The Knee - Part Three
An injury or some traumatic occurrence, perhaps minor, can be the precipitating event which kicks the knee joint into a painful state. Even a small injury can cause the joint to swell and the knee can react in complex and negative ways to the presence of minor levels of fluid in the joint. Trauma causes the synovial lining to secrete synovial fluid and this is contained inside the capsule of the joint, continual movement irritating the joint by stretching of the capsule. A swollen knee is typically held at an angle of about thirty degrees as this is the most comfy, loose position for the joint.
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.
Pathological changes which can occur behind the kneecap are a common source of knee problems, one of the commonest being chondromalacia patellae. The normal pressure of the kneecap against the surface of the femur is mild, only increasing to high levels on going down a slope or stairs and rising from a chair. A reduction of the accessory movements can cause tightness in the knee and force the kneecap more directly against the thigh. Friction developing between the two bony surfaces can be amplified by a longer leg, the presence of knock-knee or bow-leg or a degree of tibial rotation.
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 become gradually less supported as the main thigh muscle weakens and wastes, with going down slopes and stairs more difficult as these activities involve the imposition of greater forces across the patello-femoral joints. When we go downhill the quadriceps has to lengthen as it controls the body weight and this is a more stressful process than activities which involve muscle shortening.
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.