Finite-element stress analysis of the normal and osteoporotic lumbar vertebral body.

A finite-element model of an isolated elderly human L3 vertebral body was developed to study how material properties and loading conditions influence end-plate and cortical-shell displacements and stresses. The model consisted of an idealized geometric representation of an isolated vertebral body, with a 1-mm-thick end plate and cortical shell. For uniform compression, large tensile stresses occurred all around the cortical shell just below the end plate as a result of bending of the cortical shell as it supported the end plate. Large tensile bending stresses also developed in the inferior surface of the end plate. Equal reductions in both trabecular and cortical bone moduli increased displacements but did not affect peak stresses. A 50% reduction in trabecular bone modulus alone increased peak stresses in the end plate by 74%. Elimination of the cortical shell reduced peak stresses in the end plate by approximately 20%. For nonuniform, anteriorly eccentric compression, peak stresses everywhere changed by less than 11% but moved to the anterior aspect. When material properties were adjusted to represent osteoporosis with disproportionate reductions in trabecular (50% decrease) and cortical (25% decrease) bone moduli, anterior compression increased peak stresses by up to 250% compared to uniform compression. If fractures are initiated in regions of large tensile stresses, the results from this relatively simple model may explain how central end-plate and transverse fractures initiate from uniform compression of the end plate. Furthermore, for anterior compression, disproportionate modulus reductions in trabecular and cortical bone may substantially increase end plate and cortical shell stresses, suggesting a cause of age-related spine fractures.