Both reviewers independently extracted data from the full texts of included articles using specially designed forms. Data on author(s), study design, implant type/surface, any additional procedures performed, number of patients (in each group), presence of a control group, procedure and time of implant retrieval, methods of gene expression analysis, and main results, were extracted. Descriptive summaries of the studies were entered into tables, and a qualitative synthesis of evidence was planned. Any disagreement between the reviewers regarding data extraction was resolved by discussion.

Assessment of the methodological validity of the included studies was performed using criteria adapted from previous reports [30,31]. Aspects of study design, genotyping methods, and data analyses were considered using nine criteria (Table 1).

The risk of bias in the included studies was assessed using an adaptation of published guidelines for reporting systematic reviews of periodontal genetic association studies [32]. Mainly, aspects of study design and methodological validity were assessed using 15 criteria and scored as ‘yes,’ ‘no,’ or ‘unclear’ based on the information provided in the study manuscript (Table 2). Moreover, published guidance [33] regarding the qualitative and quantitative syntheses of results from genetic association studies was consulted, and heterogeneity across the included studies was assessed to explore the possibility of a meta-analysis.