There are numerous reports in the literature of osteoid osteomas occurring in patients younger than 5 years of age, but only rarely in those under 1 year of age [2
]. Habermann and Stern [5
] reported a case of OO in an 8-month-old infant whose onset of symptoms began at 4 months. A case of possible congenital OO involving a distal phalanx of the left long finger was reported by Szabo and Smith [9
]. That patient was diagnosed when she was 15 years old. However, deformity of the affected phalanx and its nail apparently had existed from birth and gone unnoticed.
Pain is the most common clinical manifestation of OO, with limp, tenderness, swelling, and atrophy the next most frequent findings in children [7
]. The symptoms may be present for periods ranging from weeks to several years before diagnosis [2
]. Although the clinical features of OO in children are similar to those in adults, greater clinical discernment is required for diagnosis, as children might not be able to accurately communicate their symptoms. In our patient, the most prominent sign was decreased use of the affected leg. Although this was first noted by the parents shortly after birth, the appropriate radiologic evaluation of the lesion was delayed until other indications of disease became evident.
The most characteristic and most common radiologic finding of OO includes a radiolucent, intracortical nidus, containing a variable amount of mineralization, surrounded by an area of dense fusiform reactive sclerosis [1
]. Approximately 20% of osteoid osteomas involve the medulla. The intramedullary lesions are often barely visible and usually induce only mild or no reactive bone formation [11
]; diagnosis of these osteoid osteomas may be challenging as shown by our case.
The nidus of an osteoid osteoma rarely exceeds 1.5 cm in the greatest dimension. The reactive periosteal bone formation is usually solid. CT scan is generally regarded as the preferred cross-sectional technique to locate the lesion. In children, an MRI study is often performed first in order to limit radiation exposure. The tumor nidus, however, may be difficult to identify on MR imaging [12
]. The great sensitivity of MR imaging to detect nonspecific, reactive changes in the bone marrow and soft tissue may result in a misleading aggressive appearance [13
]. In our patient, intramedullary location, extensive bone marrow, and soft tissue edema adjacent to the lesion, as well as lamellar periosteal new bone formation, made the diagnosis more difficult; because of this imaging appearance and the unusual age of our patient, other entities, such as osteomyelitis and aggressive tumors, were included in the differential radiologic diagnosis.
Osteoid osteoma and osteoblastoma are considered as closely related processes. Histologically, osteoid osteoma is similar to osteoblastoma and the difference between these two entities is established on the basis of the size of the nidus (>2 cm is generally considered as osteoblastoma), skeletal distribution, radiographic appearance, and clinical presentation [14
A rare complication of osteoid osteoma is localized overgrowth and deformity of bone. It has been found most frequently in children with symptom onset before 5 years of age, emphasizing the importance of prompt definitive treatment to prevent leg length discrepancy and deformity in infants and very young children [15
]. Radiofrequency ablation for the treatment of OO was first described in 1992 [16
], and is currently the therapeutic treatment of choice for OO in both adult and pediatric cases; it is minimally invasive, safe and has a high technical and clinical success rate [17
]. Successful treatment of OO with RFA was reported in a 1-year-old boy by Egstrom et al. [20
], but reviewing international medical literature our patient was the youngest child with OO to have been treated with RFA. In some studies, a higher recurrence of osteoid osteoma following treatment with RFA has been suggested in children or younger patients [17
] and association of recurrence with large nidus diameter (1.0–1.5 cm) has also been reported [18
]. Our case demonstrates potential problems with RFA in very young patients with relatively large lesions (1.2 cm in maximum diameter of the nidus). Although our patient developed a recurrence after the primary treatment, with a short follow-up after the second ablation, she is currently doing well. Whether there are any long-term consequences of the OO or the RFA procedure remains to be seen.
In summary, osteoid osteoma can be a diagnostic challenge when it presents with atypical clinical features in a very young patient. But also in very young patients, RFA is a safe procedure and should be considered as the treatment of choice. However, recurrence after the procedure is more frequently observed and close follow-up is recommended.