Septic Arthritis of the Manubriosternal Joint in an Immunocompetent Adult: A Case Report.

Septic arthritis of the manubriosternal joint (MSJ) is an exceptionally rare clinical entity, particularly in immunocompetent individuals without established predisposing risk factors. Due to its rarity and non-specific presentation, diagnosis is frequently delayed or initially mistaken for more common causes of anterior chest pain and chest wall swelling, including cellulitis, musculoskeletal pain, or cardiopulmonary pathologies.  We report the case of a 71-year-old immunocompetent man who presented with a two-week history of sharp central chest pain and progressive anterior chest wall swelling following a preceding flu-like illness. He had initially been managed in primary care with two courses of oral antibiotics for presumed cellulitis without clinical improvement. On hospital admission, examination demonstrated a tender erythematous swelling over the manubriosternal region with markedly elevated inflammatory markers (C-reactive protein 224 mg/L). Computed tomography (CT) imaging demonstrated inflammatory soft tissue changes both superficial and deep to the manubriosternal joint, raising suspicion for septic arthritis. Although blood cultures were negative. Further investigations, including magnetic resonance imaging (MRI) of the thoracic spine and transthoracic echocardiography, excluded alternative infective foci. Following multidisciplinary discussion involving acute medicine, microbiology, radiology, and cardiothoracic surgery teams, the patient was managed conservatively with prolonged intravenous and oral antibiotic therapy, resulting in significant clinical, biochemical, and radiological improvement over four weeks without surgical intervention.  This case highlights the diagnostic challenges associated with this rare condition and underscores the importance of early imaging, multidisciplinary assessment, and prompt antimicrobial therapy in achieving favourable outcomes.