Overview
Its four heads converge on the patella: rectus femoris, vastus lateralis, vastus medialis and vastus intermedius. The model shows the first three; the vastus intermedius is deep and lies on the cranial surface of the femur.
The horse has three patellar ligaments (lateral, intermediate and medial) joining the patella to the tibial tuberosity. From the medial border of the patella a parapatellar fibrocartilage continues into the medial patellar ligament and hooks around the medial trochlear ridge of the femur, which is much thicker than the lateral one.
Origin
- Rectus femoris (m. rectus femoris): body of the ilium, cranial to the acetabulum. It is the only head that crosses the hip.
- Vasti (m. vastus lateralis, medialis, intermedius): proximal femur.
Insertion
Patella and, through the patellar ligaments, the tibial tuberosity.
Action
Extends the stifle. The rectus femoris also flexes the hip.
Innervation
Femoral nerve (n. femoralis).
Clinical relevance
- Patellar lock: in the resting horse, the quadriceps lifts the patella and rotates it slightly medially so that the parapatellar fibrocartilage and medial patellar ligament hook over the medial trochlear ridge. The stifle is locked in extension with almost no muscular effort and, through the reciprocal apparatus, so is the hock. This is how the horse rests one hind limb while bearing weight on the other. To unlock, the quadriceps pulls the patella proximally and laterally.
- In upward fixation of the patella the lock fails to release: the limb is stuck rigid in extension with the fetlock flexed. It occurs in young horses, horses in poor muscular condition or after a period of rest, and is first treated with exercise to strengthen the quadriceps. Medial patellar desmotomy is kept for cases that do not respond, because it can cause fragmentation of the patella.
- Femoral nerve paralysis can follow prolonged general anaesthesia: the horse cannot bear weight on the limb and the stifle buckles.