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Incidence of dysphagia of zero-profile spacer versus cage-plate after anterior cervical discectomy and fusion

Abstract Background: The purpose of this study is to evaluate the rate of dysphagia between zero-profile spacer versus cage-plate for the treatment of multilevel cervical spondylotic myelopathy (CSM). Methods: The authors searched electronic databases for relevant studies that compared the clinical effectiveness of zero-profile spacer versus cage-plate for the treatment of patients with multilevel CSM. The following outcome measures were extracted: the Japanese Orthopaedic Association (JOA) scores, Neck Disability Index (NDI) score and fusion rate, dysphagia rate, adjacent segment degeneration, and cervical lordosis. Newcastle-Ottawa Quality Assessment Scale was used to evaluate the quality of each study. Data extraction and quality assessment were conducted, and RevMan 5.2 was used for data analysis. Results: A total of 10 studies were included in our meta-analysis. Our pooled data revealed that zero-profile spacer was associated with decreased dysphagia rate at postoperatively 1, 3, and 6 months, and the final follow-up when compared with cage-plate group. No significant difference was observed in terms of postoperative JOA score, NDI score, and fusion rate. Compared with zero-profile spacer, the postoperative adjacent segment degeneration was significant higher in cage-plate. Pooled data from the relevant studies revealed that cervical lordosis was significantly lower in zero-profile spacer compared with cage-plate. Conclusions: Our meta-analysis reveals zero-profile spacer is better than the cage-plate in terms of dysphagia. This suggests zero-profile spacer is a superior alternative invention for the treatment of multilevel CSM to reduce the risk of dysphagia.


Introduction
Cervical spondylotic myelopathy (CSM) is a clinically symptomatic condition associated with degeneration of intervertebral discs and adjacent vertebral structures. The degeneration of the intervertebral disc, uncovertebral joint, facet joint, posterior longitudinal ligament, and ligamentum flavum causes spinal cord compression and cervical myelopathy. [1] At present, patients diagnosed with single-level symptomatic CSM were often recommended to receive anterior cervical decompression and fusion (ACDF). [2][3][4][5] However, ACDF for multilevel CSM means a long cervical plate and may be associated with longer operative times, and also complications such as breakage or loosening of plate and screws, trachea-esophageal injury, neurovascular injury, and postoperative dysphagia. [6,7] To reduce the dysphagia complication, the zero-profile anchored spacer had been advocated for multilevel CSM. The device generally consists of a polyetheretherketone cage with selflocking clips or screws passing through the spacer into the endplates of the adjacent vertebral bodies. [8] Zero-profile cage utilizes an integrated, low-profile plate design to avoid plate-tosoft tissue impact, reducing dysphagia incidence and other plateassociated complications. Satisfactory clinical and fusion outcomes have been reported using this device in ACDF for multilevel CSM. [9] There is, at present, no consensus concerning the superiority of zero-profile spacer versus cage-plate regarding the incidence of dysphagia in the treatment of multilevel CSM. [10,11] To further clarify controversies in the current literature, we performed present meta-analysis to evaluate the rate of dysphagia between zero-profile spacer versus cage-plate after ACDF for the patients with multilevel CSM.

Search strategy and study selection
Electronic databases, including MEDLINE, EMBASE, Cochrane Controlled Trial Register, and ISI Web of Knowledge (all databases) were searched for relevant reports published up to May 31, 2018. The MeSH Terms and Other Term used for our searches included "anterior cervical fusion," "anterior plate," "anterior cervical discectomy and fusion," "ACDF," "interbody fusion," "low profile," "zero profile," "zero-p," "anchored fusion," "anchored spacer device," and "stand alone." The conjunctions "AND" and "OR" were used during the literature retrieval. We restricted the language to English. Reference lists of all included studies were scanned to identify additional potentially relevant studies. Two reviewers independently screened the titles and abstracts of identified papers, and fulltext copies of all potentially relevant studies were obtained.

Inclusion and exclusion criteria
Two authors reviewed the articles, including randomized controlled trials (RCTs) and retrospective or prospective studies, in detail. The inclusion criteria for this study were as follows: all patients with multilevel CSM undergoing ACDF involving 2 or more levels; studies involving 2 cervical fusion groups: zeroprofile versus cage-plate; a follow-up time of no less than 12 months. The following articles were excluded: meeting abstracts, review articles, editorial comments, letters, technical reports, case reports, biomechanical studies, and animal experiments; studies that did not meet the inclusion criteria; articles considered as duplicate publications.

Quality assessment of included studies
Risk of bias assessment was performed using the checklist proposed by Cowley [12] for nonrandomized studies. Studies included in quanƟtaƟve synthesis (meta-analysis) (n = 10 ) were scored with "yes," "no," or "unclear." A Furlan score of 6 or more out of a possible 12, or a Cowley score of 9 or more out of a possible 17, was considered to reflect "high methodological quality." These studies were independently evaluated by 2 reviewers, and any discrepancies were resolved by discussion and consensus.

