Patellofemoral Arthritis

Patellofemoral arthritis is a common cause of anterior knee pain. The patellofemoral joint is composed of the bony patella, which is a sesamoid bone embedded in the quadriceps and patella tendons and the femur. A prepatellar bursa separates the patella from the overlying skin. The patella sits within the intercondylar or trochlear groove where the lateral condyle is of slightly greater diameter than the medial condyle. There are four articular facets: the inferior, superior, middle, and medial vertical.

The patella is primarily stabilized medially by the medial patellofemoral ligament (MPFL). This ligament originates from the adductor tubercle and inserts onto the superomedial border of the patella, resisting lateral traction of the patella. The lateral patellofemoral ligament (LPFL) originates from the lateral femoral condyle and attaches to the superolateral border of the patella and resists medial traction of the patella. The medial and lateral patellotibial ligaments and retinaculum also provide static stability of the joint. Dynamic stability is provided by vastus medialis, which provides medial resistance to lateral translation, and the vastus lateralis, which provides lateral resistance to medial translation. The quadriceps tendon attaching at the proximal patella creates a Q angle with a valgus axis that creates a lateral force across the patellofemoral joint. Blood supply is from the geniculate arteries: superior, medial, inferior, lateral, anterior, and descending. Sensory innervation of the anterior knee is from the lateral and anterior cutaneous branches of the femoral nerve as well as the infrapatellar branch of the saphenous nerve.

In healthy knees, the articular cartilage of the patellofemoral joint can reach a thickness of up to 7 to 8 mm; thus, it is a potent shock absorber. A complex matrix of glycosaminoglycans lowers the friction coefficient and creates a nearly frictionless surface for flexion and extension of the knee. Chondrocytes produce the proteoglycans that balance synthesis and degrade the matrix based on the chemical and biomechanical demands of the joint space.

Patellofemoral stability is provided by both the static and dynamic anatomical supporting structures. During flexion, the patella moves within the groove acting as a lever arm, extending the functional length of the femur. In addition to a proximal-distal movement within the groove, the patella is thought to have a lateral motion with knee extension moving in a J-shaped pattern. Varus and valgus alignment of the knee, as well as any rotational component of the femur or tibia, also determine biomechanical patellofemoral function.