In vitro analysis of thoracic spinal motion segment flexibility during stepwise reduction of all functional structures.

The aim of this study was to quantify the stabilizing effect of the passive structures in thoracic spinal motion segments by stepwise resections. These data can be used to calibrate finite element models of the thoracic spine, which are needed to explore novel surgical treatments of spinal deformities, fractures, and tumours. Six human thoracic spinal motion segments from three segmental levels (T2-T3, T6-T7, and T10-T11) were loaded with pure moments of 1 and 2.5 Nm in flexion/extension, lateral bending, and axial rotation. After each loading step, the ligaments, facet capsules, and the nucleus pulposus were stepwise resected from posterior to anterior direction, while the segmental relative motions were measured using an optical motion tracking system. Significant increases (p < 0.05) in the range of motion were detected after resecting the anterior spinal structures depending on loading magnitude, motion direction, and segmental level. The highest relative increases in the range of motion were observed after nucleotomy in all motion directions. The vertebral arch mostly stabilized the thoracic spinal motion segments in flexion and extension, while the facet joint capsules mainly affected the segmental stability in axial rotation. Coupled motions were not observed. The anulus fibrosus defines the motion characteristics qualitatively, while the ligaments and the presence of the nucleus pulposus restrict the mobility of a thoracic spinal motion segment solely in a quantitative manner. The posterior ligaments do not predominantly serve for primary stability but for the prevention of hyperflexion. These slides can be retrieved under Electronic Supplementary Material.