Comparison of intervention effects in split-mouth and parallel-arm randomized controlled trials: a meta-epidemiological study

Background: Split-mouth randomized controlled trials (RCTs) are popular in oral health research. Meta-analyses frequently include trials of both split-mouth and parallel-arm designs to derive combined intervention effects. However, carry-over effects may induce bias in split-mouth RCTs. We aimed to assess whether intervention effect estimates differ between split- mouth and parallel-arm RCTs investigating the same questions. Methods: We performed a meta-epidemiological study. We systematically reviewed meta- analyses including both split-mouth and parallel-arm RCTs with binary or continuous outcomes published up to February 2013. Two independent authors selected studies and extracted data. We used a two-step approach to quantify the differences between split-mouth and parallel-arm RCTs: for each meta-analysis. First, we derived ratios of odds ratios (ROR) for dichotomous data and differences in standardized mean differences (Delta SMD) for continuous data; second, we pooled RORs or Delta SMDs across meta-analyses by random-effects meta-analysis models. Results: We selected 18 systematic reviews, for 15 meta-analyses with binary outcomes ( 28 split-mouth and 28 parallel-arm RCTs) and 19 meta-analyses with continuous outcomes ( 28 split-mouth and 28 parallel-arm RCTs). Effect estimates did not differ between split-mouth and parallel-arm RCTs ( mean ROR, 0.96, 95% confidence interval 0.52-1.80; mean Delta SMD, 0.08, -0.14-0.30). Conclusions: Our study did not provide sufficient evidence for a difference in intervention effect estimates derived from split-mouth and parallel-arm RCTs. Authors should consider including split-mouth RCTs in their meta-analyses with suitable and appropriate analysis.