Subsidence resulting from simulated postoperative neck movements - An in vitro investigation with a new cervical fusion cage

Study Design. A biomechanical in vitro subsidence test of different cervica l interbody fusion devices was performed new testing protocol that simulate s physiologic. Objectives. To investigate the effect of simulated postoperative neck movem ents on the subsidence of the new WING cervical interbody fusion cage in co mparison with two other cages and bone cement. Summary of Background Data. Cervical interbody fusion sometimes cause compl ications because of subsidence into the adjacent vertebrae with collapse of the intervertebral space. Complications such as cage dislocation or nonuni on with instability also have been reported. To prevent such complications, the new WING cervical interbody fusion cage (Medinorm AG, Quierschied, Ger many) has been developed. Its area of contact with the adjacent vertebrae i s supposed to be large to resist excessive subsidence and small enough to p revent stress protection of the tissue growing in the cage. Methods, In this study, 24 human cervical spine specimens were tested after stabilization with either a WING, BAK/C, AcroMed I/F cage or bone cement. Then, in a new testing protocol, 700 pure-moment loading cycles (+/- 2 Nm) were applied in randomized directions (lateral bending, flexion-extension, and axial rotation alone or in combination with each other) to simulate the patient's neck movements during the first few postoperative days. Measurem ents of the subsidence depth (total height loss) in combination with flexib ility tests (+/- 2.5 Nm) were performed before cyclic loading and after 50, 100, 200, 300, 500 and 700 loading cycles. Results: Cyclic loading caused subsidence in all four device groups, most d istinct with BAK/C-cages (1.63 mm after MO loading cycles) followed by the new WING (0.90 mm) and the AcroMed (0.82 mm) cages. No statistically signif icant difference could be found among the three cage designs, However, all three cage types showed a significantly higher subsidence depth than bone c ement (0.48 mm; P = 0.0.23 between each of the three cage-types and bone ce ment). A moderate correlation between bone mineral density and subsidence d epth could be found only in the BAK/C group (r(2) = 0.495). A large subside nce depth after 700 loading cycles was associated with a large flexibility increase in the WING (r(2) = 0.786) and AcroMed groups (r(2) = 0.21), but w ith a small flexibility increase in the BAK/C group (r(2) = 0.58). Conclusions. Postoperative neck movements caused subsidence in all cervical interbody implant types. The new WING cage and the AcroMed cage seemed to have a better resistance against subsidence than the BAK/C cage. However, a ll three cage types had a significantly higher subsidence tendency than bon e cement.