Many previous studies have been performed to determine the optimal plating method for ACDF.2,10,12,13,22 One issue regarding the safe plating method is the decreasing incidence of ALOD. Lee et al. reported that the plate-disc space distance should be > 5 mm to decrease ALOD incidence.10 This technique involves inserting cranial and caudal screws from the corners immediately adjacent to their respective operative-level end plate and placing the shortest plate that fits this screw placement.10,11 This technique, by limiting anterior longitudinal ligament dissection, is reported to decrease ALOD incidence with no additional complications.10,11
Other points to consider in anterior cervical plating are decreasing the amount of subsidence and the rate of pseudarthrosis. Park et al. reported that a short plate with an oblique screw trajectory construct is effective for decreasing the incidence of ALOD and subsidence.12 A screw length greater than 75% of the antero-posterior vertebral body diameter is recommended to decrease the pseudarthrosis rate.13 However, result comparisons between using the fixed and variable screws remain scarce.
The ALOD more commonly occurs at the proximal adjacent segment, and a plate-adjacent distance of < 5 mm is known as a risk factor.9,10 The ALOD limits motion at the adjacent segment and accelerates the degeneration of the level next to the adjacent level.11 This justifies the need to increase the plate-disc space distance at the cranial level. In contrast, pseudarthrosis or implant failure most commonly occurs at the caudal level.6,15,16 This can be explained by increased shear stress at the level near the cervicothoracic junction.23 However, the ALOD risk is relatively low at the distal adjacent segment, which emphasizes the need for additional stability at the caudal level, whereas there is less need to increase plate-disc space distance.9,10 Based on these previous findings, we attempted a hybrid construct using fixed screws selectively at the caudal level to increase its stability while inserting the variable screws at the cranial level.
The results of this study demonstrated that pseudarthrosis and subsidence most commonly occur at the caudal level. These results are consistent with the results of previous reports.6,15,16 However, the fusion rates of selective fixed constructs and all variable constructs did not differ significantly. Although the locking mechanism at the screw-plate interface of the fixed screw was expected to increase the stability of the caudal segment, it did not lead to increased fusion rate. The ALOD and ASD rates were not significantly different between the SF and AV groups. The cranial and caudal plate-adjacent disc space distance and rate of patients with a plate-adjacent disc space distance of < 5 mm, which is a risk factor of ALOD, did not significantly differ between the two groups. Furthermore, clinical results, such as neck and arm pain VAS and NDI scores, did not significantly differ between the two groups.
Since the rates of pseudarthrosis, subsidence, ALOD, and ASD were similar in both groups, the selective fixed screw construct did not seem to provide additional advantage over the all variable screw constructs. The variable screws are more advantageous than the fixed screws at the point where the insertion angle is freely modifiable and a longer screw can be inserted with increased angle. In contrast, the advantage of the fixed screw is that it can be easily inserted with constant angulation. Based on the results of this study, screw types can be selected based on individual patient’s anatomy and surgeon’s experience, without concern for increased pseudarthrosis or subsidence caused by screw type. Oh et al. also reported that fusion rates of using the fixed and variable screws are similar. This is consistent with the results of this study.22
Further studies should be conducted to clarify the method to decrease the rate of pseudarthrosis at the caudal level, especially for multi-level surgery, since the results of the current study did not demonstrate significant results by screw construct difference. Although Lu et al. reported that pseudarthrosis at the caudal level can be decreased by selectively using low-dose bone morphogenic protein at the caudal level, there is still concern regarding complications caused by bone morphogenic protein for anterior cervical surgery.16
Previous studies have demonstrated that the number of fusion levels, bone graft type, plating, sex, age, smoking, greater preoperative segmental motion, and greater T1 sagittal slopes are related factors associated with pseudarthrosis after ACDF.24–27 The result of the logistic regression analysis in this study also demonstrates that multi-level operation is a risk factor of pseudarthrosis, consistent with the results of previous studies. With increased fusion level, micromotion and contract stress also would have increased at the graft-bone interface, which could lead to pseudarthrosis.28
Our study has some limitations. First, this study had a limited sample size to assess the rate of ALOD or ASD. Second, there was a temporal difference in the type of operation performed. Although all operations were performed by a single surgeon at a single institute, unidentifiable factors due to time difference could have affected the results. Third, insertion angles and lengths of the screw were not considered as factors. However, a previous study demonstrated that screw insertion angle does not affect subsidence or fusion rate.22 Finally, this study has a potential bias due to the retrospective nature of this study.
In conclusion, the fusion rates, subsidence, patient-reported outcome measurements, plate-adjacent disc space distance, ALOD, and ASD were not significantly different between the selective caudal fixed screw and all variable screw constructs. The stability provided by the locking mechanism of the fixed screw did not lead to an increased fusion rate. Therefore, it would be better to select screws based on individual patient’s anatomy and surgeon’s experience without concern for increased complications caused by screw type.