Loosening at the screw-vertebra junction in multilevel anterior cervical plate constructs

Study Design. An in vitro biomechanical study of one-level and three-level corpectomy and anterior cervical plate models. Objective. To investigate the failure of the screw-vertebra interfaces in one- and three-level corpectomy models. Summary and Background Data. Although there are several biomechanical studies of strength and stability of anterior cervical plating, there has been no investigation into clinically observed failures. Methods. One- and three-level models (corpectomy, strut graft, and anterior plate) were constructed from eight cadaveric specimens (C2-T1). Multidirectional flexibility tests (1.0 Nm moments) performed before and after fatigue (1000 cycles, 1.0 Nm flexion-extension, 0.14 Hz) documented the screw-vertebra motions at upper and lower ends. Ranges of motion and neutral zones were determined. Analysis of Variance was used to evaluate significant differences between the upper and lower ends of the plates and changes caused by fatigue loading (P < 0.05). Results. Extension motion at the tower ends was more than at the upper ends in both models. Fatigue increased three-level model ranges of motion at the lower end by 171% in flexion, 164% in extension, 153% in lateral bending, and 115% in axial rotation. Similar increases were observed in neutral zones. Fatigue loading produced no significant changes in one-revel models. Conclusion. There was excessive screw-vertebra motion caused by fatigue at the lower end of the three-level corpectomy model. These findings of loosening may explain clinically observed failures at the caudal end of long anterior cervical plate constructs. Longer screws, larger diameter screws, and supplemental posterior fixation may decrease screw-vertebra loosening.