Unusually located osteoid osteomas.

The files of 12 patients (aged 12-33 years) with an equal number of surgically proven osteoid osteomas (OOs) were reviewed in attempt to find a diagnostic algorithm in cases of unusually located OOs. Plain radiography (PR) and thin collimation computed tomography (CT) had been performed in all patients, while bone scintigraphy (BS) had been performed in eight and magnetic resonance imaging (MRI) in two. The OOs were located at juxta- or intra-articular sites, except for one located at the left neck of the L4 vertebra. The diagnosis based on the MRI examinations was synovitis. BS showed increased accumulation of the radioisotope at the site of the lesions, without the 'double density' sign. PR showed the nidus of OO in only six patients, whereas CT located the nidus in all patients. In conclusion, we believe that when an OO is clinically suspected at an unusual location, CT should be performed in all cases, even when a lesion is depicted by PR and BS, because CT will not only locate the nidus but will also provide a precise anatomy of the area around the nidus and help in therapeutic decision making and surgical planning. MRI can be misleading and must not be used in the initial assessment of a possible osteoid osteoma.