INTRODUCTION: The spine is stabilized by muscles. Their exact force magnitudes are unknown even for standard motions in the main anatomical planes. Thus, simplified loads are usually applied in spine studies. Moreover, the applied forces and moments vary strongly among different studies. Therefore, the measured or calculated values of different studies are hardly comparable and the most realistic load assumption is still unknown. The aim of the study was to determine a simplified load combination (force and moment) for the simulation of axial rotation which delivers results that fit best with those measured in vivo.
METHODS: A validated finite element model of the lumbar spine was used in a sensitivity study to estimate the ranges of the output parameters, such as intervertebral rotations (IVRs), intradiscal pressures (IDP), and facet joint forces (FJF). The input parameters, like torsional moment, follower load, and boundary condition were within the ranges of published data. The input parameters which mostly affect spinal loads and kinematics during axial rotation were also determined. In a subsequent optimisation study, the load combination was calculated which delivered results that fit best with those of available in vivo measurements.
RESULTS: The calculated ranges of IVR, IDP and FJF varied widely due to different load assumptions. The IVR and FJF are mainly affected by the magnitude of the torsional moment while the follower load influences mainly the IDP. The best agreement with published results measured in vivo was found for a follower load of 720 N and a pure moment of 5.5 Nm that were applied to the unconstrained vertebra L1.
DISCUSSION: The results clearly show that in many studies the assumed loads do not realistically simulate axial rotation. The optimised load combination determined in the present study mimic best the in vivo situation. Their consequent application leads to a better comparability of different investigations.