Your kneecap (patella) sits in a groove on the front of the thigh bone (called the trochlea) and tracks up and down the groove as the knee bends and straightens. If the kneecap is displaced from this groove, it can partially or fully dislocate. This almost always happens towards the outer (lateral) side of the knee, not inwards towards your other knee.
Traumatic lateral dislocation of the patella – the kneecap is displaced to the outside of its normal position.
In a partial dislocation, it moves rapidly back into line (sometimes with a painful clunk) but with a full dislocation it can become stuck over the outer side of the knee joint with severe pain and inability to move the joint.
Immediate treatment involves getting the kneecap back into its groove. This may require a trip to the Emergency Department. X-rays are normally taken to ensure there is no fracture, and the knee may be temporarily supported in a brace or splint. Early physiotherapy is then crucial, initially to reduce swelling and pain, then to regain movement and muscle function.
Kneecap dislocation is more common in females, and often occurs first in teenage years. Usually there is a background cause that makes it more likely to happen:
- A shallower-than-normal groove (known as trochlear dysplasia)
- A high-riding kneecap that’s slow to engage as the knee bends (patella alta) or tracks poorly (lateral maltracking)
- Ligaments that are looser than normal (Joint hypermobility syndrome)
Once a kneecap has dislocated once, it becomes more likely it will happen again (recurrent dislocation) as the ligaments that normally guide its tracking are stretched or ruptured. The most crucial one of these is the medial patellofemoral ligament (or MPFL).
Any patient who has further dislocation episodes should be seen by a knee surgeon with expertise in this condition. A careful examination of kneecap position and functional movements, combined with an MRI scan, will determine which of the common predisposing factors may be present.
Comparison images above showing a normal shaped kneecap and groove vs a tilted kneecap with a limited groove.
If non-operative treatment has failed, there are operations designed to correct each of the specific problems that have been identified. These include MPFL reconstruction to keep the kneecap in the groove, and operations to move the attachment of the tendon below the kneecap on the shinbone to re-position the kneecap in the groove, known as tibial tubercle osteotomy. Both of these have a high success rate in preventing further dislocation episodes, although many weeks of careful rehabilitation are needed to ensure a good outcome.
Patella alta with poor engagement before (left) and after tibial tubercle osteotomy and fixation with plate and screws, bringing the kneecap into a more normal position.