Table 2.

Risk of bias in included studies.

Bias	Study ID
	Lundström et al. 2006 [23]	Sarikkola et al. 2011 [25]	Serrano-Aguilar et al. 2012 [24]
Random sequence generation (selection bias)	Unclear risk “The patients were randomly assigned to ISCS or to DSCS.” No further description of randomization procedure	Low risk “Randomization was performed using sealed envelopes after the preoperative examination”	Low risk “A computer-generated sequence was used”
Allocation concealment (selection bias)	Unclear risk Not described in paper	Unclear risk Patients (and staff) knew after the preoperative assessment but before the surgery to which group they belonged	Unclear risk “Random numbers were obtained for all patients on the waiting list before participants were selected on the basis of the inclusion and exclusion criteria. Randomization was performed sequentially for blocks of 200 patients.” Unclear whether those including the patients in the study were aware of the randomization status before inclusion/exclusion
Blinding of participants and personnel (performance bias)	High risk Not possible to blind patients or personnel to whether the patient had ISBCS or different date bilateral surgery	High risk Not possible to blind patients or personnel to whether the patient had ISBCS or different date bilateral surgery	High risk Not possible to blind patients or personnel to whether the patient had ISBCS or different date bilateral surgery
Blinding of outcome assessment (detection bias)	Unclear risk Not reported	Unclear risk Not reported	Unclear risk Not reported
Incomplete outcome data (attrition bias)	Unclear risk High rate of exclusion after randomization/drop-outs (8/96 = 8.3%), not possible to assess whether this influenced the outcome since the characteristics of drop-outs were not compared to non-drop-outs	Low risk 96.0% in Group 1 and 97.3% in Group 2 were treated per protocol. 491/507 randomized patients had 1 month follow-up	Low risk Low number of exclusions and drop-outs (<5% at the 1 month postoperative examination)
Selective reporting (reporting bias)	Low risk Important outcomes were reported	Low risk Important outcomes were reported	Low risk Important outcomes were reported
Other bias	Low risk Not likely in this study	Low risk Not likely in this study	Low risk Not likely in this study

Risk of bias was assessed according to the Cochrane Handbook [15].