Table 2.

Potential sources of accidental unblinding and risk mitigation strategies: post-operative phase

Potential source of accidental unblinding	Risk mitigation strategy
Interactions between the unblinded operating team and blinded post-operative staff or trial participants: The unblinded operating team could inadvertently disclose treatment allocation to the post-operative ward staff, or even the participant during routine standard inpatient and outpatient post-operative clinical reviews	Firstly, only essential personnel who are critical to safe and successful randomisation are allowed to be present during the operation. In SUcceSS, the operating team typically consists of the operating surgeon, theatre nurse/s, the anaesthetist, and occasionally a surgical trainee. After randomisation (i.e. operation), except for the unblinded operating surgeon who will only interact with the blinded participant or post-operative ward staff under explicit circumstances, all other members of the operating team do not have any further interactions with the trial participant or post-operative ward staff To limit contact between the unblinded operating team and post-operative ward staff, as well as trial participants, all participants are assigned a blinded post-operative surgeon. The blinded surgeon is trained in the trial protocol and is responsible for performing all post-operative inpatient consultations, reviewing medical records for any adverse events experienced during the hospital stay, and performing all routine standard post-operative outpatient consultations. Post-operatively, the blinded post-operative surgeon will only consult with the unblinded operating surgeon if the blinded post-operative surgeon is concerned that the participant may require further surgery. A participant will only interact with the unblinded operating surgeon if further surgery is indicated. Even if further surgery is indicated, the participant is encouraged to remain blinded to allocation, as revision surgery is a common complication of lumbar decompression surgery
Interactions between unblinded theatre staff and blinded post-operative staff: Theatre staff who are present during randomisation (surgery) may inadvertently disclose trial allocation to post-operative ward staff	When new recruitment sites are implemented, training sessions are scheduled with all theatre staff (e.g. operating surgeons and trainees, theatre nurses, anaesthetists) to ensure that staff are aware group allocation should not be discussed outside of the operating theatre. Refresher training sessions are also scheduled periodically (i.e. before new participants undergo randomisation)
Billing: In Australia, standard practice for private health insurers and the universal health care system (Medicare) is to send medical invoices, which include details of the procedures performed, directly to patients. Upon receipt, participants could accidentally become unblinded. In the SUcceSS trial, medical invoices may be related to the surgery or related investigations (e.g. blood test, imaging)	Prior to commencement of recruitment at each site, a site-specific billing protocol is established with the administration and finance departments of the partnering hospitals. Billing protocols are also established with private health insurers. The protocols ensure that medical records are kept blinded, and invoices are sent directly to the unblinded central clinical trial co-ordinator instead of the participant
Investigational images and reports: Post-operatively, the blinded surgeon, GP, or other clinicians implicated in the participant’s care may order investigations such as lumbar x-rays or MRIs. It is possible that images or reports could unblind the clinician, and/or inadvertently the participant, to group allocation	Prior to participation, participants are encouraged to discuss their involvement in the trial with their GP. If necessary, before the index surgery, the GP is alerted to the importance of maintaining participant blinding during the trial. After participants are discharged from hospital, a standardised letter is sent to their GP requesting that lumbar imaging should be avoided until completion of the 24-month follow-up, or, at least after the 3-month follow-up (i.e. primary endpoint). If further imaging is deemed necessary, the trial operations team will contact the clinician who has requested further imaging and remind them that images and reports should not be sent to the participant, and group allocation should not be revealed to the participant without approval from the chief investigator. Where feasible, the trial operations team aims to ensure that the unblinded surgeon is the only personnel with access to images or reports

GP general practitioner