Have you ever heard of knee dyplasia? I could find lots of information about hip dysplasia but nothing on knee dysplasia. This is what I've been told is wrong with my knee and why I can't keep the kneecap from popping off sideways and dislocating.
You may be referring to a fairly new factor physicians think may be contributing to chronic, recurrent patellar dislocations: trochlear dysplasia. Trochlear dysplasia refers to a groove that is shallow -- too shallow to hold the patella in place as it glides up and down. Let's back up and take a quick look at some anatomy. The patella (kneecap) is held in place by its shape and the supporting soft tissue structures such as muscle, tendon, cartilage and ligaments. As the knee moves, the patella glides up and down in front of the knee joint. There is a groove on the front of the femur (the trochlear groove) of the femur. The back of the patella has a corresponding V-shape that fits inside the groove and helps hold it in place. Any changes in the shape of the bone, alignment, ligamentous laxity (looseness), muscle weakness, or other soft tissue problems can contribute to patellar instability (dislocations). Experts in Europe have been exploring the idea of trochlear dysplasia as a possible cause of recurrent (repeated or chronic) patellar dislocation. U.S. surgeons are beginning to take a closer look at this as well. You can expect to see more on this topic in the coming years. Studies show that not very many people in the general population really have trochlear dysplasia (less than two per cent). But among patients who have patellar instability, as many as 85 per cent do have trochlear dysplasia. A shallow groove by itself may not be enough to really result in chronic patellar instability. Experts think there are multiple factors contributing to the problem. Each factor must be addressed in treatment in order to keep the patella centered in the trochlear groove. Preventing further patellar dislocations requires evaluation of the bony, soft tissue, and alignment issues. It sounds like trochlear dysplasia may be one of those issues for you.