The Knee Joint – Part Three

The changes which occur as the knee becomes troublesome and develops pain are often due to injury of some kind, perhaps minor. Swelling can occur in the joint after even a minor injury and even a small amount of fluid in the joint can lead to complex side effects within the knee. The synovial joint lining secretes fluid in response to trauma and this fluid is held within the joint capsule, stretching and irritating it further in movement. Once the fluid is present a person tends to hold their knee in the loosest and most comfortable position of slight bend, around 30 degrees.

A flexion contracture, a semi permanent loss of extension of the joint, can develop once the knee is kept in flexion for a long period. The locking function of the last few degrees of knee extension is powered by part of the quadriceps muscle and when it is blocked from this by a bent knee it can weaken and lose size. The knee is more and more difficult to straighten as the muscle becomes weaker and it suffers abnormal forces across the joint.

Chondromalacia patellae is a commonly diagnosed problem with the cartilage on the underside of the kneecap. Normally the kneecap sits lightly against the groove on the front of the femur and is only strongly pressed against it in loaded movements such as getting up from a chair or descending stairs. If the knee tightens and loses some of its accessory movements then the patella can become more tightly compressed against the femur. This can set up a frictional process between the two bony areas, particularly if there is bow leg or knock knee, where the tibia is rotated abnormally or where one leg is longer than the other.

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.

Subluxation of the patella typically occurs quickly and is very painful, causing damage to the surfaces of the cartilage and making the knee swell and become painful. The usual direction for the patella to sublux or dislocate is out away from the centre of the body, tearing the tissues on the inside edge of the kneecap and making repeated subluxation more likely as the torn tissues develop slackness. Dislocation of the kneecap recurrently can be a disabling problem and surgeons employ several operative techniques. Initially the inner knee tissues, suffering from slackness, can be reefed in to make them tight enough to hold the kneecap better.

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.

The knee can gradually lose support as the control of the main thigh muscles become weaker and the muscle loses size. The kneecap joint suffers greater forces across it when it has to manage activities such as descending slopes and stairs. The quadriceps muscle must hold the weight of the body whilst it is lengthening in these cases, an action which generates higher forces than when the muscle shortens.

A surgeon can debride the back of the joint via arthroscopy, surgically cleaning up rough areas and debris, but results of this procedure are not predictable. Manual pressures or exercises to press the surfaces together in an attempt at smoothing them can be performed by physiotherapists but this is a therapeutic technique with little support from evidence.

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.