The Human Knee - Part Three

By Jonathan Blood Smyth

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.

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.

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 joint surface of the kneecap can develop increased irritability and this limits the willingness to keep a bent knee for any time, preferring to straighten it to reduce the force. As increased forces bear on the kneecap, the articular cartilage lining it changes and becomes lined and fluffy instead of hard and smooth. Further irritation is provided by increased swelling in response to the joint surface changes, with grooves developing in the cartilage as it worsens. Subluxation of the patella, where it moves out of its groove to some degree, can occur with sudden 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.

If the less major operations are not successful then transposition of the tibial tubercle can be performed, where the prominent bony lump below the kneecap is detached from the shin bone and moved over towards the middle. The forces which the quadriceps develops are then moved more medially and pull the kneecap over towards the middle to some extent. If looked at under arthroscopy, the surface of the patella has a fissured, softened appearance as cartilage damage develops. The pain and inflammation caused by this process leads to quadriceps muscle 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. - 32188

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