Biomechanics of adjacent segments after a multilevel cervical corpectomy using anterior, posterior, and combined anterior-posterior instrumentation techniques: a finite element model study.

Abstract Background context Adjacent segment degeneration (ASD) after cervical fusion is a clinical concern. Despite previous studies documenting the biomechanical effects of multilevel cervical fusion on segments immediately superior and inferior to the operative segments, the pathogenesis of the initiation of degeneration progression in neighboring segments is still poorly understood. Purpose To test the hypothesis that changes in range of motion, disc stresses, and facet loads would be highest at the superior adjacent segment (C3–C4) after anterior C4–C7 corpectomy and fusion and that these changes would be the least in anterior fixation and the greatest in posterior or combined anterior-posterior instrumentation techniques. Study design A finite element (FE) analysis of adjacent vertebral segment biomechanics after a two-level corpectomy fusion with three different fixation techniques (anterior, posterior, and combined anterior-posterior). Methods A previously validated three-dimensional FE model of an intact C3–T1 segment was used. From this intact model, three additional instrumentation models were constructed using anterior (rigid screw-plate), posterior (rigid screw-rod), and combined anterior-posterior fixation techniques after a C4–C7 corpectomy and fusion. Motion patterns, disc stresses, and posterior facet loads at the levels cephalad and caudal to the fusion were assessed. Results Range of motion, disc stresses, and posterior facet loads increased at the adjacent segments. Use of posterior fixation, whether alone or in combination with anterior fixation, infers higher changes in segmental motion, disc stresses, and posterior facet loads at adjacent segments compared with the use of anterior fixation alone. The superior C3–C4 motion was most affected during lateral bending and the inferior C7–T1 motion was most affected during flexion, whereas both superior C3–C4 and inferior C7–T1 motions were least affected during extension. However, disc stresses and facet loads were most affected during extension. Hence, it is speculated that the most remodeling changes in discs and facets might be related to the least changes in extension motion. Conclusions Biomechanical factors such as increased mechanical demand and motion that have been associated with the development of ASD progression are highest in the segment immediately superior to the fusion. These changes are even more pronounced when the fixation technique involves the addition of posterior instrumentation, thereby supporting the hypothesis of the present study. Increased degrees of stiffening of the fused segments not only may lead to degenerative changes in the disc but may also predispose the segments to premature facet degeneration. Over subsequent time period, any remaining construct micro-motion is further eliminated with fusion of the posterior facet joints and the remaining regions in the disc space also filled in with bone, which eventually results in a circumferential type of fusion. After a circumferential fusion, authors, however, speculate that the role of instrumentation in ASD progression might not be significant. In fact, sufficient evidence to support this speculation is still lacking in the literature.