Relations of mechanical properties to density and CT numbers in human bone

Mechanical properties of cortical and cancellous bone from eight human subjects were determined using an ultrasonic transmission technique. Raw computerized tomography (CT) values obtained from scans of the bones in water were corrected to Hounsfield units. The correlations between CT numbers and mechanical property estimated from cortical bone were found to be low (r2<0.2), while these relationships for cancellous bone were found to be higher (r2>0.6). These results suggest that CT values may be useful in predicting mechanical properties only for cancellous bone. Poor correlations were found between modulus in the radial or circumferential direction and modulus in the superior-inferior direction for cortical bone, whereas good correlations were found between modulus in the anteriorposterior direction or medial-lateral direction and modulus in the S-I direction for cancellous bone. These results indicate that modulus in the radial or circumferential direction could not be predicted from modulus in the S-I direction for cortical bone, but could be predicted for cancellous bone. The predictive capabilities of linear and power models evaluated for cancellous bone alone were approximately equal. However, the power function gives a better fit of data at the low and high density values. The specific relationships, depending on the types of bone, that predict elastic modulus from density and CT numbers were suggested for human cortical and cancellous bone. These specific correlations may help a number of researchers develop more accurate models; however, these hypotheses should be proven by further study.