Table 3.

Risk of bias in included studies.

Study	Selection of participants	Confounding variables	Measurement of exposure	Blinding of outcome assessment	Incomplete outcome data	Selective outcome reporting
De Costa 2020 [11]	Low risk	Low risk	Low risk	High risk	Unclear risk	Low risk
Benomar 2018 [10]	High risk	Low risk	Low risk	High risk	Unclear risk	Low risk
Chaudhri 2016 [14]	Low risk	Low risk	Low risk	High risk	Unclear risk	Low risk
Han 2015 [15]	Low risk	Low risk	Low risk	High risk	Unclear risk	Low risk
Yang 2013 [16]	Low risk	High risk	Low risk	High risk	Unclear risk	Low risk
Chen 2012 [17]	High risk	High risk	Low risk	High risk	Unclear risk	High risk
Najafi 2012 [18]	High risk	High risk	Low risk	High risk	Unclear risk	Low risk
Mujais 2006 [19]	High risk	Low risk	Low risk	High risk	Unclear risk	High risk
Badve 2006 [19]	High risk	High risk	Low risk	High risk	Unclear risk	Low risk
Duman 2004 [21]	Low risk	High risk	Low risk	High risk	Unclear risk	Low risk
Sasal 2001 [22]	Low risk	Low risk	Low risk	High risk	Unclear risk	Low risk
Davies 2001 [23]	Low risk	High risk	Low risk	High risk	Unclear risk	Low risk