Osteoid osteoma is a benign bone tumor typically affecting the long bones in young patients. It is small but painful, and has a central nidus that rarely exceeds 1.5 cm. Vertebral localization is uncommon (about 10%), and when it occurs in the spine lumbar lesions are most common (59%). The most common presenting symptom is pain that is worse at night and relieved by aspirin. Greco et al. reported that large amounts of prostaglandin E2 and prostacyclin released from the nidus are related to the sclerosis and pain in osteoid osteoma 1 .

Although there have been several reports on osteoid osteoma, only six cases of osteoid osteoma with radiculopathy have been reported in the literature 2 3 4 . In most such cases, radiculopathy appeared to have been induced by nerve compression, and the cause of radiculopathy without nerve compression in patients with osteoid osteoma was not discussed. We report here rare cases of osteoid osteoma in the sacral lamina and the thoracic facet causing radiculopathy. We present histological findings for the soft tissue around the affected nerve root, and suggest for the first time that radiculopathy with osteoid osteoma may be induced by tumorous inflammation.

Osteoid osteoma of the spine is a relatively rare bone tumor. It is reported that 10% of osteoid osteomas occur in the spine. Patients with osteoid osteoma usually present with complaints of local pain and/or swelling. Classically, the pain associated with this lesion is worse at night and partially relieved with NSAIDs. Although patients with spinal osteoid osteoma usually exhibit painful scoliosis, presentation with isolated root irritation is relatively rare. Although our first patient presented with typical local pain, the diagnosis was delayed because of the coexisting isolated root irritation. Our second patient underwent T10 nerve root block, which was effective for the radiculopathy but not local pain. Surgical resection relieved both radicular and local pain. These findings indicated that the radicular pain was induced by tumorous inflammation. We have presented here for the first time histological findings around the nerve root suggesting that chronic inflammation may induce radiculopathy. Large amounts of prostaglandin E2 and prostacyclin released from the nidus may be related to the isolated root irritation 1 .

The diagnosis of osteoid osteoma of the spine is challenging, since conventional radiography is not useful for small lesions. MRI is useful not only for illustrating the lesion but also for demonstrating the existence of local inflammation and bone edema, especially on T2-weighted or Gd-enhanced images. In case 2, a high-intensity area including the intervertebral foramen and the nerve root were well-enhanced by Gd on T2-weighted images. In addition to MRI, radionuclide bone scanning was useful not only for localizing the lesion but also for revealing its intense activity, as described Schmitz et al. and Boretz et al 5 6 . We wish to emphasize the efficacy of radionuclide bone scanning as an aid to the early diagnosis of this painful lesion of the spine.

For surgical treatment, excision of the nidus has been reported to be effective in more than 95% of cases 7 8 . Complete resection of the nidus is a key to achieving remission of pain, with the assistance of CT imaging to localize the nidus preoperatively. Abe et al. advocated en bloc resection to facilitate precise operative procedures 9 , as have other authors 10 11 . Rates of local recurrence have been found to be as high as 28% 12 . As described by Villas et al. 3 , however, en bloc resection is not always appropriate in the spine. Recently, CT-guided radiofrequency ablation has been used to treat osteoid osteoma 13 14 . It has been proven to be an acceptable, safe, minimally invasive, and cost-effective treatment for this benign but painful tumor. Although we considered CT-guided radiofrequency ablation for the tumors in our patients, the S2 and T10 roots were very close to the nidus, and the possibility of injuring these nerve roots therefore existed. We therefore performed excision by curettage of the nidus and surrounding sclerotic bone, with complete remission of symptoms.

We have described rare cases of osteoid osteoma in the spine that induced radiculopathy. We have presented the first histological evidence of tumorous inflammation causing radiculopathy in osteoid osteoma near the intervertebral foramen.

Written informed consent was obtained from the patients for publication of this case report and any accompanying images. Copies of the written consents are available for review by the Editor-in-Chief of this journal.

The authors declare that they have no competing interests.

ZM and YI carried out the histochemical studies. TY and KI participated in the surgery and the design of the study, and KI contributed in the statistical analysis. SK and KI conceived the study and drafted the manuscript. All authors read and approved the final manuscript.