The art of radiofrequency ablation

Introduction

Radiofrequency ablation (RFA) has emerged as an important technique in the management of chronic pain, offering targeted relief in various anatomical regions. This editorial delves into the applications and efficacy of RFA, examining its role in addressing chronic pain caused by different sources such as spinal and cervical facet joints, knee and hip joints. RFA provides an alternative for patients who have exhausted conservative therapies or are unsuitable for surgical interventions. Several different variations of RFA have been developed, namely continuous, cooled, and pulsed RFA. Each form is specifically designed to yield optimal results in different clinical situations, showcasing the technique’s ability to be customized to the unique needs of individual patients.

What is RFA

The primary objective of RFA within chronic pain management is to produce sufficient heat at a specific, targeted site, thereby damaging and ablating a nerve believed to be the source of a patient’s pain. Radiofrequency waves operate within a frequency range of 3 Hz to 300 GHz. The heat generated by a radiofrequency wave varies based on several factors: the wave’s frequency, duration of application, the shape of the emitting device, and the tissue type receiving the RF wave. Small RF probes tend to produce higher energy flux resulting in greater heat production. This high energy flux at the RF (radiofrequency) probe is utilized to generate heat intense enough to cause necrosis of nearby tissue in RF ablations, aiming to ablate the targeted nerve (1).

During RFA, the RF probe’s tip induces the adjacent water molecules to align with the radiofrequency field’s direction. Throughout the procedure, this field rapidly alternates its orientation, causing the water molecules to vibrate. Through friction, this vibration generates heat. Consequently, the tissue near the probe heats up, while the probe itself remains cool. In human tissue, higher temperatures result in quicker cell death. For instance, at 45 °C, cell death occurs after roughly 20 seconds, whereas at 100 °C, it happens in less than a second (1). The required temperature and exposure duration to induce tissue necrosis play crucial roles in the development of various RFA techniques and determining their optimal usage.

Different types of RFA

Continuous RFA

In continuous RFA, a lesion is created using a frequency range of 0.1–1 MHz. The ablation process is maintained for 60–90 seconds at a temperature between 60–90 °C (1). A significant challenge of RFA arises when targeting larger areas (1,2). Ablating a larger target necessitates either a prolonged duration at a set energy level or an increased energy amount within a specific time frame. These methods result in a gradient of heat, with areas closer to the RF probe’s tip being hotter compared to regions farther away (1). As tissue nearest to the probe tip begins to necrose, heating tissue more distal from the tip becomes increasingly difficult, as the proximal necrotic tissue raises impedance to heating (1,2). To circumvent this thermodynamic limitation of continuous RFA, utilizing cooled RFA as an alternative method is a viable option.

Cooled RFA

Cooled RFA was initially employed in cardiac and tumor ablations before its adaptation for nerve ablation in chronic pain treatment. This technique addresses the limitations faced in continuous RFA, especially when targeting larger areas. In cooled RFA, a cooling fluid is circulated through the RF probe, moderating the temperature of the adjacent tissue during ablation. This process ensures a more uniform temperature distribution between the tissue closest to the probe and areas farther away, facilitating a consistent rate of tissue necrosis (1).

Pulsed RFA

In the realm of chronic pain management, pulsed RFA presents another effective technique for nerve ablation, alongside continuous and cooled RFA. Pulsed RFA operates at a lower temperature of 42 °C, in contrast to the 60–90 °C used in continuous RFA. This method involves delivering RF pulses ranging from 10 to 30 ms at a frequency of 1–8 Hz, with voltage regulation ensuring the temperature does not surpass 42 °C. The exact mechanism by which pulsed RFA enhances pain control remains a subject of debate among specialists. Several theories exist, one of which speculates that, similar to continuous RFA, pulsed RFA might create microscopic lesions primarily affecting A-delta and C pain fibers (1). Another hypothesis is based on observed reductions in cytokines such as TNF-alpha and IL6, suggesting a potential pathway for pain alleviation (1). This area continues to be a focal point of ongoing research and discussion.

Identifying good candidates for RFA

To be considered for RFA, patients must meet specific criteria. Initially, they should have unsuccessfully attempted more conservative pain control therapies (3). These typically encompass pharmacological treatments [such as Tylenol, nonsteroidal anti-inflammatories (NSAIDs), serotonin-norepinephrine reuptake inhibitors (SNRIs)] along with physical therapy. Furthermore, these conservative therapies should have been pursued for a minimum of three months (2). Additionally, candidates for RFA should undergo a diagnostic nerve block. This procedure involves identifying the nerve suspected of causing pain, often using ultrasound or fluoroscopic guidance, followed by an injection of local anesthetic around the nerve. Eligibility for RFA requires patients to experience at least a 50% reduction in pain from at least one diagnostic block (3). Many institutions mandate a second diagnostic block, with a 50% pain reduction in both, to qualify for RFA. On the other hand, patients who have previously undergone RFA with pain improvement lasting at least three months are eligible for a repeat procedure. However, it is recommended not to have more than two RFA treatments on the same nerve within a year (3).

