Validation of T10-T11 finite element model and determination of instantaneous axes of rotations in three anatomical planes

Study Design. A finite element (FE) model of thoracic spine (T10-T11) was constructed and used to determine instantaneous axes of rotation (IARs). Objectives. To characterize the locations and loci of IARs in three anatomic planes. Summary of Background Data. The center of rotation is a part of a precise method of documenting the kinematics of a spinal segment for spinal stability and deformity assessments and for implant devices study. There is little information about loci of IARs in thoracic spine. Methods. A FE model of thoracic spine (T10-T11) was developed and validated against published data. The validated model was then used to determine the locations and loci of IARs in three anatomic planes. Results. Within the validated range, The IARs locations and loci were found to vary with the applied pure moments. Under flexion and extension pure moments, the loci of IARs were tracked anterosuperiorly for flexion and posteroinferiorly for extension with rotation between the superior endplate and the geometrical center of the inferior vertebra T11. Under left and right lateral bending pure moments, the loci were detected to diverge latero-inferiorly from the mid-height of the intervertebral disc, then converge medio-inferiorly toward the geometrical center of the inferior vertebra T11. For axial rotation, the IARs were located between anterior nucleus and anulus and found to diverge in opposite direction latero-posteriorly with increasing left and right axial torque. Conclusions. The results of IARs would provide further understanding to the kinematics and biomechanical responses of the human thoracic spine, which is important for the diagnosis of disc degeneration and implant study.