TRABECULAR BONE MODULUS AND STRENGTH CAN DEPEND ON SPECIMEN GEOMETRY

We performed a series of uniaxial compression tests on wet bovine trab ecular bone to compare both modulus and strength when measured using 2 : 1 aspect ratio (10 mm long, 5 mm diameter) cylinders (n=30) and 5mm cubes (n=29). We also compared the correlation coefficients in the re sulting modulus-density and strength-density regressions and the stand ard errors of the estimate. When comparing the mean values of modulus and strength for each group, the confounding variations in apparent de nsity were accounted for with an analysis of covariance. The Fisher's Z transformation was used to compare the correlation coefficients stat istically. Results from the analysis of covariance indicated that the modulus and strength of the cubes were higher by 36% (p <0.01) and 18% (p <0.05), respectively, with respect to the 2 : 1 cylinder values. T he correlation coefficients in the modulus-density and strength-densit y regressions were not sensitive to the regression model (linear versu s power law). However, correlation coefficients for both modulus-densi ty and strength-density regressions were higher (p < 0.05) for the 2 : 1 cylinders (r = 0.90, modulus, r = 0.94, strength) than for the cube s (r = 0.57, modulus; r = 0.82, strength). In addition, the standard e rrors of the estimate in both modulus and strength were substantially lower for the 2 : 1 cylinders. These data indicate that both modulus a nd strength can depend on the specimen geometry when using conventiona l compression testing techniques. We conclude, therefore, that inter-s tudy comparisons of modulus and strength may be invalid if these confo unding effects of different specimen geometries are not addressed. Our data also indicate that density can better explain the observed varia nce in modulus and strength when 2 : 1 cylinders are used as opposed t o cubes. Using this phenomenon as a rationale for choosing a standard specimen gometry, we recommend that the 2 : 1 cylinder be used as a st andard specimen in studies designed to determine the effects of variou s treatments on the uniaxial compressive modulus and strength of trabe cular bone.