Cantwell and Eustace raised the issue of the diagnostic steps involved in localizing the recurrent osteoid osteoma. The treating interventional radiologist performed a biopsy with an 11-gauge bone biopsy needle before replacing it with the radiofrequency ablation probe to confirm the location of the twice previously resected osteoid osteoma using computed tomography (CT). Radiofrequency ablation was performed by an experienced interventional radiologist at our institution. However, at the time of the patient's treatment, radiofrequency ablation was a relatively new treatment for osteoid osteoma and therefore experience was limited. A 460,000-Hz LeVeen needle electrode (Boston Scientific Corp, Natick, MA) with an array design combined with a 200-Watt RF 3000 (Boston Scientific Corp, Natick, MA) impedance energy-controlled generator, the only available system at our institution at the time, was used for the radiofrequency ablation.
Cantwell and Eustace also suggested drilling through the lateral cortex of the tibia to approach the osteoid osteoma lesion through a longer tract with a shorter active tip probe to avoid the complication experienced by our patient. Although this suggestion might be useful to avoid the rare complication of a cutaneous skin burn our patient experienced, using this technique for treating a benign lesion also results in increased soft tissue trauma and creates an additional stress riser in the tibia.
Cantwell and Eustace inquired about our guidelines for treatment. We are unaware of specific guidelines for treatment of osteoid osteoma with radiofrequency ablation. However, Pinto et al,2 quoting Rosenthal et al,3 stated that CT-guided radiofrequency ablation should be attempted only when a definite nidus is identified on CT in a patient with an appropriate history suggestive of osteoid osteoma. Also, radiofrequency ablation is contraindicated in patients with cardiac pacemakers because the generator used can cause unwanted physiologic effects.2
Cantwell and Eustace questioned the maximum extent of the array probe that was exposed in the bone. We agree that the CT images are not technically adequate to determine the distal extent of the probe, and therefore, any additional information on the distal extent of the probe would be a gross approximation.
They also point out that the term “slow heating” seems to be a trademark. This is incorrect. As described in our case report, slow heating refers to the generator's ability to measure the increased impedance and decrease the current proportionately to limit the heating to focal osteonecrosis.1 Although the exact values are not known as this procedure was done 3 years ago, the typical initial impedance is within the range of 40 to 80 ohms. Because the generator is a constant voltage system, that is, as the tissue impedance rises the current delivered to the needle electrode is decreased, therefore the end point of the ablation occurs when the impedance increases a set percentage above the initial impedance value leading to the reduction in current.
Christian M. Ogilvie, MD;
Harish S. Hosalkar, MD;
Richard D. Lackman, MD;
Department of Orthopaedic Surgery Pennsylvania Hospital and the Hospital of the University of Pennsylvania Philadelphia, PA
Joseph L. Finstein, MD
Department of Orthopaedic Surgery Hahnemann Hospital Philadelphia, PA
1. Finstein JL, Hosalkar HS, Ogilvie CM, Lackman RD. An unusual complication of radiofrequency ablation treatment of osteoid osteoma. Clin Orthop Relat Res
. 2006; EPub Ahead of Print.
2. Pinto CH, Taminiau AH, Vanderschueren GM, Hogendoorn PC, Bloem JL, Obermann WR. Technical considerations in CT-guided radiofrequency thermal ablation of osteoid osteoma: tricks of the trade. AJR Am J Roentgenol
3. Rosenthal DI, Hornicek FJ, Wolfe MW, Jennings LC, Gebhardt MC, Mankin HJ. Percutaneous radiofrequency coagulation of osteoid osteoma compared with operative treatment. J Bone Joint Surg Am