Patellar instability and kneecap dislocation

The kneecap glides in a groove at the end of the thigh bone. It is held in that track by the shape of the groove, by the pull of the quadriceps and by ligaments on the inner side of the knee — principally the medial patellofemoral ligament, the MPFL. Patellar instability means the kneecap leaves, or nearly leaves, that track, almost always toward the outside of the knee.

A full dislocation is dramatic and often reduces itself when the knee straightens. A subluxation is subtler: a shift, a giving-way or a sense that the kneecap is about to go. Both can chip cartilage from the back of the kneecap or the outer edge of the groove, which is why a first episode still warrants imaging.

Patients describe a distinct set of complaints, and often can point to exactly which direction the kneecap moves.

Some dislocations are purely traumatic — a direct blow or an awkward landing in a knee with normal anatomy. More often, a relatively minor twisting movement dislocates a kneecap in a knee that was predisposed to it.

Predisposing features include a shallow or flat trochlear groove, a kneecap that sits higher than usual, an increased distance between the tibial tubercle and the groove, rotational alignment differences in the femur or tibia, knock-knee alignment and generalized ligamentous laxity. These occur more frequently in adolescents and in young women, and they are the reason recurrence rates after a first dislocation are meaningful in this group.

General guidance rather than individual medical advice: any kneecap that has dislocated should be evaluated, because a loose cartilage or bone fragment inside the joint changes management and is best identified early.

Evaluation is also warranted for a knee that has dislocated more than once, a kneecap that keeps slipping during sport or stairs, persistent catching or locking after an episode, or ongoing swelling and pain that has not settled with initial rest and therapy.

The examination looks at alignment, kneecap tracking through range, tenderness over the MPFL, apprehension with lateral translation and overall laxity. Quadriceps strength and hip control are assessed, since both influence how the kneecap tracks.

X-rays, including a view down the groove, show bony shape, kneecap height and any fracture. MRI shows the MPFL, the cartilage on the back of the kneecap and the groove, and any loose fragment. Measurements taken from imaging — groove depth, kneecap height and tubercle position — guide whether ligament reconstruction alone will address the problem or whether a bony procedure should be added.

A first-time dislocation without a displaced cartilage fragment is generally treated without surgery. Initial care controls swelling and protects the knee briefly, then moves quickly to motion so the joint does not stiffen.

Rehabilitation is the substance of treatment: quadriceps strengthening with attention to the inner quadriceps, hip abductor and external rotator strength, control of knee position during landing and cutting, and gradual reintroduction of sport-specific loading. A patellar stabilizing brace or taping can support confidence during that transition.

The honest limitation is that rehabilitation strengthens the muscles around a knee whose ligament is torn and whose bony shape is unchanged. It works well for many patients, particularly those with favourable anatomy, and less reliably for those with several predisposing features.

Surgery is considered after recurrent dislocations, after a first dislocation that leaves a loose osteochondral fragment requiring fixation or removal, and when instability continues to interrupt sport, work or ordinary confidence on stairs despite a committed rehabilitation program.

The purpose is to restore the restraint that keeps the kneecap tracking and, where indicated, to correct the bony factors driving it outward. As with any stabilization procedure, the aim is to reduce the risk of recurrence substantially — not to guarantee it will never happen again.

Procedures are selected from imaging measurements and examination, and are occasionally combined.

After MPFL reconstruction alone, weight-bearing usually begins early in a brace, with motion progressed under therapist supervision over the first six weeks. Strengthening builds through months two to four, running is typically reintroduced around three to four months, and cutting or pivoting sport is generally considered at about six months based on objective testing.

When a tibial tubercle osteotomy is added, weight-bearing is restricted for roughly six weeks to protect bone healing, and the overall timeline extends.

These are typical ranges provided for education. Your protocol is written for you and shared with your therapist so everyone is progressing against the same criteria.

Nolan Horner, MD is a board-certified orthopedic surgeon with sports medicine fellowship training at Rush University Medical Center and experience caring for professional athletes with the Chicago Bulls, Chicago White Sox and Chicago Fire.

Patellar instability is a condition where the same operation is not right for every knee. Measurements from imaging drive the recommendation, and those measurements are shown and explained to patients and families rather than summarized as a verdict.

This page is general educational information and is not a substitute for an in-person evaluation.