The knee can be catapulted into a painful condition by a traumatic event or injury, often minor, which sets off the process. It does not take a large injury to start up swelling within the joint and the knee is sensitive to the presence of any amounts of fluid within it. The lining of the joint capsule is made up of synovial membrane, the tissue which secretes the lubricating fluid vital to a joint. However, this swelling is maintained within the capsule and irritates the capsule as the joint is repetitively moved. Thirty degrees is the usual angle injures knees are held at.

Once the knee is held bent for a while this can develop into a flexion contracture, a small permanent bend in the joint. The part of the quadriceps muscle which performs the screwing home movement of the knee lock is then unable to perform its function and starts to atrophy. The weakness which develops in this muscle makes it gradually more difficult to straighten the knee in activity, adding to the abnormal stresses placed on the knee.

The cartilage underneath the patella can develop pathology, which is a common problem, and the diagnosis of chondromalacia patellae is a typical one. The kneecap lies gently against the femoral surface, with significant pressure only developing if we have to go down stairs, a slope or get up from sitting. A tightening and loss of the accessory movement can make the kneecap press more strongly against the femur. If friction forces develop across the two bony zones this can be worse with rotation of the shin bone, increased leg length or the development of knock knee or bow leg.

The articular surface of the patella can become more inflamed and reduce the wish to keep the kneecap against the femur such as when the knee is kept bent, with regular extension to relieve the pain. The surface of the cartilage on the back of the kneecap suffers from gradual degenerative changes as increased forces are applied to it. As the surface becomes softened and lined, the amount of swelling increases as the condition worsens. The patella can sublux, where it moves off the edge of its femoral surface to some amount, in response to unplanned vigorous movements such as turning and twisting.

If the patella subluxes this is a sudden and extremely painful event which traumatises the surfaces of the joint and can result in considerable pain and swelling of the knee. The kneecap usually subluxes or dislocates to the outside and this stretches the tissues which support the knee on the inner side, making them weaker and allowing the abnormal patellar movements to occur more commonly. In severe cases the patella can dislocate repeatedly which can be disabling and various operations are used to improve matters. The tissues can be tightened up on the medial side, known as reefing, to attempt to hold the kneecap more over to the inside.

A more major operation, performed if the more minor ones do not work, is to take the tibial tubercle, the bump centrally below the knee on the shin bone, and move it to the side, usually medially. This realigns the direction of the forces the quadriceps exerts across the kneecap and is designed to make the kneecap track more towards the inside. Arthroscopic investigation of the knee shows a softened, fissured surface under the patella as the cartilage becomes increasingly damaged. The joint inflammation and pain inhibits the quadriceps muscle from working, causing wasting.

As the quadriceps muscle wastes and become weaker the knee is less and less well supported, and the patella cartilage damage makes particular activities painful such as descending slopes and stairs, which place higher forces through the patello-femoral joint. Going downhill involves the quadriceps controlling the movement as the muscle lengthens rather than the more obvious shortening mechanism we are more familiar with.

Arthroscopic debridement of the patella, the surgical cleaning of the under surface of the kneecap, can be performed but the results are not clearly positive. Physiotherapists can use manual pressure techniques to approximate the joint surfaces and attempt to smooth out the irregularities of the joint but whether this actually occurs anatomically is not clear.

Jonathan Blood Smyth is the Superintendent of Physiotherapy at an NHS hospital in the South-West of the UK. He writes articles about back pain, neck pain, and injury management. If you are looking for physiotherapists in Oxford visit his website.