Intra-Articular Extra-Synovial Venous-Type Synovial Haemangioma of the Knee in a Young Adult: Case Report and Literature Review of This Frequently Misdiagnosed Lesion

Objective To report a rare lesion, an intra-articular extra-synovial venous-type synovial haemangioma of the knee in a young adult and to review the relevant literature on this frequently misdiagnosed entity. Case report A 21-year-old man presented with recurrent knee effusions and pain over several years. Imaging studies revealed a well-defined, intra-articular, extra-synovial soft-tissue mass located in the anteromedial compartment of the knee. Magnetic resonance imaging (MRI) demonstrated multiple tubular structures with high signal intensity on T2- and short tau inversion recovery (STIR)-weighted images and intense, homogeneous contrast enhancement, consistent with a vascular lesion. Surgical excision was performed, and histopathological analysis confirmed a venous-type synovial haemangioma, characterized by large-calibre vascular channels lined by bland endothelial cells and containing organizing thrombi. The postoperative course was uneventful, and the patient remained disease-free at 6 months of follow-up. Conclusion Synovial haemangioma, though rare, should be considered in the differential diagnosis of recurrent knee effusions in young patients. It may mimic other benign or malignant periarticular lesions. MRI plays a key role in the evaluation, and the diagnosis can be strongly suggested by imaging, but histopathological confirmation remains essential for accurate characterization and appropriate management.