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What causes my knee cap to slide off and then pop back?

Q: I'm 21 years old. I've been active all my life.  Lately I've been noticing that my left knee cap slides off to the side and then pops back. I never know when it's going to happen. I did fall while hiking last summer. Could this be the cause of the problem?

A: It is possible that you have injured some of the soft tissues around the patella (knee cap). A little bit of anatomy will help explain this injury. The patella moves up and down in front of the knee joint along a built-in track called the patellofemoral groove.

The kneecap is held in place by several ligaments on either side and by the patellar tendon (attached to the quadriceps muscle). The quadriceps muscle is the large, four-part muscle along the front of the thigh.

Although you can take your hands and passively move the kneecap from side to side, this is not an active movement you can make your patella do without assistance. We call that side-to-side (medial-to-lateral) movement accessory motion. The up-and-down and side-to-side accessory motions are referred to as patellar glide.

As part of the patellar tendon, there are slips of ligamentous fibers that help hold the patella in place and keep it from moving too far to one side or the other. On the inside of the kneecap is the medial patellofemoral ligament. On the outside is the lateral patellofemoral ligament.

Without the medial patellofemoral ligament, the kneecap dislocates laterally (in a direction sideways away from the other knee). Because the medial patellofemoral ligament is connected with other ligamentous structures, complete rupture will likely damage other areas as well. The medial patellofemoral ligament attaches above to the femur (thigh bone) and below to the tibia (lower leg bone).

A medial patellofemoral ligament injury can be treated conservatively without surgery. The knee may have to be immobilized in a splint for a number of weeks to allow for healing. Physical Therapy, taping, and a home program of exercises prescribed by the therapist begin after the period of immobilization. The rehab program must be given the good old college try: in other words, for more than a few days or weeks. It can take months to rehab this injury.

But if nonoperative care fails and the patella dislocates again, then surgery may be the next step. The surgical approach that works best depends on the underlying damage and specific patient factors. The surgeon will decide if the torn ligament can be reattached to its original insertion on the bone. This is a repair procedure.

More complex injuries with damage to the surrounding tissues may require reconstructive surgery (taking graft material and using it to replace the ruptured ligament). Overall results for repair or reconstructive surgery are good to excellent. Many active patients (teens to adults) are able to resume full physical activities, including sports participation.

Reference: Jay N. Shah, MD, MS, et al. A Systematic Review of Complications and Failures Associated with Medial Patellofemoral Ligament Reconstruction for Recurrent Patellar Dislocation. In The American Journal of Sports Medicine. August 2012. Vol. 40. No. 8. Pp. 1916-1923.