Pure moment testing for spinal biomechanics applications: Fixed versus sliding ring cable-driven test designs

Abstract

In vitro multi-axial bending testing using pure moment loading conditions has become the standard in evaluating the effects of different types of surgical intervention on spinal kinematics. Simple, cable-driven experimental set-ups have been widely adopted because they require little infrastructure. Traditionally, “fixed ring” cable-driven experimental designs have been used; however, there have been concerns with the validity of this set-up in applying pure moment loading. This study involved directly comparing the loading state induced by a traditional “fixed ring” apparatus versus a novel “sliding ring” approach. Flexion-extension bending was performed on an artificial spine model and a single cadaveric test specimen, and the applied loading conditions to the specimen were measured with an in-line multiaxial load cell. The results showed that the fixed ring system applies flexion-extension moments that are 50–60% less than the intended values. This design also imposes non-trivial anterior–posterior shear forces, and non-uniform loading conditions were induced along the length of the specimen. The results of this study indicate that fixed ring systems have the potential to deviate from a pure moment loading state and that our novel sliding ring modification corrects this error in the original test design. This suggests that the proposed sliding ring design should be used for future in vitro spine biomechanics studies involving a cable-driven pure moment apparatus.

Introduction

In vitro experimental assessment of spinal implant devices and surgical techniques is important to the development and design optimization of new clinical technologies. These studies typically compare spinal flexibility between intact and treated cadaveric specimens in multiple anatomic directions. Among the methods of acquiring such data, pure moment biomechanical conditions are preferred because they ensure uniform loading along the column of the spine easing the comparison of techniques and technologies with previous literature (Wilke et al., 2001; Wilke et al., 1998). A variety of systems are used to apply pure bending moments to the cadaveric spine, including 6-axis testing machines (Beaubien et al., 2005; Kotani et al., 2005; Wilke et al., 1994; DiAngelo et al., 2004; Schwab et al., 2006; Kotani et al., 2006; Panjabi et al., 2007a, Panjabi et al., 2007b; Panjabi, 2007; Crawford et al., 1995), suspended deadweights (Melcher et al., 2002; Puttlitz et al., 2004; Goel et al., 1988; Stanley et al., 2004), and cable-driven systems (Crawford et al., 1995; Acosta et al., 2008; Barnes et al., 2009). The cable-driven test set-up applies a couple via a continuous loop of cable that is wound around a “loading ring” attached to one end of the spinal segment. Since its development by Crawford in the mid-90s (Crawford et al., 1995), the cable-driven pure moment method has been widely adopted due to its simplicity and relatively low infrastructure requirements (it requires only a uni-axial testing frame). Recently, it has been called into question whether the cable driven pure moment test design is capable of generating a pure moment loading state (Panjabi, 2007).

The goal of this study was to determine whether the cable-driven pure moment apparatus generates accurate, consistent loading conditions for a range of possible multi-axial spinal testing scenarios. We considered differences in applied loads generated by the “fixed loading ring” design, which has been used historically for cable-driven pure moment testing(Crawford et al., 1995; Acosta et al., 2008; Barnes et al., 2009), and a modified “sliding loading ring” system that was developed by our group to address the limitations of the fixed ring design.