Validation of the Institute Mutualiste Montsouris system for the stratification of laparoscopic liver resections: an international multicenter study

Background: The Institut Mutualiste Montsouris (IMM) 3-level complexity classification has been validated for laparoscopic liver resection (LLR) in several studies with small sample size. However, it has not been well-validated in large studies down to the individual procedure type. Hence, in order to address current limitations in the studies validating the IMM complexity classification, we performed an international multicenter study to validate the IMM complexity classification across its three complexity levels and the categorization of the 11 distinct procedure types. Methods: A retrospective cohort study of 22,252 patients undergoing LLR across 64 centers worldwide between 2005 and 2021 was performed. Baseline characteristics and perioperative outcomes were analyzed across the three difficulty levels and 11 procedure types of the IMM complexity classification. Results: A total of 14,765 patients were included in our final analysis. The main indications for LLR in our study was hepatocellular carcinoma or intrahepatic cholangiocarcinoma (n=7,781, 52.7%) followed by liver metastasectomy (n=3,911, 26.5%). In terms of underlying liver pathology, 5,127 (34.7%) cases had cirrhosis, and 1,214 (8.3%) had portal hypertension. Perioperative outcomes including operative time, open conversion rate, intraoperative blood loss, need for intraoperative blood transfusion, need for Pringle's application, length of stay, postoperative morbidity, major postoperative morbidity and 90-day mortality all demonstrated a significant increasing trend with increasing IMM complexity grades (P<0.001). These trends remained significant following adjustment for baseline characteristics (P<0.001). Notably, when examining the 11 LLR procedure types, all procedures within each IMM complexity grade were individually higher than all procedures in the preceding complexity grade for operative time, blood loss, length of stay, postoperative morbidity and major postoperative morbidity. Conclusions: The three IMM complexity grades were well associated with LLR complexity as determined by key surrogate perioperative measures. Our findings also supported the categorization of the 11 distinct LLR procedures into the three complexity levels.