Anterior cervical discectomy and fusion (ACDF) decompresses compressed spinal cord and nerve root, recovers physiological lordosis, and provides stability and anatomical height of the intervertebral disc. The technique was initially developed for treating cervical spondylosis, and the efficacy and safety of using ACDF for treating patients with radiculopathy and myelopathy are excellent (1–3). Anterior cervical plate technique has been reported to increase the operation time and the risk of postoperative dysphagia (4). In contrast, non-plate interbody implants, the stand-alone (ST) cage and zero-profile (ZP) cage have been widely used for ACDF surgeries. The ST cage in ACDF has positive effects in terms of recovery of physiological disc height, rapid improvement of the cervical lordosis, and facilitation of joint fusion (1, 5). However, ST cages have been reported to cause complications like cage migration, subsidence, and revision surgery (6, 7). By contrast, immediate postoperative stabilization the core advantage of a ZP cage and an ACP system (8). Unlike the latter, the ZP cage has an additional anchoring function, which ensures less protrusion in front of the vertebral body, thereby placing less compression on the esophagus and ultimately decreasing the risk of postoperative dysphagia (9, 10). Due to the ability to overcome these disadvantages of the ST cage and ACP construct, the use of zero-profile cages in ACDF is gradually increasing.

For one-level ACDF, the ZP cage contributes to the improvements in neurological function and cervical lordosis, similar to those of the ST cage but with lower risk of implant failure (1, 10, 11). However, the number of current clinical articles comparing multi-level ACDF using ZP with that using ST is rare. Here, we aimed to explore comprehensive radiological and clinical outcomes in patients who had undergone two-level consecutive ACDF using ZP cages compared with those in whom ST cages were used.

ACDF, a gold standard for patients’ refractory to non-operative treatment, has shown efficacy to treat degenerative spine conditions (16, 17). Moreover, the ZP cage and ST cage are emphatic devices, with efficacy in disc reconstruction (15). Despite being technically easier and probably evading the risk factors of ACP, the use of ST and ZP cages in two-level ACDF has been associated with multiple intraoperative and postoperative complications. For example, decreased rigidity in extension, an increased risk of vertebral subsidence, and pseudoarthrosis have been reported (1, 15, 18). We documented the clinical and radiographic findings in two-level ACDF using the ZP and ST cages. Furthermore, we compared ZP and ST cages to clarify whether the ZP cage could achieve solid fusion and keep postsurgical clinical outcomes. Our results indicated that both groups had substantial improvements in VAS, NDI, and JOA ratings in the postoperative period, suggesting that ZP and ST could contribute to equal recovery of functional status, which is in accordance with previous evidence (19).

Cervical curvature is known to contribute to excellent clinical outcomes (10). After surgery, subsidence and the size of the cage may all have an impact on cervical curvature. A larger cage increases the intervertebral pressure and provides better axial stability of the cage, thus affecting the postoperative Cobb angle (20, 21). Nakanishi et al. (16) suggested that subsidence of the spacer affected the focal angle but did not alter the C2–C7 angle or the tilt angle. In our study, the ST and ZP groups both showed a substantial increase in the degree of cervical lordosis at the final follow-up, while both groups had substantial improvements in VAS, NDI, and JOA ratings in the postoperative period. In addition, a biomechanical study showed that cages with fixation provided greater structural strength than ST spacers for two and three levels, whereas single-level fixation provided equal stiffness (19, 22, 23). This may be the reason why there were variations in radiological measurements between single-level and mixed-level with ZP and ST cages.

The better clinical recovery was substantially more obvious in solid fusion, a condition that avoids delayed kyphotic deformity. For one-segment ACDF, several studies have found similar clinical outcomes in neurological recovery and fusion regardless of whether a ZP or ST cage was used (5, 24, 25). For two-segment ACDF, nine clinical studies included in a meta-analysis revealed that the usage of a ZP cage was associated with a 90% fusion rate after 36 months of follow-up (26). According to Sun et al. (10), radiological fusion was detected in 92.59% of the cases in the ZP group after a year, and all patients of the ZP group had complete fusion after 5 years. According to multi-level research by Li et al. (5), the usage of a ST cage resulted in a fusion rate of 91.7%. In our research, the ZP group had a higher fusion rate than the ST group, but the variations between the groups were too minor to be practically meaningful.

