Figure 2.

Risk of bias summary and graph using the Cochrane Risk of Bias tool [20]. The randomization and allocation concealment were well performed in most studies, but the method was not described in several studies. The performance bias was unclear or high in most studies, whereas the detection bias was low overall. The risk of attrition bias, reporting bias and other biases were low in most studies. Abbreviations: + = low risk of bias, ? = unclear risk of bias, - = high risk of bias.