THEORETICAL-ANALYSIS OF THE EXPERIMENTAL ARTIFACT IN TRABECULAR BONE COMPRESSIVE MODULUS

A theoretical analysis was performed to characterize potential experimental artifacts in conventional compression testing of trabecular bone, where strains are based on the relative displacements of the two loading platens. We assumed that the total experimental artifact for modulus was the SLIM of a damage and friction artifact and derived equations to describe these artifacts. The two unknown constants in these equations were found using a combination of data derived from linear finite element analyses and in vitro uniaxial compression tests. Subsequent finite element analyses allowed estimation of the artifacts for a wide range of specimens (cube, 1:4 - 3:1 aspect ratio cylinders). If friction is completely eliminated at the specimen-platen interface, the Young's modulus of a 5 mm sized (1:1 aspect ratio dimension) specimen which has a damage artifact due to machining may be underestimated by at least 45% regardless of specimen geometry; otherwise, the platens modulus may vary from less than 30 to over 175% of the Young's modulus, depending upon the specimen geometry and Poisson's ratio of the bone. Increasing the specimen size reduces the artifact only slightly. Since Poisson's ratio can be large for trabecular bone and is rarely known a priori, the precision of the conventional compression test will, therefore, be poor unless friction is completely eliminated al the specimen platen interface. However, without friction at the interface. the platens modulus will always underestimate Young's modulus, thereby reducing the accuracy of this test. There was also evidence that the strength may be affected by these artifacts. Taken together, these data suggest that the conventional compression test can be precise but is rarely accurate. and that inter-study comparisons should be made with caution. Use of other methods such as ultrasound or direct attachment of extensometers to material away from the platens may overcome problems with accuracy. Finally, a protocol that eliminates friction and uses a 2:1 aspect ratio specimen may optimize precision.