The Influence of Frailty on Total Hip Arthroplasty Outcomes: A Propensity-Matched Study of 90,660 Patients Using the Modified Frailty Index

Background: Frailty is a key predictor of adverse surgical outcomes in older adults. However, the prognostic utility of the 5-factor modified frailty index (mFI-5) for short- and long-term outcomes following total hip arthroplasty (THA) remains inadequately defined. This study assessed the association between frailty severity, measured by the mFI-5, and postoperative complications, implant survivorship, and mortality following primary THA in a large national cohort. Methods: This retrospective cohort study included 90,660 patients aged ≥50 years undergoing primary THA for osteoarthritis from 2003 to 2020 using the TriNetX research network. Patients were stratified by frailty severity based on mFI-5 scores: non-frail (0–1), moderately frail (2), and severely frail (≥3). Cases of fractures, polytrauma, or falls were excluded. Pairwise propensity score matching was adjusted for age, sex, race, and BMI. Outcomes included 90-day medical and surgical complications, healthcare utilization, and 2- and 5-year THA revision and mortality rates. Risk ratios (RRs) with 95% confidence intervals (CIs) and Bonferroni-corrected significance thresholds (p < 0.0167) were reported. Results: Severely frail patients had significantly increased risks of 90-day mortality (RR 4.41, 95% CI 2.22–8.74), acute kidney injury (RR 2.92), myocardial infarction (RR 3.61), and periprosthetic joint infection (RR 2.02) compared to non-frail patients. At five years, severely frail patients demonstrated a 58% higher revision risk (RR 1.58) and 23.0% mortality versus 6.9% in the non-frail cohort. A dose-dependent risk gradient was observed, with moderately frail patients exhibiting intermediate risks across all outcomes. Conclusions: Frailty severity, as measured by the mFI-5, was associated with a stepwise increase in short- and long-term complications and mortality following THA. The mFI-5 may serve as a practical, scalable tool for preoperative risk stratification, counseling, and resource planning in older adults undergoing primary THA.