METHODS: A non-linear ligamentous validated 3D finite element model from T1-femur (Figure 1) was developed from the CT scans of a 55-year-old normal male spine with no abnormalities, deformations, or major degenerations. The material properties were obtained from the literature and the model was assembled using Abaqus software. The intact model with ribcage was subjected to 4 Nm moments at the T1 endplate to simulate flexion, extension, lateral bending and rotation and the range of motion (ROM) was calculated. The ROM obtained was used as an input to the OpenSim thoracolumbar model that allowed the calculation of muscle forces via a static optimization tool [1, 2]. These segmental muscle forces were incorporated on the FE model via nodal forces (Figure 2) using the reference points and local datum axis to ensure the correct location and direction of muscle forces and the same ROM was applied. The annulus stress, intradiscal pressures and vertebral stresses were calculated and compared with and without muscle forces.

RESULTS SECTION: The annulus stress for the flexion, bending and rotation with muscle forces model showed an increase of 23%, 22%, and 28%(average of all levels) with 47%, 3%, and 20% increase at the lumbar region, respectively. The annulus stress for the extension model with muscle forces showed a change of less than 1%(average of all levels) decrease at the thoracic region with an increase of 67% at the lumbar region. IDPs for the flexion, extension, bending and rotation models showed an increase of 43%, 18%, 25%, and 33%(average of all levels) and a 56%, 37%, 19%, and 36% increase in the lumbar region, respectively. The vertebral stress for the flexion, extension, bending and rotation model showed an increase of 33%, 74%, 27%, and 51%(average all levels) when compared to the model without muscle forces.