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 permanent or semi-permanent bend in the knee, with a loss of extension, can appear if the knee is kept bent for too long without fully straightening. The medial part of the quadriceps muscle is responsible for rotating the knee into its locking position on full straightening, and if there is a contracture the muscle can waste and lose its strength. As the weakness progresses it becomes harder and harder to extend the knee fully.
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.
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.
The articular surface of the patella can be cleaned up surgically in a procedure known as arthroscopic debridement but the outcome is not always helpful. Approximation of the joint surfaces by exercises or manual pressures is used by physiotherapists but these techniques are not well supported by scientific evidence.
Jonathan Blood Smyth, editor of the Physiotherapy Site, writes articles about Physiotherapists, physiotherapy, physiotherapists in Leeds, back pain, orthopaedic conditions, neck pain and injury management. Jonathan is a superintendant physiotherapist at an NHS hospital in the South-West of the UK.
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