Table 2.

Newcastle-Ottawa scale of the included studies

Total	Exposure	Comparability	Selection
7	★★	★★	★★★		Sun et al.16
7	★★	★★	★★★		Jin et al.9
7	★★	★★	★★★		Rajkumar et al.10
7	★★	★★	★★★		Motiffard et al.23
7	★★	★★	★★★		Liu et al.21
7	★★	★★	★★★		Li et al.22
7	★★	★★	★★★		Ni et al.25
7	★★	★★	★★★		White et al.20
7	★★	★★	★★★		Jin et al.24
7	★★	★★	★★★		Choi et al.8
7	★★	★★	★★★		Wang et al.26
7	★★	★★	★★★		Siqueira et al.4
7	★★	★★	★★★		Cao et al.27
7	★★	★★	★★★		Bohl et al.13
7	★★	★★	★★★		Shahi et al.15
7	★★	★★	★★★		Dragosloveanu et al.12
7	★★	★★	★★★		Stephens et al.14
7	★★	★★	★★★		Lee et al.3
6	★★	★	★★★		Leopold et al.2