Compressive creep behavior of bovine trabecular bone.

There are almost no published data that describe the creep behavior of trabecular bone (at the specimen level), even though the creep behavior of cortical bone has been well documented. In an effort to characterize the creep behavior of trabecular bone and to compare it with that of cortical bone, we performed uniaxial compressive creep tests on 24 cylindrical specimens of trabecular bone taken from 19 bovine proximal tibiae. Six different load levels were used, with the applied stress normalized by the specimen modulus measured prior to creep loading. We found that trabecular bone exhibits the three creep regimens (primary, secondary, and tertiary) associated with metals, ceramics, and cortical bone. All specimens eventually fractured at strains less than 3.8%. In addition, the general shape of the creep curve was independent of apparent density. Strong and highly significant power law relationships (r2 > 0.82, p < 0.001) were found between the normalized stress sigma/E0 and both time-to-failure tf and steady-state creep rate d epsilon/dt: tf = 9.66 x 10(-33) (sigma/E0)-16.18; d epsilon/dt = 2.21 x 10(33) (sigma/E0)17.65. These data indicate that the creep behaviors of trabecular and cortical bone are qualitatively similar. In addition, the strength of trabecular bone can be reduced substantially if relatively large stresses (i.e. stresses approximately half the ultimate strength) are applied for 5 h. Such strength reductions may play a role in the etiology of progressive, age-related spine fractures if adaptive bone remodeling does not arrest creep deformations.