Intra-articular post-traumatic ankle joint mass imitating localized pigmented villonodular synovitis (LPVS), the aid of ankle arthroscopy for diagnosis and treatment—A case report

•An intra-articular posttraumatic hematoma within the ankle joint may mimic LVNS.•Both pathologies appear with hemosiderin deposits on MRI.•Ankle arthroscopy may be helpful both for diagnostic and therapeutic purposes.

Intra-articular post-traumatic ankle joint mass is a rare entity that may mimic other pathologies, mainly localized form of pigmented villonodular synovitis (LPVS) regarding the clinical and imaging characteristics. We report the case of a 16-year-old female patient that presented an intra-articular ankle joint mass 8 months after an ankle joint sprain for which magnetic resonance imaging (MRI) suggested LPVS as possible diagnosis due to the presence of hemosiderin deposits. Diagnosis of a post-traumatic hematoma of her ankle joint was made via fine needle aspiration (FNA) biopsy and anterior ankle arthroscopy. At one-year-follow-up after the arthroscopic excision of the hematoma, the patient remained asymptomatic and pain free while MRI revealed no pathologic findings. This case demonstrates that LPVS is not always the diagnosis when hemosiderin deposits are depicted on the MRI of a solitary intra-articular mass. The FNA biopsy under direct arthroscopic view assists the diagnosis and guides the treatment plan in cases that no definite diagnosis has been reached preoperatively by MRI.Level of evidence IV, case report.