Impact of outer width of the metacarpal diaphysis on the identification of low bone mass in children.

Developing a strong skeleton during growth is critical for minimizing fractures later in life. Prior work showed that bone mass varied with external bone size, a measure of the outer bone width. We tested how this association affected the identification of children with low bone mass. Radiographs of the nondominant hand of 45 White females and 54 White males, all ~ 8 years old, were assessed and second metacarpal length (Le) and the midshaft outer and inner widths were measured at the 40, 50, and 60% midshaft sites. The average total area (Tt.Ar), a measure of the area enclosed by the periosteal surface, and cortical area (Ct.Ar), a measure of bone mass, were calculated assuming a circular cross-section. Individuals were sorted into tertiles using robustness (Tt.Ar/Le). Z-scores were calculated for Ct.Ar first using the cohort mean and standard deviation and second using each robustness tertile mean and standard deviation. Females and males with Z-scores in the lower 33% range were identified for the group-average and tertile-specific average comparisons. Agreement between the two reference group approaches was determined using Cohen's kappa statistic for each sex. The percentage of individuals identified with low Ct.Ar depended on whether Z-scores were compared to the group average or tertile-specific averages. When compared to the group average, 67% of females and 56% of males identified with lower Ct.Ar were from the narrowest tertile, whereas 0% of females and 22% of males were from the widest tertile. For females and males, Cohen's kappa coefficient showed almost perfect agreement for the intermediate tertile (kappa coefficient > 0.84), but agreement was only poor to moderate (kappa coefficient < 0.53) for the narrowest and widest tertiles. Our findings provide evidence that external bone size may be a source of bias when assessing bone mass in females and males. Our analysis suggests that comparing morphological parameters from a child's metacarpal to their structural peer (i.e., reference group with a similar external size) may allow for more precision in diagnosing low bone mass and potentially lower bone strength.