Anterior cervical decompression and fusion (ACDF) has been widely used as a surgical treatment method for cervical disc degenerative diseases since it was first developed by Smith and Robinson in 1958 (1). Augmentation through the use of anterior cervical plating provides immediate stabilization and the preservation of cervical alignment, preventing graft dislodgment and enhancing fusion rates. However, the implantation of anterior plating has also been associated with complications, including postoperative dysphagia, soft tissue damage, and hardware failure (2–4). Zero-profile anchored spacers (ROI-Cs) have become popular due to reduced damage to soft-tissues, lower blood loss, and the avoidance of hardware-related complications compared with traditional cage and plate constructs (CPCs) (5–7). Moreover, after the insertion of the anchors, ROI-Cs provide immediate stability, facilitating fusion (8). Cage subsidence is a common complication following ACDF and can result in the loss of disc height, disrupting the sagittal alignment of the spine, preventing solid fusion, and introducing restenosis of the foramina (9, 10). However, the impacts of cage subsidence on clinical outcomes remain controversial for the cervical spine (11). Several factors have been proposed to contribute to cage subsidence, including aggressive endplate preparation, osteoporosis, differences in treatment levels, cage size, and cage material (11–13). However, data comparing ROI-C cages and CPCs regarding cage subsidence are scarce.

Thus, the purposes of this study were (1) to retrospectively evaluate the clinical and radiological outcomes of ACDF treatments for cervical disc degenerative disease (CDDD) using ROI-Cs compared with CPC fixation, with a focus on cage subsidence; and (2) to identify the preoperative and perioperative risk factors associated with cage subsidence and determine the impact of subsidence on clinical and radiological outcomes.

After univariate analysis, we identified the following factors as being associated with an increase in the risk of cage subsidence: male sex (P < 0.001), the use of ROI-C cage (P = 0.007), operation at multiple levels (P = 0.024), and ΔmDH (P < 0.001, Table 4). Multiple logistic regression was performed by analyzing these variables. The results revealed that the risk of cage subsidence was significantly associated with the male sex (OR = 16.767; P < 0.001) and the use of ROI-C cage (OR = 5.389; P < 0.001, Table 5).

The patient-reported outcomes in our current study results were consistent with those that have previously been published in the literature for ACDF when comparing zero-profile stand-alone locking screws and CPC (5–7). All neurological symptoms were relieved due to sufficient decompression, and no significant differences were observed in either the JOA or NDI scores between the groups at the final follow-up. However, in this study, we found that the occurrence of cage subsidence in patients using the ROI-C cage (19/26, 73.08%) was significantly higher than that for the CPC group (7/26, 26.92%) when the treatment involved multiple-level discectomies, although no significant difference was found for single-level surgeries.

After surgery, the occurrence of cage subsidence was frequently observed in ACDF surgeries, with a mean incidence of 21.1%, ranging from 0 to 83% (18). Earlier studies also reported a higher rate of cage subsidence when using ROI-C compared with CPC for the treatment of multiple-level ACDFs. In a meta-analysis conducted by Lu et al. (19) no significant differences were found between the zero-profile self-locking standalone cages (SLSA) and CPC group after performing single-segment ACDF, whereas increased subsidence was demonstrated in the zero-profile group for multi-segment ACDFs. According to Chen et al. (20) for the treatment of three-level cervical degenerative spondylopathy, cage subsidence of greater than 3 mm was observed in 14/28 patients in the SLSA group compared with 5/26 patients cage and plate fixation group at the final follow-up (P = 0.043). Similarly, Zhu et al. (21) reported 17/90 (18.8%) patients in the SLSA group experienced subsidence compared with 8/96 (8.3%) patients in the CPC group for three-level ACDF (P > 0.05). From a biomechanical view, the loading pressure that is directly delivered to endplate/cage interfaces can be shared by anterior metal plating. In multi-segment ACDFs, this effect can be more pronounced relative to that in single-level discectomies, which can attenuate the risk of cage subsidence, resulting in a higher incidence of cage subsidence when using anchored cages.

Surprisingly, we noticed a sex difference in the occurrence of cage subsidence, with significantly higher rates observed in men than in women. At our spine center, patients are recommended to wear cervical collars for at least 1 month postoperatively and to refrain from excessive movements. One possible explanation is the early removal of the cervical collar after surgery and a more aggressive range of motion among men compared with women. Indeed, aggressive cervical movement in the early postoperative period can cause larger axial and rotational stress upon the interbody/cage interface, which can result in cage subsidence before solid fusion. However, this study did not aim to collect data regarding the timing of cervical collar removal or the impact of cervical collar removal on the clinical and radiological outcomes after surgery; therefore, additional research is necessary.

