PREDICTION OF VERTEBRAL STRENGTH IN-VITRO BY SPINAL BONE DENSITOMETRYAND CALCANEAL ULTRASOUND

Spinal bone mineral density (BMD) measurements and calcaneal ultrasoun d were compared in terms of their ability to predict the strength of t he third lumbar vertebral body using specimens from 62 adult cadavers (28 females, 34 males), BMD was measured using dual X-ray absorptiomet ry (DXA) in both vertebra and calcaneus. Quantitative computed tomogra phy (QCT) was used to determine trabecular BMD, cortical BMD, cortical area, and total cross-sectional area (CSA) of the vertebral body, Bon e velocity (BV) and broadband ultrasonic attenuation (BUA) were measur ed in the right calcaneus. Vertebral strength was determined by uniaxi al compressive testing, Vertebral ultimate load was best correlated wi th DXA-determined vertebral BMD (r(2) = 0.64), Of the QCT parameters, the best correlation with strength,vas obtained using the product of t rabecular BMD and CSA (r(2) = 0.61), For vertebral ultimate stress, ho wever, the best correlation was observed with QCT-measured trabecular BMD (r(2) = 0.51); the correlation with DXA-determined BMD was slightl y poorer (r(2) = 0.44), Calcaneal ultrasound correlated only weakly wi th both ultimate load and stress with correlation coefficients (r(2)) of 0.10-0.17, as did calcaneal BMD (r(2) = 0.18), Both spinal DXA and spinal QCT were significantly (p < 0.001) better predictors of L3 ulti mate load and stress than were either calcaneal ultrasound or calcanea l 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 (r(2) = 0.67-0.76), but were only modestly correlated with the DXA and QCT measurements of the vert ebra. These data indicate that spinal DXA and spinal QCT provide compa rable prediction of vertebral strength, but that a substantial proport ion (typically 40%) of the variability in vertebral strength is unacco unted for by BMD measurements, Ultrasonic measurements at the calcaneu s are poor predictors of vertebral strength in vitro, and ultrasound d oes not add predictive information independently of BMD, These finding s contrast with emerging clinical data, suggesting that calcaneal ultr asound 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 measur ements at the calcaneus reflect factors that are related to fracture r isk but not associated with bone fragility.