Clinical Perspective
Musculoskeletal Imaging
August 7, 2018

Spinal Osteoid Osteoma: Percutaneous Radiofrequency Ablation Using a Navigational Bipolar Electrode System

Abstract

OBJECTIVE. The purpose of this article is to describe the use of a navigational bipolar radiofrequency ablation system for the treatment of spinal osteoid osteomas.
CONCLUSION. Safe and effective imaging-guided percutaneous radiofrequency ablation of spinal osteomas can be performed using a multidirectional bipolar electrode system.
Osteoid osteoma is a benign painful bone-forming lesion that typically occurs in patients younger than 30 years and accounts for approximately 12% of benign bone lesions [1]. Spinal osteoid osteomas comprise 10% of cases and are typically located within posterior elements, most commonly in the lumbar spine (up to 60% of cases), followed by cervical, thoracic, and sacral spine [2]. Typical imaging characteristics include an osteolytic nidus (often < 15 mm), with or without central mineralization, as well as surrounding osseous sclerosis and reactive marrow edema or osteitis. Clinical manifestations include focal pain, particularly at night, which improves with nonsteroidal antiinflammatory medications; painful scoliosis (≈ 70% of cases); and spinal stiffness in case of vertebral body involvement (up to 90% of cases) [3]. Clinical diagnosis of spinal osteoid osteomas may be particularly challenging, with an average reported delay from presentation to treatment of 18–24 months [4]. In the past, spinal osteoid osteomas were treated with surgical excision that involved complete resection of the nidus, with associated substantial bone loss and the potential need for postoperative augmentation at the surgical site. Incomplete surgical excision is associated with a reported recurrence rate of up to 25% [5]. In addition to cost burden, surgical management may be complicated by spinal instability, nerve or spinal cord injury, and infection [6, 7]. Over the past decade, investigators have successfully used percutaneous thermal ablation, including radiofrequency (RF) and laser ablation, for definitive treatment of spinal osteoid osteomas [814]. In the published literature to date, RF ablation of spinal osteoid osteomas has been exclusively performed using straight unipolar RF electrodes with variably prescribed ablation times, ranging from 4 to 30 minutes, after reaching a desired plateau temperature (typically 90°C) [813].
A recently introduced navigational bipolar RF ablation system has been successfully used for treatment of spinal metastases [15, 16] and has several important advantages over traditional straight unipolar RF electrodes for management of spinal lesions, particularly given the proximity of neural elements and potential risk of thermal injury. Most important, a pair of built-in active thermocouples along the electrode provides precise real-time monitoring of the ablation zone volume and geometry, beyond which tissues are safe from thermal injury, and ensures that the entire lesion (nidus in case of osteoid osteoma) is safely ablated. In addition, the articulating distal segment (tip) of the electrode can be curved in multiple directions, which allows optimal electrode positioning in several portions of the nidus from a single osseous entry site, particularly in larger lesions that would otherwise require more than one access point, and is also advantageous for challenging-to-access lesions. Finally, the bipolar electrode design eliminates the risk of skin thermal injury and obviates the placement of grounding pads.
In this report, our initial experience with the use of a navigational bipolar RF ablation electrode system for ablation of spinal osteoid osteoma is reported, focusing on implications for patient care, including equipment advantages and implemented procedure safety measures. The Mallinckrodt Institute of Radiology institutional review board approved the study.

