Income Inequality Quantified by the Gini Index Is an Indicator for Adverse Total Knee Arthroplasty Outcomes

Postoperative outcomes following total knee arthroplasty (TKA) may vary due to nonmedical factors, including social determinants of health. While composite indices such as the Area Deprivation Index and Social Vulnerability Index link community disadvantage to adverse outcomes, the role of income inequality is underexplored in orthopaedics. The Gini Index (GI), a measure of income disparity, may provide a focused approach to assess socioeconomic influence on surgical outcomes. This study investigates whether a patient's GI is associated with short-term complications after TKA.

A large national database was queried to identify adults who underwent primary TKA from 2017 to 2021 via International Procedural Code-10th Edition codes. Patients were categorized by GI into low (< 0.40), average (0.40 to 0.49), and high (≥ 0.50) groups. Outcomes included revision rates, mechanical complications, and infectious complications within one year postoperatively. Propensity score matching was performed to control for demographic differences.

Patients in the high-GI cohort had significantly increased rates of all-cause revision at 90 days (0.9 versus 0.6%, odds ratio (OR) 1.42, P = 0.01) and one year (1.5 versus 1.1%, OR 1.46, P < 0.01) compared to the average-GI group. Dislocation rates were also elevated at 90 days (2.6 versus 1.7%, OR 1.54, P < 0.001) and one year (3.3 versus 2.1%, OR 1.58, P < 0.001 in the high-GI group. There were no differences observed in revisions or mechanical complications in the low-GI group. Infection rates were similar across all GI categories.

Patients assigned a GI index greater than the national average demonstrated a higher risk for TKA complications, while there were no differences in patients who had a GI lower than the national average. The degree of income inequality in a community may be an influential social determinant of health driving variance in orthopaedic outcomes.