As the knee approaches the position where it is going to lock the femur rotates inwards, a movement which is only small but vital to the normal function of the knee, indicating its great complexity beyond a simple hinge. The minor movements which occur internally in the joint are restricted in the knee and it cannot manage to lose any of these small motions without functional loss as a consequence. Accessory movements are the name given to these limited gliding and sliding movements which normally occur during activity but which can't be done on their own.
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 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.
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.
The patella can also suffer as a consequence of changes which occur in other joints nearby. The arches of the feet can weaken from bearing our weight for many years, losing strength and collapsing to some degree towards flat foot. When the foot bears weight the arch collapses inwards and takes the ankle and the lower leg with it, increasing the degree of knock knee. This can predispose towards anterior knee pain as the patella is forced medially against the femur. Corrective orthotics to be worn in the shoes can be a useful treatment, allowing heel correction as well as support for the arches in weight bearing.
