Applied biomechanics of the patella.

Although numerous prominent orthopaedists of the twentieth century considered the patella to be useless, even detrimental, it now is clear that the patella serves an important biomechanical function. It is a complex lever that magnifies the moment arm of the extensor mechanism. The patellofemoral contact area (the fulcrum of the lever) shifts along a proximodistal axis through the knee's arc of motion. As the knee flexes, the force within the patellar tendon diminishes relative to that of the quadriceps tendon. One's interpretation of patellar tracking is dependent on the choice of coordinates. When assessing tracking by way of anatomic coordinates, patellas are seen to be slightly lateralized at 0 degrees flexion and to follow similar paths down the trochlea. The tracking pattern is the result of an elaborate interplay between the quadriceps muscles, patellofemoral ligaments, the geometry of the trochlea, and the quadriceps angle. The articular cartilage of the patella is the thickest in the human body and does not follow the contour of the subchondral bone. Patellar cartilage is softer and more permeable than that of the trochlea. It is insensate. In size, nature, and number, the facets of the patellar articulation vary from person to person.