The knee is a large hinge joint comprised of the femur (thigh bone), tibia (shin bone), and patella (kneecap). The muscles in the front of the thigh, the quadriceps act to extend or straighten the knee. The hamstrings in the back of the thigh act to flex or bend the knee (and extend the hip). The patella sits in a groove in the femur (trochlear groove). The quadriceps tendon attaches to the patella and the patella links the quadriceps tendon to the patella tendon which inserts into the tibia. From a biomechanics standpoint, the patella acts as an anatomic pulley for the quadriceps and increases the lever arm of the extensor mechanism allowing the quadriceps to function more effectively to extend the knee.
Articular cartilage lines the under the surface of the patella and the surface of the trochlear groove. Normally, the cartilage is quite thick, with a smooth surface and quite dense. Chondromalacia is a term given to the condition when the articular cartilage starts to soften and develop fissures.