Radiofrequency Ablation Equipment and Procedure

The procedures are typically performed under total IV anesthesia for pediatric patients and under conscious sedation with fentanyl and midazolam for adult patients under CT guidance, after written informed consent is obtained. The CT scanning technique should be optimized to minimize radiation exposure while providing adequate image quality for guidance. The implemented measures to reduce patient radiation dose include decreasing the tube current and gantry rotation time (milliampere-seconds), decreasing the x-ray beam energy (kilovoltage peak), increasing the pitch (at the set tube current), using central patient positioning at the CT iso-center, and limiting the scan length to the volume of interest. For CT guidance in percutaneous RF ablation of spinal osteoid osteomas, we typically use 80 mAs, 80 kVp, a pitch of 1.0–1.2, and a section thickness of 2 mm. In addition, placement of thermal protection needles and the ablation electrode should be performed simultaneously to reduce scanning time. Furthermore, no immediate postablation scan is necessary.
Local and periosteal anesthesia are achieved with a combination of 1% lidocaine and 0.25% bupivacaine. The osteoid osteoma nidus is accessed using a coaxial battery-powered hand drill with a 10-gauge introducer cannula and 12-gauge inner diamond-tipped needle (OnControl, Vidacare). The inner needle is then exchanged for a 12-gauge hollow biopsy needle, and a single bone core biopsy specimen of the nidus is obtained. RF ablation is performed using the STAR Tumor RF Ablation System (Merit Medical Systems), which includes the Spine STAR ablation device and the MetaSTAR generator (Fig. 1). The ablation device is an articulating navigational bipolar RF electrode (the 5/10 STAR electrode), which contains a pair of active thermocouples positioned 5 and 10 mm from the center of the ablation zone (Fig. 1). According to the manufacturer's thermal distribution curves, the dimensions of the ellipsoid ablation volume are 20 × 15 × 15 mm when the thermocouple located 10 mm from the center of the ablation zone (proximal thermocouple) reaches 50°C, and 15 × 7 × 7 mm when the thermocouple located 5 mm from the center of the ablation zone (distal thermocouple) reaches 50°C. The RF energy automatically stops when the proximal thermocouple registers at 50°C, which is a valuable safety feature. There are laser etchings along the device that demarcate the exiting point of the electrode from the noninsulated working cannula to reduce the risk of ablating along the introducer tract (Fig. 1). The MetaSTAR generator provides 3-, 5-, 7.5-, and 10-W power settings, which allow slow ablations, thereby improving efficacy and reducing undesired heat dispersion, and displays ablation time, impedance, and the two thermocouple temperature readings, which permit precise real-time monitoring of the ablation zone geometry (Fig. 1).
Fig. 1A —Navigational bipolar radiofrequency (RF) ablation system, STAR Tumor RF Ablation System (Merit Medical Systems), consisting of Spine STAR ablation device and MetaSTAR RF generator.
A, Photograph of Spine STAR ablation device. Multidirectional distal tip of electrode (short arrow) can be articulated up to 90° by turning gray knob (long arrow) at handle. Laser etchings (arrowheads) along device shaft mark exit point and deployment point of electrode from introducer cannula. After desired positioning within nidus, RF electrode tip is fully extended by turning white knob (curved arrow) at handle.
Fig. 1B —Navigational bipolar radiofrequency (RF) ablation system, STAR Tumor RF Ablation System (Merit Medical Systems), consisting of Spine STAR ablation device and MetaSTAR RF generator.
B, Graphic shows that there are two active thermocouples embedded within electrode (arrows) located 5 mm and 10 mm from center of ablation zone, which allow precise real-time monitoring of ablation zone geometry.
Fig. 1C —Navigational bipolar radiofrequency (RF) ablation system, STAR Tumor RF Ablation System (Merit Medical Systems), consisting of Spine STAR ablation device and MetaSTAR RF generator.
C, Photograph of MetaSTAR RF generator. Generator screen shows temperature readings of distal and proximal thermocouples, impedance, duration of RF energy delivery, and power output level.
The 10-gauge introducer cannulae of the coaxial biopsy and RF ablation device are then exchanged over a K-wire. The navigational bipolar RF ablation electrode is subsequently positioned into the center of the nidus and articulated in different orientations, if needed, to ensure that the entire nidus is ablated (Fig. 2). The limitation of the STAR RF ablation system includes the size of the introducing cannula (10-gauge) and the ablation electrode (11-gauge), which may preclude use of this system for small children, whose pedicles or laminae cannot accommodate needles of these calibers.
Fig. 2A —18-year-old woman with low back pain and hypomobility of lumbar spine.
A, Axial unenhanced CT image shows partly mineralized nidus in posterior L2 vertebral body abutting posterior wall cortex and resulting in cortical thinning and indentation. Note surrounding reactive medullary sclerosis (arrows).
Fig. 2B —18-year-old woman with low back pain and hypomobility of lumbar spine.
B, Axial CT image obtained during radiofrequency (RF) ablation with patient in prone position shows articulation of distal tip of navigational RF electrode placed via lateral transpedicular approach for optimal access to nidus, which would be challenging to reach with traditional straight RF electrodes. Note epidural injection of carbon dioxide and 5% dextrose in water (diluted in iodinated contrast agent) for active thermal protection. Somatosensory and motor evoked potential monitoring was performed during procedure.

