A biomechanical study on single rod spinal instrumentation system in an unstable thoracolumbar injury model: A finite element analysis

ABSTRACT

OBJECTIVES: To develop three dimensional computer models of the anterior thoracolumbar spine implants or constructs (the novel single rod-screw implant and the standard implants) and to evaluate its biomechanical properties through a graphically reconstructed testing standard. METHODS: We developed a finite element modeling technique based on actual geometry of the implant constructs and mechanical property data from standard biomechanical studies on anterior thoracolumbar spinal instrumentation systems. Seven constructs were mounted on simulated vertebral bodies. Axial load sharing was measured through a range of applied axial loads from 100 N to 1600 N. The static destructive tests were conducted. The bending strength of each construct was calculated with a full length corpectomy graft in place, simulating reconstruction of the anterior column, and with no graft in place, simulating catastrophic graft failure. RESULTS: Static testing parameter demonstrated highly significant differences between devices. The plate construct formed the highest subset in bending strength of 1000-1100 N, whereas the single rod showed the lowest value of 300-400N. However, the bending strengths of single rod and dual rod both without bone grafts were not significantly different. With the graft in place, bending strength of the constructs significantly increased beyond the maximum set of load of 1600N, underlying the importance of the graft in overall construct strength. CONCLUSION: The 3-D finite element models for anterior thoracolumbar instrumentation system were designed with mechanical properties comparable to the actual biomechanical testing results. Although single rod construct has the lowest value, its bending strength is comparable to the standard dual rod system under static axial loading. Bone graft contributed to overall construct stiffness.