Table 1.

Risk of bias within the studies

Study	Year of publication	Random sequence generation	Allocation concealment	Blinding	Incomplete outcome data	Selective reporting	Other bias
Deppe et al. [22]	2007	?	?	?	-	+	+
Roos-Jansåker et al. [23]	2007	?	?	+	-	?	+
Roos-Jansåker et al. [24]	2007	?	?	?	-	?	+
Schwarz et al. [25]	2008	+	?	+	-	?	+
Romanos et al. [26]	2008	?	?	?	-	-	-
Schwarz et al. [27]	2009	+	?	+	-	?	+
Schwarz et al. [28]	2010	?	?	+	-	?	+
Roccuzzo et al. [29]	2011	?	?	+	+	?	+
Froum et al. [30]	2012	?	?	+	-	-	-
Aghazadeh et al. [31]	2012	+	-	+	+	+	+
Wohlfahrt et al. [32]	2012	+	+	+	+	+	+
Wiltfang et al. [33]	2012	?	?	?	-	-	+
Schwarz et al. [34]	2013	+	?	+	?	-	+
Matarasso et al. [35]	2014	?	?	?	-	?	+
Roos-Jansåker et al. [36]	2014	?	?	+	-	?	+
Jepsen et al. [37]	2015	+	+	+	+	+	+
Froum et al. [38]	2015	?	?	+	+	+	-
Roccuzzo et al. [39]	2016	?	?	-	+	+	+

+ = low risk; ? = unclear risk; - = high risk.