The Knee Joint – 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.

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 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.

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.

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.

Jonathan Blood Smyth, editor of the Physiotherapy Site, writes articles about Physiotherapists, physiotherapy, physiotherapists in Glasgow, 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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