All 318 participants included in the original trial were eligible for cohort follow-up. Participant characteristics and details of the original surgical technique have been previously published [3]. Original participants were contacted approximately 2 months before their 5-year surgical anniversary to inform them of the follow-up study and to seek consent to repeat participation. Given that the 5-year follow-up window occurred during the COVID-19 pandemic, the cohort was developed with primary data collection being collected and managed using REDCap electronic data capture tools hosted at the Clinical Research Unit (CRU), Cumming School of Medicine, University of Calgary, Calgary, Alberta, Canada [12, 13]. For those participants who preferred to complete paper copies of the questionnaires, these were delivered and returned by postal mail with subsequent data entry into REDCap. This approach avoided in-person contact as there was a moratorium on in-person research visits within the relevant university and health care institutions. The original MUST RCT utilized an in-person examination to objectively measure the presence or absence of urine loss using a standardized cough-stress test physical examination [3]. Given the aforementioned COVID-19-related organizational directives to minimize unnecessary physical contact, we were unable to include this outcome in this study. However, results from previous MUS trials demonstrate high correlation between patient-reported and clinician-measured outcomes of SUI [3, 7, 14], supporting the decision that patient-reported outcomes in isolation would generate important data relevant to clinical decision making.

To ensure that all surgical interventions related to the original MUS insertion were included in the analysis, and to limit recall and attrition biases, administrative health data were linked to all original 318 trial participants. Administrative data sources included the Discharge Abstract Database (DAD), which captures all inpatient surgical procedures and hospital admissions, and the National Ambulatory Care Reporting System (NACRS), which captures all same-day surgeries and emergency room visits. These datasets adhere to International Classification of Diseases, Canadian Version-10, and the Canadian Classification of Health Intervention standards of coding [15, 16], and have previously been utilized to study longer term outcomes of MUS surgery in Canada [5, 17, 18].

Participants maintained classification by the randomization groups from the original study. Group demographics were compared by absolute standard difference (ASD) to test for a continued balance between randomization groups [19]. In keeping with the original trial, the primary outcome for the 5-year follow-up was bothersome versus nonbothersome SUI symptoms, defined by a score of ≥ 2 on Question 3 of the Urinary Distress Inventory (UDI-6) [3, 20]. Secondary outcomes included bothersome versus nonbothersome OAB symptoms, as defined by a score of ≥ 2 on either Question 1 or Question 2 of the UDI-6; summary scores of the UDI-6, Incontinence Impact Questionnaire (IIQ-7) [20], and International Consultation on Incontinence Questionnaire Female Lower Urinary Tract Symptoms Long Form (ICIQ-FLUTS LF) module questionnaires [21]; as well as the rate and timing of repeat surgery related to SUI symptoms and/or complications from the original mid-urethral sling. Mean difference (MD) and 95% confidence interval (CI) were calculated for proportional and numerical data; repeat surgeries were expressed as both incidence rate and cumulative incidence, as defined by the time between the original MUS surgery and the subsequent surgery [5].

Subsequent surgery for SUI and/or mesh complications was determined through patient report and administrative data. Validated Canadian Classification of Health Intervention (CCI) codes, which indicated a subsequent surgery for SUI treatment and/or to remove or revise implanted surgical devices or mesh, removal of a urethral foreign body, urethral dilation, retropubic or transvaginal urethrolysis, or repair of a urethrovaginal fistula were used to flag subsequent surgery in administrative data [5]. After identification of subsequent surgery by either participant report and/or occurrence of a validated code in the administrative data sources, the specific case was confirmed by chart review to determine the nature of the procedure confirmed. Descriptive statistics reported the frequencies of diagnostic International Statistical Classification of Diseases and Related Health Problems, 10th Revision, Canada (ICD-10-CA) codes and the CCI codes represented by these procedures.

A manual review of all mesh erosion cases recorded in the dataset was performed, in response to questions that have arisen since the original MUST paper as to explanation of mesh erosions in the original Babcock group that were not sustained after conclusion of the RCT. Given that Babcock was a novel technique for the study surgeons, we classified each Babcock case relative to the surgeon’s first Babcock case in the trial to explore for any temporal relationship in mesh erosion and introduction of the Babcock technique into a surgeon’s practice.

An a priori power calculation was performed for this study’s primary outcome, assuming a follow-up rate of 75% at 5 years (n = 238 over both arms) based on prior MUS follow-up studies [7], as well as an SUI cure rate of 69% at 5 years using Question 1 of the UDI-6 [7] and a non-inferiority limit of 15% [22], which has previously been determined as the threshold for surgeons to favor one surgical approach to the treatment of SUI over another [1, 14]. Calculations revealed that a sample of this size would have > 80% power with alpha = 0.05 to exclude a difference in participant-reported SUI cure.

Although all the original trial participants are biologically female, the original MUST trial methodology did not collect self-identified gender as part of demographic characteristics. Realizing this oversight, at the 5-year follow-up we did collect self-reported gender identity, as well as a measure of gender expression, in alignment with expert opinion that incorporation of the complexities of gender should be improved in health research [23, 24]. Given that within a gender identity, an individual may report differing levels of femininity and masculinity, measurement of gender expression is an analytic approach to better understanding how the nuances of gender are connected to experiences of health [25]. A secondary analysis of gender expression as a binary variable of gender polar with exclusively feminine traits (only feminine scores reported, masculine score of zero) or diversity in gender expression with mixed feminine and masculine traits (≥ 1 feminine score, ≥ 1 masculine score) was planned given that gender expression has previously been associated with patient perceptions in pelvic floor disorders [25]. Additionally, sensitivity analyses using a more conservative definition of bothersome SUI and OAB symptoms as defined by a score of ≥ 1 on corresponding questions of the UDI-6 were planned to ensure robust findings.

The original study protocol was registered at ClinicalTrials.gov (NCT02480231), and inclusion/exclusion criteria and technique details have been previously described [1, 3]. This study obtained ethics approval from the relevant university ethics board. The Strengthening the Reporting of Observational Studies in Epidemiology (STROBE) statement for reporting of observational studies was followed [26].