Axial compression force measurement acting across the strut graft in thoracolumbar instrumentation testing

Objective. Current recommendations for spinal implant testing do not consider the determination of axial compression forces of the overbridging implant on the strut graft. No direct data exist on the influence of load transfer through the strut graft and of the kind of instrumentation, especially in thoracolumbar corpectomy models. Design. Therefore in this biomechanical in vitro study a method for measurement of the axial compression force acting across the strut graft in different thoracolumbar instrumentations was developed. Methods. In this in vitro study, a corpectomy model was simulated and anterior, posterior and combined short fixation devices currently available were tested under pure moments to evaluate their biomechanical stabilizing characteristics. Range of motion, neutral zone and the axial compressive force acting on the strut graft were measured continuously in the three primary directions. Results. Without loads, the combined stabilization and followed by anterior instrumentation created a higher axial compression force than the dorsal instrumentation on the strut graft. Especially during maximal extension there was no axial compression of the dorsal instrumentation on the strut graft, which resulted in an increase of the range of motion. Conclusion. The feasibility of the new method was demonstrated in this study. For the purpose of standardization and comparison it should be considered in spinal implant testing.