Kinematics of the Thoracic Spine in Trunk Rotation In Vivo 3-Dimensional Analysis

Study Design. In vivo 3-dimensional (3D) study of the thoracic spine. Objective. To demonstrate axial rotations (ARs) and coupled motions of the thoracic spine. Summary of Background Data. In vivo 3D kinematics of the thoracic spine in trunk rotation with intact thorax and soft tissues has not been well-known. There were no quantitative data of AR in the consecutive thoracic spinal segments. Patterns of coupled motion with AR have been controversial. Methods. Thirteen healthy volunteers underwent 3D computed tomography of the thoracic spine in 3 positions; neutral, right, and left maximum trunk rotation. Relative motions of vertebrae were calculated by automatically superimposing the vertebrae in a neutral position over images in rotational positions, using voxel-based registration. Motions were represented with 6 degrees of freedom by Euler angles and translations on the local coordinate system. Results. Mean (+/- SD) relative rotational angles of T1 with respect to L1 to 1 side were 24.9 degrees +/- 4.9 degrees in maximum trunk rotation. AR of each thoracic segment with respect to the inferior adjacent vertebra to 1 side was 1.2 degrees +/- 0.8 degrees at T1-T2, 1.6 degrees +/- 0.7 degrees at T2-T3, 1.4 degrees +/- 0.9 degrees at T3-T4, 1.6 degrees +/- 0.8 degrees at T4-T5, 1.8 degrees +/- 0.7 degrees at T5-T6, 1.9 degrees +/- 0.6 degrees at T6-T7, 2.3 degrees +/- 0.7 degrees at T7-T8, 2.5 degrees +/- 0.8 degrees at T8-T9, 2.7 degrees +/- 0.6 degrees at T9-T10, 2.6 degrees +/- 0.8 degrees at T10-T11, 1.3 degrees +/- 0.7 degrees at T11-T12, and 0.5 degrees +/- 0.4 degrees at T12-L1. Significantly larger segmental AR was observed at the middle thoracic segments (T6-T11) than at the upper (T1-T6) and lower (T11-L1) segments. At the upper thoracic segments, coupled lateral bending with AR was observed in the same direction as AR. However, at the middle and lower thoracic segments, coupled lateral bending occurred both in the same and opposite directions. Conclusion. In vivo 3D ARs and coupled motions of the consecutive thoracic spinal segments in trunk rotation were investigated accurately for the first time.