Cages are inserted to maintain the clinical efficacy of decompression. However, overly distracted disc space can increase the risk of cage subsidence. Yang et al. (22) confirmed that a larger anterior intraoperative distraction increased the risk of cage subsidence and recommended that interbody distraction be performed before anterior longitudinal ligament resection. Similarly, Yamagata et al. (23) demonstrated that using a titanium cage for ACDF with a size of 6.5 or 7.5 mm had a higher rate of subsidence than when using a titanium cage for an ACDF with a size of 4.5 or 5.5 mm. According to an in vitro biomechanical study performed by Truumees et al. (24) the insertion of larger grafts results in higher distractive forces and increases the subsequent compressive forces delivered to the endplate-cage interface. These authors further proved that distractive force and the subsequent compressive forces were strongly correlated in an in vivo ACDF model (25). After the restoration of disc height via cage insertion, the increase in disc height causes the surrounding ligaments and muscles to absorb and resist distraction forces, contributing to the immediate compression of the graft. In the present study, the ΔmDH and ΔFSH in the CS group were significantly higher than those in the NCS group. The results of our study, combined with those of other studies, suggested that excessive distraction should be avoided to reduce the risk of potential cage subsidence in ACDF surgery.

Cage subsidence has been reported to cause loss of fused segment height, further leading to disruption of cervical stability (13, 20). Similarly, we found that the loss of cervical lordosis and fusion segment cobb angle were more pronounced in CS group (Figure 4). However, the impact of subsidence on loss of general and local lordosis was mainly observed in the treatment of multiple segments. Whereas, in single level surgeries, cage subsidence had little or no effect (Figure 3). This suggests that the increase in the number of fused segments would increase the effect of subsidence upon both local and general curvature. However, the overall clinical outcomes, fusion rates, and ASD rates were not associated with cage subsidence in our study. Previous studies have proposed that cage subsidence may represent an inherent process that occurs during the fusion of the bony endplates with the interbody cage, which includes the resorption and remodeling of the bone until rigid arthrodesis occurs (26, 27). Fujibayashi et al. (9) further divided cage subsidence into two types: a transient subsidence type demonstrates 1–3 mm subsidence without further change, whereas a progressive subsidence type results in nonunion. A systemic review by Noordhoek et al. (18) was unable to conclude that subsidence impacts clinical outcomes and fusion. Wu et al. (28) reported that cervical lordosis, rather than cage subsidence, had the most effect on long-term clinical and radiological outcomes. However, from a biomechanical standpoint, progressive subsidence is likely to result in the recompression of nerves after initial decompression. Surgeons should be highly aware of the risk factors for subsidence to avoid its occurrence. Previously reported risk factors and those identified in the present study include and are not limited to age, sex, bone density, endplate preparation, cage material and position, over-distraction, and multi-segment fusion (22, 29–31).

The limitations of the present study include its retrospective nature and the lack of randomization between procedures. Second, this study did not include osteoporosis as a risk factor that may influence cage subsidence. Third, although postoperative CT (93.2%) were taken for most patients, an inconsistency among the imaging techniques used for evaluation was present during follow-up, which may cause variations in radiographic measurements. Longer follow-up period and larger number of patients remains necessary to evaluate changes in subsidence over time and to determine its impact on clinical outcomes and cervical alignments.

In our study, ACDF with ROI-C cage achieved comparable clinical outcomes and cervical stability compared with the use of a CPC. However, our study demonstrated that the occurrence of cage subsidence was considerably higher in the ROI-C group when multiple-level surgeries were performed compared with that in the CPC group. We further identified that male sex, the use of a ROI-C cage, multiple-level discectomies, and over-distraction were significant risk factors for cage subsidence. Despite no correlation between cage subsidence and clinical outcomes was observed in our study, the potential drawbacks of cage subsidence should be considered when using the ROI-C cage in multiple-level ACDFs.

The original contributions presented in the study are included in the article/supplementary material, further inquiries can be directed to the corresponding author/s.

The studies involving human participants were reviewed and approved by Medical Ethics Committee of the First Affiliated Hospital of Soochow University (IRB#2021-119). The patients/participants provided their written informed consent to participate in this study. Written informed consent was obtained from the individual(s) for the publication of any potentially identifiable images or data included in this article.

M-fG, Y-jL, and H-lY contributed to the study concept and design. Z-yJ, YT, and H-zW contributed to the acquisition of data and analysis and interpretation of data. Z-yJ contributed to the drafting of the manuscript. All authors have read and approved the final manuscript.

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

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