Thermal Protection Techniques

Neural thermal monitoring is always performed for all patients because of the close proximity of the neuroforamen or central canal to the margins of the ablation zone (Figs. 2 and 3). Active and passive neural thermal protection techniques include epidural or neuroforaminal injection of carbon dioxide or cooled 5% dextrose in water, achieved by placement of an 18-gauge spinal needle in the epidural space or in the neuroforamen, connected to a hemostasis valve (Passage, Merit Medical), and coaxial placement of a thermocouple into the neuroforamen to measure temperatures. Carbon dioxide is preferred because of its advantages, including its lower thermal conductivity compared with cooled 5% dextrose in water and its superior solubility in blood compared with air, which minimizes the risk of embolism in case of intravasation. The thermoprotective agents are injected prophylactically into the neuroforamen or epidural space immediately before ablation to minimize resorption or redistribution. A range of 5–20 mL carbon dioxide is injected either into the epidural space or neuroforamen. The solution of 5% dextrose in water is cooled in a refrigerator and subsequently aliquoted. A combination of carbon dioxide and 5% dextrose in water enhances thermal protection. It is emphasized that the use of saline solutions should be avoided with RF ablation because the electric conductivity may result in expansion of the ablation zone and creation of a plasma field [17]. It should be noted that before injection of thermoprotective agents, needle position is confirmed by injection of 1–5 mL of nondiluted iohexol (Omnipaque 300, GE Healthcare). Intraprocedural somatosensory and motor-evoked potential monitoring is performed when procedures are performed under total IV anesthesia for early detection of potentially impending thermal nerve or spinal cord injury. In addition, immediate postablation prophylactic ipsilateral nerve root block is performed when the nidus abuts the neuroforamen with no intact cortex. This is achieved by neuroforaminal injection of 10 mg of dexamethasone to improve postablation inflammation. No anesthetic is injected to allow precise evaluation of potential immediate postablation neurologic deficit. Given the bipolar design of the RF electrode, the risk of current-related skin thermal injury is eliminated, obviating the need for grounding pad placement.
Fig. 3A —13-year-old boy with painful scoliosis.
A, Unenhanced axial CT image shows centrally mineralized nidus within right pedicle of T10 extending to right lamina with indistinctness of posterolateral cortex along central canal.
Fig. 3B —13-year-old boy with painful scoliosis.
B, Axial CT image obtained during radiofrequency (RF) ablation shows placement of navigational bipolar RF electrode within nidus. Note that 18-gauge spinal needle is placed in right T10-11 neuroforamen, and carbon dioxide and cooled 5% dextrose in water (diluted in iodinated contrast agent) are injected into neuroforamen (not shown) and epidural space (arrows) for thermal protection. Immediately after RF ablation, prophylactic right T10-11 nerve root lock was performed through existing neuroforaminal spinal needle by injection of 10 mg of dexamethasone.
Fig. 3C —13-year-old boy with painful scoliosis.
C, Photograph obtained during RF ablation shows coaxial placement of thermocouple within neuroforamen for temperature monitoring. Cooled 5% dextrose in water is injected through second lumen for active thermal protection.

