Treatment of Osteoid Osteoma: How to Optimize Impedance during Radiofrequency Ablation

Abstract

Introduction

Osteoid osteomas (OOs) are benign bone lesions that predominantly affect young individuals and are characterized by persistent, nocturnally exacerbated pain, which typically responds to nonsteroidal anti-inflammatory drugs (NSAIDs). While traditional surgical excision was once the standard treatment, percutaneous CT-guided radiofrequency ablation (RFA) has emerged over the past decades as the preferred minimally invasive approach.

Objectives

This article explores the technical aspects of RFA in OO treatment, with a particular focus on the challenges posed by elevated impedance during the procedure and strategies for optimization.

Materials and Methods

The RFA procedure involves precise CT-guided placement of an RF electrode within the nidus, followed by thermal ablation at 90°C for 5 to 6 minutes. Procedural technical success is determined by achieving effective coagulation necrosis, indicated by stable impedance values (350–400 ohms) and the occurrence of a “roll-off” effect, as set by the manufacturer. However, abnormally high impedance can hinder energy delivery, compromising treatment efficacy.

Results

Elevated impedance commonly arises due to electrode misplacement, contact with cortical bone or the sclerotic rim of the OO, or tissue charring around the electrode tip. Strategies to mitigate these issues include meticulous electrode positioning, incremental energy delivery, continuous temperature monitoring, and techniques such as repositioning the electrode, cleaning or replacing the radiofrequency system, “overdrilling,” or saline injection to enhance conductivity.

Conclusion

Managing impedance variations is crucial for optimizing technical RFA outcomes in OO treatment. By understanding and addressing impedance-related challenges, interventional radiologists can improve procedural success, minimize complications, and enhance patient outcomes in this minimally invasive approach.

 

Scroll to top

Keywords

osteoid osteoma - radiofrequency ablation - impedance

Introduction

Osteoid osteomas (OOs) are the third most common benign bone lesion, predominantly affecting young people, particularly during the first two decades of life.[1] They are characterized by persistent bone pain, often exacerbated at night, with symptoms usually relieved by nonsteroidal anti-inflammatory drugs (NSAIDs) or salicylates. The lesions most frequently occur in the metaphysis or diaphysis of long bones, particularly in the lower extremities (femur and tibia), followed by the posterior spinal elements and small bones of the hands and feet.[1] [2] [3]

Diagnosis of OOs relies heavily on clinical history and imaging. Plain radiographs, being the first line of imaging, demonstrate a cortical-based lucent lesion (nidus) surrounded by sclerosis or cortical thickening without aggressive radiographic features. Computed tomography (CT) remains the gold standard in diagnosing OOs, providing precise localization of the nidus, which typically measures less than 1.5 cm in diameter, with surrounding sclerosis. Magnetic resonance imaging (MRI) is particularly beneficial in identifying the surrounding bone marrow edema-like signal on fluid-sensitive sequences, aiding in the definitive diagnosis when the nidus is medullary, periosteal, or lacks typical sclerosis on radiographs or CT. Ancillary imaging modalities, such as bone scans or single-photon emission CT, may be used in equivocal cases, often displaying the “double-density” sign, with intense central uptake surrounded by a rim of lesser but still increased uptake.[1] [2] [3]

While OO-related pain may regress and resolve spontaneously over 6 to 15 years, the continuous use of salicylates and/or NSAIDs has been shown to significantly shorten this period to approximately 2 to 3 years.[1] [4] [5] [6] [7] [8] Historically, surgical excision of the nidus was the standard treatment for prolonged and severe cases, either nonresponding or intolerant of medical management. However, this has largely been superseded by percutaneous CT-guided thermal ablation over the past decades, in accordance with current NICE guidelines published in 2004.[1] [2] [3] [9]

 

Scroll to top

Objectives

In our tertiary centre, CT-guided radiofrequency ablation (RFA) serves as the primary curative treatment for OOs. This article explores the technical aspect of OO RFA, particularly focusing on the challenges posed by abnormally elevated impedance, causing automatic and premature discontinuation of treatment, leading to suboptimal coagulation necrosis. Here, we outline strategies for optimization of impedance to ensure an appropriate treatment duration and therefore increase the rate of successful treatment.

 

Scroll to top

Materials and Methods

Radiofrequency Ablation—Procedure and Technique

The RFA procedure begins with a comprehensive review of imaging and multidisciplinary consensus. Patients are counselled regarding the procedure and the expected outcome, risks, and potential complications.

Under sedation or general anesthesia, a localized CT is performed with a grid placed over the skin to mark the entry site ([Fig. 1]). Following aseptic preparation and local anesthetic administration, a coaxial penetration and bone biopsy system is advanced under CT guidance to establish a tract and obtain a nidus sample for histopathological analysis ([Figs. 2] and [3]). In our center, we use the Bonopty Penetration Set 14G 9.5 cm and Bonopty Biopsy Set 15G 16 cm for access and biopsy, respectively.

