G Lowet

Prediction of vertebral strength in vitro by spinal bone densitometry and calcaneal ultrasound

Spinal bone mineral density (BMD) measurements and calcaneal ultrasound were compared in terms of their ability to predict the strength of the third lumbar vertebral body using specimens from 62 adult cadavers (28 females, 34 males). BMD was measured using dual X‐ray absorptiometry (DXA) in both vertebra and calcaneus. Quantitative computed tomography (QCT) was used to determine trabecular BMD, cortical BMD, cortical area, and total cross‐sectional area (CSA) of the vertebral body. Bone velocity (BV) and broadband ultrasonic attenuation (BUA) were measured in the right calcaneus. Vertebral strength was determined by uniaxial compressive testing. Vertebral ultimate load was best correlated with DXA‐determined vertebral BMD (r2 = 0.64). Of the QCT parameters, the best correlation with strength was obtained using the product of trabecular BMD and CSA (r2 = 0.61). For vertebral ultimate stress, however, the best correlation was observed with QCT‐measured trabecular BMD (r2 = 0.51); the correlation with DXA‐determined BMD was slightly poorer (r2 = 0.44). Calcaneal ultrasound correlated only weakly with both ultimate load and stress with correlation coefficients (r2) of 0.10–0.17, as did calcaneal BMD (r2 = 0.18). Both spinal DXA and spinal QCT were significantly (p < 0.001) better predictors of L3 ultimate load and stress than were either calcaneal ultrasound or calcaneal DXA. Multiple regression analysis revealed that calcaneal ultrasound did not significantly improve the predictive ability of either DXA or QCT for L3 ultimate load or stress. Calcaneal DXA BMD, bone velocity, and BUA correlated well with each other (r2 = 0.67–0.76), but were only modestly correlated with the DXA and QCT measurements of the vertebra. These data indicate that spinal DXA and spinal QCT provide comparable prediction of vertebral strength, but that a substantial proportion (typically 40%) of the variability in vertebral strength is unaccounted for by BMD measurements. Ultrasonic measurements at the calcaneus are poor predictors of vertebral strength in vitro, and ultrasound does not add predictive information independently of BMD. These findings contrast with emerging clinical data, suggesting that calcaneal ultrasound may be a valuable predictor of vertebral fracture risk in vivo. A possible explanation for this apparent discrepancy between in vivo and in vitro findings could be that current clinical ultrasound measurements at the calcaneus reflect factors that are related to fracture risk but not associated with bone fragility.

Prediction of Vertebral and Femoral Strength In Vitro by Bone Mineral Density Measured at Different Skeletal Sites

The aim of the present study was to investigate the prediction of vertebral and femoral strength in vitro by bone mineral density (BMD) measured at different skeletal sites. The third lumbar vertebral body, the right proximal femur, and the right calcaneus were removed from 38 male and 32 female cadavers (mean age 69 years, range 23–92 years). Areal BMD of all bone specimens was determined by dual‐energy X‐ray absorptiometry (DXA). The failure load of the vertebral body and the femur was determined by mechanical testing. Vertebral and femoral strength were both greater in males than females (p < 0.01), as was BMD at all sites (p < 0.01). Vertebral strength correlated well with vertebral BMD (r2 = 0.64) but was only moderately correlated with BMD measured at the femur (r2 = 0.36) or the calcaneus (r2 = 0.18). Femoral strength showed the highest correlations with femoral BMD (r2 = 0.88) and somewhat weaker relationships with BMD at the vertebra (r2 = 0.50) and the calcaneus (r2 = 0.54). BMD values at the vertebra, femur, and calcaneus were only moderately interrelated (r2 = 0.31–0.65), and vertebral strength correlated only modestly with the strength of the femur (r2 = 0.36). These in vitro results support the concept that optimal prediction of vertebral or femoral strength by DXA requires site‐specific assessments.

Effects of anteversion on femoral bone mineral density and geometry measured by dual energy x-ray absorptiometry : A cadaver study

The effect of femoral neck anteversion on bone mineral density (BMD) and geometry as measured by dual energy X-ray absorptiometry (DXA) was assessed using 64 right proximal femora from 36 male and 28 female cadavers. The anteversion angle was measured on computed tomography (CT) images, and DXA measurements were made both in the neutral position (i.e, at 0 degree anteversion, femoral neck axis parallel to the table) and in the simulated anteverted position (i.e., femoral shaft axis parallel to the table, greater and lesser trochanters in contact with the table, and femoral neck free). The mean anteversion angle measured by CT was 19.3 degrees (range 6 degrees-38 degrees). Anteversion was associated with a significant elevation in femoral neck BMD of +2.8% (range -5.3%-(+)9.8%) (p < 0.05), and the femoral neck BMD increased with increasing anteversion (p < 0.01). Trochanteric BMD was less affected by anteversion, with an average increase of only 0.2% (range -5%-5.9%) (p = n.s.) in the anteverted position, but there was a significant positive association between the change in trochanteric BMD and the anteversion angle (p < 0.01). Anteversion produced a mean reduction of -2.4% (range -7.6%-(+)4.3%) (p < 0.001) in apparent femoral neck axis length, while femoral neck width remained generally unaffected. These data confirm that femoral BMD as measured by DXA is affected by femoral anteversion with a lesser magnitude than previously reported. The use of trochanteric BMD may minimize the influence of anteversion. While the mean changes in BMD and neck axis length attributable to anteversion are modest, the considerable interindividual variability in the magnitude of the effects demonstrates that other factors, such as, the complex geometry of femoral neck modifies the effect of anteversion on BMD measurements. The error in BMD introduced femoral anteversion may represent a significant confounding influence in cross-sectional and longitudinal studies. Careful repositioning of the foot and leg is essential in monitoring changes in BMD longitudinally. Knowledge of the effects of femoral anteversion may assist in understanding the relation of femoral BMD and neck axis length to hip fracture.

Ultrasonic measurement of the calcaneus: investigation of some error sources, and correlation with femoral strength

Ultrasonic signal velocity, phase velocity and broadband ultrasonic attenuation (BUA) have been measured in 64 calcanae in vitro. Femoral strength (FS) was determined for specimens from the same cadavers in a mechanical test simulating a fall. Femoral bone mineral density (BMD) was measured using X-ray absorptiometry. Signal velocities were highly technique-dependent, and were significantly greater than phase velocities. Negative dispersion was observed. In a subset (n=23) of the calcanae, there were highly significant reductions in velocity and BUA when the cortical shell was removed. BMD was the best predictor of FS (r/sup 2/=0.88). The best ultrasonic predictor was BUA (r/sup 2/=0.47). Multiple regression analysis indicated that ultrasonic parameters did not significantly improve the ability of BMD to predict FS. The results suggest that ultrasound has only a limited ability to predict hip fracture strength. However, investigation of error sources and new measurement techniques can be expected to lead to improvements in the clinical performance of bone ultrasound, as well as advancing ones understanding of the underlying interaction between bone and ultrasound.