Patient demographics
The 10 studies enrolled 626 patients (315 in the ZP group and 311 in the CP group) tolally, which included 380 males and 236 females. The ZP used in the studies included Zero-P (Synthes GmbH, Oberdorf, Switzerland), ROI-C, ROI-MC+ (LDR, Troyes, France), PREVAIL (Medronic Sofamor Danek, Memphis, TN, USA), and Fidji cervical cage (Abbott Spine, Bordeaux, France). The control group was treated with anterior plating system and bone graft materials. Mean age, gender, follow-up durations, surgical levels and other informations of patients in each study was listed in Table 1.
Operative Data
Seven studies consisted with 447 patients (ZP group: 221; CP group: 226) noted operation time [9,11-13,24,26,27]. Mean operation time was greater for the CP group in six studies, and the operation time was significantly greater in the CP group compared to 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, senven studies included 447 patients (ZP group: 221; CP group: 226) were reported [9,11-13,24,26,27]. The ZP group was noted to have a lower blood loss in six studies. Overall, the CP group had a comparable amount of blood loss versus to the ZP group (WMD -5.51, 95% CI -11.69 to 0.67, P = 0.08) (Fig. 3).
Clinical evaluation
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]. Mean difference of JOA scores at final follow-up between ZP and CP groups were not significant (WMD 0.07, 95% CI -0.12 to 0.25, P = 0.48). In addition, pooled data of NDI scores at final follow-up did not reveal significant difference between the two groups (WMD -0.16, 95% CI -0.47 to 0.16, P = 0.33). Figs. 4 and 5 describe above informations in forest plots.
Radiological assessment
The results of radiographic fusion were described in nine studies [2,9-13,24,26,27], with fusion rate varying from 71 to 100%. Successful bone union was achieved in 348/377 cases (92.3%) in the ZP group, and 359/379 cases (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 between the ZP and CP groups 3 months postoperatively (WMD -0.53, 95% CI -0.98 to -0.09, P = 0.02). Nevertheless, Mean difference of cervical alignment between the two groups at the final follow-up were 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.
Dysphagia
The incidence of dysphagia was reported in seven studies [9-13,24,27], with results of 2.0%-57.1% in ZP group and 10.6%-73.1% in CP group at early period, respectively. In the early postoperatively (< 1 month), 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). Besides, the ZP group also had a lower incidence of dysphagia at 3 month postoperatively (OR 0.17, 95% CI 0.06 to 0.48, P = 0.0008) and final follow-up (OR 0.11, 95% CI 0.01 to 0.91, P = 0.04). Post-operative and follow-up dysphagia forest plots are described in Fig. 8.
Subsidence
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. It was significant higher in the patients following the zero-profile device (OR 1.97, 95% CI 1.34 to 2.89, P = 0.0005). The forest plot analysis of subsidence and funnel plot for publication bias are presented in Figs. 9 and 10, respectively.
We performed a subgroup analysis stratified by 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) in the definition of ≥ 2mm and ≥ 3mm, respectively. Correspondingly, it was 10.2% (26/254) and 7.4% (17/231) in the CP group. The results showed that there was higher risk of subsidence in the ZP group, while the definition of subsidence ≥ 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 quantity of operative segment was also performed, which included single-level surgery in three studies [2,12,25] and multi-level surgery in five studies [9,11,24,26,27]. In the operation of single-segment, incidence of subsidence between the ZP and CP groups were not significant (OR 1.43, 95% CI 0.61 to 3.37, P = 0.41). As for the multi-level surgery, 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 of subsidence for subgroup analysis are described in Figs. 11 and 12, respectively.