Discussion

This report illustrates that the unique features of the navigational bipolar STAR Tumor Ablation system, which has been used to successfully treat spinal metastases and primarily appendicular osteoid osteomas [15, 16, 18], can also be used for safe and effective treatment of spinal osteoid osteomas. RF ablation of spinal osteoid osteomas poses a unique challenge because of the proximity of neural elements, particularly given the location of most lesions within the posterior spinal elements. However, intact spinal cortical bone, flow of CSF, and small vessels in the epidural space have been described as protective measures against undesired propagation of RF energy [19]. In one of the initial reports on the percutaneous use of RF technology, Rosenthal et al. [20] recommended that RF ablation of osteoid osteomas should be considered only when at least a 1-cm margin exists between the lesion and the closest neural elements because of the potential risk of neural thermal injury; however, other investigators have successfully treated spinal osteoid osteomas with margins as close as 1 mm to neural structures using unipolar RF ablation systems [10, 13]. For example, Vanderschueren et al. [10] successfully treated 24 spinal osteoid osteomas using 5-mm non–cool tip unipolar electrodes (28 RF ablation procedures) and suggested that RF ablation should be the treatment of choice for lesions at least 2 mm distant from the closest neural structure. Our experience highlights the importance of precise real-time monitoring of the ablation zone volume, which is made possible by two active embedded thermocouples along the RF ablation electrode, a feature that is a recent development in RF ablation electrode design technology. This allows the operator to ensure that a sufficiently high temperature is generated over a volume that encompasses the entire nidus, while minimizing the risk of undesired thermal injury, and is vital when ablating close to spinal cord or nerve roots. It should also be recognized that vessels, such as the vertebral artery when treating cervical spine lesions, may act as a heat sink that hinders RF energy deposition within the nidus. In our practice, we routinely RF ablate spinal osteoid osteomas that are less than 1 mm from the closest neural element or with no intact interposing cortex (Figs. 2 and 3) with no complications. We recommend that both passive and active neural thermal protective measures be implemented for RF ablation of spinal osteoid osteomas, particularly with lesions closer than 5 mm to the spinal cord or nerve roots. Albisinni et al. [13] successfully treated 61 patients with spinal osteoid osteomas using monopolar non–cool tip electrodes with no neural thermal protection and reported a single case of transient neural thermal injury characterized by lower extremity radiculopathy. The authors reported a mean distance between the nidus and the closest neural element of 7.4 mm (range, 1–35 mm).
In addition, shorter cumulative ablations may be achieved by knowledge of temperature along the periphery of the ablation zone. Osteoid osteomas are typically RF ablated for 4–6 minutes at a target temperature of 90°C in the center of the ablation zone [21], to account for heat dissipation toward the periphery of the nidus, because tissue necrosis and thermal coagulation occur above 50°C [22]. However, a wide range of ablation times at a typical target temperature of 90°C is used in practice, ranging from 4 to 30 minutes according to the nidus size and length of active electrode tip [813]. These reported times exclude the ablation period passed to reach the desired plateau temperature, which adds to the total ablation time. Therefore, direct temperature measurement at the periphery of the nidus may be a more accurate indicator of adequate treatment and may obviate the need to ablate for a specific duration. In our anecdotal experience, a subgroup of spinal osteoid osteomas can be successfully treated for a total RF ablation time of 2–3 minutes using the described RF ablation system. However, the need for RF ablation of spine osteoid osteomas for a specific duration and temperature remains a potential topic for future research.
Because the entire nidus must be ablated to ensure definitive cure and a nidus of larger than 12 mm is an independent risk factor for recurrence [23], a larger nidus may require placement of more than one straight RF electrode to achieve sufficient overlapping ablation volume through separate osseous access sites, further compromising bone integrity and predisposing to potential fracture [24]. The articulating distal segment of the navigational system makes possible placement of the electrode within any desired location within the nidus from a single bone entry site, a characteristic that is particularly useful in larger lesions. This feature also facilitates access to difficult-to-reach nidus locations, particularly in the cervical spine and adjacent to vertebral body posterior wall (Fig. 2), where it would be otherwise challenging to access using traditional straight RF electrodes. Furthermore, the risk of skin burn inherent to the unipolar RF ablation electrodes caused by inadequate dispersion of RF energy on the skin surface at the region of grounding pad placement is eliminated by using bipolar RF electrode designs.
In conclusion, safe and effective CT-guided RF ablation of spinal osteoid osteomas can be achieved using a targeted navigational bipolar electrode system.

Footnote

A. Tomasian is a consultant for Medtronic Inc. J. W. Jennings is a consultant for, serves on the Interventional Oncology Advisory Board of, and is a member of the Speaker Panel for Medtronic and BTG (Galil Medical).