Data extraction
The data were extracted by 2 reviewers independently from each included study based on the following items: basic characteristics, including country, study design, age, enrolled number, and length of follow-up; function outcomes including postoperative Japanese Orthopedic Association (JOA) score, Neck Disability Index (NDI) score, and percentage of clinical success; radiological outcomes including postoperative total and segmental cervical lordosis, and disc height; complication types and complication rates. Any disagreement between the reviewers was resolved by discussion.

Data analysis
We performed all meta-analyses with the Review Manager software (RevMan Version 5.2; Cochrane Collaboration, Oxford, UK). Heterogeneity was tested using chi-square test and quantified by calculating I 2 statistic, for which P < .1 and I 2 < 50% was considered to be statistically significant. For the pooled effects, weighted mean difference (WMD) or standard mean difference (SMD) was calculated for continuous variables according to the consistency of measurement units, and odds ratio (OR) was calculated for dichotomous variables. Continuous variables are presented as mean differences and 95% confidence intervals (CIs), whereas dichotomous variables are presented as ORs and 95% CI. Random-effects or fixed-effects models were used depending on the heterogeneity of the studies included.

Results
The process of identifying relevant studies is summarized in Fig. 1. From the selected databases, 436 references were obtained. By screening the titles and abstracts, 415 references were excluded due to duplicates, irrelevant studies, case reports, not comparative studies, and review articles. The remaining potentially relevant 25 studies underwent a detailed and comprehensive evaluation. Finally, 10 studies were included in our meta-analysis. [13][14][15][16][17][18][19][20][21][22] The characteristics of these studies are summarized in Table 1.

Quality assessment
Newcastle-Ottawa Quality Assessment Scale was used to evaluate the quality of each study. This scale for nonrandomized case controlled studies and cohort studies had a maximum of 9 points, which included the quality of selection, comparability, exposure, and outcomes for study participants. Of these studies, 6 scored 8 points and 4 scored 7 points. Therefore, the quality of each study was relatively high ( Table 2).

Postoperative cervical lordosis.
Postoperative cervical lordosis was reported in 10 studies. Pooled data from the 10 relevant studies revealed cervical lordosis was significantly lower Table 1 Characteristics of included studies.

Discussion
Anterior cervical decompression and fusion has been commonly regarded as 1 of the gold-standard operation for CSM when conservative treatment fails. [23] An anterior cervical plate is widely used in ACDF to enhance the interbody fusion rate, and increase the cervical stability and avoid subsidence or graft dislocation. [24,25] However, the application of anterior cervical plate is closely associated with several postoperative complications Table 2 Quality assessment of included studies in the meta-analysis according to NOQAS.
Our meta-analysis demonstrates that there was no significant difference in terms of postperative JOA score, NDI score, and fusion rate between zero-profile and cage-plate. Compared with cage-plate group, postoperative dysphagia rate was significantly lower in the zero-profile spacer group. Pooled data from the relevant studies revealed a significant difference in postoperative adjacent segment degeneration and cervical lordosis.
The JOA and NDI scores were often used to evaluate the improvement of nerve function. Our study shows that there was no statistically difference in JOA scores, and also NDI scores between zero-profile and cage-plate. Our findings are in line with previous studies confirming that surgical management of multilevel CSM by zero-profile or cage-plate shows no significant differences in terms of achieved nerve improvement. [26,27] Hence, these results suggest that both procedures can have sufficient decompression and improve the patients' neurological function.
Dysphagia is 1 of the most concerning postoperative complication after ACDF with anterior plate fixation, especially after multilevel surgeries. The exact etiology of dysphagia remained unknown. Previous study reported that esophageal injury, postoperative soft tissue edema, adhesive formations around implanted cervical plates, and postoperative hematoma may be the possible reasons for dysphagia-related symptoms. [28,29] In addition, with the raise of fused segment, the risk of dysphagia increases. [10] Cho et al [30] found that the rate of postoperative dysphagia could be as high as 71% within the first 2 weeks after surgery, which gradually decreased during the following months. The results of this meta-analysis suggested that the zero-profile group was associated with lower incidence of dysphagia at postoperative 1, 3, and 6 months, and final followup, when compared with the cage-plate group. Recently, a metaanalysis performed by Tong et al [27] revealed that significantly lower dysphagia rates were observed in the zero-profile group at postoperative 2, 3, and 6 months. Our finding was comparable with previous studies. [27] However, our study revealed no significant difference between the 2 groups at postoperative 48 hours. At the early stage (2 weeks postoperatively), it could partly be attributed to longer intraoperative esophagus retraction time and greater retraction extent to fix the anterior plates.
We believe that our result of meta-analysis is affected by several reasons. Firstly, in this meta-analysis, most of the studies selected were not RCTs, although it did not influence the credibility of the

Conclusions
In conclusion, our meta-analysis reveals zero-profile spacer is better than the cage-plate in terms of dysphagia. This suggests zero-profile spacer is a superior alternative invention for the treatment of multilevel CSM to reduce the risk of dysphagia. This requires further validation and investigation in larger sample-size prospective and randomized studies.