Relation of vertebral deformities to bone density, structure, and strength

Abstract

Because they are not reliably discriminated by areal bone mineral density (aBMD) measurements, it is unclear whether minimal vertebral deformities represent early osteoporotic fractures. To address this, we compared 90 postmenopausal women with no deformity (controls) with 142 women with one or more semiquantitative grade 1 (mild) deformities and 51 women with any grade 2-3 (moderate/severe) deformities. aBMD was measured by dual-energy X-ray absorptiometry (DXA), lumbar spine volumetric bone mineral density (vBMD) and geometry by quantitative computed tomography (QCT), bone microstructure by high-resolution peripheral QCT at the radius (HRpQCT), and vertebral compressive strength and load-to-strength ratio by finite-element analysis (FEA) of lumbar spine QCT images. Compared with controls, women with grade 1 deformities had significantly worse values for many bone density, structure, and strength parameters, although deficits all were much worse for the women with grade 2-3 deformities. Likewise, these skeletal parameters were more strongly associated with moderate to severe than with mild deformities by age-adjusted logistic regression. Nonetheless, grade 1 vertebral deformities were significantly associated with four of the five main variable categories assessed: bone density (lumbar spine vBMD), bone geometry (vertebral apparent cortical thickness), bone strength (overall vertebral compressive strength by FEA), and load-to-strength ratio (45-degree forward bending ÷ vertebral compressive strength). Thus significantly impaired bone density, structure, and strength compared with controls indicate that many grade 1 deformities do represent early osteoporotic fractures, with corresponding implications for clinical decision making.

Fig. 1

To illustrate the biomechanical effects of spatial variation in bone density, transverse cross sections of the finite-element model of a vertebra are shown for a subject with relatively strong bone (top row) and one with relatively weak bone (bottom row). Each finite element is assigned material properties based on vBMD data from the QCT scan for that element, ranging from high-density (red) to low-density (gray) bone. For each subject, cross sections are shown for four models: the unaltered vertebra (“STANDARD”); the vertebra with all elements assigned the average vBMD value for that model (“HOMOGENIZED”); a model consisting only of the trabecular compartment, in which the outer 2 mm of bone is virtually removed (“TRABECULAR”); and a homogenized version of that trabecular model with all elements assigned the average vBMD of the trabecular compartment (“TRAB-HOM”). In each case, the resulting finite-element models are virtually compressed to failure to estimate compressive strength, resulting in multiple strength outcomes for each subject.