The kneecap, or patella, is a very mobile bone in the front of the knee. It is part of the extensor mechanism of the knee and is involved in extending or straightening the knee. The patella connects the large quadriceps muscles in the front of the thigh to the patellar tendon, which is then connected to the tibia, or shin bone. When the quadriceps muscles contract they pull on the patella, which transmits the force to the patellar tendon and then to the tibia, allowing the knee to straighten. The patella acts as a fulcrum to increase the force of the quadriceps muscles.
Because the patella is so mobile, it can be prone to dislocation. The patellofemoral joint is part of the knee and comprises the patella and the groove in the femur bone that the patella glides in (the trochlea). Normal patellofemoral motion is the result of a combination of active and passive restraints.
Active restraint of the patella is provided by the quadriceps muscles. If the inside part of the quadriceps muscle (the vastus medialis obliquus or VMO) is weak or does not contract in phase with the outside part, the patella may track abnormally in the groove and cause pain. This is usually treated very effectively by physical therapy.
Passive restraint of the patella is provided by the ligaments around the knee and the bony anatomy of the knee and whole lower extremity. Problems that can lead to patellofemoral instability include a shallow trochlear groove on the femur, an abnormal angle between the femur and tibia where they come together at the knee joint (Q Angle, see Figure X), and an injury to the ligament on the inside of the knee that connects the patella to the femur (the medial patellofemoral ligament or MPFL). Most people that develop chronic instability of the kneecap, or patellar instability, remember sustaining a traumatic first dislocation of the patella, after which the instability developed. During this first dislocation, the MPFL is usually torn. Multiple repeated dislocations of the patella can lead to injury of the cartilage underneath the patella and can lead to early arthritis.
Treatment for patellofemoral instability initially focuses on trying to strengthen the quadriceps muscles and VMO; a brace is often used to help stabilize the patella. If these treatments are unsuccessful, then surgical stabilization can be considered. The individual procedure performed depends on the underlying cause of the instability. A newer procedure that has been shown to be very effective for the treatment of patellofemoral instability is reconstruction of the MPFL with one of the hamstring tendons near the knee (hamstring autograft).
MPFL reconstruction involves several components. During the first part of the operation an arthroscopy of the knee is performed to assess and treat any damage that has occurred because of the dislocations. At this point, any tight tissues on the outside of the kneecap can be released (a lateral release). During the second part of the operation, one of the hamstring tendons (either the gracilis or the semitendinosis) is harvested through a small incision on the inside of the knee. The MPFL is then carefully reconstructed through two more small incisions on the inside of the knee, creating a very strong tether to prevent further lateral dislocation of the kneecap. Depending on the bony anatomy of the knee, the attachment of the patellar tendon to the tibia bone (tibial tubercle) may also need to be shifted by performing a tibial tubercle osteotomy. This involves making a bony cut of the tibia at the tibial tubercle, sliding it medially, and fixing it into place with screws. This part of the operation is being done much less commonly now, however, given how successful simple reconstruction of the MPFL is.
Postoperatively, a brace is usually worn for 6 weeks. Physical therapy is usually started after 1-2 weeks and continued for several months. Full return to athletic activity typically occurs between 4-6 months after surgery.