Growth tracking of femoral and humeral strength from infancy through late adolescence

Aim: To determine to what degree femoral and humeral strengths "track", or remain at the same ranked position relative to other individuals during the entire growth period. Methods: Radiographs of 20 individuals, equally divided by sex, were measured at 6-mo or annual intervals from 6 mo to 17 y of age. The section modulus, a measure of bending/torsional strength, was derived from cortical and subperiosteal breadths taken at the femoral midshaft and at 40% of the bone length from the distal end of the humerus. Body size was also assessed as the product of body weight and bone length. Growth tracking was evaluated in two ways: as Spearman rank-order correlations between strengths at each age point and strengths at 17 y of age, and as the number of individual changes in rank over specified age intervals. All analyses were carried out within sex. Results: The degree of growth tracking varied by sex, skeletal location, and age. Correlations were higher for the humerus (r = 0.56-0.83) than for the femur (r = 0.10-0.63). Males showed particularly poor tracking for the femur, most likely due to late and variable adolescent growth spurts in body size. Standardizing for body size improved tracking for the male femur, but not for the humerus. Over the entire growth period, individuals averaged 5-8 out of a possible 9 changes in rank. Early childhood (<6 y) was the least stable period. Conclusion: Previously documented growth tracking of skeletal parameters over relatively short (1-6-y) time periods can not necessarily be extrapolated to longer time intervals, nor can results from one skeletal region or time period be applied to another region or time period. Variation in timing of growth events, and body size for weight-bearing elements, may have important influences on growth tracking.