Spinal loads after osteoporotic vertebral fractures treated by vertebroplasty or kyphoplasty

Vertebroplasty and kyphoplasty are routine treatments for compression fractures of vertebral bodies. A wedge-shaped compression fracture shifts the centre of gravity of the upper body anteriorly and generally, this shift can be compensated in the spine and in the hips. However, it is still unclear how a wedge-shaped compression fracture of a vertebra increases forces in the trunk muscle and the intradiscal pressure in the adjacent discs. A nonlinear finite element model of the lumbar spine was used to estimate the force in the trunk muscle, the intradiscal pressure and the stresses in the endplates in the intact spine, and after vertebroplasty and kyphoplasty treatment. In this study, kyphoplasty represents a treatment with nearly full fracture reduction and vertebroplasty one without restoration of kyphotic angle although in reality kyphoplasty does not guarantee fracture reduction. If no compensation of upper body shift is assumed, the force in the erector spine increases by about 200% for the vertebroplasty but by only 55% for the kyphoplasty compared to the intact spine. Intradiscal pressure increases by about 60 and 20% for the vertebroplasty and kyphoplasty, respectively. In contrast, with shift compensation of the upper body, the increase in muscle force is much lower and increase in intradiscal pressure is only about 20 and 7.5% for the vertebroplasty and kyphoplasty, respectively. Augmentation of the vertebral body with bone cement has a much smaller effect on intradiscal pressure. The increase in that case is only about 2.4% for the intact as well as for the fractured vertebra. Moreover, the effect of upper body shift after a wedge-shaped vertebral body fracture on intradiscal pressure and thus on spinal load is much more pronounced than that of stiffness increase due to cement infiltration. Maximum von Mises stress in the endplates of all lumbar vertebrae is also higher after kyphoplasty and vertebroplasty. Cement augmentation has only a minor effect on endplate stresses in the unfractured vertebrae. The advantages of kyphoplasty found in this study will be apparent only if nearly full fracture reduction is achieved. Otherwise, differences between kyphoplasty and vertebroplasty become small or vanish. Our results suggest that vertebral body fractures in the adjacent vertebrae after vertebroplasty or kyphoplasty are not induced by the elevated stiffness of the treated vertebra, but instead the anterior shift of the upper body is the dominating factor.