Overview
The patella (patella) is the largest sesamoid bone in the body. It lies in the tendon of insertion of the quadriceps femoris and glides over the femoral trochlea, changing the line of pull of the muscle to extend the stifle. In the horse it is pyramid-shaped, with a proximal base and a distal apex, and it is part of the stay apparatus of the pelvic limb.
Key features
- Articular surface (facies articularis): caudal, fits the femoral trochlea.
- Parapatellar fibrocartilage (fibrocartilago parapatellaris): extends the patella medially; with the medial patellar ligament it forms a loop that grips the medial ridge of the trochlea.
- Patellar ligaments: three, lateral, intermediate and medial (ligamentum patellae laterale, intermedium et mediale), running from the patella to the tibial tuberosity. They are easily palpated on the cranial aspect of the stifle.
- Locking mechanism: when the horse is at rest, the patella rotates medially and its loop of fibrocartilage and medial patellar ligament hooks over the tubercle of the medial ridge of the trochlea. The stifle is locked in extension with almost no muscular effort and, through the reciprocal apparatus (peroneus tertius and superficial digital flexor), so is the hock. This lets the horse doze standing, with its weight on one pelvic limb while resting the other.
Clinical relevance
- Upward fixation of the patella: the patella stays hooked and the horse cannot release it; the limb locks in extension and the animal drags the toe. It can be intermittent, with a momentary catch at walk, and is more common in young horses, horses in poor condition or with an underdeveloped quadriceps. It is treated first with exercise to strengthen the muscle; medial patellar desmotomy is kept for persistent cases, because it can cause fragmentation of the patellar apex.
- Patellar fractures: from kicks or hitting obstacles when jumping.