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.

The symptoms of a patellar subluxation include:

• pain at the front of the knee
• a sensation that the kneecap is slipping outwards and that the knee is unstable

Most commonly, the kneecap dislocates because of congenital developmental problems of the knee. In this situation we speak of patellar instability, which appears chiefly in children and adolescents, but can also affect adults if these problems are not treated in time.

Patients often come to the orthopaedic surgeon not because of the sensation of instability — which some of them get used to — but because of pain at the front of the knee and clicking as the knee moves.

Patellar instability causes the kneecap to track incorrectly in its groove, the femoral trochlea, which destroys the cartilage and later leads to osteoarthritis. The first signs of cartilage destruction are clicking sensations as the knee bends and straightens; pain follows later.

Factors leading to patellar instability

The main causes of patellar dislocation are:

• trochlear dysplasia (image 1 below) — 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 (image 2 below)
• weakening of the medial patellofemoral ligament, which stabilises the kneecap on the inner side (image 3 below)

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, where the dislocation will be reduced.

A patellar dislocation is diagnosed by the treating doctor. Investigations include reviewing the patient's medical history, followed by clinical examination of the knee and of other parts of the lower limb such as the foot and the hip.

A diagnosis of patellar dislocation is established on the basis of clinical and radiological examination. When the trauma that caused the dislocation was not severe, or when the patient has had previous dislocations (recurrent dislocation), imaging plays an important role in establishing the causes.

In general, the following investigations are carried out:

• MRI — this allows the doctor to determine whether the dislocation has damaged the ligaments, tendons or cartilage of the knee. The magnetic waves create a clear image of the tissues in this area.
• X-ray — allows the doctor to check the bony structure of the joint, particularly in a severe dislocation, when swelling of the knee is so pronounced that the kneecap can only be examined with an X-ray. It also allows the doctor to determine whether the kneecap has been fractured by the dislocation, or whether it sits higher than the normal limit.
• CT scan — provides further information about the factors causing the patellar instability.

How a patellar dislocation is treated and what recovery involves

Patellar dislocations are usually lateral and can be treated without surgery. When a patellar dislocation occurs, the medial patellofemoral ligament tears.

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 (Rest), ice packs (Ice), an elastic bandage for gentle compression (Compression) and elevation of the leg (Elevation).
• sometimes the kneecap can be returned to its anatomical position by applying pressure to its outer edge while the knee is flexed
• 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 (images below)
• 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. When surgery is not carried out in good time, cartilage damage can develop on the kneecap. In that situation, cartilage reconstruction at that site is performed during the same operation.

After surgery, the aim of rehabilitation is to protect the knee until healing is complete.

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, methods for relieving pain, and wearing a brace. The orthopaedic surgeon will also recommend a period of rest for a full recovery. To return to sport safely, the patient must then pass a number of specific tests.