Objective: To compare arthroscopic synovectomy with deposition of intra-articular corticosteroids (DIACS) versus intra-articular injection of corticosteroids (IACSI) in RA and PsA patients with mono- or oligoarthritis of the wrist that is refractory to cDMARD therapy. Study design: Multi-centre randomized controlled trial conducted in the Maasstad Hospital and Spijkenisse Medisch Centrum (SMC). Study population: Patients with active RA or PsA and bDMARD-naive, who develop a localized flare of disease activity (mono- or oligoarthritis) that involves the wrist, defined as an increase in DAS28 \> 1.2 or \> 0.6 if DAS28 ≥ 3.2 compared to the last DAS28 measurement (maximum six months before) and that is refractory to systemic cDMARD for at least three months, defined as no response on the European League Against Rheumatism (EULAR) response criteria. Intervention: This study will randomize between IACSI of the wrist (control) and arthroscopic synovectomy of the wrist combined with DIACS (intervention). During arthroscopy synovial biopsies will be collected and stored for later analysis of the functional and histological characteristics of the synovium (beyond the scope of this study). Main study parameters/endpoints: Primary outcome is the change in Patient-Rated Wrist Evaluation (PRWE) score from randomization to three months of follow-up. The PRWE is a validated, fifteen-item self-reported questionnaire rating wrist pain and function. Secondary outcomes are resolution of wrist arthritis measured by ultrasound, standard wrist radiographs, DAS28, EULAR response rate, Visual Analogue Scale (VAS), EQ-5D quality of life questionnaire, iMTA Productivity Cost Questionnaire (iPCQ), iMTA Medical Consumption Questionnaire (iMCQ), cost effectiveness analyses (CEA), physical examination, adverse events (AE) and laboratory results. Follow-up visits are scheduled at three, six and twelve months after intervention. Nature and extent of the burden and risks associated with study participation: Both study arms include standard treatment of care. Wrist arthroscopy is a standard treatment for wrist arthritis and often implemented for other intra-articular wrist pathology. The risks include infection, neurovascular damage and articular surface damage. Nevertheless, wrist arthroscopy is a well-established and safe technique. Reduction of risks will be done according to inclusion and exclusion criteria. If complications arise, the treating physician will proportionate the adequate treatment according to the current protocols based on the published literature. Patients will be asked to return at three, six and twelve months. These visits are standard of care following the rheumatic arthritis protocol. Patients will be asked to complete questionnaires at baseline and at three follow-up moments. These will take 160 minutes in total. The arthroscopy group will return between ten to fourteen days for wound inspection. All patient will be contacted by telephone at two, four and six weeks for VAS pain scores. Expected results: We expect that arthroscopic synovectomy followed by DIACS will lead to significantly more improvement in PRWE scores compared to IACSI three months after intervention. Furthermore, we anticipate that wrist arthroscopy will result in lower pain scores, better joint mobility, better response on EULAR score, sustained resolution of arthritis on ultrasound, less joint damage on radiographs and is more cost-effective after one-year analysis.