Retrospective Cohort. The objective of this study was to analyze postoperative complications in different mFI-11 groups after surgery for odontoid fractures in a geriatric population. A single center retrospective review of odontoid fractures surgery (between 2013 and 2022) in patients aged 65years and older was conducted. The primary outcome was the occurrence of a major complication (Calvien-Dindo ≥4) within 30days post-surgery. The secondary outcome was the occurrence of a major complication within 3months after surgery, and death within 1-month post-surgery. Survival curve, multi-variate analysis was performed and adjusted receiver operating characteristic curves were generated. There were 92 patients included in this study, with a mean age of 80.5years. Serious complication occurred for 16 patients (17%) during hospitalization. Multivariate analysis demonstrated an mFI 11 >.27 was strongly and independently associated with serious complications within 1-month post-surgery (OR = 16.7, 95% CI = 4.50-83), as well as serious complications within 3months post-surgery (OR = 11.8, 95% CI = 3.48-49.1) and death within 1month post-surgery (OR = 11.7; 95% CI = 3.02-60.4). The Receiver Operator Characteristics (ROC) curves for the three models all have an Area Under the Curve (AUC) value greater than 0.7. The mFI-11 is a straightforward and validated tool that can be used during the preoperative period to identify the patient's level of frailty and assess their risk of postoperative complications. Patients with mFI-11 ≥.27 are at greater risk of serious complications within 1 and 3months' post-surgery and death within 1month post-surgery.