Finite Element Analysis of Cage Subsidence in Cervical Interbody Fusion
Cages were used frequently in cervical spine interbody fusion to get better fusion rate. However, the fusion cage would subside in adjacent vertebral body. Three dimensional cervical spine model of C4-C6 was constructed referring to our previous model procedure (FEM). The suitable location of fusion cage had discussed with experienced surgeons and then cervical interbody fusions were simulated by finite element analysis. Factors caused subsidence, including (1) different moment loadings; (2) cage geometry; (3) cage material and (4) bone mineral density were also simulated. The results showed that extension of cervical spine caused remarkable subsidence than average value about 31.8%, next were flexion (13.8%), lateral bending (10.8%), and torsion. Cylindrical shape cage (BAK) appeared a higher risk of subsidence (6.6%) and von-Mises stresses (10.1%) than ring-liked shape cage (SOLIS). Comparing the cage material, the BAK cage made the settlement higher to 11% and the stress increased to 33.8%. The subsidence increased to 21.2% related to intact model in both cages while osteopenia of bony density were simulated. In conclusion, in the ROMs, extension motion caused remarkable subsidence after cages fusion. The geometry of the cage should keep from the cylinder or sharp angle, and the composed material of the cage is suggested to be closed to bone density for avoiding stress shielding. Patients with osteopenia will increase subsidence in cage interbody fusion.