Table 1. Methodological quality assessment (risk of bias) of included studies by Newcastle-Ottawa Scale.

For each domain, either a "star" or "no star" is assigned, with a "star" indicating that study design element was considered adequate and less likely to introduce bias. A maximum of two stars can be given for Comparability. A study could receive a maximum of ten stars.

Study	Selection				Comparability	Outcome			Data analysis	Total score
	Exposed cohort	Non-exposed cohort	Ascertainment of exposure	Outcome of interest		Assessment of outcome	Length of follow-up	Adequacy offollow-up
Chee et al., 2008	*	*	*	-	*	*	*	-	-	6
Cermak et al., 2009	-	*	*	-	**	*	*	*	-	7
Park et al., 2011	*	*	*	*	**	*	*	*	*	10
Kikuchi et al., 2012	*	*	*	-	**	*	*	*	*	9
Komrokji et al., 2012	*	*	*	-	*	*	-	*	*	7
Li et al., 2013	*	*	*	-	*	*	*	*	*	8