Anterior cervical discectomy and fusion has been an effective procedure for treating symptomatic cervical spondylotic myelopathy since it was initially described by Robinson and Smith19. It has produced good results in the treatment of many degenerative cervical lesions20–22. With the development of surgical instruments and surgical techniques, titanium plate has been routinely used in ACDF surgery. Reasonable use of titanium plate can provide immediate stability in early postoperative period, which can prevent bone graft subsidence, bone graft extrusion and improve fusion rates, and reduce the need for external immobilization22–24. However, it has been reported that the use of short PDD plate will accelerate the degeneration of adjacent segments, eventually result in symptomatic ASD25.
ASD as a complication of ACDF has been reported to cause 17.4% of ACDF patients to have to undergo a second surgery9, which has seriously affected patient's quality of life and increased the economic burden on families and society.
Various scholars have different opinions regarding PDD in ACDF. Chung et al.6 reported that 177 patients who underwent anterior cervical discectomy and fusion using cervical plates, with follow-up periods of at least 10 years. They found most clinical adjacent-segment degeneration appeared on the patients with a PDD less than 5 mm. So they considered that to prevent adjacent segment degeneration, the PDD should be 5 mm or more if possible. In addition, Yu et al.25 agreed that PDD < 5 mm is a risk factor for ASD by the logistic regression analysis based on 138 patients. However, Yang et al.26 retrospectively reviewed 218 patients who underwent anterior cervical arthrodesis with plating and considered that there is no correlation between PDD and the incidence of ASD, but PDD > 5 mm could avoid the development of adjacent segment ossification. In a cadaver study, Raj D found that there was no statistical difference at intervertebral disc and intervertebral motion of adjacent segments with plate or not. Therefore, it remains controversial whether different PDD plate will affect the incidence of ASD.
FEA have been widely used for a biomechanical analysis of the cervical spine because they can analyze various results quantitatively without any invasion. In the present study, we constructed 10 three-dimensional finite element C4-C7 models based on cervical CT images of 10 volunteers. Considering that cervical disc lesions mostly occur in the C5/C6 segments, we selected the C5/C6 segment as the surgical segment so that the results could be suitable for more patients.
In our study, no significant differences were found in the adjacent intervertebral disc stress in the three different PDD groups. This result indicates that PDD does not affect the adjacent intervertebral disc stress. It has been reported that excessive loading can induce degeneration of intervertebral discs. So, we can conclude that the titanium plates of different PDD will not promote the degeneration of adjacent segments by increasing the intervertebral disc stress of adjacent segments. This can further validate the clinical findings of Yang et al.26 that there is no correlation between PDD and the incidence of ASD.
As for the bone graft stress, we found that the bone graft stress decreased as the PDD decreased. This may be related to the fact that the short PDD plate increases the stiffness of the surgical segment and provides better stability. Although the most appropriate stress on bone graft is not clearly, excessive stress on the bone graft may result in fusion failure because of graft dislodgement and endplate fracture27. Furthermore, non-fusion is an important reason for the failure of instruments28. Considering bone graft stress and fusion rate, shorter PDD plate is safer because that shorter PDD is helpful to prevent bone graft subsidence and instrument failure from a biomechanical point of view.
The stress of titanium plate increased as the PDD decreased, which shows that as the length of the plate increases, the stress increases accordingly. This is due to the longer torque of the long plate. Of course, this result also explains that long titanium plates can carry more stress, which provides better stability. Although the increase of stress of the plate may cause the plate to break, the increase of the plate stress among different PDD groups is not obvious in the average value.
The screw stress decreased as the PDD decreased. As the length of the steel plate increases, the screw stress decreases. This may be caused by that the most of the overall stress is mostly carried by the plate. Screw loosening and breakage are associated with metal fatigue via pseudarthrosis, which were most dangerous complications in cervical anterior plating fixation. So shorter PDD plate plays an important role in preventing screws breakage and loosening. Yang et al.26 found when the end point of plate violates the adjacent space, the incidence of ASD is the lowest among different PDD groups. They considered that this may be because the formation of ossification improves the stability of adjacent segments. However, they do not advocate the use of plates that can invade adjacent space. Considering that the strength of the screw is weaker than that of the plate28, we believe that choosing a slightly shorter PDD plate is more conducive to postoperative recovery. Of course, we also don't recommend that the PDD is too small to violate the adjacent space.
In addition, we find that the maximum stress of each part occurred was mostly in the conditions of rotation and lateral bending, so we recommend that patients should avoid excessive rotation and lateral bending during early postoperative period to prevent screws breakage, fusion failure, endplate fracture and other complications.
The present study has a number of limitations: 1) Muscles and other soft tissue were not constructed in the models, however, these structures are extremely important for spine biomechanics research; 2) In addition, the screws were designed as solid cylinders bound to the cage or plate, and the threads on the screws were not modeled; 3) The model was based on only 10 persons, which may limit the present study’s applicability to a wider population; 4) Some simplifications were carried out in the prosthesis geometry, for example, we simplify the Cancellous bone as a solid structure which may affect the distribution and geometric deformation of the load. Although completely duplicating the result of in vivo studies in FE analysis was impossible, this study effectively shows the biomechanical differences among different PDD plate groups.