Fig. 1 Axial sequence of the planning CT performed preprocedure; note the grid placed over the skin surface to aid localization for skin entry. The osteoid osteoma nidus (arrowhead) is surrounded by a thick sclerotic rim of dense bone.

Fig. 2 A subsequent axial CT showing the Bonopty penetration set in use; the cannula component (arrow) is placed against the cortex with the drill (arrowheads) placed through this to prevent damage to the surrounding soft tissues. This is used to drill to penetrate the dense sclerotic rim of bone up to the edge of the nidus.

Fig. 3 Axial CT maximal intensity projection (MiP) slice after a tract has been created to the edge of the nidus; the drill is exchanged for the biopsy needle (arrowheads), while the cannula remains in place to maintain access to the drilled tract. The biopsy needle is inserted through the nidus and into the distal cortical bone to ensure the sample is securely anchored for retrieval.

Once the sample is acquired, the outer co-axial cannula is retained and kept in a fixed position while the inner biopsy needle is replaced with a radiofrequency (RF) cannula containing an RF electrode ([Fig. 4]). This features a 0.5- to 1.0-cm active tip (cathode), which is positioned within the nidus, and a grounding pad (anode) is applied to the patient's trunk or thigh; in our center, we use the Abbott 20G, 15 cm RF electrodes. The outer sheath is retracted to prevent heat conduction away from the treatment site and complications such as soft tissue or skin necrosis. The nidus is then subjected to thermal ablation for 6 minutes at 90°C. Postprocedure, the RF system is withdrawn, and local anesthetic is injected into the tract to mitigate postprocedural discomfort.[2] [10] [11]

Fig. 4 Axial CT maximal intensity projection (MiP) slice shows the RF electrode (arrowhead) now in place with the tip in the center of the nidus. Note how the access cannula has been withdrawn slightly to prevent heat conduction and reduce the risk of inadequate treatment and soft tissue burn (arrow).

To ascertain that the lesion has adequately undergone coagulation necrosis during the procedure, we ensure that the RF electrode tip temperature is maintained at approximately 90°C with observed impedance values ranging between 350 and 400 ohms. The occurrence of “roll-off,” marked by a significant increase in impedance resulting in the loss of AC flow—at the 6-minute mark, signals successful ablation.[12] However, any rise in impedance over a certain value set by the manufacturer may reflect ineffective coagulation necrosis and can hinder the procedure's efficacy ([Video 1], [Fig. 5]). Understanding the causes of impedance and implementing appropriate remedies are crucial for successful ablation outcomes.

Video 1 Video showing RFA monitor during ablation with roll off and stoppage of ablation with an impedance of over 850 ohms.

Fig. 5 Images of the RFA machine monitor showing normal impedance (A) and high impedance (B). Impedance values highlighted by a red oval.

 

Scroll to top

 

Scroll to top

Results

Causes of Elevated Impedance in OO RFA

Several factors can lead to abnormally high impedance during OO RFA, potentially compromising procedural efficacy.

One common cause is the misplacement of the RF electrode tip, particularly when it contacts cortical bone or the sclerotic rim of the OO. These structures exhibit higher impedance compared to the nidus, which interferes with effective energy delivery. Additionally, during heating, tissue changes such as vaporization, carbonization, and charring of the tissue surrounding the electrode tip can create an insulating layer within the nidus. This layer increases electrical resistance and leads to elevated impedance levels. In such cases, when impedance exceeds the manufacturer's predefined threshold, the radiofrequency current is interrupted earlier than expected, causing premature “roll-off,” and hindering effective ablation.[13] [14] [15] [16]

 

Scroll to top

Strategies to Optimize Impedance in OO RFA

To achieve optimal impedance and temperature levels during RFA of OO, we employ several strategies to ensure procedural success.

Accurate positioning of the electrode tip is paramount and is achieved using CT guidance to confirm that it lies within the nidus rather than the cortical bone or sclerotic rim surrounding the nidus. If the tip is found in an unsuitable position, repositioning is essential to optimize energy delivery and to ensure effective tissue necrosis.

Energy delivery is carefully managed by administering radiofrequency energy in incremental stages, as defined by the manufacturer, to prevent rapid charring around the active electrode tip and to help maintain impedance values within the desired range. The temperature at the electrode tip is also continuously monitored, allowing for timely adjustments if deviations occur. These parameters are usually preset and monitored by the specialist RF generator; for example, our institute uses the Abbott IonicRF Generator to deliver and monitor the correct RF supply.

When impedance spikes arise due to boiling or carbonization of the tissue, adjusting/withdrawing the electrode tip slightly, cleaning it, or even replacing the RF system electrode with a new one can help bypass nonconductive areas. If this method proves insufficient, the tract can be “overdrilled” slightly beyond the nidus ([Figs. 6] and [7]). Injecting saline through the coaxial cannula before reintroducing the RF electrode further reduces the effects of tissue charring and maintains consistent impedance levels throughout the ablation process.

