Consultant Orthopaedic Knee and Trauma Surgeon
Skiing Knee Injury

Patella Instability (kneecap dislocation)

The kneecap joint is the connection between the kneecap (patella) over the thigh bone (femur). This joint transmits the forces of the thigh muscles (quadriceps) over the front of the knee down to the front of the shin bone (tibia). The lever arm of the quadriceps muscle is lengthened by the patella which decreases the amount of energy required to straighten the knee. This joint has the thickest cartilage of any joint in the body (up to 7mm thick) due to the forces it has to deal with. The stability of the kneecap is dependant upon the shape of the joint, the ligaments and the muscles. Because of the orientation of the quadriceps muscles and the patella tendon there is a constant force trying to move the patella outwards.

Dislocation means complete separation of a joint when the two bones move away from each other. A subluxation is a partial dislocation with some contact remaining. When the kneecap dislocates it moves from the anterior (frontal) position to translate laterally (outer side of the knee).

Usually it is either a traumatic event e.g. sporting injury with a lot of force transmission or it's down to the underlying shape of the joint being slightly abnormal. Luckily a traumatic dislocation usually occurs in an entirely normally shaped joint and seldom occurs again. If the joint is slightly abnormal in shape the patella can often dislocate again and can be troublesome particularly in teenage years.

Immediately following a first dislocation patients report symptoms of pain, swelling, weakness, giving way and difficulty walking. The patella may relocate (reduce) spontaneously or may need to be reduced by a medical practitioner in hospital.

As with most knee problems, the diagnosis is mostly made from the history of symptoms and is confirmed by examination of the knee and investigations. Kneecap dislocation can be confused with an ACL rupture due to the similar mechanism of injury, but examination should distinguish between the two. An x-ray is necessary to exclude any associated bony injury and an MRI scan is needed if symptoms persist and surgery is being planned or there is diagnostic uncertainty.

Treatment will initially focus on recovery from the acute injury with Rest, Ice, Compression and Elevation (RICE). Painkillers and anti inflammatories will undoubtedly also be necessary. Surgery in the early phase following a dislocation is rarely appropriate although the kneecap may need to be manipulated back into place. Occasionally a loose piece of bone may need to be reattached or removed. Physiotherapy is essential to restore muscle control and strength particularly the strong muscles on the inside of the thigh (the VMO).

Later treatment will depend upon the progress of symptoms, the type of dislocation and the severity of any underlying shape abnormality.

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