A quantitative analysis of strain at adjacent segments after segmental immobilization of the cervical spine

Study Design: A biomechanical study on human cadaveric cervical spines with segmental fixation. Objectives: To quantify the strains across all segments of the spine after simulated fusion. Summary of Background Data: Clinical evidence suggests that degenerative changes occur at adjacent levels after cervical fusion. This may, in part be due to increased stress and motion at the adjacent segments. Materials and Methods: Seven fresh frozen human cervical cadaveric spines were used. The spines were mounted onto frames at C2 and C7. Biomechanical testing was performed on a modified NITS tester. The specimens were tested in rotation control. To simulate fusion, a block was used to replace the disc. Fixation was enhanced using an anterior plate and stainless steel wire through the spinous processes. Testing was then performed with the same displacement magnitudes used for the intact spine. Displacement across 5 disc spaces was recorded using extensometers. The same preparation and testing was done for 1, 2, and 3-level simulated fusions. All data were normalized to the individual intact specimen. Results: After I-level simulated fusion at C5-6, flexion-extension rotation increased by 60% at the superior adjacent level (C4-5) and by 15% at the adjacent inferior level (C6-7). Lateral bending increased by 51% at C4-5 and by 16% at C6-7. Axial rotation increased by 25% at C4-5 and by 200% at C6-7. Flexion-extension, lateral bending and axial rotation increased at all other segments, not only at adjacent segments, after 1, 2 and 3-level fixation. Conclusions: Cervical fusion results in increased strains at adjacent levels, and to all other levels, inferiorly and superiorly. This study represents the first to quantify the increased strain at all adjacent levels.