Effects of neck movements on stability and subsidence in cervical interbody fusion: an in vitro study

Object. The aim of this in vitro study was to determine the influence of si mulated postoperative neck movements on the stabilizing effect and subsiden ce of four different anterior cervical interbody fusion devices. Emphasis w as placed on the relation between subsidence and spinal stability. Methods. The flexibility of 24 human cervical spine specimens was tested be fore and directly after being stabilized with a WING, BAK/C, AcroMed I/F ca ge, or with bone cement in standard flexibility tests under 50 N axial prel oad. Thereafter, 700 pure moment loading cycles (+/- 2 Nm) were applied in randomized directions to simulate physiological neck movements. Additional flexibility tests in combination with measurements of the subsidence depth were conducted after 50, 100, 200, 300, 500, and 700 loading cycles. In all four groups, simulated postoperative neck movements caused an increa se of the range of motion (ROM) ranging from 0.4 to 3.1 degrees and of the neutral zone from 0.1 to 4.2 degrees. This increase in flexibility was most distinct in extension followed by flexion, lateral bending, and axial rota tion. After cyclic loading, ROM tended to be lower in the group fitted with AcroMed cages (3.3 degrees in right lateral bending, 3.5 degrees in left a xial rotation, 7.8 degrees in flexion, 8.3 degrees in extension) and in the group in which bone cement was applied (5.4 degrees, 2.5 degrees, 7.4 degr ees, and 8.8 degrees, respectively) than in those fu;ed with the WING (6.3 degrees, 5.4 degrees, 9.7 degrees, and 6.9 degrees, respectively) and BAK c ages (6.2 degrees, 4.5 degrees, 10.2 degrees, and 11.6 degrees, respectivel y). Conclusions. Simulated repeated neck movements not only caused an increase of the flexibility but also subsidence of the implants into the adjacent ve rtebrae. The relation between flexibility increase and subsidence seemed to depend on the implant design: subsiding BAK/C cages partially supported st ability whereas subsiding WING cages and AcroMed cages did not.