Stifle, Stifle Joint
Four bones form the stifle joint: the femur, the tibia, the fibula, and the patella (kneecap). The major bones of this hinge joint are the femur and the tibia. In the middle of this joint and attached to these two bones are the cruciate ligaments. Two small cartilages, the menisci, are also interpose between the joint surfaces of the two bones. The fibula is a small bone situated on the outside of the tibia and has little function. It is often formed frow two or more separate ossification centers which sometimes never unite and this should not be confused with a fracture. The patella slides up and down the bottom end of the femur.
Attached to the inside of the patella is a large cartilaginous extension. Three ligaments, the medial, middle and lateral patellar ligaments, run from the bottom of the patella to the tibia. This unique arrangement of the patellar cartilage and ligaments allows the horse to hook the patella over the top of the trochlear ridges of the femur and fix the stifle in extension. This means that the horse can stand with little muscular effort, so can sleep standing up. The stifle joint cannot flex independently of the hock because of another unique arrangement of muscles and tendons, the reciprocal apparatus, so stifle and hock lameness can appear similar.