Purpose: The disease activity score including 28 joints (DAS28), the simplified disease activity index and the clinical disease activity index (CDAI) were developed in order to provide a quantifiable measure of rheumatoid arthritis (RA) activity. Although inflamed hip joints greatly impact activities of daily living (ADL) and walking ability, the hip joint was not included in the DAS28, SDAI or CDAI assessments. Although excellent clinical results have been reported for total hip arthroplasty (THA) in RA patients, correlations between disease activity and hip function in RA patients after THA remain unknown.
Methods: We analysed the effect of RA disease activity on a hip function score in an observational cohort of RA patients after THA. Twenty-five registered RA patients who had undergone THA (33 joints) were included. Hip function was recorded and RA disease activity was measured on the same day. The mean age of the patients was 65.17 years. They were followed up for a mean of 5.24 years after surgery. The mean duration of disease following RA diagnosis for this patient group was 19.47 years. The Japanese Orthopaedic Association (JOA) hip score was used as a clinical outcome measure for hip dysfunction. RA disease activity and health-related quality of life were measured using the DAS28, SDAI, CDAI and the modified health assessment questionnaire (MHAQ).
Results: The mean JOA score for hip function was 80.48 at the final follow-up. The mean DAS28-ESR, DAS28-CRP, SDAI, CDAI and MHAQ measuring RA disease activity levels were 3.38, 2.65, 9.59, 8.63 and 0.44, respectively, at the final follow-up. There was a significant negative correlation between the JOA hip score and all disease activity assessments observed after THA (DAS-ESR [P = 0.0067], DAS-CRP [P = 0.0008]), SDAI [P = 0.0034], CDAI [P = 0.0003]) and MHAQ [P = 0.0002]).
Conclusion: We found significant negative correlations between JOA hip scores and all disease activity assessments in RA patients treated with THA.