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this has largely been superseded by percutaneous CT-guided thermal ablation over the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]
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.
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.
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]
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.
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.
