Construction of the Femoral Neck During Growth Determines Its Strength in Old Age

Study of the design of the FN in vivo in 697 women and in vitro in 200 cross-sections of different sizes and shapes along each of 13 FN specimens revealed that strength in old age was largely achieved during growth by differences in the distribution rather than the amount of bone material in a given FN cross-section from individual to individual. Introduction: We studied the design of the femoral neck (FN) to gain insight into the structural basis of FN strength in adulthood and FN fragility in old age. Materials and Methods: Studies in vivo were performed using densitometry in 697 women and in vitro using high-resolution μCT and direct measurements in 13 pairs of postmortem specimens. Results: The contradictory needs of strength for loading yet lightness for mobility were met by varying FN size, shape, spatial distribution, and proportions of its trabecular and cortical bone in a cross-section, not its mass. Wider and narrower FNs were constructed with similar amounts of bone material. Wider FNs were relatively lighter: a 1 SD higher FN volume had a 0.67 (95% CI, 0.61–0.72) SD lower volumetric BMD (vBMD). A 1 SD increment in height was achieved by increasing FN volume by 0.32 (95% CI, 0.25–0.39) SD with only 0.15 (95% CI, 0.08–0.22) SD more bone, so taller individuals had a relatively lighter FN (vBMD was 0.13 [95% CI, 0.05–0.20 SD] SD lower). Greater periosteal apposition constructing a wider FN was offset by even greater endocortical resorption so that the same net amount of bone was distributed as a thinner cortex further from the neutral axis, increasing resistance to bending and lowering vBMD. This was recapitulated at each point along the FN; varying absolute and relative degrees of periosteal apposition and endocortical resorption focally used the same amount of material to fashion an elliptical FN of mainly cortical bone near the femoral shaft to offset bending but a more circular FN of proportionally more trabecular and less cortical bone to accommodate compressive loads adjacent to the pelvis. This structural heterogeneity was largely achieved by adaptive modeling and remodeling during growth—most of the variance in FN volume, BMC, and vBMD was growth related. Conclusions: Altering structural design while minimizing mass achieves FN strength and lightness. Bone fragility may be the result of failure to adapt bone's architecture to loading, not just low bone mass.