Cement augmentation restores the strength of treated vertebrae, but leads to increased endplate 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.

IntroductionIntervertebral cages in the lumbar spine have been a promising advancement in spinal fusion. The final goal of the procedure is to relieve low back pain, one of today's major and most expensive health care problems. A large number of cages exist, which are different in design, material and surgical implanting approach. Several clinical follow-up studies have been published, mostly reporting high fusion rates, even though the criteria for success are inconsistent [1,5,17,18,23,25,30,31]. Additionally, there have been reports of mechanical failure, subsidence and migration [23,39,42]. The success of a cage insertion, which is regarded as fusion, and thereby stabilisation, of the spine, in addition to the biological factors, may be dependent on other parameters, including material properties of the vertebrae or the cage, geometry of the implant, and the interface between the bone and the cage [23,25,39,43].Hence, experimental and finite element studies have been carried out to investigate the influence of these facAbstract Intervertebral cages in the lumbar spine have been an advancement in spinal fusion to relieve low back pain. Even though initial stability is accepted as a requirement for fusion, there are other factors. The load transfer and its effect on the tissues adjacent to the cage may also play an essential role, which is not easily detectable with experimental tests. In this study the effects of an intervertebral cage insertion on a lumbar functional spinal unit were investigated using finite element analyses. The influences of cage material, cancellous bone density and spinal loading for the stresses in a functional spinal unit were evaluated. Three-dimensional (3D) finite element models of L2-L3 were developed for this purpose. An anterior approach for a monobloc, boxshaped cage was modelled. Models with cage were compared to the corresponding intact ones. The results showed that inserting a cage increased the maximum von Mises stress and changed the load transfer in the adjacent structures. Varying the cage material or the loading conditions had a much smaller influence than varying the cancellous bone density. The denser the cancellous bone, the more the stress was concentrated underneath the cage, while the remaining regions were unloaded. This study showed that the density of the underlying cancellous bone is a more important factor for the biomechanical behaviour of a motion segment stabilized with a cage, and its eventual clinical success, than the cage material or the applied load. Inserting an intervertebral cage markedly changed the load transfer. The altered stress distribution may trigger bone remodelling and explain damage of the underlying vertebrae.

Cement augmentation restores the strength of treated vertebrae, but leads to increased endplate 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.