Teaching Case

A 36-year-old man presented with increasing right hip pain for four months. The pain showed increasing severity at night and worsened with impingement manoeuvers. There were no walk complaints and no trauma history.

A MR examination was performed first. The most striking anomaly is the presence of marked bone marrow oedema in the right acetabulum and iliac bone (Fig. 1). We depicted an apparent osteochondral lesion in the right acetabulum (Fig. 2) which seems to be in the centre of the oedema. It seems to present a tiny mineralised element (Fig. 3) and shows intense peripheral gadolinium enhancement (Fig. 4). These findings are suggestive of an intra-articular osteoid osteoma. A small Brodie’s abscess could present some of these characteristics, although they are usually hyperintense on T2 images and there are no other findings suggesting infection/osteomyelitis. Small articular fluid that could be due to synovitis was found.
A CT examination was subsequently performed which allowed the detection of an area of high attenuation centrally, representing mineralised osteoid (intracortical nidus) (Fig. 5). Also, reactive sclerosis was apparent, and left hip articulation was unremarkable.

A.
Osteoid osteoma (OO) is a benign bone tumour most frequently found in men between 7 and 25 years of age, and is composed by a nidus with a central variable amount of calcification, fibrovascular rim and surrounding reactive sclerosis [1].
Recently OOs are categorised as intracortical (most common), subperiosteal, endosteal or intramedular. Femur and tibia are the most common locations (50%), with approximately 30% occurring in the spine, hands or feet. Skull, scapula, ribs and pelvis are rare locations [2].
Intra-articular OO, which occurs within or near a joint, is considered a rare separate clinical entity [3]. The most commonly involved joint is the hip. Ankle, elbow, wrist, and knee are less commonly affected.

B.
Clinical manifestations of intraarticular OO may be misleading. Pain is not necessarily worse at night and joint tenderness and effusion may be prominent, mimicking an inflammatory arthropathy or synovitis.
An accelerated growth may occur if the OO is located near the growth plate in kids.

C.
Typical radiographic findings include a small (<2cm) radiolucent round intracortical nidus, accompanied by cortical thickening and reactive sclerosis [4, 5].
At CT the nidus is a well-defined round area of low attenuation or/with centrally high attenuation due to mineralised osteoid [6]. It presents low-to-intermediate SI on MR T1 and variable SI on T2-weighted images.
Enhancement of a hypervascular nidus may be seen at CT and MRI [7].
Several studies have shown that, compared with CT, MR imaging is of limited value in depicting the nidus [8, 9], although oedema, joint effusion and synovitis are better appreciated.
With intra-articular OO, reactive cortical thickening is typically minimal [10].
Scintigraphy generally shows focal uptake and sometimes reveals a highly specific double density sign.
On ultrasound a focal cortical irregularity with adjacent hypoechoic synovitis may be present. The nidus may be depicted as a round hypoechogenicity, hypervascular on Doppler.
Stress fracture, intracortical abscess, cortical desmoid and chondro/osteoblastomas may mimic OO.

D.
OO is benign, and treatment includes surgical resection or percutaneous radiofrequency ablation under CT or US guidance.
There is growing evidence that osteoid osteoma finally resolves spontaneously over time (average 33 months) and can be treated conservatively with NSAIDs [11].

E.
Intra-articular OO is rare and considered a separate clinical entity. The unusual location may complicate the diagnostic process. The most commonly involved joint is the hip and clinical manifestations may be puzzling. Enhanced CT is the modality of choice since MRI is of limited value in depicting the nidus, although it can identify bone oedema, joint effusion and synovitis.