Biomechanics of Artificial Disc Replacements Adjacent to a 2-Level Fusion in 4-Level Hybrid Constructs: An In Vitro Investigation

Background: The ideal procedure for multilevel cervical degenerative disc diseases remains controversial. Recent studies on hybrid surgery combining anterior cervical discectomy and fusion (ACDF) and artificial cervical disc replacement (ACDR) for 2-level and 3-level constructs have been reported in the literature. The purpose of this study was to estimate the biomechanics of 3 kinds of 4-level hybrid constructs, which are more likely to be used clinical-ly compared to 4-level arthrodesis. Material/Methods: Eighteen human cadaveric spines (C2–T1) were evaluated in different testing conditions: intact, with 3 kinds of 4-level hybrid constructs (hybrid C3–4 ACDR+C4–6 ACDF+C6–7ACDR; hybrid C3–5ACDF+C5–6ACDR+C6–7ACDR; hybrid C3–4ACDR+C4–5ACDR+C5–7ACDF); and 4-level fusion. Results: Four-level fusion resulted in significant decrease in the C3–C7 ROM compared with the intact spine. The 3 dif-ferent 4-level hybrid treatment groups caused only slight change at the instrumented levels compared to in-tact except for flexion. At the adjacent levels, 4-level fusion resulted in significant increase of contribution of both upper and lower adjacent levels. However, for the 3 hybrid constructs, significant changes of motion in-crease far lower than 4P at adjacent levels were only noted in partial loading conditions. No destabilizing ef-fect or hypermobility were observed in any 4-level hybrid construct. Conclusions: Four-level fusion significantly eliminated motion within the construct and increased motion at the adjacent seg