Kneecap dislocation (patella dislocation)

The patellofemoral joint

The knee joint is made up of three separate and distinct articulations. The patellofemoral joint is the articulation of 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 kneecap increases the lever arm of the quadriceps thereby reducing the force needed to straighten the knee (the longer the lever the lighter the load). The added advantage is that the kneecap forms a joint with the thigh bone that is lined with cartilage which reduces friction. This joint has the thickest cartilage of any joint in the body (up to 7mm thick) due to the forces transmitted.

The stability of the kneecap is dependant upon the shape of the joint, the ligaments and the muscles with an overall general tendency for the kneecap to move outwards due to the alignment of the thigh bone on the shin bone.

What is a dislocation?

Dislocation means complete disruption of a joint where the two bone surfaces move away from each other. A subluxation is a partial dislocation.

When the kneecap dislocates it moves off the front of the thigh bone to sit on its outside.

When does it occur?

The kneecap will generally dislocate in one of two ways: A traumatic dislocation involves a significant force often during sport knocking the kneecap out of joint. Occasionally the kneecap will dislocate with a powerful twisting movement of the knee similar to that which often causes an ACL rupture (link to that section please) and the diagnosis can therefore be missed. Traumatic dislocation usually occurs in an entirely normally shaped joint and rarely occurs again. More commonly the kneecap will dislocate with very much less force ( atraumatic dislocation) due to an underlying abnormality of the shape of the joint (similar to a clicky or dislocated hip in a newborn). This will usually become a recurrent problem often in the mid to late teens. The most common abnormality is a shallow or non-existent groove (trochlea) on the front of the thigh bone (called trochlea dysplasia), with less commonly a high kneecap (patella alta) or twisted thigh and shin bones (femoral and tibial torsion). Frequently there will be a history within the family and generalised flexibility of the joints.

What are the symptoms?

Immediately following a first dislocation, the symptoms are of pain, swelling and weakness, difficulty weight bearing and a feeling of instability. The kneecap may relocate (reduce) spontaneously or may need to be reduced by manipulation with the knee held straight. Following a traumatic dislocation a full recovery is expected although on return to sporting activities a feeling of instability or untrustworthiness may persist.

Following a non traumatic dislocation in a knee with underlying abnormality of shape, it is very common for symptoms of instability, swelling and pain to persist and around half of people will have further true dislocations. The persistent symptoms can be extremely disabling but are sadly often overlooked.

How is it diagnosed?

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. The apprehension test (picture)will suggest a dislocation.

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.

What is the treatment?

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 invaluable in restoring muscle strength and control.

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

There are three operations commonly used for persistent symptoms either alone or in combination. A fourth operation called a lateral release is also used by many surgeons but in my opinion is rarely appropriate used alone.

Medial patellofemoral ligament reconstruction
The medial patellofemoral ligament is a fibrous structure that runs from the inner aspect of the kneecap to the inner aspect of the thigh bone. It is said to provide up to 60% of the stability to the kneecap in a normal shaped knee and has to rupture to allow the kneecap to dislocate.

I reconstruct this ligament using keyhole surgery and use a single hamstring tendon as the graft. Repair of the ligament alone is seldom successful. This is an extremely effective operation in the right situation. I have the largest series of this operation in the UK and have presented and published my results.

Trochleoplasty
In people in whom the groove on the front of the thigh bone has failed to develop, a bump rather than groove will be present resulting in a high chance of recurrent dislocations of the kneecap off the outside of the bump. The logical solution is to create a groove rather than try to hold the kneecap on top of the bump. This operation to create a groove is called a trochleoplasty. It is a highly specialised operation done by a handful of surgeons across the world. I have the second largest world series of this operation and have again published my results. 93% of patients would have the operation again and would recommend it to others. A number of different operations have been grouped under the umbrella term of trochleoplasty and care must be taken when researching these.

Tibial tubercle osteotomy
The tibial tubercle is the lump on the front of the shin bone where the tendon from the kneecap joins the shin bone. An osteotomy is a detachment of the lump of bone and its reattachment in a different place using screws to hold it in the new position. The bone will then heal in around 6 weeks taking the load off the screws. This operation is usually appropriate in people who have a high kneecap or twist in the thigh and shin bones. It is important that this operation is not used in all people with kneecap dislocation as we know that it can increase the risk of arthritis if used incorrectly.