Case report: Atypical presentation of rheumatoid arthritis that initially presented as osteoarthritis

Background: Rheumatoid arthritis (RA) is well-known for its inflammatory characteristics of pain, stiffness and swelling of the joint. Although it may result in any joint involvement, it usually affects the small joints in the hand, foot, wrist and shoulder. This case highlights an atypical presentation of RA which initially manifested as primary osteoarthritis (OA) with the absence of classic synovitis and emphasizes the importance in considering RA in the differential diagnoses especially, when symptoms involve joints that are atypical for OA. Case description: In this case report, a man came for an outpatient assessment with pain and stiffness of bilateral knees, for which he was being treated for chronic OA. Two total knee replacements were performed resulting in satisfactory relief of symptoms. Surprisingly, he developed new symptoms of OA involving atypical joints for OA in the form of glenohumeral joint OA and OA at the distal radioulnar joint. Although the examination findings of synovitis were initially absent, the serological diagnosis of RA was later made. Conclusion: This case serves as a reminder that RA can mimic primary OA, and serological testing should be considered in cases with unusual joint involvement even in the absence of inflammatory signs.