Swollen, but not tender joints, are independently associated with ultrasound synovitis: results from a longitudinal observational study of patients with established rheumatoid arthritis.

Objectives: Joint swelling and tenderness are considered a proxy for inflammation in patients with rheumatoid arthritis (RA). With ultrasound-detected inflammation as reference, our objectives were to explore on patient and joint level the associations between ultrasound synovitis and joint swelling, tenderness and patient-reported joint pain (PRJP).

Methods: 209 patients with established RA were examined six times during 12 months with assessment of 32 joints in upper/lower extremities for joint swelling/tenderness and Grey scale (GS)/power Doppler (PD) synovitis. PRJP was assessed on a manikin. Correlations between different sum scores were at each examination calculated using Spearman's rho (r), agreement at joint level was examined by Cohen's kappa and logistic regression models were used to explore the associations between joint assessment and GS/PD scores.

Results: At patient level, swollen joints were strongly correlated with GS/PD sum scores (r=0.64-0.88), while tender joints were primarily associated with PRJP (r=0.54-0.68). At joint level, GS/PD pathology had higher agreement with swelling (kappa 0.54-0.57) than tenderness (kappa 0.20-0.21) or PRJP (0.23-0.25). Higher percentages of joints were swollen according to increasing GS/PD scores, independently of joint tenderness. However, joints being tender, but not swollen, were not associated with GS/PD scores. Receiver operating curves showed swollen but not tender joints to be associated with GS/PD scores.

Conclusions: Swollen joints were strongly associated with ultrasound detected synovitis at both patient and joint level, while this association was not found for tender joints. These results may question if tender joints reflect ongoing inflammation in established RA.