4. Bone age vs height age

Of crucial importance is the relationship between retardation in height and in 'maturity'. If both are delayed to exactly the same extent, we can view this relativistically as one where time has simply run more slowly for the child; the growth yet to come, the time available for that growth and the potential may all be adequate for full catch-up to occur. If, on the other hand, maturity is more advanced with the 'biological clock' running ahead of attained growth, the time available for growth is restricted, its intensity is likely to be inferior and catch-up much less likely. Martorell has specifically addressed this point (Martorell et al., 1979). He contends chat the bone age (maturity?) of stunted children is much less delayed than height growth. However, this interpretation is largely a function of the way in which the data are expressed. They are expressed as "relative retardation", defined as the difference between the age- and sex-specific means of the standard and the stunted populations, divided by the standard deviation of the standard. This is, in effect, a Z score of the mean, rather than a mean of the Z score. Expressed in this way, the bone age of Guatemalan children at age 36 months was only -1.7 SD units, when height was -3.3 SD units. It would appear that stunting was twice as severe as the delay in bone maturation. But the curves for bone age and height have quite different shapes; the standard deviations of the normal population are much larger for bone maturity (number of wrist ossification centres) than for height. A 36-month-old child with a height of -3.3 Z has the height of a normal child of 21 months. The same child with a bone age of -1.7 Z has a maturity of between 21 months (girl) and 26 months (boy). The length of the delays are not really different, although they appear so when expressed as Z scores. In Fig. 2, I plot the standards for bone age used by Martorell (Garn & Rohman, 1960; Yarbrough et al., 1973) and the bone maturity score of similar Guatemalan children (Blanco et al., 1972). What is quite clear is that bone age is very markedly delayed in these children as well as height.

Fig. 2. Bone age of Guatemalan children in relation to American standards. Bone age is from data of Blanco et al., (1972). The standards, showing the mean and one standard deviation, for girls (upper plot) are from Garn & Rohman (1960) and for boys (lower plot) from Yarbrough et al. (1973) derived from the Fels data set. The boys' standards given by Garn & Rohman are almost the same as those presented except that the standard deviations are somewhat wider.

This method of assessing 'maturity', and hence potential, is open to question. The number of ossification centres in the wrist is not necessarily closely related to the maturity of the long bones and spine which determine height. Certainly, there is a poor relationship between the delay in the maturity of the long bones and that of the skull or the teeth; furthermore, the different individual ossification centres are affected by malnutrition to different degrees (Dreizen et al., 1954; Dreizen, Spirakis & Stone 1964; Garn, 1981; Gupta et al., 1988). In malnourished children there is neither a steady advance in bone maturity nor in height; indeed, the two seem to be completely out of step seasonally; in Alabama height gain is least over the summer months when bone maturity rapidly advances, whereas when the height gain is most rapid few ossification centres appear (Dreizen et al., 1959).

There is some support for the proposition that bone maturity is much less retarded than height in malnourished children (Reichman & Stein, 1968; Adams & Berridge, 1969); however, most authors report gross retardation in bone maturity (Jones & Dean, 1956; 1959; Ghosh, Varma & Bhardawaj, 1967; Dreizen et al., 1956; 1958) that is either about the same (Briers, Hoorweg & Stanfield, 1975; Graham, 1972; Keet et al., 1971) or more severe (Alvear et al., 1986) than the delay in height age.

Given the general conclusion that the delays in maturity and in height are not significantly different when expressed in units of time, it seems that most malnourished children retain their capacity for full catch-up.