position. Horses usually hyperextend the pelvic limb while they are trotting down a hill or sliding to a stop (particularly in muddy conditions). The pelvic limb wants to flex but cannot due to the position of the patella (knee cap) over the medial (inside) trochlear ridge of the femur. This "upward fixation of the patella" temporarily locks the limb in extension, thereby disallowing normal flexion.
If you would like to learn more about INTERMITTENT UPWARD PATELLAR FIXATION click HERE.
As the hind limb tries to flex, the ligaments attached to the (fixated) patella hyperextend (stretch), usually resulting in acute severe lameness. Lameness is often characterized by an unwillingness to drop the heel (i.e. toe-touching) during movement and a hypometric (or toe-dragging) gait. In many cases, visible (biomechanical) stifle interference becomes more obvious pursuant to this type of injury.
See what this looks like:
In some cases, palpable swelling and pain associated with the stifle area is evident immediately following the injury. Interestingly, the horse's comfort level usually improves dramatically within 72-96 hours post-injury. It is very important to note that this is one of the few problems that results in a severe hindlimb lameness that resolves very quickly. After 4-5 days following the injury, approximately 50% of the horses maintain some degree of residual lameness. The characteristics of the lameness are similar (i.e. a hypometric toe-dragging gait with a hip drop), but the severity is much less than initially observed. Residual lameness is most commonly a result of persistent inflammation within the femoropatellar (stifle) joint (as a consequence of the distal patellar hyperextension) and/or an exacerbation of biomechanical stifle interference.
If you would like to learn more about JOINT INFLAMMATION click HERE.
At The Atlanta Equine Clinic, we usually elect to treat persistent stifle joint inflammation (synovitis) with a single injection of high-viscosity hyaluronan combined with a weak steroid (e.g. Betamethasone). The injection is usually successful at eliminating the residual joint pain. Further injections are rarely necessary unless distal patellar hyperextension recurs.