The Knee Joint-Part 3

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 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.

If the patella subluxes this is a sudden and extremely painful event which traumatises the surfaces of the joint and can result in considerable pain and swelling of the knee. The kneecap usually subluxes or dislocates to the outside and this stretches the tissues which support the knee on the inner side, making them weaker and allowing the abnormal patellar movements to occur more commonly. In severe cases the patella can dislocate repeatedly which can be disabling and various operations are used to improve matters. The tissues can be tightened up on the medial side, known as reefing, to attempt to hold the kneecap more over to the inside.

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.

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.