When one joint matters: the monoarticular blind spot in inflammatory arthritis

Monoarthritis is a familiar-yet systematically under-theorised-scenario in rheumatoid arthritis and psoriatic arthritis, occurring both at onset and as residual synovitis that persists despite apparently successful treat-to-target care. Contemporary composite indexes, trial eligibility criteria, and guideline algorithms were built for polyarticular disease; as a result, these tools can numerically dilute the functional and prognostic weight of a single inflamed knee, ankle, or dominant wrist. This situation results in an uncomfortable paradox: overall disease activity can meet the predefined therapeutic target (eg, remission or low disease activity), while the patient remains substantially symptomatic or functionally limited because of persistent inflammation in a single joint. When the burden is focal, the usual reflex of escalating systemic immunosuppression deserves scrutiny, because the hazards of escalation do not shrink with joint count, whereas the incremental benefit often does. In this Viewpoint, we argue that monoarthritis is not merely a measurement nuisance but a decision problem. We examine why monoarthritis remains poorly captured by current measurement systems, trial paradigms, and treatment frameworks. We also highlight key research priorities to make monoarthritis visible again-both epidemiologically and methodologically-including index-joint phenotyping, joint-centred outcomes, and comparative evaluation of local versus systemic treatment strategies.