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Located in the knee, the medial patellofemoral ligament links the medial epicondyle of the femur, or thigh bone, to the patella, or kneecap. Its function is to stabilize the patella, the disk-shaped bone that covers the front side of the knee joint, against lateral tracking or sliding toward the outside of the knee. The medial patellofemoral ligament alone is said to provide the majority of the resistance against patellar translation in this direction, with studies citing anywhere from 50 to 80 percent. As such it is the structure commonly torn in lateral patellar displacement injuries, or those in which the kneecap is dislocated to the outside.
At the knee joint are found both intra-articular and extra-articular ligaments. Those within the joint between the femur and the tibia bone of the shin are the intra-articular ligaments and include the anterior cruciate ligament (ACL), among others. Extra-articular ligaments are situated to the outside of the knee joint capsule; the medial patellofemoral ligament is among these. Most of the extra-articular ligaments on the front side of the knee function to hold the kneecap in place in front of the knee joint.
Just as the patellar ligament holds the kneecap in place from above and below, the medial patellofemoral holds it in place from the inside by running horizontally to attach to the medial epicondyle of the femur. This is the large bony prominence at the lower inside end of the thighbone; the lateral epicondyle is that at the lower outside end of the bone. The medial patellofemoral attaches, specifically, to the anterior or front side of the epicondyle, which lies immediately behind the ligament and lateral to the patella. Also in contact with the medial patellofemoral ligament is the vastus medialis obliquus, a muscle of the quadriceps group in the front of the thigh.
Lateral displacement injuries of the patella are a common cause of medial patellofemoral ligament ruptures. In short, when the kneecap is forced to the outside, the ligament is abruptly stretched, resulting in a tear. Studies have shown that this injury occurs most often when the knee is flexed or bent to 20 degrees and that in the vast majority of cases the tear involves the ligament pulling away from the femur bone and not from the patella. Often confused with a medial collateral ligament (MCL) tear, this injury may result in pain, swelling, and tenderness felt just to the inside of the kneecap. A ligament tear of this type will often require surgery to reattach the tissue to the bone.