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movement, and thus are collectively referred to as the reciprocal apparatus.

Function in Movement

The extension of the stifle (when the leg straightens to push off) is achieved by a large fan-shaped muscle group (comprised of four parts) which originates on the lower pelvis and along the femur. All four muscles in this group insert on the patella. When these muscles contract, they pull the patella upwards acting on the patellar ligaments and, in turn, straighten the tibia in relation. Since the hock and stifle essentially act on the same purpose, the power is transferred downwards and the horse pushes off.

The flexion of the stifle (when the angles close towards the horse's body) is created by a large number of muscles acting from a distance as well as some which directly activate the joint. One of these, the politeus muscle, which originates on the lateral (outside) condyle of the femur and inserts on the inside of the tibia primarily flexes the stifle but is also responsible for slightly rotating the leg inward.

The stifle requires strength to be powerful, but unfortunately, its design predisposes it to injury. The muscles that cover and activate the joint primarily run across the side of the joint, but not over it. This means that there is really only ligaments which cover the front of the joint. Over-exertion or any problems in those ligaments can drastically hinder the horse's ability.

Problems in the Stifle

Probably one of the most common disorders of the stifle is upward fixation or displacement of the patella (also referred to as a locked stifle). Horses with straight hind limbs are most commonly affected, although horses with trauma to the patella or muscle deterioration are also affected. This disorder is when the patella, the kneecap, gets stuck within the condyles of the femur when the leg is in an extended position. The stifle and, coincidentally, the hock joint will get locked in a straightened position, though the fetlock is still movable.

The locked stifle can either be a permanent fixation (which requires veterinary attention) or, more difficult to assess, is the periodical upward fixation of the patella. In the periodical, the stifle joint will intermittently lock and release, much like the joint continuously "popping," as the horse moves (most predominantly noticeable on a circle). This causes a great deal of pain and any manipulation of the stifle joint will likely cause a reaction.

Treatment for mild cases is trotting exercises and hacks to develop the muscle that supports the side of the joint. Avoidance of sharp turns is best to prevent further periodical fixation of the patella. Anti-inflammatory medication will help relieve the pain associated with the disorder. For severe cases where the joint is locked in extension, surgically cutting the medial patellar ligament will resolve it.

Overview

A healthy stifle is essential for maximum performance. Proper conditioning to ensure adequate muscling alongside the joint is paramount to prevent avoidable lameness and will help you to get the most out of your horse's potential.

Learn more about this author, Sheri Spencer.
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