Patellofemoral Instability

What is patellofemoral instability?

The patella, or kneecap, is one of the three bones that make up the knee joint. All three are covered with cartilage where their surfaces come into contact. The patella is a sesamoid bone — it sits within the thickness of a tendon, the quadriceps tendon.

Patellofemoral instability comes in two forms:

• Primary
• Secondary

Primary patellofemoral instability is a condition in which the extensor mechanism of the knee — made up of the quadriceps muscle, the quadriceps tendon, the kneecap and the patellar tendon — is malaligned, meaning it does not follow the normal anatomical axes. The consequence is that subluxation or dislocation of the kneecap becomes possible. Secondary patellofemoral instability is the instability that appears after a knee dislocation, when multiple ligament injuries occur.

What is a patellar dislocation?

The kneecap glides in a groove at the lower end of the femur as the knee bends. This groove is called the trochlea. In normal anatomy the kneecap is centred in this groove and glides evenly. In patients with patellofemoral instability, because the extensor mechanism is malaligned, the kneecap is subjected to outward-directed forces. When those forces cause the articular surface of the kneecap to lose contact with the femoral trochlea, a patellar dislocation occurs.

What is a patellar subluxation?

A patellar subluxation is the situation in which the kneecap is not centred on the femoral trochlea and the articular surfaces are only partly in contact. Unlike a dislocation, in a subluxation the articular surfaces remain in contact. This too happens under outward-directed forces produced by a malaligned extensor mechanism.

What causes it?

Primary patellofemoral instability is caused by conditions that disturb the extensor mechanism:

• A kneecap positioned too high (patella alta)
• A tibial tubercle positioned too far laterally
• Trochlear dysplasia

Another situation in which patellofemoral instability appears, but without malalignment of the extensor mechanism, is ligamentous laxity at the knee.

Secondary patellofemoral instability is caused by rupture of the medial patellofemoral ligament — the main stabiliser of the kneecap — during a patellofemoral dislocation.

What are the symptoms?

The patient may describe:

• pain at the front of the knee, below and around the kneecap
• difficulty going up or down stairs
• discomfort on the first few steps after sitting for a longer period
• joint locking
• joint instability — the sensation that the kneecap "runs away"

Conservative treatment

This is the same for both primary and secondary patellofemoral instability and consists of:

• An extension brace for immobilisation — if the knee is swollen and tender
• Strengthening the quadriceps, particularly the VMO (vastus medialis obliquus) — this can stabilise the kneecap by taking over the function of the medial patellofemoral ligament, the main stabiliser of the kneecap
• Patellar taping techniques — these can help the kneecap glide centrally in the trochlea

Surgical treatment

For primary patellofemoral instability the following surgical solutions exist:

• An extension brace for immobilisation — if the knee is swollen and tender
• Medialisation, distalisation or advancement of the anterior tibial tubercle — this involves moving the bony prominence into which the patellar tendon inserts; doing so reduces the outward-directed forces acting on the kneecap
• Trochleoplasty — surgical reshaping of the trochlea to restore an anatomical form

For secondary patellofemoral instability, repair of the medial patellofemoral ligament — the main restraining structure opposing dislocation — is recommended when it is torn.