0733 - Fatigue Behavior of Human Vertebral Trabecular Bone

Introduction Over half of the 700,000 vertebral body fractures suffered annually by elderly Americans are unrelated to any single traumatic event (1) and the precise mechanisms of the fracture are not well understood. One possibility is that the fracture is due to a series of isolated, relatively high overloads (2); another is that it is due to lower level, more highly repetitive loads (3,4). In between is the possibility that the vertebral fractures develop from repetitive moderately high loads — low cycle fatigue. To date, the only studies on fatigue behavior of trabecular bone have been performed either on bovine bone (4, 5), or human trabecular hard tissue (6). While the latter are difficult to interpret at the whole trabecular bone level, the former studies have shown that modulus reductions can occur during fatigue loading of trabecular bone, and that strain accumulation from creep may dominate those from fatigue. It remains to be shown, however, if any of these trends can be extended to human vertebral trabecular bone, and if so, to what extent. The goal of this study was to investigate the fatigue behavior of human vertebral trabecular bone using in vitro mechanical test methods. Our specific objectives were to: 1) measure the cycles-to-failure, modulus reductions, and strains at failure for high-stress, low-cycle fatigue loading conditions; and 2) explore the sensitivity of the resulting behavior to specimen stiffness. The results provide a first glimpse at the fatigue behavior of human vertebral trabecular bone, and provide insight into possible mechanisms of progressive age-related vertebral fracture.