What is Patellar Instability?
The knee joint is composed of three bones: the kneecap (patella), thigh bone (femur), and shin bone (tibia). The back of the kneecap has a ridge on it called the patellar ridge and the front of the thigh bone has a groove in it, called the trochlea. The ridge of the patella slides up and down in the trochlea as the knee flexes and extends. The shape of the ridge and groove helps to stabilize the kneecap during knee range of motion.
In addition to the bony anatomy of the kneecap and the thigh bone, the kneecap is also stabilized in the center of the groove by soft tissues, including ligaments and muscles. The main ligamentous soft tissue stabilizer of the knee is called the medial patellofemoral ligament (MPFL). The kneecap is also stabilized by the dynamic actions of the quadriceps muscle and tendon; the quadriceps is a group of four muscles including the vastus medialis, vastus intermedius, vastus lateralis, and rectus femoris.
Patellar instability can occur following an acute traumatic episode, such as a sudden twisting injury to the knee. Patellar instability can also occur in the absence of an injury in a patient who has an anatomic predisposition to patellar instability. Factors which predispose to patellar instability include weakness or atrophy of the vastus medialis muscle, a shallow trochlear groove, an abnormally shaped patella with no patellar ridge, ligamentous laxity or soft-tissue hypermobility disorders (such as Ehlers-Danlos syndrome), or other abnormal anatomic configurations involving the interaction between the patella, femur, and tibia.