Fig. 2

Risk of bias summary: Review authors’ judgments about each risk of bias item for each included study. White boxes: Low risk of bias (questions in Table 1 answered by ‘yes’). Blue boxes: Unclear risk of bias (questions in Table 1 could not be answered). Red boxes: High risk of bias (questions in Table 1 answered by ‘no’). Other bias/comments for which the authors assessed a consibarable risk of bias are marked with grey and further discussed in the text. Level of evidence was scored according to guidelines from Habour and Miller [16]: ¹Observational studies, case-series. ²Qasi experimental studies, case-control, or cohort studies