We performed a test for heterogeneity of included studies using REVMAN, version 5.1 software (The Nordic Cochrane Centre, The Cochrane Collaboration, Copenhagen, Denmark). We expressed each study result as a relative risk (RR) for dichotomous data, with 95% confidence intervals (CI). A fixed-effect model was used if the studies were homogeneous, which was determined by chi-squared test (p < 0 1, I 2 > 50%), and a random-effect model was used if the groups need to be merged by our clinical judgment when the studies were heterogeneous. When we could not determine the source of heterogeneity, we used descriptive analysis.