Objective: To study socio-economic inequalities in patient-reported outcomes in primary hip and knee arthroplasty (THA/TKA) patients for osteoarthritis, using two analytical techniques. Methods: We obtained data from 44,732 THA and 30,756 TKA patients with preoperative and 12-month follow-up PROMs between 2014 and 2020 from the Dutch Arthroplasty Registry. A deprivation indicator based on neighborhood income, unemployment rate, and education level was linked and categorized into quintiles. The primary outcome measures were the EQ-5D-3L index and Oxford Hip/Knee Score (OHS/OKS) preoperative, at 12-month follow-up, and the calculated change score between these measurements. We contrasted the most and least deprived quintiles using multivariable linear regression, adjusting for patient characteristics. Concurrently, we calculated concentration indices as a non-arbitrary tool to quantify inequalities. Results: Compared to the least deprived, the most deprived THA patients had poorer preoperative (EQ-5D −0.03 (95%CI −0.02, -0.04), OHS −1.26 (−0.99, −1.52)) and 12-month follow-up health (EQ-5D −0.02 (−0.01, −0.02), OHS −0.42 (−0.19, −0.65)), yet higher mean change (EQ-5D 0.02 (0.01, 0.03), OHS 0.84 (0.52, 1.16)). The most deprived TKA patients had similar results. The higher mean change among the deprived resulted from lower preoperative health in this group (confounding). After accounting for this, the most deprived patients had a lower mean change. The concentration indices showed similar inequality effects and provided information on the magnitude of inequalities over the entire socio-economic range. Conclusion: The most deprived THA and TKA patients have worse preoperative health, which persisted after surgery. The concentration indices allow comparison of inequalities across different outcomes (e.g., revision risk).