E066 Does chemical synovectomy with osmic acid have a place in current rheumatology practice?

Background/Aims Chronic synovitis, characterised by persistent inflammation of the synovial membrane, often results in pain, swelling, and progressive joint dysfunction. When conservative management fails, synovectomy may be considered to reduce synovial proliferation and improve joint function. Chemical synovectomy using osmium tetroxide (osmic acid) is a minimally invasive alternative to surgical synovectomy. With the advent of conventional, biologic and targeted synthetic disease-modifying anti-rheumatic drugs, the role of chemical synovectomy is uncertain. This study evaluates the role of intra-articular osmium acid injection in patients with chronic synovitis in current rheumatology practice. Methods A retrospective review was undertaken of patients with chronic synovitis who received osmic acid knee injections between June 2015 and August 2025 at the Royal Wolverhampton NHS Trust. Patients were identified by analysing the local patient electronic database, and their clinical notes were subsequently reviewed. The procedure involved the injection of 10 mL of 1% osmium tetroxide intra-articularly after aspiration of joint effusion and injection of 15 ml of 1% lignocaine, followed by 40mg triamcinolone acetonide. Patients were monitored for immediate adverse effects and followed up at 2 weeks, 1 month, 3 months, and 6 months post-injection. A positive clinical response was determined by resolution of the patient’s pain, absence of swelling on clinical examination, and improvement. Results We had 10 patients over the last decade diagnosed with chronic synovitis refractory to conventional therapy who underwent osmic acid injections. There were 6 females and 4 males with a mean age of 47.5 years (30-90). Affected joints included knees (n = 9 ) and shoulder (n = 1). Following first administration of osmic acid, 3 patients had no further relapses and a further 7 out of 10 patients showed no recurrence at 6 months. 2 patients had multiple injections in the same joint. The procedure was well tolerated with no significant side effects reported apart from reactive effusion in a couple of patients and skin burn in one patient. The underlying diagnosis was seronegative inflammatory arthritis or psoriatic arthritis in 9 patients, with synovial lipomatosis in 1. 5 of the patients had failed arthroscopic synovectomy. 2 of the patients moved onto biologic therapy when they developed polyarthritis and improved. Conclusion Intra-articular osmium tetroxide synovectomy is a safe and effective minimally invasive procedure for managing chronic synovitis unresponsive to conservative measures. It offers sustained symptomatic relief, improved joint mobility, and a low complication rate, making it a practical alternative to surgical synovectomy, particularly in elderly or high-risk patients who are unfit for surgery or not appropriate for biologic therapy. It also has a role in women of reproductive age with chronic synovitis in whom there is contraindication for yttrium synovectomy.