Modified 5-Item Frailty Index Is Associated with Postoperative Delirium but Not Survival in Patients Undergoing Surgery for Oral Squamous Cell Carcinoma

Background/Objectives: Frailty is increasingly recognized as a clinically relevant marker of reduced physiological reserve in surgical oncology. The modified 5-item frailty index (mFI-5) is simple and practical, but its clinical significance in oral squamous cell carcinoma (OSCC) remains incompletely defined. This study evaluated the association between mFI-5-defined frailty and postoperative complications, especially postoperative delirium, as well as survival outcomes in patients undergoing surgery for OSCC. Methods: We retrospectively analyzed 127 patients who underwent surgical resection for OSCC with postoperative high care unit (HCU) management between 2013 and 2021. Frailty was defined as an mFI-5 score of ≥2. Clinical characteristics, postoperative outcomes, HCU stay, length of hospital stay, overall survival (OS), and disease-free survival (DFS) were compared between frail and non-frail groups. Univariable logistic regression was used to estimate odds ratios (ORs) and 95% confidence intervals (CIs) for postoperative complications. An exploratory multivariable logistic regression analysis for postoperative delirium was performed using age ≥65 years and sex as covariates. Delirium was retrospectively assessed from clinical documentation considered consistent with DSM-5 criteria. Results: Twenty-one patients (16.5%) were classified as frail. Postoperative delirium occurred more frequently in frail patients than in non-frail patients (42.9% vs. 19.8%; p = 0.023). In a multivariable logistic regression model adjusted for age ≥65 years and sex, mFI-5-defined frailty was significantly associated with postoperative delirium (adjusted OR, 3.07; 95% CI, 1.08–8.60; p = 0.035). No significant association was observed for pneumonia, surgical site infection, or free-flap reoperation. Frailty was not significantly associated with HCU stay, length of hospital stay, OS, or DFS. Conclusions: mFI-5-defined frailty was associated with postoperative delirium but not with survival outcomes in this OSCC cohort. Because this retrospective study was limited by sample size, comorbidity-driven mFI-5 scoring, non-standardized delirium screening, and potential residual confounding, mFI-5 should be interpreted as a convenient screening marker rather than a stand-alone predictor. Comprehensive perioperative assessment incorporating frailty, nutrition, sarcopenia, cognition, tumor burden, and treatment-related factors may better identify patients at risk.