Title : Sterile inflammatory synovitis mimicking septic arthritis following intra-articular polyacrylamide hydrogel (arthrosamid) injection: A case report
Abstract:
Background: Arthrosamid® is a non-biodegradable 2.5% cross-linked polyacrylamide hydrogel used as a long-acting intra-articular implant for knee osteoarthritis. Regulated as a medical device and considered biocompatible, serious adverse events are reported in under 1%. Sterile inflammatory reactions mimicking septic arthritis are rarely described.
Case Presentation: A 56-year-old woman with Kellgren–Lawrence grade III osteoarthritis presented with right knee pain and swelling ten days after an intra-articular Arthrosamid® injection, symptoms having begun four days post-injection and progressively limiting ambulation. She had received three prior cycles of hyaluronic acid injections and had no diabetes, immunosuppression or recent infection. She was afebrile with a circumferential effusion, positive patellar tap and medial joint-line tenderness, but no erythema or warmth; flexion was limited to 90° with a 10° extension deficit. Leucocytes were 9.5×10?/L (76% neutrophils), CRP 337 mg/L and ESR 78 mm/hr. Two arthroscopic washouts one week apart revealed turbid fluid and hypertrophic synovium without purulence. Aerobic, anaerobic and fungal cultures were repeatedly sterile and polarised microscopy excluded crystal arthropathy. Histology demonstrated chronic lymphoplasmacytic synovitis with foreign-body-type giant cells. Empirical intravenous flucloxacillin was given pending results, with CRP normalising by four weeks. MRI showed persistent synovial thickening and mild effusion. Following multidisciplinary review, antibiotics were discontinued and the patient referred to immunology, with synovectomy planned if symptoms recurred.
Conclusion: Markedly raised inflammatory markers following polyacrylamide hydrogel injection may reflect a foreign-body-type immune reaction rather than infection. Sterile cultures combined with giant-cell synovitis on histology should prompt consideration of aseptic synovitis, avoiding prolonged unnecessary antibiotic exposure. As intra-articular hydrogel use expands, particularly in private outpatient settings, awareness of this mimic and early multidisciplinary input are essential.
Keywords: Polyacrylamide hydrogel, Arthrosamid, Synovitis, Septic arthritis, Foreign-body reaction, Knee osteoarthritis.
