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 is subject to conflicting demands for both mobility and stability, needing to perform as a strong and predictable support and also to function to provide mobility very quickly. In the example of gait the knee has at one point to stabilise the body under its whole weight and next release the stability and move forward as a mobile segment. Walking then consists of a cycle of the knees locking to bear weight and then unlocking with predictable regularity, allowing a person to walk rapidly and make considerable progress without falling. As a knee gives early problems this may involve the loss of accessory movements.
The knee is very powerful but also capable of very fine movements in response to changes such as uneven ground. It has the power to allow us to squat down and stand up again without missing a beat. Side to side accessory movement in the knee is limited to a small range but this may help with adapting to an uneven surface, with a gapping of the inside of the knee joint the larger of the two movements due to the natural outward angulation of the lower limb and the weaker ligamentous support.
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.
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 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.
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