Introduction:
Infection of the sternoclavicular joint is a rare condition, representing less than 1% of all bone infections. Most patients who develop this condition have known risk factors, such as immunosuppression, diabetes, intravenous drug use, or rheumatoid arthritis1. The acquisition of MRSA-positive septic arthritis can be attributed to various factors and sources. Staphylococcus aureus, including methicillin-resistant strains, is a predominant pathogen responsible for a significant proportion of septic arthritis cases, with MRSA accounting for 6% to 22% of S. aureus isolates2. Particularly injection drug users are at a higher risk for MRSA infections and joint complications, including septic arthritis, though this association is relatively rare and lacks proper documentation3. This case report details a rare presentation of MRSA-positive septic arthritis in a patient with a history of multiple intravenous (IV) and intramuscular (IM) injections. The rarity of this case is the presence of a mass in the left sternocleidomastoid muscle extending into the left superior mediastinum with associated lymphadenopathy, giving the impression of a mediastinal malignancy. Radiology revealed sternoclavicular septic arthritis. The involvement of the sternoclavicular joint in septic arthritis is uncommon, and when coupled with atypical presentations mimicking other conditions like mediastinal masses, the diagnosis becomes challenging4. The combination of MRSA-positive septic arthritis, sternoclavicular joint involvement, mediastinal mimicry, and a history of multiple injections creates a unique and rare clinical scenario that requires careful evaluation and management5.