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Patellar Dislocation

The patella, or knee-cap, sits in front of the knee. Simply put, it is a "bone within the tendon of the quadriceps muscle". The patella functions as a pulley allowing the contraction of the quadriceps muscle of the thigh to straighten the leg.

The patella runs up and down the centre of the knee in a groove (the trochlea) on the front of the femur. Stabilizing the patella in the groove are bony factors. Principally the depth of the groove. There are also ligamentous restraints to stabilize the patella in the groove. These are the soft tissue thickenings or ligaments on either side of the knee which are to prevent it moving out of this groove to either the medial or lateral side of the knee. These are the medial patella-femoral ligament (MPFL) and the lateral patella-femoral ligament (LPFL).

Anatomy of the Knee Joint

A patient can be predisposed to the patella shifting out of the groove, called “dislocation”. Predisposing factors include a shallow groove, which is a bony abnormality. Abnormal alignment of the leg affects the direction in which kneecap is pulled when the muscle works. This pulls the patella outside the line of the groove creating relative instability. Together with this, there may be increased laxity or instability of the ligamentous constraints, which is more common in patients who have generalized hyperlaxity of the ligaments.

The patella can dislocate involving a complete loss of congruity between the patella and the trochlea groove. The patella dislocates laterally tearing the MPFL. A chondral injury to the cartilage lining of the patella can occur simultaneously.


The decision on whether to treat a patellar dislocation conservatively (without surgery) or surgically depends on

  1. Whether this is the first patellar dislocation. If there have been multiple events conservative management is likely to fail.
  2. Age. Teenagers and younger children will become less supple as they mature physically thus 'stabilizing' the patella.
  3. High or low energy injury event. In the case of a single high energy injury in a mature non-hyperlax individual the MPFL is likely to be ruptured. This will require reconstruction.
  4. A chondral injury to the cartilage lining of the patella can necessitate surgery.

Conservative Management

This involves protection in a patella brace and physiotherapy. The final phase of recovery is guided by a Biokineticist, to regain balance, power and control.


The goal of the surgery is to stabilize the MPFL, and thereby, the patella. This is done by reconstructing the MPFL using a graft. The gracilis, one of the 5 hamstring tendons is used.