Baker's Cyst with Atypical Superior Extension in a Patient with Gout: A Case Report

Background Baker's cyst is a synovial fluid-filled distension of the gastrocnemius-semimembranosus bursa, commonly associated with inflammatory and degenerative disorders of the knee. It typically extends inferiorly into the calf, whereas superior extension into the posterior thigh is rare and may mimic soft tissue neoplasms or infective collections. Case Presentation A 55-year-old man with chronic kidney disease and poorly controlled gout presented with a painless swelling over the posterior aspect of the left knee extending into the posterior thigh for one month. Serum uric acid was 9.2 mg/dL, while inflammatory markers (ESR and CRP) were elevated. Synovial fluid analysis demonstrated negatively birefringent monosodium urate crystals, whereas cytology, Gram stain, bacterial culture, and Cartridge-Based Nucleic Acid Amplification Test (CB-NAAT) for tuberculosis were negative. Magnetic resonance imaging demonstrated a large multiloculated Baker's cyst communicating with the knee joint and extending superiorly for approximately 25 cm into the posterior thigh. The patient was treated with ultrasound-guided therapeutic aspiration, oral prednisolone, and febuxostat. At three-month follow-up, the swelling had markedly reduced, inflammatory markers normalized, serum uric acid decreased to 7.0 mg/dL, and the patient remained symptom-free. Conclusion This case highlights the rare occurrence of a gout-associated Baker's cyst with extensive superior extension into the posterior thigh. MRI plays a pivotal role in defining the extent of the lesion and excluding important differential diagnoses. Early recognition and treatment of the underlying inflammatory disorder can result in excellent clinical outcomes without surgical intervention.