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

One-anastomosis gastric bypass is a metabolic-bariatric procedure in which a long gastric pouch is connected to the small bowel by a single gastrojejunostomy. The length of the biliopancreatic limb (BPL) changes the length of bowel exposed to biliopancreatic secretions before mixing with ingested nutrients and may therefore influence weight loss, metabolic response, nutritional risk, and bile/acid reflux. Existing clinical practice includes different BPL lengths, and the available evidence does not establish one universally optimal length for every patient.

This study is designed as a prospective, multicentre, observational cohort for development and external validation of an explainable clinical decision-support model. The study will enrol adults undergoing primary laparoscopic or robot-assisted OAGB as part of routine clinical care. The BPL length will not be assigned by the AI system during model development. The actual clinically selected BPL length will be recorded as an observed exposure together with operative configuration, centre, surgeon, and relevant patient characteristics.

The primary prespecified factor is P_JI, calculated as 100 × FI/JI. JI is the full measured distance from the internal left-hypochondrial trocar point J to the ligament of Treitz at point I. FI is the portion of the J-I instrument segment covered or occupied by the displaced visceral/omental fat layer. P_JI ranges from 0% to 100% and is a laparoscopic geometric coverage metric, not a direct volumetric measurement of total visceral fat.

The second factor is TSBL, measured in centimetres from the ligament of Treitz to the ileocecal junction. TSBL will be used to estimate residual absorptive bowel for each candidate BPL length. A 4-10 m interval may be used as an operational study range, but it is not a universal biological normal range; measurements outside this range will trigger quality review rather than automatic truncation.

The third factor is AII, calculated as (CD/AC) + (CG/EG) + (JI/FI). AII is a secondary geometric and technical feature. When FI equals zero, JI/FI is structurally undefined and must be handled by a prespecified missingness and sensitivity-analysis strategy rather than by substituting an arbitrary value.

The statistical baseline will be multivariable regression appropriate to each endpoint, with centre and surgeon effects handled explicitly. Nested models will compare the incremental contribution of P_JI, TSBL, and AII. Explainable AI will be used as a calibrated prediction and visualization layer, not as an autonomous decision maker. Candidate BPL actions of 0.5, 1, 2, and 3 m will be represented as research actions for analysis. Longer candidates will be subject to residual-bowel, nutritional, anatomical, uncertainty, governance, and follow-up safety constraints.

The proposed hierarchy P_JI greater than TSBL greater than AII is a falsifiable hypothesis. It may be supported, rejected, or revised after out-of-sample validation. The study will report effectiveness and harm endpoints separately and will not use an unvalidated composite outcome as the sole definition of the best BPL length. The final surgeon decision, any model override, and the reason for abstention will be documented.

Inclusion criteria

• Adults aged 18 years or older at the time of consent.
• Patients who meet the institution's independent clinical criteria for primary one-anastomosis gastric bypass.
• Planned laparoscopic or robot-assisted OAGB at a participating centre.
• Ability to undergo standardized intraoperative measurement of the J-I segment, P_JI components, and total small-bowel length when clinically and ethically appropriate.
• Ability to provide informed consent according to local requirements.
• Willingness and reasonable ability to complete the prespecified postoperative follow-up.

Exclusion criteria

• Revisional, conversion, or non-primary gastric bypass surgery.
• Operative anatomy that prevents reliable identification or measurement of the ligament of Treitz, the J-I segment, or the ileocecal junction.
• Unaddressed altered gastrointestinal anatomy that is outside the development protocol.
• Severe pre-existing malnutrition or another condition for which experimental variation in BPL length would be considered clinically inappropriate by the treating team.
• Inability to provide informed consent when consent is required.
• 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.
• Any intraoperative or postoperative circumstance in which the treating surgeon determines that protocol measurement would create unacceptable risk.