Study Of The Lumbar Spinal Fusion Of Single And Double Segments And The Degeneration Of Their Adjacent Facet Joints By Finite-Element Analysis

Objective: To establish finite-element models of the lumbar spinal fusion of double(L3-5,L4-S1) and single(L4-5) segments with posterior decompression and internal fixation and analyze the stress of lumbar facet joints in all kinds of loading. To explain the reason why double segments are more likely to cause adjacent facet joints to degenerate, compared with single segment in the biomechanics point of view. To draw some theoretical conclusions to suggest how many segments should be fused and fixed, and guide patients how to do functional exercise.Methods: We first used the finite element analysis software called Super-Sap to create a finite element model of normal L1-S1 motion segments from the lumbar spine CT scans of the donor. Then based on the normal model, we built three different models (two lumbar spinal double segment fusion models: L3-5,L4-S1, one single segment fusion model : L4-5) with posterior decompression and internal fixation. By changing the material properties of disc tissues, we applied five different loadings, including plumb, anteflexion, back extension, lateroflexion, and torsion, on these three models respectively. We measured and compared the stress changes on the adjacent facet joints between these three models.Results: 1 Compared with single segments fusion, double segment fusion may cause the Von Mises stresses of the adjacent facet joints (L3-4,L5-S1) to dramatically increase (P<0.05) at double sides of the joint, when doing back extension, the same side of the joint, when doing lateroflexion, and opposite side of the joint, when doing torsion. However, under other circumstances, such as at the same side of the joint, when doing torsion, at the opposite of the joint, when doing lateroflexion, the P values have no statistical meanings. 2 In the double segment fusion model, back extension has the most increase of adjacent facet joint Von Mises stress, and lateroflexion and torsion have less (P<0.05) than back extension.Conclusion: 1 Under the condition of fusion and internal fixation, double segments are more likely to cause adjacent facet joints to degenerate, compared with single segment. 2 After the surgery of fusion and internal fixation of double segments, patients should avoid functional exercises, such as back extension, lateroflexion, and torsion, to reduce the possibility of neighboring joint degeneration.