Biomechanical analysis of the range of motion after placement of a two-level cervical ProDisc-C versus hybrid construct.

Study Design: The study design was that of an in vitro human cadaveric biomechanical analysis.

Objectives: The objective of this study was the biomechanical analysis of the range of motion (ROM) of a 2-level intact spine control versus total, then operative- and adjacent-segment ROM after (1) 2-level ProDisc-C placement (PP), (2) anterior cervical discectomy and fusions (ACDFs), and (3) hybrid constructs of both. Follower load and multidirectional testing were performed in each instance.

Summary Of Background Data: With in vivo cervical arthroplasties gaining in popularity, limited biomechanical data are available, which highlight changes in the adjacent-level biomechanics after multilevel procedures.

Methods: Biomechanical testing for ROM was performed using 7 cadaveric C4-T1 spine specimens. Moments up to 2 Nm with a 100 N follower load were applied in flexion/extension (F/E), right and left lateral bending (LB), and right and left axial rotation (AR). Specimens were tested in the intact state and then with a combination of ProDisc-C arthroplasty and ACDF at C5-C6 and C6-C7.

Results: In the 2-level PP group, the increase in ROM in F/E, LB, and AR of C4-T1 occurs due to an increased ROM at the operative level. The ROM of the level adjacent to the operative levels showed no significant change, except at C4-C5 in LB. For the latter level, the ROM of C4-C5 in each direction showed increases for each parameter. In the 2-level fusion C5-C6 and C6-C7 fusion (FF) group, the ROM in F/E, LB, and AR of C4-T1 was decreased because of a decrease in ROM primarily at the fused levels, and the ROM of adjacent levels was increased. In the ProDisc-C/Fusion (PF) and Fusion/ProDisc-C (FP) groups undergoing placements of a 1-level ProDisc-C/1-level fusion with cage and plate, both groups showed no significant ROM change of C4-T1 when compared with the control and no significant change at adjacent levels, with the exception of C4-C5 in LB.

Conclusion: (1) Two-level ACDFs decrease whereas 2-level PPs increase the entire C4-T1 ROM. (2) ACDF/ProDisc-C hybrid operations do not alter the C4-T1 ROM. (3) For the ACDF/ProDisc-C hybrid operative groups, the combined ROM of the operative levels showed no significant difference when compared with that of the intact spine. (4) Regarding adjacent-level ROM, a 2-level ACDF increases ROM, but 2-level ProDisc-C and hybrid ACDF/PPs do not show significant change except for LB at C4-C5. (5) When the segmental distribution of C4-T1 ROM is plotted as the percentage of total motion, it demonstrates that for PF and FP groups, the combined ROM of the C5-C6 and C6-C7 operative levels are similar to that of the intact spine in EF and LB. For the PP group, the combined ROM of the operative levels increased, whereas the combined ROM for the FF group is decreased. The decrease or increase of the adjacent C4-C5 or C7-T1 level ROM compensates for the operative levels.