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Bone

By Dr Kenneth Backhouse OBE

Bone is the hardest material in the body, except for the non-living enamel of the teeth. It is made up of complex crystalline calcium (with magnesium) salts, which gives it the necessary hardness. In addition, this hard material is interspersed with strong fibrous strands in a living matrix, which gives resilience and some elasticity to the hard, but otherwise brittle calcium component. It is important to understand that bone is a living tissue and, as such, has cells (osteocytes) which maintain its functional integrity. It also has a rich blood supply and bleeds like any other tissue when injured. It is a popular misconception to think of bone in the body as being essentially the same as that seen in prepared skeletons used for teaching, or bones dug up from the ground of people or animals long dead. These are dead bones from which the living elements have decayed and been lost. They have essentially the same form but are little more than the calcium component and hence quite brittle. They would be incapable of withstanding even a minute proportion of the stresses and strains to which living bones are subjected. These stresses can be enormous in such physical activities as dance.

Living bone responds to a great extent to the stresses habitually imposed upon it, so long as raw materials (calcium, vitamins etc) are available in the diet and the individual is otherwise in good physical health. Bone reacts to work in much the same way as muscle does. Work leads to increase in bone strength; lack of work leads to withdrawal of calcium from the bone, with consequent loss of strength. Thus a fit dancer can subject bones to stresses and strains in the course of activity, which could lead to fractures (breaks) in an unfit individual. Bone also tends to become weaker with increasing age, particularly in females following the reduction in their sex hormonal output, so that fractures are more common in older women, following quite mild trauma such as a simple fall. However, even here, bone strength is to some extent related to the amount of work to which it is habitually subjected.

The condition of loss of calcium (osteoporosis) is unfortunately not only a condition of the unfit and the elderly. It is regrettably seen far too often in young female dancers (and athletes) who have responded to the present-day enthusiasm for slimming to what may seem a convenient, but in fact can become, an unhealthy extent. In spite of their light weight, they become vulnerable to stress and other fractures. It is a phenomenon associated with reduced menstrual activity and female hormonal (oestrogen) output, linked with the reduced diet.

In dancers and other athletic people, work normally increases the strength of the bones to the extent needed. However, a period of rest, particularly after an injury, very rapidly leads to a reduction in bone strength and the need for slow redevelopment of, not only the muscles but the bones as well. In females, in particular, an adequate balanced diet is vital, regardless of the need or desire to slim. A slim body with strong bones is possible if just a little care is taken with the diet. However, it is important to be aware that extra calcium or vitamins in the diet is of little value unless normal healthy work and a correct hormonal environment are available.

Structure of Bone
Bones are designed in the body in direct relation to their use. It is common to classify bones according to their shapes, i.e. long, short, flat or irregular, but with little reference to use. Long bones are those found particularly in the limbs where they act as the central supports and levers; i.e. the bones of the thigh, leg, upper and lower arms and also, on a smaller scale, in the hands and fingers, feet and toes. Short bones are found making up the complex of bones at the wrist (carpals) and the hind foot (tarsals). Flat bones are found in the skull as well as the plate-like scapulae and part of the pelvis (iliac bones) while the vertebrae are examples of irregular bones. But such a classification is nothing more than a description of shape. The wide range of shapes and sizes of the bones in the body is directly linked to their function.

The long bones are of particular structural interest. They act as the major axial supports and levers in the limbs where, in the legs especially, they are subjected to enormous stresses (both compressional and tensile) as when jumping and landing, or from the powerful pull of muscles. If they were solid they would be heavy while the bone in the centre of the shaft would give little extra strength. Hence they are made of tubes of hard compact bone. This gives maximal strength compared with weight, much as is achieved in tubular metal furniture. The centre of the bone is filled with fatty marrow. At the bone ends, however, the stresses are much more variable. Here the bones have thinner, hard compact bone at the surface with plates of bone within, the plates being laid down in the directions of stresses. This produces a honeycomb-like arrangement called cancellous bone; a multiple box-like construction, the boxes being filled by the essentially incompressible bone marrow. This arrangement gives remarkable compressional strength, in spite of the relatively small amount of actual bone.

The Skeleton as a Whole
From the point of view of dance, fine details of the skeleton and its form are of relatively little importance. What is needed is an understanding of how the skeleton supports the body and gives the levers on which the muscles work to generate movement.

Growth of the Skeleton
As we have seen, the quality of bone in the body is maintained by a complicated balance between a variety of factors, including diet, hormonal control and work. During the growing phases of life the bones also have to adjust, not only to growing in length but also in bulk and at the same time maintain the same basic architecture.

Many of the bones in the body start off, in the early embryo, as tiny hyaline cartilaginous structures, with essentially the same shape as the eventual adult bone. As early as 8 weeks after conception, in the case of some of the long bones of the limbs, bone starts to be laid down, in the centre of the cartilage of the shaft, and gradually extends towards the ends. The ends remain as cartilage until just before birth or, in many cases, until some time later. Then, secondary bone centres appear at the ends (epiphyses) but do not join with the bone in the shafts: an intervening area of cartilage remains. This is known as the growth (epiphyseal) cartilage and is the region where the bone grows most in length. It does, of course, grow in thickness and is remodelled by the bone cells as required. Growth continues, and the epiphyseal cartilage persists, in many of the long bones, until well after puberty; in girls until 16-17 and in boys a year or two later.

In many of the shorter and irregularly shaped bones, ossification (i.e. laying down of bone) in the cartilaginous precursor does not occur until well after birth, gradually increasing in amount over the growing period. The bones of the wrist (carpals) and of the hind foot (tarsals) are examples of this.

Not all bones start as cartilage i.e. cartilaginous ossification; some are laid down directly as bone in the existing tissue. These are said to be ossified in membrane. Examples are the bones of the vault of the skull, where rapid growth is required commensurate with that of the brain. The bones of the face are also similarly ossified.

The rate of ossification varies somewhat from person to person, much as their rate of growth in height varies. This variation is sometimes made use of in the dance world, particularly where a child wishes to train in a field where height is important. The eventual size of a child is, to some extent, dependent upon the size of the parents but this may present problems, particularly when parents are of markedly different sizes and especially of different races. Who will the child take after? Some children grow quickly and then slow down, or visa versa. Examination of the degree of ossification of the bones can often help with the prediction. A radiograph (X-ray photograph) may be taken of the wrist region and the degree of ossification assessed against averages for children of the same age and sex.

Bone growth is materially affected by puberty. In females bone growth slows down after puberty, leading to the earlier fusion of the epiphyseal cartilages. In boys, however, the advent of the secretion of the male sex hormone, testosterone, usually leads to a rapid increase in bone growth, not immediately matched by that of the controlling muscles or the brain's adjustment to coping with the increased length of the limbs. The gangling youth is an important concept to be aware of in dance, until the rapid increase in growth is matched by the development of the strength and control of the muscles.

It is often said that girls should not start point work in classical ballet before about 12 years of age because their bones would be too weak and liable to bend in growth. This is hardly an acceptable argument though often a convenient one for calming down the enthusiasm of student or parent. There is no magic change in the character of bone growth at this stage. The real decision of when a child should or should not go on to point depends on the time at which she has developed adequate muscle control over her feet and body to give her the necessary strength.