Age / Sex : 9 / M

Chief complaint : Left hip pain for 3months

What is your diagnosis?

Two weeks later, you can see the final diagonosis with a brief discussion of this case (Please submit only one answer).

Courtesy : Eun Jung Shim/Hye Won Chung, University of Ulsan College of Medicine, Asan Medical Center

Discussion

Answer:

Osteoid osteoma  

Findings:

Plain radiograph shows subtle regional osteopenia with cortical obliteration at left acetabulum.

Coronal T2-weighted fat-suppressed and T1-weighted MR images show large amount of left hip joint effusion and extensive bone marrow edema at left acetabulum. A small nodular bony lesion with central dark signal intensity is suspected at the left ischium, near the triradiate cartilage. On gadolinium-enhanced T1-weighted fat-suppressed MR image shows diffuse enhancement at left acetabulum, adjacent left obturator internus muscles and synovial enhancement at left hip joint. These findings suggest the possibility of osteomyelitis with infectious arthritis. In joint fluid analysis, the cell differentiation of the aspirated fluid showed nucleated cells count of 1342cells/uL (3% of neutrophils). C-reactive protein(CRP) was less than 0.2mg/L(normal:<0.5mg/L) and erythrocyte sedimentation rate(ESR) was 5mm/hr (normal range:<20mm/hr). Because these findings did not suggest infection, CT was performed additionally. Coronal reformatted pelvis CT image shows focal bony lesion of left ischium, which suggests a hypoattenuating nidus with central mineralization and reactive periosteal bone formation. Final imaging diagnosis was osteoid osteoma of left acetabulum. Patient underwent arthroscopic mass excision and intraoperative arthroscopic view shows ovoid-shaped intraosseous nidus at acetabulum. The lesion was confirmed pathologically as osteoid osteoma.

Differential Diagnosis:

Osteomyelitis

Infectious arthritis including tuberculous arthritis

Diagnosis:

Osteoid osteoma  

Discussion:

Osteoid osteoma is a benign bone tumor that occurs most frequently in men and boys between 7 and 25 years old. Most patients experience pain that worsens at night and is promptly relieved by the administration of salicylates. They usually occur in the shaft of the long bones, especially the femur and tibia. Intra-articular osteoid osteoma accounts for approximately10% of all osteoid osteomas and mainly arises in the elbow, ankle or hip joints. Osteoid osteoma is not common at acetabulum.

Typical radiographic findings of osteoid osteoma include an intracortical nidus, which may display a variable amount of mineralization, accompanied by cortical thickening, reactive sclerosis and bone marrow edema. In intra-articular osteoid osteoma, standard radiographs only provide subtle findings due to the absence of any perilesional sclerosis or periosteal reaction, unlike extra-articular locations

MR imaging depicts not only the nidus and accompanying sclerosis but also adjacent bone marrow and articular abnormalities. The nidus has low to intermediate signal intensity on T1-weighted images and variable signal intensity on T2-weighted images, depending on the amount of mineralization present in the center of the nidus. Edema in adjacent bone marrow and soft tissue and joint effusion also may be seen. However, sometimes nidus can be masked by the severe bone marrow edema or combined joint effusion on MR images. In such cases, CT imaging is helpful to detect nidus.

At CT, the nidus is well defined and round or oval with low attenuation. An area of high attenuation may be seen centrally, a finding that represents mineralized osteoid.

However, osteoid osteoma may display imaging findings that can be misleading, and it can be difficult to differentiate osteoid osteoma from other conditions such as infection, inflammatory and noninflammatory arthritis, and other tumors. In addition, stress fracture, intracortical abscess, intracortical hemangioma, chondroblastoma, osteoblastoma, and compensatory hypertrophy of the pedicle may mimic osteoid osteoma.

References:

Jee Won Chai et al. Radiologic Diagnosis of Osteoid Osteoma: From Simple to Challenging Findings. RadioGraphics 2010;30:737–749

J.Scalici et al. Intra-articular osteoid osteoma of the hip misdiagnosed by MRI: An unusual cause of unexplained hip pain. orthopaedics & Traumatology: Surgery & Research (2011)97, 881—885