Patella Instability & Dislocations
The knee joint is made up of the femur (thigh bone) on the tibia (shin bone) with the patella (knee cap) gliding over the end of the femur. Usually, the patella sits in the trochlear groove, which is a groove in the end of the femur. This forms the patellofemoral joint.
Dislocation of the patella occurs when the patella moves out of the patellofemoral groove. It is most commonly caused by a twisting injury or a direct blow to the knee. Factors that increase the chance of the patella dislocating include patella alta (when the patella is too high, bringing it out of the groove), trochlear dysplasia (in which the trochlear groove is shallow or domed the other way), weakness or abnormalities of the muscles that help to hold the patella in place and an abnormal alignment of the bones of the leg. When the patella dislocates, all of the soft tissue on the inner side of the patella is stretched or torn, including a ligament called the medial patellofemoral ligament (MPFL) which usually helps to hold the patella in. Up to half of people who dislocate their patella will have further dislocations. They may also feel that the patella is unstable, even if it does not completely dislocate.
The initial treatment includes RICE (rest, ice, compression and elevation) and painkillers as required. Physiotherapy is important because it helps to strengthen the muscles that stabilise the patella and regain range of movement. Braces can help to reduce pain but should be removed as soon as possible because they delay the rehabilitation and strengthening of the knee.
Surgical treatment is recommended for those individuals who have recurrent patellar dislocations. Some of the surgical options include:
- Medial Patellofemoral Ligament Reconstruction - In this procedure, a hamstring tendon is used to create a new MPFL. Tunnels are drilled into the patella and femur and the graft is inserted and fixed with a metal button and screw.
- Tibia Tubercle Transfer - The piece of bone that the patella tendon inserts into on the tibia is shifted downward to bring a high patella down to a normal position. This piece of bone is fixed with two screws.
- Trochleoplasty - If the trochlear groove is very abnormal and has a domed shape, the cartilage of the trochlea is lifted up and the bone underneath is reshaped to create a new groove. The cartilage is then laid back down and fixed in place with a dissolvable tape.
The choice of which procedure to perform is based on the specific abnormality that is causing the patella dislocations.