Fig. 1.

The anatomy of the left stifle joint (cranial view). When locked, the patella (1) with parapatellar fibrocartilage (2) is shifted behind the top of the prominent medial ridge of the femoral trochlea (3). Bending of the stifle joint is now prevented by the medial (4), intermediate (5) and lateral (6) patellar ligaments. The quadriceps femoris muscle, inserting to the patella, serves to extend the stifle joint. It may assist in keeping the patella in locked position. The direction of pull of its three main portions, the vastus medialis (7), rectus femoris (8) and vastus lateralis (9) is shown by arrows.