Table 3.

Quality assessment of included case-controlled studies.

Study	Selection	Comprability	Exposure	NOS stars
Bagcioglu et al. (2012)	***	*	**	6
Fernie et al. (2015)	**	**	**	6
Mutlu et al. (2018)	***	**	**	7
Toffalini et al. (2015)	***	**	**	7

NOS, Newcastle Ottawa Scale. Each study can be awarded of a maximum of nine NOS stars.