Fig. 6 After an unsuccessful ablation attempt, which was terminated by the RF generator prematurely because of increased impedance, the decision was made to “overdrill” the tract. (A) Reintroduction of the drill into the cannula to continue drilling a short distance into the distal cortex. Due to the previous ablation attempts, there was charred material in the tract, which caused the drill to become stuck within the access cannula when it was attempted to be removed. As such, the cannula had to be removed and replaced as shown in (B): this image more clearly demonstrates the extent of the overdrilled tract and the space which has been created (arrowhead).

Fig. 7 Magnified axial CT slices show a comparison in the electrode position (arrowhead) before (A) and after (B) the “overdrilling” process: note the subtle difference in electrode tip position and greater space distal to the electrode created by the overdrilling in [Fig. 6B] (arrowheads). It is critical to ensure that the ablation zone around the electrode tip completely encompasses the nidus after the overdrilling process; please note that the cannula was withdrawn from the cortical surface in [Fig. 6B] before recommencing ablation.

 

Scroll to top

 

Scroll to top

Conclusion

Managing impedance variation is an integral component of ensuring technical success during OO RFA. By understanding the underlying factors contributing to elevated impedance and its impact on treatment success, interventional radiologists can optimize energy delivery, minimize complications, and improve patients' clinical outcomes. An appreciation of the difficulties one can encounter when performing this procedure and how to overcome them can help improve clinical outcomes and prevent the need for a repeat procedure or surgical alternative.

 

Scroll to top

 

Scroll to top

Conflict of Interest

None declared.

Data Availability Statement

Data are available to share on request.

Patient's Consent

Consent to participate was obtained.

• References

• 1 Boscainos PJ, Cousins GR, Kulshreshtha R, Oliver TB, Papagelopoulos PJ. Osteoid osteoma. Orthopedics 2013; 36 (10) 792-800
• 2 Bianchi G, Zugaro L, Palumbo P. et al. Interventional radiology's osteoid osteoma management: percutaneous thermal ablation. J Clin Med 2022; 11 (03) 723
• 3 Refaat R, Niazi G. Factors affecting time to pain relief in patients with osteoid osteoma treated by computed tomography (CT)-guided percutaneous radiofrequency ablation (RFA). Egypt J Radiol Nucl Med 2015; 46 (02) 397-404
• 4 Simm RJ. The natural history of osteoid osteoma. Aust N Z J Surg 1975; 45 (04) 412-415
• 5 Golding JS. The natural history of osteoid osteoma; with a report of twenty cases. J Bone Joint Surg Br 1954; 36-B [phone]
• 6 Bottner F, Roedl R, Wortler K, Grethen C, Winkelmann W, Lindner N. Cyclooxygenase-2 inhibitor for pain management in osteoid osteoma. Clin Orthop Relat Res 2001; (393) 258-263
• 7 Carpintero-Benitez P, Aguirre MA, Serrano JA, Lluch M. Effect of rofecoxib on pain caused by osteoid osteoma. Orthopedics 2004; 27 (11) 1188-1191
• 8 Kneisl JS, Simon MA. Medical management compared with operative treatment for osteoid-osteoma. J Bone Joint Surg Am 1992; 74 (02) 179-185
• 9 National Institute for Clinical Excellence. Computed tomography-guided thermocoagulation of osteoid osteoma. National Institute for Clinical Excellence Web site. Interventional procedures guidance. Reference number:IPG53. Published: 24 March 2004 . Accessed September 1, 2025 at: https://www.nice.org.uk/guidance/ipg53/chapter/1-Recommendations
• 10 Uldin H, Kanbour I, Patel A, Botchu R. Image-guided musculoskeletal interventional radiology in the personalised management of musculoskeletal tumours. J Pers Med 2024; 14 (12) 1167
• 11 Al-Omari MH, Ata KJ, Al-Muqbel KM, Mohaidat ZM, Haddad WH, Rousan LA. Radiofrequency ablation of osteoid osteoma using tissue impedance as a parameter of osteonecrosis. J Med Imaging Radiat Oncol 2012; 56 (04) 384-389
• 12 Hoffmann R-T, Jakobs TF, Kubisch CH. et al. Radiofrequency ablation in the treatment of osteoid osteoma-5-year experience. Eur J Radiol 2010; 73 (02) 374-379
• 13 Cantwell CP, Eustace S. An unusual complication of radiofrequency ablation treatment of osteoid osteoma. Clin Orthop Relat Res 2006; 451 (451) 290-291 , author reply 291–292
• 14 Lanza E, Thouvenin Y, Viala P. et al. Osteoid osteoma treated by percutaneous thermal ablation: when do we fail? A systematic review and guidelines for future reporting. Cardiovasc Intervent Radiol 2014; 37 (06) 1530-1539
• 15 Singh DK, Katyan A, Kumar N, Nigam K, Jaiswal B, Misra RN. CT-guided radiofrequency ablation of osteoid osteoma: established concepts and new ideas. Br J Radiol 2020; 93 (1114) 20200266
• 16 Chen B, Shi Y, Li J. et al. Tissue recognition based on electrical impedance classified by support vector machine in spinal operation area. Orthop Surg 2022; 14 (09) 2276-2285

Publication History

Article published online:
16 September 2025