Osteoid osteoma is a common benign bone lesion in the pediatric and young adult populations. Typical age of patients ranges from five to 40 years old, with reports of patients from nine months to 77 years of age reported in the literature.6
Males are more commonly affected at a 1.6–4 ratio.3
Long bones are the most common location for these lesions with the majority occurring in the lower extremity.7
Presenting symptoms in children are often pain, which is worse at night and relieved with salicylates,3
and a noticeable limp.6
The location of these lesions within the bone may vary. The classic and most common location is within the cortex.7,8
These lesions have also been found in the periosteal region of long bones.7
Children can have lesions involving the epiphyses of long bones. In all patients, the intramedullary location within bone is relatively infrequent, 2/38 in one study and 6/67 in another.7,8
In our review of the literature, only one report of two intramedullary osteoid osteomas in young pediatric patients was described, in the French literature.5
Similar to our patient, both of these patients in the literature were under 3 years of age, and the lesions were in the distal tibia. However, unlike our patient, the two in the literature had reactive bone formation more characteristic of cortical lesions.
The differential diagnosis of an osteoid osteoma includes osteoblastoma, osteomyelitis, eosinophilic granuloma, osteogenic sarcoma, Ewing sarcoma or healing fracture. Typical radiographic findings are of a dense ivory-like sclerotic mass attached to the bone with sharply demarcated borders.4
When localized in the cortex, these lesions often evoke a reactive sclerosis; however, intramedullary lesions often fail to elicit such a response.8
Further delineation can be obtained using computerized tomography (CT) scan or MRI. CT scans generally show a small lucent nidus surrounded by reactive sclerosing bone, sometimes seen on plain films.2
Visualizing the nidus by MRI can be difficult, and intramedullary soft tissue changes may produce a misleading aggressive appearance.1
Typically, an intermediate signal is seen on T1 and a high-intensity signal on T2-weighted images. Low-intensity signal within the nidus on T2 images suggests calcification.2
Treatment options for an osteoid osteoma include prolonged non-steroidal anti-inflammatory drug (NSAID) use, CT-guided ablation and surgical excision.3
In this case, we chose surgical excision. While prolonged NSAID use has been shown to be effective, we were uncomfortable placing a two-year-old on long-term non-steroidal medication due to skeletal and renal complications. Likewise, CT-guided ablation could have been performed and this is done regularly at our institution. However, performing this on a two-year-old would have required anesthesia during the procedure. Therefore, we proceeded with an open excisional biopsy performed under the controlled environment of the operating room. This allowed for an adequate tissue sample to be sent for pathology and direct visualization of the nidus, helping to ensure complete excision.
In this case, we present a rather common pathologic lesion, osteoid osteoma, found at an atypical age, 30 months, in an uncommon location, within the medullary canal of the distal tibia. Because this lesion did not have the common reactive sclerosis often seen in cortical osteoid osteomas, diagnosis is more difficult.
The cause of pain in a limping child can often be difficult to discern. Osteoid osteoma should be kept in mind as a source of pain in the pediatric age group, with or without "classic" radiographic features.