The Knee – 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 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.
A patellar subluxation is of sudden onset and often exceptionally painful for the short time it occurs, damaging the joint surfaces and bringing on swelling and pain in the knee. Dislocation or subluxation of the patella mostly occurs towards the outside of the knee and subjects the inner knee tissues to a stretch. This slackness permits the pathological movements to recur more easily. Recurrent dislocation of the patella is common and can cause significant disability, with several orthopaedic procedures typically employed, such as medial reefing, where the inside tissues of the knee are tightened up to draw the kneecap more firmly in towards the middle.
After an attempt at minor interventions has not been successful then the surgeon can progress to tibial tubercle transposition, the moving of the bony prominence on the upper shin bone towards the inner side of the knee. This brings the line of pull of the quadriceps muscles into a more inwards line and draws the kneecap in away from the side where the pressure is greatest. Investigation by arthroscopy can show an appearance of fissures and softened cartilage in worse cases of damage. Wasting of the quadriceps muscle can occur in response to the inflammation and pain of this process.
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.
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, editor of the Physiotherapy Site, writes articles about Physiotherapists, physiotherapy, physiotherapists in Glasgow, back pain