Atypical location of an osteoid osteoma with atypical anterior knee pain

INTRODUCTION: An osteoid osteoma is a painful tumor that most commonly affects the extra-articular parts of the long bones. An intra-articular location of an osteoid osteoma is rare. Various differential diagnoses may arise in connection with such an unusual location because it causes atypical clinical signs. PRESENTATION OF CASE: A 24-year-old male developed pain in the central region of the right knee. Magnetic resonance imaging (MRI) showed no clear pathology in the knee joint. A technetium bone scan and computed tomography (CT) were then ordered and confirmed the presence of an osteoid osteoma in the knee joint. The patient was treated through an anteromedial approach to the knee, and the lesion was removed by excisional biopsy under fluoroscopy. DISCUSSION: The diagnosis of intra-articular osteoid osteoma is challenging because the clinical presentation can be misleading. MRI is often requested as the first imaging method when dealing with knee symptoms, and radiologists are often unaware of the clinical presentation. Edema seen on MRI can be misleading with respect to the location of the nidus. CT is considered to be the best imaging method because it usually allows for clear visualization of the nidus. Different treatments have been proposed, ranging from open excision to arthroscopic resection. CONCLUSION: Osteoid osteoma should be considered in young adult patients with chronic knee pain and no history of trauma.