Patellar (kneecap) instability

The patella (kneecap) sits within the quadriceps tendon at the front of the knee and glides in a groove — the trochlea — at the end of the thigh bone as the knee bends and straightens. Soft tissues on the inner side, along with the shape of the groove and the alignment of the leg, help keep the kneecap centered.

Patellar instability means the kneecap moves partly or completely out of the trochlear groove, usually toward the outer (lateral) side, or feels as though it might. It can follow a specific injury, or relate to the underlying anatomy of the knee — often a combination.

The medial patellofemoral ligament (MPFL) is a soft-tissue restraint on the inner side of the patella that helps resist the patella sliding too far to the outer side. The MPFL is commonly injured when the patella dislocates, which is one reason a first dislocation can make later instability more likely.

A patellar dislocation occurs when the kneecap comes completely out of the groove; it may go back into place on its own or need to be reduced. A subluxation is a partial slip in which the kneecap shifts but does not fully dislocate. Both can injure the structures that stabilize the kneecap and can leave the knee feeling unreliable.

A first-time dislocation is evaluated to confirm the kneecap is back in place and to look for associated injuries. Many first-time events are treated without surgery. Recurrent instability — repeated dislocations or subluxations, or a knee that keeps feeling like it may give way — is a different situation, and the ongoing apprehension it causes is part of what is discussed when weighing options.

Some instability begins with a clear traumatic event, such as a twist or a direct blow. In other people, features of the underlying anatomy make the kneecap more prone to slipping. These factors can include the alignment of the leg, the height of the kneecap, the shape and depth of the trochlear groove, skeletal maturity (whether growth plates are still open), and any previous injury to the bone or cartilage. Understanding which factors are present helps individualize treatment.

Symptoms depend on whether the kneecap has fully dislocated and on how often instability happens.

• The kneecap visibly slipping out of place, or a sense that it might
• Pain and swelling, especially after a dislocation
• A feeling of the knee giving way or catching
• Apprehension with certain movements, such as twisting or deep bending
• Difficulty returning to sports and activities

Evaluation begins with the history of what happened and a physical examination of how the kneecap tracks and how the knee is aligned. X-rays are used to assess alignment, the position of the kneecap, and bony injury. Advanced imaging, such as an MRI, is used selectively — for example to look at the MPFL, the cartilage surfaces, loose bodies, or the shape of the groove when that information will affect the plan.

After a dislocation, part of the evaluation is checking for a fracture, a loose piece of bone or cartilage within the joint, and cartilage injury. In younger patients whose growth plates are still open, those growth-plate considerations also affect how the knee is evaluated and treated.

Nonsurgical treatment commonly focuses on settling pain and swelling and then rehabilitation to restore motion, strength — particularly of the quadriceps and hip — and control of the kneecap. Activity modification, and sometimes bracing or taping, may be used while the knee recovers. Many people regain dependable function this way, particularly after a first-time dislocation.

Surgery is not automatic. It may become part of the conversation when instability keeps happening despite rehabilitation, when there is a loose body or a repairable injury inside the knee, or when anatomy or patient-specific factors that can increase recurrence risk are present. Which operation is considered depends on the individual — soft-tissue reconstruction of the MPFL addresses the restraint that resists the kneecap sliding outward, while some knees also have alignment or other bony factors that a surgeon may address with an additional procedure. These decisions are made together, based on your examination, imaging, and goals.

Whether instability is likely to return is influenced by many factors, including age and skeletal maturity, activity and sport, the number of previous events, whether tissue or cartilage was injured, and features of the underlying anatomy. No single factor determines treatment, and the trade-offs are weighed individually with your surgeon.