The technical term for the kneecap is the patella. The patella is a small yet very hard and dense bone that sits right in front of the knee. The simplest way to explain its function is to say that it helps our thigh muscles work better to control the position of our knee. The patella moves up and down on the end of the thigh bone, also called the femur, when we straighten and bend our knee. This is normal patellar motion. However, in some people, the kneecap will move off the outside, or lateral, aspect of the knee. This is not normal and is called an unstable kneecap or patellar instability.
Patellar instability can be a debilitating condition that can take someone out of their sports or activities in the short term and lead to arthritis in the long term. Some people have their kneecap pop of place, or dislocate, as a result of a violent twisting injury to the knee. This typically happens during sports. However, a lot of other people may dislocate their kneecap doing everyday activities without ever sustaining an injury. A lot of these people have an underlying issue that predisposes the patella to dislocate. Identification of what caused the patella to become unstable is key to determining treatment.
Several factors have been identified that can cause the kneecap to dislocate, and most of them are issues with underlying anatomy, particularly the way the femur is shaped. The most common anatomic difference is called trochlear dysplasia. The trochlea of the femur is a “V’ or “U” shaped groove at the end of the femur that the patella rests in when it moves back and forth when the knee bends and straightens. The trochlea provides excellent stability to the kneecap because the undersurface of the kneecap has a similar “V” or “U” shape that allows the patella and trochlea to fit together like two puzzle pieces. For many people with patellar instability, the trochlea does not have a normally shaped groove and is thus termed “dysplastic.” A dysplastic trochlea can have several different shapes. The groove can still be present but it can be more shallow. It can also become completely flat or even become the opposite of a groove; it can be a bump.
For several people with dysplasia, physical therapy and bracing may not be adequate to help keep the kneecap in place and surgery may be required to keep the kneecap in place. Milder cases of dysplasia can be successfully managed with a medial patellofemoral ligament (MPFL) reconstruction. Higher levels of dysplasia typically require a trochleoplasty procedure to obtain an optimal outcome.
There are other less common anatomic differences that can lead to patellar instability such as a patella that sits too high in the knee or abnormal bends or twists in the bones of the thigh or leg. These can also be successfully treated, and a thorough examination should always be undertaken to evaluate all possible risks factors for patellar instability.