Anterior knee pain.

Troublesome anterior knee pain is a common problem in active adolescents and young adults and the complex of symptoms is well known. From a clinical viewpoint it is convenient to consider the causes under the headings of ‘Distinct’ and ‘Obscure’ (Table I). The first group consists mainly of focal lesions which can be clinically and radiologically defined and for which local treatment can be applied and outcomes predicted. The second includes dynamic problems, such as maltracking and the excessive lateral pressure syndrome, as well as idiopathic chondromalacia and psychogenic pain. The frequency of anterior knee pain is related to the very considerable compression and sheer forces which are transmitted through the patellofemoral joint. One important function of the patella is to displace the patellar tendon away from the centre of rotation of the knee and so increase its moment arm. The contact point between the patella and the trochlear groove is a fulcrum and on the patellar side of the articulation an area of contact sweeps up from the inferior to the superior pole as the knee flexes from full extension to 90° of flexion. Static experiments show that at 60° of knee flexion over twice the body-weight is transmitted through the patellofemoral joint, while dynamic studies in serious weight-lifters suggest a patellar tendon tension of 17.5 times the body-weight when pushed to the extreme. It is therefore not surprising that the patellofemoral joint has the thickest articular cartilage of any joint in the body and that peripatellar failure of the extensor mechanism is such a common problem. Acute failure is of course manifest as ‘fracture’ or ‘rupture’ while subacute and chronic failure give rise to anterior knee pain. The origin of the pain is a subject of debate since articular cartilage is devoid of nerve endings. Cartilage also has very limited powers of repair and regeneration once fibrillation and ulceration have occurred. Overuse and repeated minor trauma commonly play a part in the production of anterior knee pain, especially when sufficient time is not allowed for the resolution of subliminal damage. Paradoxically, underuse can also be incriminated as a cause of damage to the articular surface, an example being the chondromalacia which is seen on the medial or odd facet of the patella. This small area does not come into contact with the femur until the knee flexes past 130° and therefore in most patients is a very underused part of the articular surface. Osteoporosis of the patella often accompanies anterior knee pain although it is difficult to know if it is a cause or effect. It reduces the stiffness of the patella and causes thinning of the subchondral bone plate, thus decreasing the ability to cope with the huge forces already referred to.