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

Femoral neck shape and the spatial distribution of its mineral mass varies with its size: Clinical and biomechanical implications

The femoral neck (FN) is a cantilever with external and internal dimensions determining its size, shape, the spatial distribution of the mineralized cortical and trabecular bone tissue mass, and its strength. Geometric indices of FN strength are often derived using FN dimensions estimated in vivo from dual X-ray absorptiometry (DXA) assuming that the FN cross section approximates a circle or a square. As DXA does not measure FN depth, we examined whether circular, square, or elliptical models of FN cross sections predict FN depth, and so its external volume, shape, volumetric bone mineral density (vBMD), and geometric indices of strength. We studied paired FN specimens from 13 Caucasian female cadavers (mean age 69 years, range 29 to 85) using DXA, micro-computed tomography (μ-CT), and direct calliper measurements. DXA accurately measured FN width (supero-inferior diameter) but models assuming a circular and a square cross section overestimated FN depth (antero-posterior diameter) and volume, and so underestimated vBMD by 15.0 ± 5.8% (circular cross section) and by 33.2 ± 4.6% (square cross section) (both P < 0.05). As depth was less than the width, an elliptical model with a constant depth/width ratio of 0.75 reduced the accuracy error in vBMD to 14.0 ± 8.5% (P = 0.10). However, as FN width increased, FN depth increased relatively less. An elliptical model using a quadratic equation to mimic this changing in shape with increasing size reduced the error in vBMD to 4.4 ± 7.7% (NS). Circular cross-section models overestimated section modulus at the mid-FN by about 51%. The elliptical models reduced the error two- to three fold. Images from micro-CT scanning show that the FN cross-sectional shape resembles an ellipse with the long axis and the maximum moment of inertia (Imax) oriented in the supero-inferior direction, and the cortical mass concentrated inferiorly. The larger the cross section, the more elliptical the shape, and the greater the Imax supero-inferiorly, while Imin (in the antero-posterior direction) remains relatively constant. The shape, spatial distribution of bone, and moments of inertia are likely to be adaptations to bending moments during bipedalism. Assuming the FN cross section approximates a circle or square produces errors in FN depth, volume, vBMD, and geometric indices of bone strength. Studies are needed to determine the effects of age, sex, and race on FN size and shape in health and disease

Differences in the degree of bone tissue mineralization account for little of the differences in tissue elastic properties

Mini-abstract: Study of postmortem samples of cortical bone from the trochanters of 12 Caucasian females revealed that tissue mineral density (TMD) and tissue elastic modulus correlate weakly within and between individuals. Other material properties nee

The heterogeneity in femoral neck structure and strength

Most measures of femoral neck strength derived using dual-energy X-ray absorptiometry or computed tomography (CT) assume the femoral neck is a cylinder with a single cortical thickness. We hypothesized that these simplifications introduce errors in estim