The Human Knee – Part Two
The inward rotation of the femur which occurs as the knee comes close to its locking position of extension is not large but very important to knee function, making the knee much more complex than a simple hinge joint. The small internal movements of the knee are limited in the knee joint and the joint cannot afford any losses of these motions without losing some of its function. These small movements are called accessory movements and are small gliding and sliding movements which occur within the joint during functional activity but which cannot be performed in isolation.
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 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.
If a degree of bow leg develops in the knee the pull of the quadriceps on the kneecap tends to bowstring it inwards to some extent, making the kneecap grate against the inner side of the femoral groove, potentially causing pain. This also forces increased stresses onto the lateral compartment of the knee, increasing the degenerative processes on that side of the joint. Slight knock knee is more common, and this predisposes the person to lateral kneecap pain and degenerative changes in the medial compartment of the knee.
A lack of the ability to extend the knee completely can also lead to patellar pain, as the residual bend of the knee requires the quadriceps muscle to keep the knee held against gravity, forcing the patella against the femoral groove. Over time these increased forces can lead to the development of anterior knee pain, a very common presentation. As treatment a small heel wedge can be placed under the outer side of the heel, correcting the line of forces through the leg from below and so changing the forces going 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.
Jonathan Blood Smyth is the Superintendent of Physiotherapy at an NHS hospital in the South-West of the UK. He writes articles about back pain, neck pain, and injury management. If you are looking for physiotherapists in Brighton visit his website.