The locking position close to full extension of the knee is a small internal movement of the joint involving an inward twisting of the thigh bone, a small joint motion but crucial to knee function and illustrating the knee is much more than a normal hinge. The knee has unobtrusive movements which occur within the joint and because these are minor the knee loses significant function if any of this ability is lost. The small movements of joint slide and joint glide are known as accessory movements, occurring during normal joint motion but incapable of isolated performance.

The knee has to satisfy the competing demands of stability and mobility, it has to be a powerful and reliable prop for the body weight and it has to move into varied positions with great speed. In the case of walking the knee must at one moment bear the entire body weight and then the next unlock so that the leg can be lifted to step forward. As the cycle progresses the knees lock and unlock regularly and reliably, enabling rapid walking and the covering of great distances without fear of falling. The slide that occurs during the knee lock and unlock is vital to this function and can give problems early on in knee pathology.

The knee is controlled by very powerful musculature and can perform fine coordinated actions as well as power movements. We can do a full knee bend and then get right up again without any delay in the movement. The amplitude of the accessory knee movements is not large but may be useful in managing uneven surfaces. The medial side of the knee gaps more significantly under stress as the medial ligament is looser than the lateral and the slight natural knock knee alignment tends to stress the knee that way.

The first article about the knee covered the idea that the knee moves backwards and forwards and tends to stick in that plane, so if an abnormal stress such as to the side is added this changes the balance in the joint. The kneecap and the main knee compartments can experience wear changes if the knee suffers from bow-leg or knock knee. The knee is divided into two compartments, the medial and the lateral side, both with their own meniscus, ligament, femoral and tibial condyles. The stresses which are transmitted across the compartments vary with changes in the sideways angle of the knee.

The development of an amount of bow leg at the knee changes the quadriceps pull so the kneecap is pulled to the inside, pushing it more forcefully against the inner edge of the groove it sits in, which can result in a painful condition. Along with this there are increased loads on the lateral compartment and this can hasten degenerative changes on that side. Normal knee joints naturally have some knock knee but if this amount is increased then the outside of the kneecap is likely to suffer from impingement pain.

If the knee is not capable of full extension then the kneecap can develop problems due to the persistent flexion which makes the quadriceps overactive to hold the knee in place, increasing patellar compression. This increased compression can cause a very common condition known as anterior knee pain or patellofemoral pain. To correct the lateral misalignment a small wedge can be placed under the outer edge of the heel to correct alignment of the shin and so influence the stresses which are passing through the knee joint above.

The patella can also give problems in response to abnormal changes in other joints. As we get older our foot arches can become less strong and so less pronounced, sometimes leading towards a degree of flat foot. As the feet rotate inwards on weight bearing the whole foot and shin move inwards to some extent, introducing an amount of knock knee effect at the knee. This can cause the kneecap to glide more outwards along the groove than normal and lead to patello-femoral pain. An effective treatment can be to wear orthotics in the shoes, which can combine restoration of the foot arches with the necessary level of medial wedging of the heel.

Jonathan Blood Smyth, editor of the Physiotherapy Site, writes articles about Physiotherapy, back pain, orthopaedic conditions, neck pain, injury management and Physiotherapists in Coventry. Jonathan is a superintendant physiotherapist at an NHS hospital in the South-West of the UK.

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