A rare case of deep-seated angioleiomyoma causing fixed flexion contracture of the knee: a case report.

Introduction and importance: Articular angioleiomyoma (ALM) is a rare benign tumor originating from the vascular smooth muscle layer, with very few documented cases involving joint structures. Its deep anatomical location and atypical presentation pose significant diagnostic challenges, often mimicking chronic joint pathology. This case highlights the importance of considering articular ALM in patients presenting with unexplained joint contractures and persistent pain. Case presentation: A 40-year-old female presented with a 4-year history of progressive right knee pain and fixed flexion deformity. Physical examination revealed severe medial joint line tenderness, marked muscle atrophy, and a 120° fixed flexion deformity of the right knee. She was using cane support for walking. No palpable mass was identified. Ultrasound failed to detect the lesion, which was subsequently discovered intraoperatively inside the knee joint beneath the deep medial collateral ligament. Histopathological analysis confirmed ALM. Postoperative recovery led to the complete disappearance of the pain, marked improvement in joint mobility, and the ability to walk normally without the use of an assistive device (cane). Clinical discussion: ALM typically manifests as a painful subcutaneous nodule, most frequently in the lower extremities. Articular involvement is rare and may result in significant morbidity due to delayed recognition. In such cases, standard imaging modalities like ultrasound may be insufficient, whereas magnetic resonance imaging can offer superior diagnostic ability and localization. Surgical excision remains the definitive treatment. Conclusion: Articular ALM should be considered in the differential diagnosis of unexplained joint pain and contracture. Early use of advanced imaging and heightened clinical suspicion are essential for accurate diagnosis and timely intervention.