Internal and External Responses of Anterior Lumbar/Lumbosacral Fusion: Nonlinear Finite Element Analysis

Study Design Determination of external and internal responses of the human lumbosacral spine using a validated 3-dimensional finite element model. Objective The objective of the present study was to evaluate the range of motion, disc stress, and facet joint pressure owing to anterior fusion at L4-L5 or L5-S1 level and compare with the intact spine. Summary of Background Data A significant majority of finite element models of anterior lumbar interbody fusion are primarily focused on upper and middle levels, whereas lower spinal levels are most commonly treated with surgery. Methods A 3-dimensional L4-S1 finite element model, validated in the entire nonlinear range of the moment-rotation response, was used to determine ranges of motion, disc stress, and facet joint contact pressure under normal and 2 surgical conditions with bone graft and porous tantalum. Biomechanical responses were compared under flexion and extension loading between the 2 fusions and fusion masses and at the fused and intact segments. Results Moment-rotation responses were nonlinear under all conditions. The range of motion at the caudal level was greater than the range of motion at the rostral level in the intact spine. The range of motion of the L4-S1 spine decreased more with the caudal than rostral fusion and more with the tantulum than bone under both loading modes. Facet joint pressures increased more with the rostral than caudal fusion. Stresses in the adjacent disc were greater with the caudal than rostral fusion under both modes of loading. Conclusions At the fused level, the caudal fusion imparted additional rigidity under flexion to the lumbosacral joint. Both fusion masses added flexibility to the adjacent segment. Under both fusion masses, increased facet joint pressure in the lumbosacral joint indicates the susceptibility of this transitional joint to long-term biomechanics-induced consequences. Increased facet joint pressures with the rostral fusion indicate that the posterior complex responds with increased load sharing, and may predispose the spine to facet-related arthropathy. Increased stresses in the adjacent disc with the caudal fusion under both modes of loading imply the potential to disc-related changes owing to long-term physiologic loading.