Table 2.

Study assessment using the Newcastle-Ottawa Scale

Study	Patient selection	Comparability	Exposure	Total stars
Belmont 1997 [32]	4*	1*	2*	7*
Kuhn 1998 [22]	3*	1*	1*	5*
Gong 2002 [33]	4*	1*	1*	6*
Xu 2003 [34]	3*	1*	2*	6*
Zheng 2006 [35]	4*	1*	1*	6*
Bollain 2009 [36]	4*	1*	2*	7*

* indicates item achieves 1piont in Newcastle-Ottawa Scale