The Knee - Part Two
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''s function is to complete two contradictory demands, the ability to move the body quickly into a newly desired position and the ability to keep the body stable and controlled in a chosen position. In the walking cycle the knee has to be a mobile limb for moving into the next position at one moment then at the next function as a reliably stable support. In the gait cycle the knees go through a repeated process of unlocking to move and locking to bear weight, permitting a human to walk significant distances with safety and effectiveness. Loss of the accessory movements may be involved in early knee problems.
The knee has very strong muscles but can also respond to changes such as an uneven surface by finer controlled reactions. The knee is strong enough to achieve full squatting and then stand our body weight up again without pause. The knee''s accessory movements are small in distance with side to side more limited than front to back, both however contributing to coping with uneven ground. The inside of the knee joint can gap open more than the outside due to the natural angle of the lower leg to the knee.
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.
If the knee becomes bow-legged to some degree the quadriceps pulling on the patella levers it towards the inside and can cause the patella to be compressed against the inside of the femoral groove, with painful results. The lateral knee compartment then suffers increased forces and is subject to accelerated wear changes on that side. Typically people have a small degree of knock knee, and any exaggeration of this can make kneecap pain more likely on the outside and cause increased wear of the inside compartment.
Patellar problems can also occur if the knee does not typically extend fully, as the knee remains slightly flexed and the quadriceps has to maintain knee stability, pushing the patella strongly against the femoral groove. These increased forces can be a cause of patello-femoral pain which is a very common complaint. If the knee has some abnormal lateral alignment then a small wedge under one side of the heel can realign the foot and shin bone from below and thereby make a very small but important change to the stresses through the knee.
Difficulties with other joints can cause changes in the patella stresses also. The arches of the foot suffer from time and body weight and so reduce as they get weaker, moving towards a flatter foot. On weight bearing the foot turns inward as it flattens, bringing the ankle and the shin towards the centre with it, exaggerating any knock knee amount. Patello-femoral pain can then occur as the kneecap is pressed more forcefully towards the lateral side of the knee. Prescribing individual orthotics for the shoes can be an effective management, both providing an amount of medial heel wedging plus correcting the collapse of the foot arches.