Zero-profile implant versus conventional cage-plate implant in anterior cervical discectomy and fusion for the treatment of degenerative cervical spondylosis: a meta-analysis

Since anterior cervical discectomy and fusion (ACDF) was first described by Smith and Robinson [1], the procedure has become the gold-standard operation for single- or multiple-level degenerative cervical spondylosis. The application of stand-alone cage with a titanium plate (conventional cage-plate (CCP)) in ACDF has become more and more popular since studies [2‐5] showed that CCP had many advantages compared with the stand-alone cage: improving sagittal alignment, interbody fusion rate, and stability and preventing interbody graft dislocation. However, complications related to the plates were not rare, such as postoperative dysphagia, adjacent level degeneration, and soft tissue injury [6‐8]. And, sometimes, it seems to be inescapable.

Thus, Zero-profile implant was invented to reduce part of the complications. It consisted of a cage and a plant with locking screws which could be fixed into the intervertebral space to keep away from the front tissue [9, 10]. In the past few years, Zero-p implant has been used more and more wildly since studies [11‐13] showed that it had many advantages over CCP implant.

The purpose of this analysis was to conduct a meta-analysis of relevant studies directly comparing the Zero-profile implant (Zero-profile group) with CCP implant (CCP group) in ACDF for degenerative cervical spondylosis.

We divided studies into two subgroups, retrospective study subgroup and prospective study group. All data were conducted with RevMan 5.0 analysis software (The Cochrane Collaboration, Copenhagen, Denmark). Odds Ratios (ORs) and 95 % confidence intervals (CIs) were used for the analysis of dichotomous outcomes. For continuous data, weighted mean differences (WMDs) were calculated with 95 % CI as the summary statistics. A chi-square test and I ² test were used to calculate the statistical heterogeneity. We considered I ² values of 25, 50, and 75 % as low, medium, and high heterogeneity, respectively. If I ² <50 %, we used the fixed-effects model; otherwise, the random-effects model was used. We performed such sensitivity analyses only if there were three or more studies included in the comparison.

In the past several decades, there were many studies which suggested that CCP implant had many advantages compared with stand-alone cage. But, the application of anterior cervical plate may face with some complications. Esophageal injury, soft tissue compression, and adhesion caused by the anterior plant could lead to neck pain, hoarseness, and dysphagia [18‐20]. Bazaz et al. [21] found a high dysphagia incidence of 50.2, 32.2, 17.8, and 12.5 % at 1, 2, 6, and 12 months postoperative, respectively. Lee et al. [22] found that a thinner and smoother plant could reduce dysphagia incidence postoperative significantly by means of less touch to the soft tissue precervical.

To lessen potential complications while maintaining the benefits of anterior cervical plate, the Zero-profile implant was invented. Zero-profile implant is a kind of device that contains a stand-alone cage and several angle-controlled screws, which can be implanted into the intervertebral space. The screws could get into the vertebral body through the endplate, and they provide adequate stability and avoid the implant contact to the soft tissue precervical. These unique structures offer a fixation mechanism that is similar to the function of a plate and screws. Since Zero-p implant of Synthes GmbH Switzerland was approved by the US Food and Drug Administration in 2008, many studies have engaged in comparing the outcomes between the Zero-profile implant and CCP implant in order to pursue the optimal implant in ACDF for degenerative cervical spondylosis patients. However, the inconclusive debate still remains.

In this analysis, we focused on objectively comparing the operation time, blood loss, and postoperative dysphagia between the Zero-profile implant (both contain the Zero-p of Synthes GmbH Switzerland and ROI-C of France) and CCP implant in ACDF.

The design of Zero-profile implant avoids the need for any additional internal fixation implants and theoretically circumvents the aforementioned complications associated with anterior plate while providing the segmental rigidity necessary for cervical spine fusion. In recent years, some studies have evaluated the safety and efficacy of Zero-profile implant. Njoku et al. [23] reported that Zero-profile implant had comparable pain outcomes, functional outcomes, and radiographic fusion rates with CCP implant. Barbagallo et al. [24] found that Zero-profile implant had a significant improvement on outcomes such as NDI scores, postoperative dysphagia, and fusion time. And, Scholz et al. [10] showed that all patients had a reduction on VAS pain, and the incidence of chronic dysphagia was lower than the current literature, which indicated good outcomes of Zero-profile implant. Meanwhile, some studies [11‐17, 25] directly compared the clinical and radical outcomes of Zero-profile implant with CCP implant in ACDF, which could provide better evidence for comparing Zero-profile implant with CCP implant.

Thus, in this meta-analysis, we summarized the current evidences about the operation time, blood loss, and postoperative dysphagia comparing Zero-profile implant with CCP implant. After integrating the results, we found that Zero-profile implant had advantages in reducing the incidence of postoperative dysphagia at 2 weeks, 6 months, and 1 year (p = 0.0002, p = 0.008, and p = 0.001, respectively) compared with CCP implant.

Although the exact pathophysiologic mechanism of dysphagia remains unknown, according to Fountas et al. [8], esophageal injury, postoperative soft tissue edema, adhesive formations around implanted cervical plates, and postoperative hematoma may be the possible explanations for dysphagia-related symptoms. The Zero-profile implant can be completely contained in the decompressed intervertebral space, not placed across the anterior vertebral body, avoiding the stimulus to the esophagus and other prevertebral soft tissues, preserving as many normal anatomical tissues as possible. Although it might not be clinically relevant, we still can find significant difference in blood loss (p = 0.0001) between the Zero-profile implant and CCP implant. This is possibly because fewer steps are needed to insert the Zero-profile implant, which has a one-step locking mechanism with simple insertion of the cage and tightness of the screws. The Zero-p device has a smaller volume than the CCP implant, which allows a smaller incision. The smaller incision allows more limited resection and less exposure and avoids mechanical stimulus to the related structures [13].

To the best of our knowledge, our analysis is the first meta-analysis to evaluate the efficiency and safety of Zero-profile implant compared with CCP implant in ACDF. It combines the results of existing studies and makes the results more practical. In this way, it may reduce the effect of publication bias. Moreover, the results are encouraging. Zero-profile implant may be a viable alternative in ACDF for degenerative cervical spondylosis patients.

We acknowledge limitations such as the following: (1) the inclusion of both retrospective and prospective studies, random and nonrandom studies; (2) only seven studies were included, and their sample sizes were relatively small; and (3) not all main outcomes were analyzed in the analysis, such as JOA scores, Cobb angles, NDI scores, and fusion rate. These all could lead to bias and reduce the level of evidence in our analysis. (4) As listed in Fig. 1, our study contained two kinds of Zero-profile devices. Although they were of similar mechanism and structure, it may affect the results.

Based on the results of our analysis, Zero-profile implant can reduce the incidence of postoperative dysphagia at 2 weeks, 6 months, and 1 year and reduce blood loss compared with CCP implant. It cannot reduce the operation time. In summary, the application of Zero-profile implant is superior to CCP implant in ACDF. Given possible biases in our study, more rigorous and adequately powered prospective randomized controlled trials with long-term follow-ups are required to elucidate a more objective outcome.