Patellar Dislocation

Patellar dislocation or subluxation — specific symptoms and causes

Patellar dislocation occurs when the kneecap is pulled out of the femoral groove (the trochlea) as a result of trauma or a heavy blow, but also because of a strong muscle contraction or a fall onto the knee.

More specifically, patellar dislocation occurs in two situations:

• traumatic — after a direct, forceful blow to the knee
• in patellar instability — when the kneecap dislocates spontaneously as the knee moves, or after minor trauma

The symptoms of a patellar dislocation are:

• the kneecap is out of its groove and sits on the outer side of the knee
• a sensation of the knee popping
• severe pain
• inability to straighten the leg or to walk
• the knee may swell

Patellar subluxation, by contrast, occurs when the kneecap slips slightly and is no longer centred in the femoral groove, but is not completely dislocated, because the joint surfaces remain in contact.

Patellar instability causes the kneecap to track incorrectly in its groove, which damages the cartilage and later leads to osteoarthritis.

Factors leading to patellar instability

The main causes of patellar dislocation are:

• trochlear dysplasia — the groove for the kneecap is not deep enough, so the kneecap can easily slip outwards
• a high-riding kneecap (patella alta), which may be congenital or the result of an injury
• a laterally positioned tibial tubercle
• weakening of the medial patellofemoral ligament, which stabilises the kneecap on the inner side

How patellar dislocation is diagnosed

In most cases, when the kneecap dislocates it returns to position on its own or is put back by the patient. When this does not happen, the patient must be taken to hospital urgently.

A diagnosis of patellar dislocation is established on the basis of clinical and radiological examination. Investigations include:

• MRI — this allows the doctor to determine whether the dislocation has damaged the ligaments, tendons or cartilage of the knee.
• X-ray — allows the doctor to check the bony structure of the joint, particularly in a severe dislocation.
• CT scan — provides further information about the factors causing the patellar instability.

Treatment

Patellar dislocations are usually lateral and can be treated without surgery.

Non-surgical treatment of a patellar dislocation consists of:

• anti-inflammatory drugs or painkillers to reduce severe pain and swelling
• the RICE protocol: rest through immobilisation of the knee, ice packs, an elastic bandage, and elevation of the leg
• immobilising the knee in a cast or brace to hold the kneecap in the correct position and allow the medial patellofemoral ligament to heal
• using crutches to reduce pressure on the knee until healing is complete

Surgical treatment

This is recommended for recurrent patellar dislocations or for patellar instability. Depending on the factors responsible for the instability or recurrent dislocation, surgical treatment may consist of:

• deepening the femoral trochlea (the groove for the kneecap) by trochleoplasty
• reconstruction of the medial patellofemoral ligament
• distalisation and/or medialisation of the tibial tubercle (where the patellar tendon attaches to the tibia)

It is worth noting that the operations listed above can be performed either on their own or in combination.

Recovery after a patellar dislocation

Whether or not the patient needed surgery to treat the dislocation, rehabilitation is necessary.

After surgery, physiotherapy, kinesiotherapy and massage sessions are recommended to strengthen the joint and muscles, so that the knee regains its mobility and the muscles regain the strength they need. Recovery usually takes several months before the patient is ready for sporting activity.

Rehabilitation involves an individualised set of specific exercises, tailored to the patient's needs, and wearing a brace. To return to sport safely, the patient must pass a number of specific tests.