Influence of jig design on biomechanical evaluation of spinal construct stability

Abstract

The biomechanical evaluation of spinal construct stability is of clinical importance as the results of such an evaluation provide information about the implant's performance in vivo in stabilizing the spine. The flexion/extension test is critical for the evaluation of spinal stability. However, it is more technically demanding to flex the spinal construct with the more structurally complex jig assembly than the assembly used in the compressive or twisting stability tests. A review of the literature shows that a variety of jig assemblies have been used to investigate the mechanical and kinematic behaviors of spinal constructs. However, there is little consensus about the influence of jig design on the investigated behaviors. The current study used the elastic beam‐column theory to predict the load and deformation of the spinal construct among four possible jig assemblies. The suitability of these four jig assemblies for the short‐ and long‐segment spinal construct was evaluated in detail with three indices: lateral deflection, loading condition, and failures at the bone‐screw interfaces. Among the four assemblies, the jig assembly that generates the distally increasing flexion moment and free cephalic motion was suitable for short‐segment fixation. The jig assembly that generates the constant flexion moment seemed to be more appropriate for long‐segmentation cases with physiologically acceptable deflection. This study aims to provide an insight into what jig‐related factors are associated with physiologically reliable outcomes of the flexion stability test for lumbar implants.