Systematic electronic literature surveillance will be conducted in the PubMed, Embase (OVID interface), and Cochrane Central Register of Controlled Trials (CENTRAL) databases utilizing TAPB-related text words and medical subject headings (MeSH) to obtain relevant studies published through July 31, 2023. All references of the included literature will be further retrieved to identify potential eligibility. Identifying some relevant studies through hand searching is planned to supplement searching whenever necessary. To review trials in progress, the ClinicalTrials.gov and the International Clinical Trials Registry Platform (ICTRP) databases will also be searched. The search will be confined to human subjects and the English language. The detailed and specific search strategy and syntax for the PubMed database are formulated ( Table 1 ).

Records obtained following the search strategy will be collected and imported in Mendeley software (RELX Group, Amsterdam, Netherlands). A team of three reviewers (WY, TY, and ZC) will independently screen the searched titles and abstracts against the eligibility criteria. For potentially eligible studies, full texts will be reviewed thoroughly. If there are a series of reports on one clinical trial, the latest publication containing the most sufficient data is suitable for inclusion. For further information on eligibility, corresponding authors of the studies will be contacted via e-mail whenever necessary. Thereafter, the above-mentioned three reviewers will reevaluate the entire texts post initial identification and document the reasons for some records to be excluded. All disputes among the three reviewers in this process will be settled via a consultation with a senior author (XL, LY). A structured PRISMA 2020 flowchart will be drawn to display the overview of the study identification procedure ( Figure 1 ).

The review author pair (QM, HZ) will then extract general characteristic data of selected articles using a standardized electronic form designed by all authors ( Table 2 ) for pooled analyses. A data set of first author’s name, publication year, country (or region), study design, study period, sample size, general characteristics of study population, TAPB technique and perioperative analgesia protocol, and all outcomes of interest will be collected from all eligible studies. Moreover, e-mails will be sent to the corresponding author to request adequate raw data in order to ensure the accuracy of the meta-analysis. If no effective response is received in 2 weeks, individual trials with missing data will be omitted from pooled analyses of the outcomes of interest. For high-quality management and synthesis, the cross-checked data will then be entered into Stata 17.0 software (StataCorp LLC, College Station, Texas, USA).

The included studies will be meticulously evaluated by a team of three reviewers (WY, TY, and ZC) for methodological quality. The Cochrane Risk of Bias version 2 (RoB 2) tool is supposed to validate the risk of bias for included RCTs based on bias arising from the randomization process, bias due to deviations from intended interventions, bias due to missing outcome data, bias in measurement of the outcome, and bias in selection of the reported result (31). The RoB 2 Excel Tool (available at: https://www.riskofbias.info/welcome/rob-2-0-tool/current-version-of-rob-2 ) will be applied to implete RoB 2 for primary outcomes (31, 32). To quantify the risk of bias in each study, the Cochrane Handbook for Systematic Reviews of Interventions will be adopted (28). For non-randomized studies (NRSs), on the other hand, the Risk of Bias in Non-randomized Studies of Interventions (ROBINS-I) tool will be utilized (33). Due to potential risk of bias in the selected studies, the findings generated from this meta-analysis will be interpreted with caution.

Stata 17.0 and Review Manager 5.4.1 software (The Nordic Cochrane Centre, The Cochrane Collaboration, Copenhagen, Denmark) will be employed to conduct the present meta-analysis. For continuous outcomes (opioid consumption at 24 hours postoperatively, pain scores at 24 hours postoperatively, and LOS), pooled weighted (WMDs) or standardized mean differences (SMDs) with their respective 95% confidence intervals (CIs) will be calculated due to the uniformity of scales used in studies. In addition, pooled odds ratios (ORs) will be worked out with corresponding 95% CIs for dichotomous variables (TAPB-related adverse events, overall postoperative 30-day complications, POI, SSI, and PONV). As a way to summarize the findings across the studies, statistical significance level will be set at a p value of less than 0.05. Given that NRSs with large sample sizes could dominate and reverse the pooled effect estimates, data synthesis for the RCT group and NRS group will be performed separately. We will identify the statistical heterogeneity among studies using the χ² test and quantify it with Cochrane’s Inconsistency (I²)-statistic. We set 50% as a cutoff value, such that substantial heterogeneity is defined as I² exceeding 50% and/or p value less than 0.10. It is preferable to adopt a random-effect model (REM) if heterogeneity is considerable, or else a regular fixed-effect model (FEM) will be the alternative. Meanwhile, to investigate the potential sources of substantial heterogeneity, sensitivity analysis and significative subgroup analyses will be conducted. Whenever clinical heterogeneity is considerable, we will undertake a narrative review rather than a meta-analysis.

A leave-one-out sensitivity analysis will be carried out, which aims to verify the robustness of the primary outcomes’ measure effects regarding study design, sample size, heterogeneity qualities, and non-informative prior distribution for heterogeneity parameters.

The subgroup analyses listed below will be arranged if homogeneous outcomes are reported in multiple studies within the matched subgroups:

To ascertain the possibility of publication bias, we will firstly check whether the RCT protocol was published prior to the enrollment of patients for the study. Studies published after July 1, 2005 will be checked at the World Health Organization-affiliated ICTRP. The presence of outcome reporting bias (selective reporting of outcomes) will also be evaluated. The visual symmetry of a funnel plot will be considered as the primary predictor of publication bias when more than ten studies are included (34).

In order to grade the certainty of evidence for each outcome, the Grading of Recommendations, Assessment, Development, and Evaluation (GRADE) working group approach will be chosen, which mainly contains the dimensions of study limitations, publication bias, imprecision, inconsistency, and indirectness (35). The evidence’s strength will be ranked as four levels: high (very confident that the effect estimate lies close to the true effect), moderate (moderately confident in the effect estimate), low (limitedly confident in the effect estimate), and very low (very little confident in the effect estimate) (36). In order to make the table and the process easier to be understood, all decisions to downgrade or upgrade the certainty of evidence will be accompanied by clear arguments in footnotes whenever necessary.

Owing to the nature of secondary analysis of existing data, there will be no patients involved in this study, and ethical approval will be not needed. High-quality peer-reviewed publications and presentations at international conferences will facilitate disseminating the results of this study.