Thoracic spinal kinematics is affected by the grade of intervertebral disc degeneration, but not by the presence of the ribs: An in vitro study.

Abstract

Thoracic spinal three-dimensional kinematics is widely unknown. For the evaluation of surgical treatments and the complete validation of numerical models, however, kinematic data of the thoracic spine are essential. To identify possible effects of rib presence and grade of intervertebral disc degeneration on thoracic spinal kinematics including three-plane helical axes and instantaneous centers of rotation. Radiological grading of intervertebral disc degeneration and in vitro tests using n=8 human thoracic functional spinal units of the segmental levels T1-T2, T3-T4, T5-T6, T7-T8, T9-T10, and T11-T12, respectively, were performed with as well as without ribs to analyze the specific kinematic properties. Specimens were loaded with pure moments of 5 Nm and constant loading rates of 1°/s in flexion/extension, lateral bending, and axial rotation. Optical motion tracking was performed to visualize helical axes and instantaneous centers of rotation on three-plane X-rays and to evaluate primary ranges of motion (ROMs) and coupled motions. Motion segments with no or mild disc degeneration showed reproducible kinematics in all motion planes, whereas medium or severely degenerated specimens offered high variations and shifts of the rotational axes to the distal direction as well as lower ROM. Coupled motions were generally not detected. With progressing disc degeneration, the rotational axes show higher variation and tend to shift in distal direction, especially in flexion/extension with a shift to the anterior direction, whereas rib resection does not affect thoracic spinal kinematics but its stability. Rib resections as part of spinal deformity treatment destabilize the thoracic spine, but do not alter its kinematics. Young and healthy discs, however, could be affected by surgical treatments of the thoracic spine regarding thoracic spinal kinematics.