IntroductionWe have limited knowledge about the underlying disease mechanisms and causes of pain in hand osteoarthritis (OA). Consequently, no disease-modifying drug exists, and more knowledge about the pathogenesis of hand OA is needed, as well as a validation of different outcome measures. Our first aim of this study is to explore the validity of various imaging modalities for the assessment of hand OA. Second, we want to gain a better understanding of the disease processes, with a special focus on pain mechanisms.Methods and analysisThe Nor-Hand study is a hospital-based observational study including 300 patients with evidence of hand OA by ultrasound and/or clinical examination. The baseline examination consists of functional tests and joint assessment of the hands, medical assessment, pain sensitisation tests, ultrasound (hands, acromioclavicular joint, hips, knees and feet), CT and MRI of the dominant hand, conventional radiographs of the hands and feet, fluorescence optical imaging of the hands, collection of blood and urine samples as well as self-reported demographic factors and OA-related questionnaires. Two follow-up examinations are planned. Cross-sectional analyses will be used to investigate agreements and associations between different relevant measures at the baseline examination, whereas the longitudinal data will be used for evaluation of predictors for clinical outcomes.Ethics and disseminationThe protocol is approved by the Norwegian Regional Committee for Medical and Health Research Ethics (Ref. no: 2014/2057). The participants receive oral and written information about the project and sign a consent form before participation. They can, whenever they want, withdraw from the study, and all de-identified data will be safely stored on the research server at Diakonhjemmet Hospital. Results will be presented at international and national congresses and in peer-reviewed rheumatology journals.Trial registration numberNCT03083548; Pre-results.

Objective To examine the association of body mass index (BMI) with pain in people with hand osteoarthritis (OA), and explore whether this association, if causal, is mediated by systemic inflammatory biomarkers. Methods In 281 Nor‐Hand study participants, we estimated associations between BMI and hand pain, as measured by the Australian/Canadian Osteoarthritis Hand Index (AUSCAN; range 0–20) and Numerical Rating Scale (NRS; range 0–10); foot pain, as measured by NRS (range 0–10); knee/hip pain, as measured by the Western Ontario and McMaster Universities Osteoarthritis Index (WOMAC; range 0–20); painful total body joint count; and pain sensitization. We fit natural‐effects models to estimate natural direct and natural indirect effects of BMI on pain through inflammatory biomarkers. Results Each 5‐unit increase in BMI was associated with more severe hand pain (on average increased AUSCAN by 0.64 [95% confidence interval (95% CI) 0.23, 1.08]), foot pain (on average increased NRS by 0.65 [95% CI 0.36, 0.92]), knee/hip pain (on average increased WOMAC by 1.31 [95% CI 0.87, 1.73]), generalized pain, and pain sensitization. Mediation analyses suggested that the effects of BMI on hand pain and painful total body joint count were partially mediated by leptin and high‐sensitivity C‐reactive protein (hsCRP), respectively. Effect sizes for mediation by leptin were larger for the hands than for the lower extremities, and were statistically significant for the hands only. Conclusion In people with hand OA, higher BMI is associated with greater pain severity in the hands, feet, and knees/hips. Systemic effects of obesity, measured by leptin, may play a larger mediating role for pain in the hands than in the lower extremities. Low‐grade inflammation, measured by hsCRP, may contribute to generalized pain in overweight/obese individuals.

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