RESUMO
OBJECTIVES: The objective of this paper is to present the results of a biomechanical study of an original system of internal fixation of the anterior cervical spine after discectomy. MATERIAL AND METHODS: The system includes an intersomatic cage fixed to an anterior plate. The cage is placed in the intersomatic space after discectomy and is fixed to the plate with a screw. The plate is fasted to the vertebral bodies with two screws with an original blocking system. A polyethylene model of vertebral unit was tested after the placement of a 6 mm cage and 32 mm plate with 14 mm screws (plate-cage system). In a series of static studies the stiffness, the strength in the elastic limit and the ultimate or failure strength in extension, flexion, compression and torsion were calculated. The fatigue study was done in extension reaching 2.5 million cycles. The model was studied also in extension with the plate and intersomatic iliac crest grafts (plate-bone system). RESULTS: The stiffness in extension calculated for the plate-cage system was 4.16 Nm/mm, in flexión 0.56 Nm/mm, in compressión 822 N/mm and in torsión 0.46 Nm/degree. The extension stiffness of the plate-bone system was 4.29 N/mm. The plate-cage system had a strength in the elastic limit of 386.3 N in extension reaching 295 N in the plate-bone system. The ultimate strength in extension for the plate-cage system was 673.3 N and 585 N in the plate-bone system. The strength in the elastic limit and the ultimate strength were 223 N y 336 N in flexion respectively, 1976 N y 3374 N in compression and, finally, 295.7 N y 385 N in torsión. The plate-cage system reached a total of 2.5 million cycles before it broke or began its deformity. CONCLUSIONS: The results of this biomechanical study are compared in a favourably manner with those obtained in similar experiments for the intact functional cervical unit, Morscher plate and a monoblock plate-cage prototype.
