Abstract
Areal BMD (aBMD) is relatively poor at discriminating those patients at risk of hip fracture. This study tested the hypothesis that a measure of bending resistance, cross section moment of inertia (CSMI) and section modulus, derived from 3D peripheral quantitative computed tomography (pQCT) images made ex-vivo, would discriminate cases of hip fracture from controls better than areal bone mineral density.
The biopsies were from (n = 20, F) subjects that had suffered an intracapsular hip fracture. The control material (n = 23, F) was from post-mortem subjects. Serial pQCT 1mm thick cross-sectional images using the Densiscan 1000 pQCT clinical forearm densitometer were obtained, and matched for location along the neck. The image voxels were converted to units of bone mass, which were then used to derive the mass weighted CSMI (MWCSMI), section modulus and areal bone mineral density, (see Table).
The aBMD results showed that the difference between the means of the fracture cases compared to the controls was 9.9% (−0.061g/cm2; +0.0055g/cm2, −0.127g/cm2; 95% confidence interval). However, the MWCSMI was 29.5% (−5966mm4; −8868mm4,−3066mm4; 95% confidence interval) lower in the fracture cases compared to the controls, while section modulus was 32.5% (−242mm3; −133mm3, −352mm3 95% confidence interval) lower. When presented as Z scores the fracture cases had considerably lower section modulus Z scores (mean −1.27 SD, p=0.0001) than aBMD – Z scores (mean −0.5 SD, p=0.07). To simulate the forces experienced during a sideways fall, the model’s neutral axis was rotated by 210°. The results were similar for section modulus to those at 0°.
This study suggests that biomechanical analysis of the distribution of bone within the femoral neck may offer a marked improvement in the ability to discriminate patients with an increased risk of intracapsular fracture. Progress towards implementing this form of analysis in clinical densitometry should improve its diagnostic value.
Correspondence should be addressed to Carlos Widgerowitz, Honorary Secretary BORS, Division of Surgery and Oncology, Section of Orthopaedic and Trauma Surgery, Ninewells Hospital and Medical School, Tort Centre, Dundee DD1 9SY, Scotland.
