Biomechanical comparison of cervical spine interbody fusion cages

Study Design. An in vitro biomechanical, study of cervical spine interbody fusion cages using a sheep model was conducted. Objectives. To evaluate the biomechanical effects of cervical spine interbo dy,fusion cages, and to compare three different cage design groups. Summary and Background Data. Recently, there has been a rapid increase in t he use of cervical spine interbody fusion cages as an adjunct to spondylode sis. These cages can be classified into three design groups: screw, box, or cylinder designs. Although several comparative biomechanical studies of lu mbar interbody fusion cages are available, biomechanical data for cervical spine constructs are lacking. Additionally, only limited data are available concerning comparative evaluation of different cage designs. Methods. In this study, 80 sheep cervical spines (C2-C5) were tested in fle xion, extension, axial rotation, and. lateral bending with a nondestructive stiffness method using a nonconstrained testing apparatus, Three-dimension al displacement was measured using an optical measurement system (Qualysis) . Complete discectomy (C3-C4) was performed. Cervical spine interbody fusio n cages were implanted according to manufacturers' information. Eight spine s in each of the the following groups were tested: intact, autologous iliac bone graft, two titanium screws (Novus CTTi; Sofamor Danek, Koln, Germany) , two titanium screws (BAK-C 8 mm; Sulzer Orthopedics, Baar, Switzerland), one titanium, screw (BAK-C 12, mm; Sulzer Orthopedics), carbon box (Novus C SRC; Sofamor Danek), titanium box (Syncage; Synthes, Bochum, Germany), tita nium mesh cylinder (Harms; DePuy Acromed, Sulzbach, Germany), titanium cyli nder (MSD; Ulrich, Ulm, Germany), and titanium cylinder (Kaden; BiometMerck , Berlin, Germany). The mean apparent stiffness values were calculated from the corresponding load-displacement curves. Additionally, cage volume and volume-related stiffness was determined. Results. After cervical spine interbody fusion cage implantation, flexion s tiffness increased, as compared with that of the intact motion segment. On the contrary, rotation stiffness decreased after implantation of a cervical spine interbody fusion cage, except for the Novus CSRC, Syncage, and Kaden -Cage. If two screws were inserted (Novus CTTi and BAK-C 8 mm), there was n o significant difference In flexion stiffness between screw and cylinder de sign groups. If one screw was inserted (BAK-C 12 mm), flexion stiffness was higher for cylinder designs (P<0.05). Extension and bending stiffness were always higher with cylinder designs (P<0.05). Volume-related stiffness for flexion extension and bending was highest for the Harms cage (P<0.05). The re was no difference for rotation volume-related stiffness between Harms an d Syncage, Conclusions. The biomechanical results, indicate that design variations in screw and cylinder design groups are of little importance. In this study, h owever, cages with a cylinder design were able to control extension and, be nding more effectively than cages with a screw design.