Table 1.

Quality assessment for observational studies.

	Domains
	Bias due to confounding	Bias in selection of participants in the study	Bias in classification of interventions	Bias due to deviations from intended intervention	Bias due to missing data	Bias in measurement of outcomes	Bias in selection of the reported result	Overall risk of bias
Eggli et al. 1996	Critical	Critical	Serious	Moderate	Low	Moderate	Moderate	Critical
Alfaro-Adrián et al. 1999	Critical	Critical	Serious	Moderate	Low	Moderate	Moderate	Critical
Parvizi et al. 2006	Critical	Critical	Serious	Moderate	Low	Moderate	Moderate	Critical
Berend et al. 2007	Critical	Critical	Serious	Moderate	Low	Moderate	Moderate	Critical
Hooper et al. 2009	Critical	Critical	Moderate	Moderate	Low	Serious	Moderate	Critical
Aghayev et al. 2010	Critical	Critical	Moderate	Moderate	Low	Moderate	Moderate	Critical
Saito et al. 2016	Critical	Critical	Moderate	Moderate	Low	Moderate	Moderate	Critical
Johnston et al. 2011	Critical	Critical	Moderate	Moderate	Low	Moderate	Moderate	Critical
Lindberg-Larsen et al. 2013	Serious	Critical	Moderate	Moderate	Low	Moderate	Moderate	Serious
Rasouli et al. 2014	Moderate	Moderate	Moderate	Moderate	Low	Moderate	Moderate	Moderate
Garland et al. 2015	Moderate	Moderate	Moderate	Moderate	Low	Low	Low	Moderate
Quadri et al. 2015	Critical	Critical	Serious	Moderate	Low	Serious	Moderate	Critical