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

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.