BIOMECHANICAL EFFECT OF THORACIC POSTERIOR VERTEBRAL ELEMENTS ON PATTERNS OF THE LOCI OF INSTANTANEOUS AXES OF ROTATION IN SAGITTAL PLANE

Abstract

The objective of this study was to investigate the effect of the thoracic posterior vertebral elements on the kinematics of T10–T11 motion segment in sagittal plane by assessing the locations and loci of the instantaneous axes of rotation (IARs) under flexion and extension pure moments using finite element (FE) method. The IAR has proven to be a useful parameter of vertebral motion and it is an indicator of spinal instability. An anatomically accurate FE model of thoracic T10–T11 functional spinal unit (FSU) was used to characterize the loci of centers of rotation for the intact T10–T11 FSU and disc body unit (without posterior vertebral elements) under flexion and extension pure moments. The centers of rotation predicted by the intact model and disc body unit of thoracic T10–T11 for both flexion and extension were directly below the geometrical center of the moving vertebra. However, the loci of the IARs were significantly affected by the posterior vertebral elements. The loci of instantaneous axes of rotation for the intact model were tracked superoanteriorly and inferoposteriorly for flexion and extension with rotation, respectively. While, for the disc body unit, the loci were detected to diverge lateroinferiorly from the mid-height of the intervertebral disc, they converge medio-inferiorly toward the superior endplate of the inferior vertebra T11 with increased moment. These findings may offer an insight to better understanding the kinematics of the human thoracic spine and provide clinically relevant information for the evaluation of spinal stability and implant devices functionality.