A Rare Case of Spontaneous Elbow Osteomyelitis Presenting With Aseptic Effusion.

Spontaneous osteomyelitis of the capitellum is an exceptionally rare condition, particularly in the absence of identifiable risk factors or inciting events. This case report describes a previously healthy 20-year-old male who presented with progressive elbow pain and swelling, initially raising suspicion for septic arthritis. Point-of-care ultrasound (POCUS) identified a significant joint effusion, and subsequent arthrocentesis revealed purulent fluid with a high white blood cell count, strongly suggestive of a septic joint. The patient was treated empirically with antibiotics and underwent surgical debridement, but cultures from both the arthrocentesis and the surgical washout were negative. Further imaging with MRI ultimately revealed findings consistent with osteomyelitis of the capitellum. This case highlights the diagnostic challenges associated with musculoskeletal infections, particularly in atypical sites like the capitellum, and underscores the importance of advanced imaging modalities such as MRI when initial diagnostics are inconclusive. The report also emphasizes the utility of POCUS in identifying joint effusions and guiding arthrocentesis in the emergency setting. Early recognition and appropriate management of osteomyelitis are crucial to avoid serious complications such as chronic infection, joint dysfunction, or limb deformity. This case contributes to the limited literature on capitellum osteomyelitis, particularly in the setting of an aseptic effusion, and advocates for the integration of multidisciplinary approaches and advanced diagnostic tools in the evaluation of pediatric and young adult musculoskeletal infections. Further research is needed to better understand the pathophysiology and management of rare presentations like this one.