The kneecap (patella) lies at the front of the knee joint within the tendon of the quadriceps muscle (the large muscle group on the front of the thigh). The function of the kneecap is to increase the leverage from the quadriceps when straightening the knee. The kneecap lies within a groove (patellofemoral groove) in which it slides vertically during knee flexion and extension (bending and straightening of the knee).
Kneecap dislocation occurs when the patella moves completely outside of its groove. This can occur as a result of a high energy injury. In some individuals the kneecap can dislocate without trauma for example after twisting awkwardly. This may be because the grove that the kneecap sits in is very shallow, the ligaments are slack, or the quadriceps muscle does not pull the kneecap centrally within the groove.
If you dislocate your kneecap there is severe pain and almost immediate swelling of the knee. You may have the feeling of something “popping out”, and might notice the kneecap has jumped sideways. The dislocation usually reduces spontaneously on straightening of the knee, however this is often extremely uncomfortable, requiring strong pain relief. Diagnosis is usually made by clinical examination although X-rays are performed at various angles to be sure that the kneecap is back in joint and that there has not been a fracture.
After the kneecap has been relocated, treatment is with physiotherapy, to regain the movement in the joint and to stretch and strengthen the leg muscles. Rehabilitation has good results but a few people will go on to have recurrent dislocation of the kneecap. In these cases, surgery can often help. There are various surgical options, depending on the cause of the instability. For example surgery can deepen the groove the kneecap sits in, centralise the patellar within the groove or repair the ligaments which support the kneecap.