Initial stability of cervical spine fixation: predictive value of a finite element model. Technical note.

The purpose of this study was to generate a validated finite element (FE) model of the human cervical spine to be used to analyze new implants. Digitized data obtained from computerized tomography scanning of a human cervical spine were used to generate a three-dimensional, anisotropic, linear C5-6 FE model by using a software package (ANSYS 5.4). Based on the intact model (FE/Intact), a second was generated by simulating an anterior cervical fusion and plate (ACFP) C5-6 model in which monocortical screws (FE/ACFP) were used. Loading of each FE model was simulated using pure moments of +/- 2.5 Nm in flexion/extension, axial left/right rotation, and left/right lateral bending. For validation of the models, their predicted C5-6 range of motion (ROM) was compared with the results of an earlier, corresponding in vitro study of six human spines, which were tested in the intact state and surgically altered at C5-6 with the same implants. The validated model was used to analyze the stabilizing effect of a new disc spacer, Cenius (Aesculap AG, Tuttlingen, Germany), as a stand-alone implant (FE/Cenius) and in combination with an anterior plate (FE/Cenius+ACFP). In addition, compression loads at the upper surface of the spacer were investigated using both models. As calculated by FE/Intact and FE/ACFP models, the ROM was within 1 standard deviation of the mean value of the corresponding in vitro measurements for each loading case. The FE/Cenius model predicted C5-6 ROM values of 5.5 degrees in flexion/extension, 3.1 degrees in axial rotation (left and right), and 2.9 degrees in lateral bending (left and right). Addition of an anterior plate resulted in a further decrease of ROM in each loading case. The FE/Cenius model predicted an increase of compression load in flexion and a decrease in extension, whereas in the FE/Cenius+ACFP model an increase of graft compression in extension and unloading of the graft in flexion were predicted. The current FE model predicted ROM values comparable with those obtained in vitro in the intact state as well as after simulation of an ACFP model. It predicted a stabilizing potential for a new cage, alone and in combination with an anterior plate system, and predicted the influence of both loading modality and additional instrumentation on the behavior of the interbody graft.