Histomorphometric evaluation of skeletal development.

Hylonome dimensional relationships change little 2,3. Concomitant with the increase in bone size, the internal structure is modified. The average thickness of trabeculae in the proximal femoral metaphysis increases by about a third during the second half of gestation 4. Cortical thickness at the femoral midshaft approximately doubles during the same period 2. Histomorphometric studies of rib and iliac bone have yielded the expected result that periosteal bone formation is much more active in children than in adults 5-7. However, there may be a more fundamental difference between periosteal bone metabolism in children and in adults. In children, bone formation is continuous, which is the hallmark of modeling 7. In adults, periosteal bone may undergo cyclical resorption and formation, which is characteristic of remodeling 8. It is clear that wider bones must have higher mid-shaft periosteal apposition rates, because this is how they become wider. For example, during male puberty the estimated peak periosteal apposition rate of the metacarpal is about 0.5 Ìm/day, but it is close to 2 Ìm/day at the mid-shaft humerus 1. What is less widely appreciated is that periosteal growth is not necessarily synchronized between bones. For example, in 3-month-old babies, the humerus grows in width by a third faster than the femur 1. At the age of one year, the two bones expand at about the same rate, whereas at 33 months of age periosteal apposition is almost four times as fast at the femur as it is at the humerus. At 5 years of age, this difference in periosteal appo-sition rate between the two bones has shrunk to 25% in favor of the femur. These differences in bone growth in width between the humerus and femur mirror the mechanical usage of these extremities during development between 1 and 4 years of age 1. When infants start to walk, the femur is exposed to much higher forces and gets stronger quickly. At the same time, the humerus is used less and less for locomotive purposes and, accordingly, humerus strength increase is slow. During skeletal development, external bone shape undergoes marked changes through growth in length and in width. Whereas the former process has received a great deal of scientific attention, much less is known about bone growth in width. However, bone growth in width determines cross-sectional bone size, which is a major determinant of bone strength throughout life 1. At a typical mid-diaphyseal site, a …