References

1.
Jaffe H. “Osteoid osteomas,” a benign osteoblastic tumor composed of osteoid and atypical bone. Arch Surg 1935; 31:709–728
2.
Morassi LG, Kokkinis K, Evangelopoulos DS, et al. Percutaneous radiofrequency ablation of spinal osteoid osteoma under CT guidance. Br J Radiol 2014; 87:20140003
3.
Pettine KA, Klassen RA. Osteoid-osteomas and osteoblastoma of the spine. J Bone Joint Surg Am 1986; 68:354–361
4.
Cové JA, Taminiau AH, Obermann WR, Vanderschueren GM. Osteoid osteomas of the spine treated with percutaneous computed tomography-guided thermocoagulation. Spine 2000; 25:1283–1286
5.
Zileli M, Cagli S, Basdemir G, Ersahin Y. Osteoid osteomas and osteoblastomas of the spine. Neurosurg Focus 2003; 15:E5
6.
Hempfing A, Hoffend J, Bitsch RG, Bernd L. The indication for gamma probe-guided surgery of spinal osteoid osteomas. Eur Spine J 2007; 16:1668–1672
7.
Laus M, Albisinni U, Alfonso C, Zappoli FA. Osteoid osteomas of the cervical spine: surgical treatment or percutaneous radiofrequency coagulation? Eur Spine J 2007; 16:2078–2082
8.
Rehnitz C, Sprengel SD, Lehner B, et al. CT-guided radiofrequency ablation of osteoid osteoma and osteoblastoma: clinical success and long-term follow up in 77 patients. Eur J Radiol 2012; 81:3426–3434
9.
Rybak LD, Gangi A, Buy X, La Rocca VR, Wittig J. Thermal ablation of spinal osteoid osteomas close to neural elements: technical considerations. AJR 2010; 195:[web]W293–W298
10.
Vanderschueren GM, Obermann WR, Dijkstra SP, Taminiau AH, Bloem JL, van Erkel AR. Radiofrequency ablation of spinal osteoid osteoma: clinical outcome. Spine 2009; 34:901–904
11.
Martel J, Bueno A, Nieto-Morales ML, Ortiz EJ. Osteoid osteoma of the spine: CT-guided mono-polar radiofrequency ablation. Eur J Radiol 2009; 71:564–569
12.
Klass D, Marshall T, Toms A. CT-guided radiofrequency ablation of spinal osteoid osteomas with concomitant perineural and epidural irrigation for neuroprotection. Eur Radiol 2009; 19:2238–2243
13.
Albisinni U, Facchini G, Spinnato P, Gasbarrini A, Bazzocchi A. Spinal osteoid osteoma: efficacy and safety of radiofrequency ablation. Skeletal Radiol 2017; 46:1087–1094
14.
Tsoumakidou G, Thénint MA, Garnon J, Buy X, Steib JP, Gangi A. Percutaneous image-guided laser photocoagulation of spinal osteoid osteoma: a single-institution series. Radiology 2016; 278:936–943
15.
Anchala PR, Irving WD, Hillen TJ, et al. Treatment of metastatic spinal lesions with a navigational bipolar radiofrequency ablation device: a multicenter retrospective study. Pain Physician 2014; 17:317–327
16.
Hillen TJ, Anchala P, Friedman MV, Jennings JW. Treatment of metastatic posterior vertebral body osseous tumors by using a targeted bipolar radio-frequency ablation device: technical note. Radiology 2014; 273:261–267
17.
Tsoumakidou G, Koch G, Caudrelier J, et al. Image-guided spinal ablation: a review. Cardiovasc Intervent Radiol 2016; 39:1229–1238
18.
Wallace AN, Tomasian A, Chang RO, Jennings JW. Treatment of osteoid osteomas using a navigational bipolar radiofrequency ablation system. Cardiovasc Intervent Radiol 2016; 39:768–772 [Erratum in Cardiovasc Intervent Radiol 2018; 41:984]
19.
Dupuy DE, Hong R, Oliver B, et al. Radiofrequency ablation of spinal tumors: temperature distribution in the spinal canal. AJR 2000; 175:1263–1266
20.
Rosenthal DI, Springfield DS, Gebhardt MC, Rosenberg AE, Mankin HJ. Osteoid osteoma: percutaneous radio-frequency ablation. Radiology 1995; 197:451–454
21.
Motamedi D, Learch TJ, Ishimitsu DN, et al. Thermal ablation of osteoid osteoma: overview and step-by-step guide. RadioGraphics 2009; 29:2127–2141
22.
Zhou YF. High intensity focused ultrasound in clinical tumor ablation. World J Clin Oncol 2011; 2:8–27
23.
Roqueplan F, Porcher R, Hamze B, et al. Long-term results of percutaneous resection and interstitial laser ablation of osteoid osteomas. Eur Radiol 2010; 20:209–217
24.
Earhart J, Wellman D, Donaldson J, Chesterton J, King E, Janicki JA. Radiofrequency ablation in the treatment of osteoid osteoma: results and complications. Pediatr Radiol 2013; 43:814–819

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Information & Authors

Information

Published In

American Journal of Roentgenology
Pages: 856 - 860
PubMed: 30085840

History

Submitted: December 2, 2017
Accepted: January 28, 2018
First published: August 7, 2018

Keywords

  1. navigational bipolar radiofrequency electrode
  2. osteoid osteoma
  3. radiofrequency ablation
  4. spine

Authors

Affiliations

Anderanik Tomasian
Department of Radiology, University of Southern California, 1520 San Pablo St, Los Angeles, CA 90033.
Jack W. Jennings
Mallinckrodt Institute of Radiology, St. Louis, MO.

Notes

Address correspondence to A. Tomasian ([email protected]).

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