Nonspecific Knee Synovitis Caused by Foreign Bodies Undetectable on Imaging Studies: Four Cases and Literature Review.

Diagnosing and managing acute knee monoarthritis in children presents a complex challenge, and when an etiology is not clear, it might be the result of an occult presence of a radiolucent foreign body. We report on four instances of acute monoarticular knee arthritis in pediatric patients. Despite employing standard imaging techniques, patient history, and laboratory tests, no foreign bodies (FBs) were initially identified. However, arthroscopy conducted in all cases, unveiled FBs that might have otherwise gone unnoticed and led to proper treatment of the problem. Despite their rarity, a unique challenge arises in this population due to the deceptive presentation of symptoms. The often-inconspicuous nature of the injury, combined with negative radiographs and nonspecific soft-tissue inflammation, on MRI frequently leads to delayed diagnosis and treatment. Our study and broader literature highlight the limitations of standard imaging in detecting FBs, especially in children. In cases of persistent or recurring knee pain, a high index of suspicion for undetected FBs and prompt arthroscopic evaluation are paramount for achieving optimal outcomes in children with otherwise unexplained monoarticular knee arthritis.