MULTISEGMENTAL SPINAL BIOMECHANICAL TESTING USING A NOVEL 3-D SLIDING RING PURE MOMENT APPARATUS

INTRODUCTION: Reports of in vitro experimental assessment of spinal implants and surgical techniques are now commonplace given the growing development of motion preservation devices. These studies typically compare spinal flexibility between intact and treated cadaveric specimens in all bending directions. Pure moment boundary conditions are preferred because they ensure uniform loading along the length of the spine. Various systems have been developed to induce pure moment loading conditions, including pneumatic actuators and sliding rails [1] and simple deadweight set-ups [2]. The cable-driven pure moment setup pioneered by Crawford et al [3] has been widely adopted due to its cost efficiency and ability to integrate with commercially-available uniaxial mechanical test frames (Figure 1). Recent studies have called into question the accuracy of current cable-driven pure moment designs [3, 4], asserting that the stationary loading ring affixed to the superior end of the specimen (“fixed ring”) imposes non-physiologic kinematic constraints. In response, we developed a novel “sliding ring” apparatus that permitted free motion of the superior end of the specimen while maintaining a uniform pure moment loading state [5]. The goal of this study is to develop and validate a cable-driven pure moment system for multi-axial testing applications. This “3-D sliding ring” design will be validated against a standard “fixed ring” set-ups using a cadaveric model. The results of this study are important in the development of accurate test standards for orthopaedic spinal devices, particularly new motion preserving technologies such as total disc and facet replacement systems.