Literature Search and study characteristics
A total of 221 related records were obtained by searching the databases above. After removing 26 duplicated studies and 22 non-English language articles, 173 studies remained for screening and 163 records were excluded according to the selection criteria. As a result, ten controlled trials [2,9-13,24-27] were included in this meta-analysis. The literature search procedure is shown in Fig. 1. All clinical trials came from different research centers. The study data and baseline characteristics of both treatment groups are presented in Table 1. Considering that the eight of nine nonrandomized studies scored more than 7 points and that the one RCT scored 10 points, the quality assessment of each study was relatively reliable (Tables 2, 3).
The 10 studies enrolled a total of 626 patients (315 in the ZP group and 311 in the CP group), which included 380 males and 236 females. The ZPs used in the studies included the Zero-P (Synthes GmbH, Oberdorf, Switzerland), ROI-C, ROI-MC+ (LDR, Troyes, France), PREVAIL (Medronic Sofamor Danek, Memphis, TN, USA), and the Fidji cervical cage (Abbott Spine, Bordeaux, France). The control group was treated with an anterior plating system and bone graft materials. The mean age, sex, follow-up durations, surgical levels and other patient information from each study are listed in Table 1.
Seven studies consisting of 447 patients (ZP group: 221; CP group: 226) noted the operation time [9,11-13,24,26,27]. The mean operation time was greater for the CP group in six studies, and after meta-analysis, the operation time was significantly greater in the CP group than in the ZP group (WMD -15.87, 95% CI -30.62 to -1.11, P = 0.04) (Fig. 2).
For the outcome regarding intraoperative blood loss, seven studies including 447 patients (ZP group: 221; CP group: 226) reported this variable [9,11-13,24,26,27]. The ZP group was noted to have lower blood loss in six studies. Overall, the CP group had a comparable amount of blood loss versus the ZP group (WMD -5.51, 95% CI -11.69 to 0.67, P = 0.08) (Fig. 3).
Data regarding JOA and NDI scores postoperatively were documented in six studies consisting of 436 patients (ZP group: 217; CP group: 219) [9-11,13,24,27]. The mean difference in JOA scores at the final follow-up between the ZP and CP groups was not significant (WMD 0.07, 95% CI -0.12 to 0.25, P = 0.48). In addition, pooled NDI score data at the final follow-up did not reveal a significant difference between the two groups (WMD -0.16, 95% CI -0.47 to 0.16, P = 0.33). Figs. 4 and 5 describe the above information in forest plots.
The results of radiographic fusion were described in nine studies [2,9-13,24,26,27], with fusion rates varying from 71 to 100%. Successful bone union was achieved in 348/377 patients (92.3%) in the ZP group, and 359/379 patients (94.7%) in the CP group. The forest plot analysis showed no significant difference between the two groups (OR 0.66, 95% CI 0.36 to 1.20, P = 0.17) (Fig. 6).
There was a significant difference regarding cervical alignment 3 months postoperatively between the ZP and CP groups (WMD -0.53, 95% CI -0.98 to -0.09, P = 0.02). Nevertheless, the mean difference in cervical alignment between the two groups at the final follow-up was not significant (WMD -0.75, 95% CI -1.76 to 0.25, P = 0.14). The corresponding forest plot analysis is shown in Fig. 7.
The incidence of dysphagia was reported in seven studies [9-13,24,27], with results of 2.0%-57.1% in the ZP group and 10.6%-73.1% in the CP group at the early period, respectively. In the early postoperative period (< 1 month), the rate of dysphagia was noted to be lower in the ZP group (OR 0.39, 95% CI 0.24 to 0.64, P = 0.0002). In addition, the ZP group also had a lower incidence of dysphagia at 3 months postoperatively (OR 0.17, 95% CI 0.06 to 0.48, P = 0.0008) and at the final follow-up (OR 0.11, 95% CI 0.01 to 0.91, P = 0.04). Forest plots for the postoperative and final follow-up dysphagia evaluations are presented in Fig. 8.
A total of ten studies were included in the comparison of the incidence of subsidence between the ZP and CP groups [2,9-13,24-27]. The subsidence rates were 15.1% (89/588) in the ZP group and 8.8% (51/581) in the CP group. The subsidence rate was significantly higher in the ZP group patients (OR 1.97, 95% CI 1.34 to 2.89, P = 0.0005). The forest plot analysis of subsidence and the funnel plot evaluation of publication bias are presented in Figs. 9 and 10, respectively.
We performed a subgroup analysis stratified by the definition of subsidence, which included subsidence ≥ 2mm in five studies [2,9,12,13,26] and ≥ 3mm in four studies [10,11,24,25]. In the ZP group, the incidence was 16.5% (42/255) and 12.3% (30/243) with the definition of ≥ 2mm and ≥ 3mm, respectively. Correspondingly, it was 10.2% (26/254) and 7.4% (17/231) in the CP group. These results showed that there was a higher risk of subsidence in the ZP group, regardless of whether the definition of subsidence was ≥ 2mm (OR 1.78, 95% CI 1.03 to 3.06, P = 0.04) and ≥ 3mm (OR 1.98, 95% CI 1.00 to 3.91, P = 0.05).
A subgroup analysis stratified by the quantity of operative segments was also performed, which included single-level surgery in three studies [2,12,25] and multilevel surgery in five studies [9,11,24,26,27]. In the single-segment operations, the incidence of subsidence between the ZP and CP groups was not significant (OR 1.43, 95% CI 0.61 to 3.37, P = 0.41). For the multilevel surgeries, the ZP group had a higher incidence of subsidence (OR 2.61, 95% CI 1.55 to 4.40, P = 0.0003). The forest plots for the subgroup analysis of subsidence are described in Figs. 11 and 12, respectively.