Patellar Instability

The kneecap (patella) sits in a groove at the end of the thigh bone called the trochlea, held in place by ligaments, tendons, and muscles. Patellar instability occurs when the kneecap slips partially (a subluxation) or fully (a dislocation) out of the groove, almost always toward the outside of the knee. Most patients describe this as their kneecap “popping out.”

The most important soft tissue stabilizer is the medial patellofemoral ligament (MPFL), a ligament on the inner side of the knee that acts as a tether, preventing the kneecap from shifting outward in early knee bending before it is seated in the groove. In most first-time dislocations, this ligament is torn, and it often heals in a stretched position that no longer provides reliable restraint.

The bony anatomy matters just as much. A deep groove holds the kneecap securely, while a shallow or flat groove offers little resistance. When the underlying anatomy is abnormal, the risk of repeat instability increases substantially.

A first-time dislocation usually happens during a twisting or pivoting motion with the foot planted, such as cutting in soccer, landing from a jump, or dancing. Less commonly, it results from a direct blow to the side of the knee.

Many patients, however, have anatomic features that make the kneecap easier to dislocate in the first place.

• Trochlear dyplasia: An abnormally shaped groove. These can be shallow, flat, or sometimes with a bony “bump” at its entrance, that provides little bony support for the kneecap. This is the most significant anatomic risk factor for instability.
• Patella alta: A high-riding kneecap that sits above the groove and engages it later during knee bending, leaving it unsupported for longer.
• Lateralized tibial tubercle: When the bony attachment of the patellar tendon sits too far toward the outside of the knee, the pull of the tendon draws the kneecap outward. This is measured as the tibial tubercle to trochlear groove (TT-TG) distance.
• Rotational malalignment: Excessive inward rotation of the thigh bone (femoral anteversion) or outward rotation of the shin bone changes the direction of pull on the kneecap.
• Knock knees (valgus alignment): Increases the outward force on the kneecap.
• Generalized ligamentous laxity: Naturally loose joints provide less soft tissue restraint.
• Young age, skeletal immaturity, and a history of dislocation in the opposite knee also increase the likelihood of future episodes. Understanding which of these factors are present is the foundation of an effective treatment plan.

During a dislocation, the kneecap may visibly shift to the outside of the knee and either pop back into place as the leg straightens or need to be put back. Common symptoms include:

• Pain and swelling: Significant swelling typically develops quickly after a dislocation, often from bleeding within the joint.
• Tenderness on the inner side of the kneecap: Pain where the MPFL has torn.
• Giving way: A feeling that the knee is unstable or might buckle, especially with twisting, cutting, or going down stairs.
• Apprehension: A sense of fear or guarding when the kneecap is pushed outward or during activities that previously caused an episode.
• Catching or locking: A dislocation can break off a piece of cartilage or bone, which can float in the joint and catch. This requires prompt evaluation.
• Pain at the front of the knee: Aching around the kneecap with stairs, squatting, or prolonged sitting, which may reflect cartilage injury.

Dr. Rynecki Baker encourages patients not to dismiss repeated episodes. Each dislocation can damage the cartilage on the back of the kneecap and in the groove, and this damage accumulates over time.

Evaluation begins with a detailed history, including how the first episode occurred, how many episodes have followed, and whether there is a family history of instability or a prior episode in the opposite knee. Dr. Rynecki Baker then performs a thorough exam, including how far the kneecap can be shifted outward, apprehension, how the kneecap tracks as the knee straightens (the J-sign), overall joint laxity, and the rotational alignment of the hips and legs.

Imaging is essential for identifying the anatomic factors that drive treatment:

X-rays are used to assess the shape of the groove, the height of the kneecap, how the kneecap sits within the groove, growth plate status in younger patients, and any bone fragments from the dislocation. Full-length standing x-rays are obtained when knock-knee alignment is suspected. An MRI evaluates the MPFL, the cartilage on the kneecap and groove, loose fragments, the depth and shape of the groove, and the TT-TG distance. A special CT may be obtained in specific circumstances to measure rotation of the thigh and shin bones when rotational malalignment is suspected on exam. Together, these findings allow Dr. Rynecki Baker to identify exactly why a kneecap is unstable, rather than treating every dislocation the same way.

Patellar instability is a complex condition, and treatment success depends on accurate diagnosis, appropriate patient selection, and surgical precision.

Dr. Rynecki Baker’s training at both NYU Langone Health and Hospital for Special Surgery, her work with the JUPITER Study Group, and her focused research in patellofemoral instability, including a 10-year outcomes study of MPFL reconstruction and evaluation of which patients are best served with surgical treatment after their first dislocation, mean patients are cared for by a surgeon dedicated to understanding this condition at the highest level. This is why she was selected to be on a panel of expert speakers at NYU’s 2026 Patellofemoral Instability: Current Concepts and Controversies Course.

She also understands injury from the patient’s side. A former collegiate soccer player, Dr. Rynecki Baker returned to play after multiple knee injuries and knows the physical and mental challenges of coming back. She is committed to honest communication, precise diagnosis, and a plan built around each patient’s anatomy and goals.