The effect of cement augmentation on the load transfer in an osteoporotic functional spinal unit - Finite-element analysis

Study Design. The effect of cement augmentation on an osteoporotic lumbar functional spinal unit was investigated using finite-element analysis. Objective. To evaluate the influence of cement augmentation on load transfer, stresses, and strains. Summary of Background Data. Osteoporosis is the most frequent skeletal disease of the elderly, leading to weakness of the bony structures. Cement injection into vertebral bodies has been used to treat osteoporotic compression fractures of the spine. The clinical results are encouraging. Experimental biomechanical studies showed significant increases in stiffness and strength of treated bodies. However, little is known about the consequences for the adjacent, nontreated levels. Methods. Three-dimensional finite-element models of L2-L3 were developed and the material properties adapted to simulate osteoporosis. The influence of augmentation level as well as uni- and bipedicular filling with polymethylmethacrylate were investigated. Compression, flexion, and lateral bending were simulated. Results. Augmentation increased the pressure in the nucleus pulposus and the deflection of the adjacent end-plate. The stresses and strains in the vertebrae next to an augmentation were increased, and their distribution was changed. Larger areas were subjected to higher stresses and strains. The treatment clearly altered the load transfer. Changes to the overall stress and strain distribution were less pronounced for unipedicular augmentation. Conclusions. Cement augmentation restores the strength of treated vertebrae, but leads to increased end-plate bulge and an altered load transfer in adjacent vertebrae. This supports the hypothesis that rigid cement augmentation may facilitate the subsequent collapse of adjacent vertebrae. Further study is required to determine the optimal reinforcement material and filling volume to minimize this effect.