Table 2.

Quality assessment of included studies

Author	Selection				Comparability		Outcome			Total score (Max: 9) (risk)
	Is the case definition adequate?	Representiveness of the cases	Selection of controls	Definition of controls	The subjects in different outcome groups are comparable, based on the study design or analysis	Confounding factors controlled	Ascertainment of exposure	Same method of ascertainment for cases and control	Non-response rate
Cohort studies (n = 1)
Gijbels et al. [33]	1	0	1	1	1	1	1	1	1	8 (low)
Case–control studies (n = 6)
Ahmed et al. [32].	1	1	1	1	1	1	1	1	0	8 (low)
Al‐Omoush et al. [19]	1	1	1	1	1	1	1	1	1	9 (low)
Dierickx et al. [11]	1	1	1	1	1	0	1	1	0	7 (medium)
Gonçalves et al. [34]	1	1	1	1	1	1	1	1	0	8 (low)
Gurbuz et al. [24]	1	1	0	1	1	1	1	1	0	7 (medium)
Khaimook et al. [35]	1	1	1	1	1	1	1	1	0	8 (low)
Author	Selection				Comparability		Outcome		Total score (Max. 10) (risk)
	Representative-ness of sample	Sample size	Non-respondents	Ascertainment of exposure	The subjects in different outcome groups are comparable, based on the study design or analysis.	Confounding factors controlled	Assessment of outcome	Statistical test
Cross-sectional studies (n = 10)
Alabdulwahhab et al. [27]	1	1	0	2	1	0	2	1	8 (low)
Al-Rawi et al. [23]	1	1	0	2	1	1	1	1	8 (low)
Chopra et al. [28]	1	1	1	2	1	1	2	1	10 (low)
Daud et al. [29]	1	1	0	2	1	1	1	1	8 (low)
Gabrielle et al. [30]	1	1	0	2	1	1	2	0	8 (low)
Lopes et al. [25]	1	1	1	2	1	0	0	1	7 (medium)
Shetty et al. [31]	1	1	1	2	1	1	2	1	10 (low)
Theodoroff and Folmer [20]	1	1	0	2	1	1	2	1	9 (low)
Willershausen et al. [26].	1	1	0	2	1	1	2	1	9 (low)
Wilson et al. [18]	1	1	0	2	1	1	0	1	7 (medium)