Cartilage

By Dr Kenneth Backhouse OBE

Anyone suffering from damaged cartilage, common enough in both young and old, can testify to the pain and disability associated with the condition.

The body is made up of many tissues, each with differing character and functions. Of these, bone is the hard material which gives protection to vital structures as the skull protects the brain. It also forms rigid levers on which muscles act to produce movement. Although bone is often said to be the hardest material in the body, this is not strictly true. That substance is the enamel, the outer layer of teeth but this is a non-living crystalline substance, laid down as the tooth is formed and not replaceable if damaged. Bone is the hardest vital substance, dependent for its efficiency on stresses of use, available structural substances (calcium etc) and a supporting system of living cells (osteocytes). It requires a rich blood supply and, having nerves, pain can be induced from bone.

Cartilage is also an important component of the firm skeletal system. It could be compared with plastics relative to harder structural materials such as wood or steel. Like bone it has an extensive supporting matrix which gives it the strength and resilience required for its particular function. It contains living cells (chondrocytes) but these have to be supplied with nutrients through the substance of the cartilage, as it has no direct blood supply and is also without nerves. Sustenance and nervous stimulae must come from the surrounding tissues. As such it has a low metabolic rate (unlike bone) and having no nerves, can be injured without producing pain unless this in induced indirectly from the surrounding tissues. Due to its low metabolic rate it has a poor capacity for repair and so any injury may be permanent.

Whereas bone has a hard calcareous matrix, cartilage has one of a series of mucoproteins (glycosaminoglycans or proteoglycans), complex c ompounds of proteins and glycogen. These have a high propensity to attract and hold water (about 75% of weight) which, where the cartilage is load bearing as on the bone surface of joints, can be s queezed out into the joint space to assist synovial fluid in lubrication (weeping lubrication). The water is then reabsorbed when the load is reduced.

As with plastics there are differences in hardness and resilience (elasticity). The hardest cartilage is that lining the load bearing surfaces of bone ends in joints. This is hyaline cartilage, which in the living state is translucent, bluish white in colour and has a smooth glassy surface. It has very fine fibrils running through its matrix but these are only visible under high magnification, hence the glassy appearance. Think of it as the equivalent of a plastic such as nylon. It is also found where this degree of hardness and resilience are required as in the larynx, tracheal and bronchial rings and part of the nose. It also forms the early skeleton in the embryo, later to be replaced by bone but being retained as the epiphyseal growth plates of the bone to near adulthood.

In other situations, more resilient flexible cartilage is required with a greater fibrous component, fibro-cartilage and even more so elastic-cartilage. The costal cartilages of the chest wall are of fibro-cartilage. (Feel how the lower anterior part of the rib cage moves under pressure). Of particular interest to dancers, the vulnerable semilunar cartilages (menisci) of the knee joint are also of fibro-cartilage.

The intervertebral discs are usually described as being fibro-cartilaginous but in fact they are rather more complex in structure than that name implies.

The hyaline cartilage covering the load bearing ends of bone in synovial joints is subject to considerable stress, particularly in the legs in dance. Without reasonable care in the control of joint movement while under load, the stresses can result in irreversible damage, often leading to severe disability. The function of the hyaline cartilage is to give the bone a smooth, slippery, hardish, plastic surface in the joint. It has a low coefficient of friction being three times as slippery as smooth ice. In this it is assisted by synovial fluid, a viscous substance (hyaluronic acid), produced by the synovial membrane, to produce a relatively friction free surface for movement. With reasonably correct training patterns the stresses should not lead to cartilage damage in a healthy person. Admittedly some people seem to havegreater likelihood of age change leading to cartilage wear than others, but this does not have any direct link with correct exercise loading.

Cartilage and the Knee Joint

In the knee the bone ends of both femur and tibia are covered by hyaline cartilage, as is the deep surface of the patella, where it is related to the femur. In addition, the two fibro-cartilaginous menisci (semilunar cartilages) run around the periphery of the tibio-femoral joint spaces. The main load and hence friction in movement, should not fall on these cartilages but on the more central regions of the two tibial tables, i.e. the hyaline cartilage. As the load is frequently heavy, lubrication could be a problem and one important function of the semilunar cartilages is to assist in the control of the lubrication of the joint space. They should not be unduly subject to the major stresses in the joint but can become so in poorly controlled and particularly in abnormal movements, such as rotation under load. Rotation should not be possible in the near extended knee other than a small controlled rotation, the so-called locking into extension and a reverse at the beginning of flexion. In these movements the lateral cartilage is controlled by the activating muscles and associated ligaments, adjusting its position as needed, with the medial cartilage largely static. A degree of rotation of the knee is permitted, maximally at about 60¼ of flexion but this should not normally be under heavy load. However, if this occurs the cartilages, particularly the medial, can be trapped and split producing partially loose flaps which are free to intervene between the load surfaces, leading to locking of the joint, usually at the most inconvenient moment.

In classical ballet, where turn-out is used to allow a wide range of movement at the hip joint, it has become fashionable to expect a flat turn-out as displayed by the feet. For some dancers with a highly mobile hip joint and/or a suitably directed socket, this is possible. For others (and I speak with feeling) a flat turn-out is impossible but not affecting the prime object of the action or in most cases the artistic presentation of the dance. The latter are often forced by mechanically orientated teachers to achieve a flat turn-out by developing a lateral twist at the knee, producing a less controllable joint, so increasing the risk of lower leg injuries and particularly cartilage ruptures. Many dance careers have been destroyed by this manoeuvre as well as inducing long term disability. How often one sees the foot and patella facing in different directions and the scar of surgery for a ruptured cartilage. Although surgery is less interfering now when carried out by arthroscope, nevertheless there is still a greater likelihood of early osteoarthrosis after such
injuries.

Another important focus of cartilage damage in the knee is to that on the joint surface of the patella (chondromalacia patellae), leading to pain in flexion/extension movements of the joint, particularly under load. When the knee is flexed towards 90¼ or beyond, the patella is carried round the end of the femur. On powerful extension, as on rising from a grand plié, the stresses on the patellar cartilage are great, gradually reducing as the knee straightens. If for any reason this is not under perfect control and health, damage to the cartilage can occur. For this reason there have been medical recommendations that the grand plié should be banned from the dance curriculum. "Bunny hopping", a former popular exercise for quadriceps training of footballers has disappeared for a similar reason. But where would the range of dance be without the practical equivalent of the grand plié? The importance is that in practice it is not a repetitive activity and this is where much of the danger lies. Hence it should be remembered that as a class exercise it be practiced when fully warmed up: certainly not as a repetitive exercise at the beginning of a cold class as often happens. Furthermore, illness can materially affect the health of the cartilage and aspects of lubrication of the joint. Alcohol can lead to dehydration with similar problems on the joint so that after the evening alcoholic party, not only should there be an effective rehydration (water to get over a hangover!) but also a slow warm-up in order to protect the joint cartilage.