Prospective Study of an Explainable AI Tool to Support Biliopancreatic Limb Selection in One-Anastomosis Gastric Bypass

This prospective observational cohort study will develop and externally validate a human-supervised, explainable artificial-intelligence decision-support model for analysing factors associated with biliopancreatic limb length selection during one-anastomosis gastric bypass (OAGB). The study will record three prespecified factors: the percentage of the laparoscopic instrument segment J-I covered by visceral or omental fat (P\_JI = 100 × FI/JI), the measured total small-bowel length (TSBL), and the abdominal integral index (AII = (CD/AC) + (CG/EG) + (JI/FI)). The study will evaluate weight-loss effectiveness, nutritional and malabsorptive outcomes, bile/acid reflux, major complications, and technical feasibility. During model development, the AI system will not autonomously assign a BPL length. The operating bariatric surgeon will make the clinical decision, and the model will be evaluated for calibration, external validity, factor contribution, safety constraints, and abstention.

Inclusion Criteria: 1. Adults aged 18 years or older at the time of consent. 2. Patients who meet the institution's independent clinical criteria for primary one-anastomosis gastric bypass. 3. Planned laparoscopic or robot-assisted OAGB at a participating centre. 4. Ability to undergo standardized intraoperative measurement of the J-I segment, P\_JI components, and total small-bowel length when clinically and ethically appropriate. 5. Ability to provide informed consent according to local requirements. 6. Willingness and reasonable ability to complete the prespecified postoperative follow-up. Exclusion Criteria: 1. Revisional, conversion, or non-primary gastric bypass surgery. 2. Operative anatomy that prevents reliable identification or measurement of the ligament of Treitz, the J-I segment, or the ileocecal junction. 3. Unaddressed altered gastrointestinal anatomy that is outside the development protocol. 4. Severe pre-existing malnutrition or another condition for which experimental variation in BPL length would be considered clinically inappropriate by the treating team. 5. Inability to provide informed consent when consent is required. 6. Inability to complete the minimum follow-up required by the approved protocol, unless the participant is retained for an approved missing-data or safety analysis. 7. Any intraoperative or postoperative circumstance in which the treating surgeon determines that protocol measurement would create unacceptable risk.