Exploring Dimensions of Stiffness in Rheumatoid and Psoriatic Arthritis: The ARAD and OMERACT Stiffness Special Interest Group collaboration

Objective It is not known how the experience of stiffness varies between diagnoses or how best to measure stiffness. The aims of this study were to: 1) compare stiffness in psoriatic arthritis (PsA) and rheumatoid arthritis (RA) using patient-reported outcomes, 2) explore how dimensions of stiffness are associated with each other and reflect the patient experience, 3) explore how different dimensions of stiffness are associated with physical function. Methods An online survey was sent to Australian Rheumatology Association Database participants (158 PsA, and 158 age- and sex-matched RA), assessing stiffness severity, duration, impact, importance, coping and physical function (mHAQ). Scores were compared between diagnoses and correlations among stiffness dimensions calculated. Multivariate regression was performed for stiffness severity, impact and duration on mHAQ, adjusting for age, sex, disease duration, obesity and pain. Cognitive debriefing was conducted via semi-structured telephone interviews. Results Overall, 240/316 (75.9%) responded (124/158 RA (78.5%) and 116/158 (73.4%) PsA), with no significant difference in stiffness ratings between diagnoses. Scores for all stiffness dimensions were strongly correlated (r=0.52-0.89) and severity and impact were associated with mHAQ in both diagnoses. Stiffness duration was not associated with mHAQ in RA. In cognitive debriefing, participants described stiffness severity and impact by the effect on daily activities (10/16 and 14/16 participants respectively). Conclusion Stiffness ratings were similar between PsA and RA. Different dimensions of stiffness were strongly correlated. Stiffness severity and impact both independently predicted mHAQ. Stiffness was important to participants, however, measuring multiple dimensions of stiffness may have minimal additive value.