What is Patella Dislocation?

Patella Dislocation: The knee joint is composed of the thigh bone, (femur) and leg bone (tibia) and a small floating bone at the front, commonly known as the kneecap (patella). The interaction between these bones allows for smooth movement of the knee as it bends and straightens.

During movement, the kneecap sits in a groove at the front of the knee and acts as a mechanical see-saw. This protects the knee joint and improves the efficiency of the muscles working to move it. When the patella moves out of this groove it is called a subluxation. If the patella moves far enough out of this groove it becomes a Patella dislocation.

What are the symptoms of Patella Dislocation?

The first occurrence of patella dislocation is usually the most traumatic and painful. The knee may give way, and a visible lump can be noticed where the patella has dislocated. There will often be bruising, swelling and the knee may feel unstable. First time patella dislocation may also cause a hemarthrosis or bleeding within the knee joint. If there is damage to the ligaments of the knee, subsequent dislocations can happen more easily, and from everyday activities, causing the knee to give way suddenly.

What are the causes?

First time dislocations often occur due to a traumatic event. The most common cause of patellar dislocation is a non-contact injury to the knee with a twist of the leg (the thigh bone rotates internally on a fixed leg and foot). In addition, a direct blow to the side of the knee can also dislocate the patella.

If there is some instability of the joint, dislocations can occur more regularly and from smaller forces. Dislocations usually occur when the knee is bent and the kneecap slips back into place when the knee is straightened again. While the kneecap can be dislocated in both directions, it usually dislocates towards the outside of the knee.

Certain factors can make dislocation more likely, including overall hyper-flexibility, damage to the ligaments of the knee and muscular imbalance of the quadriceps. The structure and angle of the knee joint itself can also make dislocation more likely. This can be seen in the increased prevalence of dislocations for women as they have a slightly different angle of femur compared to the tibia than men. A traumatic dislocation can cause instability that can lead to future dislocations.