Teaching Case

1 year of pain in right thigh. Right leg 1cm longer with muscle wasting. Normal ESR.

Presentation with a 1 year history of limping and pain in the right thigh. The onset of symptoms coincided with a fall and the child had been taking regular ibuprofen for the pain with some benefit. No past history of note. On examination there was limitation of movement at the right hip and muscle wasting in the right thigh and calf. The right leg was also noted to be 1cm longer than the left. A full blood count and ESR were both normal. A plain radiograph (fig 1) of the right hip showed features highly suggestive of osteoid osteoma in the femoral neck and CT (fig 2) confirmed this.

The differential diagnosis in the limping child with proximal leg pain is wide eg. septic arthritis, transient synovitis, osteomyelitis, JCA, slipped capital femoral epiphysis, fracture, Legg-Calve Perthes disease, metabolic bone disease, secondary neoplasms such as leukaemia, lymphoma, neuroblastoma, and malignant and benign primary neoplasms. In this case, the radiographic abnormalities would be in keeping with a differential diagnosis of osteoid osteoma, osteomyelitis, and perhaps osteoblastoma. However, the history in this case is against infection although osteomyelitis can cause confusion with osteoid osteoma as both conditions may cause a raised ESR. Osteoblastomas can sometimes cause a purely sclerotic appearance, but are less common than osteoid osteoma in the long bones, and are particularly rare in the upper femur. As such, the combined clinical and radiological features of this case are highly suggestive of osteoid osteoma. The classical radiographic features of osteoid osteoma are of an area of cortical thickening and sclerosis, containing a lucent nidus of less than 1cm. When these features are present, the diagnosis is almost certain although the nidus may sometimes calcify and become invisible. When the lesion is subperiosteal, most commonly in the neck of femur, the sclerotic response may be much more limited making diagnosis difficult. Occasional periosteal osteoid osteomas cause marked periosteal reaction. Typically osteoid osteoma occurs under 30 years of age and cases have been described where there is associated limb overgrowth. Confirmation of the diagnosis is best achieved by CT, which accurately demonstrates the low attenuation nidus within the area of cortical thickening. Isotope bone scanning shows increased activity in all three phases but is non specific unless there is the classical ‘double density sign’ of very intense central nidus uptake with a peripheral area of less marked activity (fig 3). In addition, it cannot provide the information for surgical planning that CT can. MRI is less useful than CT in detection of the nidus. MRI also shows marrow and soft tissue oedema which can cause confusion with malignant tumours and osteomyelitis.