Patellar dislocation

Anatomy of the knee

Function and location of the patella

The knee is a joint which connects the thighbone to the bones of the lower leg. It is the largest, most complex and most vulnerable joint of the human body. Dislocation of the patella is often caused by trauma such as a blow or a sudden change in direction when the leg is planted on the ground. It is a common injury in young people and tends to occur during sports or dancing. In normal circumstances the kneecap is located over the front of the knee. It glides up and down over a groove in the joint when the leg is bent or straightened, without making any noise or causing a popping sensation.

When the patella dislocates, however, it comes out of its groove and the supporting tissues can be stretched or torn, causing a lot of discomfort such  as swelling and an inability to walk. Misalignment of the patella is a common cause of pain and instability of the knee joint.

Dislocated patella

Normally, the kneecap fits neatly into the recess on the front surface of the thighbone so both sides bear equal weight. If the shape of the kneecap or the thighbone is different, the pressure is distributed unevenly, which can lead to damage to the cartilage.

The kneecap is held in place by strong bands of fascia. Anatomical abnormalities or trauma can cause the patella to become misaligned in relation to the centerline. This is most common in lateral (outward) direction.

Patellar dislocation

What are the symptoms of a dislocated kneecap?

The most common symptom to accompany a patellar dislocation is of the problem is pain at the front of the knee or behind the kneecap, mostly felt when the knee is in a bent position:

  • walking downhill
  • squatting
  • walking on uneven ground
  • running

It is a typical symptom of kneecap dislocation when long periods of sitting cause pain in the knee.

How is patellar dislocation diagnosed?

During the initial physical examination, your doctor will look for signs of patella dislocation and associated damage to the muscle and ligament. If the kneecap is still misaligned at the time of the examination, it can easily be seen and felt.

Sometimes an imaging test (X-ray, ultrasound or MRI) is needed to confirm the diagnosis and identify damaged structures surrounding the kneecap.

Is there a surgical procedure to correct a dislocated patella?

Accessing the patellar cartilage is no easy task, since the space is narrow and the bone beneath is rather thin (about 2 cm). Lateral release (or kneecap realignment) is a minimally invasive surgery, usually performed  arthroscopically. It is used to correct an excessive tilt of the patella. It involves cutting through a tight retinaculum (a fibrous bands of fascia) so that the kneecap can go back to its proper location, restoring its normal alignment. As a result of the operation the patella can move around more freely and the pressure on the surface of the cartilage is also reduced. The use of arthroscopic techniques (with “keyhole” incisions) minimizes complications and shortens recovery time.