Effects of end boundary conditions and specimen geometry on the viscoelastic properties of cancellous bone measured by dynamic mechanical analysis.

The viscoelastic properties of cancellous bone can be measured nondestructively in compression testing using a dynamic mechanical analyzer. In this study, we examined the effects of end boundary conditions and specimen geometry on the viscoelastic properties of cancellous bone measured by dynamic mechanical analysis. During dynamic compression testing, the cancellous bone specimens may be mechanically fixed (e.g., glued) to the loading platens or they may be free to expand across the platen surface. When specimens of cancellous bone were tested between platens with gluing, the dependence of loss tangent on frequency was not consistent with previously observed strain-rate-dependent mechanical behavior of cancellous bone. When long specimens of cancellous bone (length = 10 mm, diameter = 8 mm) were tested without gluing, the relationship between loss tangent and frequency depended on the level of load applied. For short specimens (length = 5 mm, diameter = 8 mm) tested without gluing, however, the frequency dependence of loss tangent agreed with existing data reported for the strain-rate-dependent behavior of cancellous bone and also with the frequency dependence of cortical bone viscoelasticity. Therefore, we recommend that short cancellous bone cylinders with a length of 5 mm and a diameter of 8 mm should be used without gluing in the dynamic mechanical analysis of cancellous bone. This is consistent with the American Society for Testing and Materials testing recommendations for plastics, but different from current practice for unimodal mechanical testing of cancellous bone.