Patellar Instability & Dislocation

Patellar instability is your kneecap not staying where it belongs. It runs from a kneecap that just slips slightly off track (a subluxation) all the way to one that pops fully out of the groove at the end of your thigh bone (a dislocation). First dislocations usually happen to teens and young adults during a sudden pivot or a direct blow to the side of the knee. When the kneecap pops out — almost always toward the outside of the leg — it tears the inner ligament that holds it in place (the medial patellofemoral ligament, or MPFL) and often chips cartilage off the back of the kneecap and the side of the thigh-bone groove.

Once it's happened, it tends to want to happen again — about 15-40% of adults dislocate a second time, and the rate is higher in teens. The kneecap is more likely to keep popping out if your groove is shallow (trochlear dysplasia), your kneecap rides high on the joint (patella alta), or the bump on your shin where the patellar tendon attaches sits too far to the outside relative to the groove.

A sudden giving-way of the knee during a pivot, twist, or direct blow — often with a tearing sensation on the inner side of the kneecap. The kneecap visibly shifts to the outside of the knee, and the joint fills with blood and swells rapidly.

After the kneecap pops back in (which usually happens on its own when you straighten the leg), the inner edge of the kneecap is tender and the knee feels unstable — as though it could slip again at any moment. Subsequent episodes may feel less dramatic but still leave the knee swollen and unsteady.

An acute dislocation looks dramatic — the kneecap visibly sits off to the outside of the knee — though it usually pops back in on its own when you straighten the leg. After it goes back in, the knee fills with blood and is tender along the inner edge where the ligament tore. Your provider checks for the apprehension sign — pushing the kneecap toward the outside while the leg is straight makes you brace and pull away because it feels like it's about to slip again. X-rays include a sunrise view that shows the kneecap sitting in (or out of) its groove. An MRI shows whether the inner ligament is torn, whether any cartilage chipped off, and whether the groove itself is shallow.

If a chunk of cartilage and bone broke off when the kneecap dislocated, that piece needs to be put back surgically — soon, before it can do more damage. For someone whose kneecap keeps dislocating — especially with a shallow groove or other anatomy that stacks the deck against them — surgical reconstruction is the way to make it stable for good.

External patient-education references and related OSI pages for additional background:

• Britannica — Dislocation
• Britannica — Knee (anatomy)
• AAOS OrthoInfo — patient education library
• OSI glossary — orthopedic terminology in plain language