Subsidence occurs when the measured height at any of the two-disc heights decreases by more than 2 mm. Recent statistics reveal that postsurgical subsidence occurs most frequently within the first 3 months after the treatment (27, 28). The gap under the disc surface might decline due to natural or pathological processes during the fusing process. In general, the estimated range of the percentage of patients with subsidence following ACDF is between 0% and 40% (2, 29, 30). Bartels et al. (27) reported a subsidence rate of 19.1% after the usage of ZP cage. Wu et al. (18) reported a subsidence rate of up to 29.2% after utilizing ST cages. Even when the cage subsided, Opsenak et al. (28) showed that subsidence had no serious influence on the clinical outcome. A number of additional trials have shown comparable results (2, 30, 31). In our study, the incidence of interbody fusion cage subsidence in the ZP and ST groups was 7.7% and 9.5%, respectively. A higher rate of subsidence may explain the differential disc height loss in the ST group. Although the loss of disc height significantly differed between the groups, the patients’ mid-long-term clinical outcomes were, surprisingly, still rather excellent in both groups. Additionally, the disc height in patients with subsidence was substantially higher than before. Progressive subsidence of the cage might be caused by surgery, such as cervical fusion, as well as by the movement caused by the natural procedure itself, and poor bone mineral density.

Dysphagia is a frequently reported complaint of patients after ACDF; however, it is often temporary and self-limiting (10). Persistent and chronic dysphagia poses substantial medical nutritional concerns and may increase mortality. In recent studies (32, 33), it has been shown that dysphagia was more common in the ZP group than in the ST group. The frequency of post-ACDF dysphagia varies from 0% to 76% (34, 35), likely depending on how thoroughly investigators check for dysphagia following surgery. According to a prior study, there is a notable incidence of postoperative dysphagia (40.7%) and chronic dysphagia (3.7%) after ACDF with a ZP cage (10). The occurrence of dysphagia in our study is similar to that reported in the literature (2, 36, 37). The ST group showed a lower incidence of dysphagia both in the early postoperative period (<3 months) and at the final follow-up than the ZP group. It may becasuse the removal of the anterior plating in ST group reduces the irritation to the anterior cervical tissue and esophagus. It has been proved that the thickness of the cervical plate is related to dysphagia. Therefore, the ZP cage might not be a strategy to diminish dysphagia, which agrees with the reports by Albanese and Scholz. According to Albanese et al. (32), the prevalence of dysphagia was 20.8%; likewise, according to Scholz et al. (33), among patients with a ZP cage, 15% (3/20) reported a history of mild dysphagia. When compared with the documented occurrence of chronic dysphagia following placement of the ZP cage (33, 36), our occurrence of this condition was somewhat low. A variety of variables have been implicated in the development of dysphagia following ACDF, but the exact reasons remain unknown. According to prior research, dysphagia is more common with older age (3, 38), unfavorable postoperative C2–C7 angle (39), pre-pneumonectomy (40), two-level surgery (9, 41), long operation time (36), and greater prevertebral soft tissue swelling (9, 42). Further research on dysphagia mechanisms and techniques for reducing the occurrence of dysphagia are necessary.

The current study is limited by its retrospective nature and small sample size. Besides, the follow-up time was not long enough to demonstrate the mid-long-term impact of the ZP and ST cages for cervical degenerative disc disease. However, given there are few papers describing outcomes after ZP and ST cages for two-level ACDF, we believe this study provides helpful information about surgical intervention for cervical radiculopathy and myelopathy. To overcome the limitations and identify whether the ZP cage is better than the ST cage for ACDF, larger and prospective, randomized studies with long-term follow-up periods are needed.

ACDF with ZP cages is an effective and safe procedure for two-level cervical spondylosis. Compared with ST cages, ZP cages generate less disc height loss and fewer negative issues associated with cervical curvature.