The effect of posterior instrumentation following PLIF with BAK cages is most pronounced in weak bone

Background. The use of BAK-cages for lumbar fusion has become very popular but complications such as cage subsidence and settling occurred. To treat these complications posterior instrumentation was used to improve segmental stability. It is, however, poorly understood, why some patients require additional posterior instrumentation, whereas the majority does not. The objectives of the study presented were first to determine the influence of bone mineral density (BMD) to the initial compressive stiffness of a segment that underwent posterior lumbar interbody fusion (PLIF) with two BAK-cages. Second, to estimate the importance of additional posterior instrumentation for compressive stiffness with respect to bone mineral density. Methods. A validated finite element model (FEM) including posterior decompression and stabilisation by two BAK-cages (BAK_FEM) was used to predict the initial compression stiffness in axial loading of 600 N. This model was used to predict the influence of various grades of BMD on compression stiffness, A second FEM was generated in which additional posterior screw-rod instrumentation was simulated (BAK + PI_FEM) and this model used to predict the influence of BMD in axial loading. Findings. The responses of all FEM suggested that initial compressive stiffness will increase if there is an increase of BMD. The stiffness as predicted by BAK + PI_FEM was always superior to FEM_BAK. This difference was most pronounced for weak bone quality. Interpretation. Compression stiffness following PLIF with BAK-cages depends on BMD. Additional posterior instrumentation results in an additional increase of compression stiffness. This effect is most pronounced in simulated soft bone quality. These results may help to select patients for combined stabilisation.