Table 2.

Quality of the studies evaluated by the Newcastle-Ottawa Quality Assessment for Assessing Non-randomized Studies.

References	Selection				Comparability	Outcome/Exposure			Quality category
	Representativeness of the exposed cohort	Selection of the non-exposed cohort	Ascertainment of exposure	Outcome of interest not present at start of study	Comparability of cohorts on the basis of the design or analysis	Ascertainment of outcome	Adequacy of duration of follow-up	Adequacy of completeness of follow-up
Zhang et al., 2022 (2)	★		★			★	★	★	Poor
Alabsawy et al., 2021 (5)	★		★			★	★	★	Poor
Ji et al., 2021 (6)	★		★			★	★	★	Poor
Torii et al., 2018 (10)	★		★			★	★	★	Poor
Jung et al., 2018 (3)	★		★			★	★	★	Poor
Fujio et al., 2015 (8)	★		★			★	★	★	Poor
Justino et al., 2010 (9)	★		★			★	★	★	Poor
Jankowska et al., 2007 (4)	★		★			★	★	★	Poor
Jonas et al., 2005 (11)	★		★			★	★	★	Poor

Solid star represents one score, and the blank represents zero score.