The mechanics of polymethylmethacrylate augmentation.

Osteoporosis frequently leads to vertebral compression fractures. Percutaneous cement augmentation, one recent technique, may alter the biomechanics of the vertebral body and spinal segment. These alterations reportedly predispose the spinal segment to additional vertebral compression fractures. We investigated the changes in segment stiffness and strength after polymethylmethacrylate augmentation. Twelve thoracic segments consisting of five vertebral bodies were divided into two groups, a pure moment group (Group 1) and an eccentric compression group (Group 2). Baseline measurements of stiffness were taken on each segment followed by the creation of an initial vertebral compression fracture during which stiffness and strength were measured. After augmentation, stiffness was again measured. Finally, a second vertebral compression fracture was created measuring stiffness and strength again. Augmentation did not alter stiffness before and after augmentation in either group. Augmentation also did not result in any difference in strength measured at subsequent fracture when compared with strength measured at initial fracture in either group. The augmentation of vertebral compression fractures by kyphoplasty does not alter the stiffness or the strength of the multilevel segments and eccentric compression in contrast to pure moments leads to a lower strength during mechanical testing.