Development of a More Physiological Loading Protocol for Spine In Vitro Flexibility Testing

Abstract

Abstract With motion preserving systems, whose behavior is dependent on the loading applied, it is becoming more important to produce a loading environment that better simulates the situation in vivo. Several studies show that the spine experiences high compressive loads that change as a function of position. The purpose of this study was to apply a high compressive dynamic follower load and determine the moment required to produce a physiological range of motion in vitro. Six human specimens (L2-L3) were subjected to a pure moment, in combination with a high compressive dynamic follower load. Appropriate compressive loads were obtained from literature based on in vivo intradiscal pressure measurements. The moments necessary to produce pre-defined angles of rotation in flexion, extension, lateral bending, and axial rotation (in vivo literature values) were recorded. The follower load was attached laterally in flexion-extension and axial rotation and anterior-posteriorly in lateral bending. Tests were also conducted using two traditional loading protocols for comparison: ±10 Nm (no follower load); and ±10 Nm with a 600 N constant follower load, in terms of range of motion (ROM), helical axis of motion (HAM), and flexibility coefficients.The new loading protocol resulting from this study consisted of a compressive follower load of 800 N in the neutral position, a flexion moment of 35 Nm combined with a maximum compressive follower load of 2000 N, an extension moment of 10 Nm combined with 900 N, a moment of ±15 Nm in lateral bending with 1100 N, and a moment of ±20 Nm in axial rotation with 1250 N. The anterior-posterior follower load fixation in lateral bending allowed more unrestrained movement. The moments necessary to produce physiological motion under a dynamic compressive follower load are higher than what is currently used and are comparable to calculated in vivo moments.