Specific RFA targets for various types of pain

There are many different targets for RFA in the treatment of chronic pain. Due to the limited length, this editorial will focus on just a few of the most common RFA targets among these patients. We will explore spinal and cervical facet joints, knee joints, and hip joints RFA. There are numerous other potential targets for RFA including thoracic facet joints, ankle joints, and shoulder joints.

Lumbar facet joints

Low back pain is a rampant issue in patients suffering from chronic pain. Treating spinal facet joints with RFA presents a viable treatment option for this condition. Specifically, a lumbar medial branch nerve RFA can be considered for patients with lower back pain. Numerous studies have underscored the effectiveness of RFA in such cases. For instance, Rosenblum et al. reported that “60% of patients undergoing lumbar medial branch RFA experienced 90% pain relief at 12 months, and 87% experienced 60% relief” (3).

In a lumbar medial branch block, the procedure is executed under fluoroscopic guidance with the patient in the prone position (4,5). Optimal visualization of the facet joints is achieved by angling the C-arm 25–30 degrees caudally. The skin over the target area is anesthetized with a local anesthetic. The key objective during the procedure is to position the RFA probe “between the superior margin of the transverse process and the superior articular process of the facet” (4). Typically, the probe is advanced over the transverse process’s superior margin to align it adjacent to the medial branch nerve (4,6). The procedure involves targeting multiple spinal levels, placing an RFA needle at each level. Once the needles are positioned, testing occurs first at a frequency of 50 Hz and less than 0.5 V; this should produce a tingling sensation in the patient without causing motor stimulation in the lower extremities (4,7). Then, a local anesthetic, usually 0.5 mL of 1–2% lidocaine, is injected through the RFA needles. Finally, continuous RFA is conducted at a temperature of 80 °C for 90 seconds (4).

Cervical facet joints

Patients with chronic neck pain, like those with low back pain, experience a statistically significant reduction in pain following cervical medial branch RFA. MacVicar et al. reported that 60–75% of patients undergoing cervical medial branch RFA noticed a lasting improvement in their chronic pain, with a median duration of 17–20 months (3,8). Similar to lumbar medial branch RFA, cervical facet RFA is performed in the prone position under fluoroscopic guidance. During a cervical medial branch RFA, the medial branch nerves at the articular pillar level are targeted. The procedure begins with the injection of local anesthetic at the skin’s surface, followed by advancing the RFA needle towards the articular pillar’s midpoint at the desired cervical level. Once the needle contacts the bone, a lateral fluoroscopic image is taken to confirm its placement. Sensory and motor stimulation assessments then proceed, with frequency gradually increasing from 0 to 50 Hz. Needles are placed at a minimum of two spinal levels. It is crucial that there is no muscle stimulation in the upper extremities; if such stimulation occurs, the needle must be repositioned or withdrawn. Subsequently, local anesthetic is administered through the RFA needles, and continuous RFA is performed at 80 °C for 90–150 seconds (9).

Knee joints

Chronic knee pain and knee osteoarthritis, like chronic back and neck pain, are common in patients visiting pain clinics. Many such patients are initially considered for total knee arthroplasty. However, some are deemed unsuitable for surgery due to medical comorbidities (10). Notably, about 44% of patients continue to experience chronic knee pain even after undergoing total knee arthroplasty (10-12). For these patients, genicular nerve RFA presents an alternative treatment option.

Candidates for genicular nerve RFA are those who have responded positively to two diagnostic genicular nerve blocks. The procedure is typically carried out under fluoroscopic guidance, with the patient lying supine and the knee flexed at 30 degrees (10). The objective of genicular nerve RFA is to ablate the superomedial, inferomedial, and superolateral genicular nerves. Fluoroscopy assists in targeting these nerves as they “run across the periosteum near the femur’s bilateral epicondyles and the tibia’s medial epicondyle” (10).

After administering local anesthetic, RFA needles are carefully positioned under fluoroscopic guidance towards the superomedial, inferomedial, and superolateral genicular nerve targets. The needle placement is verified with a lateral image, ensuring the depth is “midway down the diaphysis of the femur and tibia” (10). Following this, local anesthetic is injected through the RFA needles. As with previous RFA procedures, sensory and motor stimulation tests are performed before creating a lesion. Traditionally, continuous RFA has been the method of choice for genicular nerve RFA. However, recent studies have begun exploring the efficacy of cooled RFA instead. Although the data are still emerging, these studies indicate promising outcomes, with up to 74% of patients experiencing a 50% reduction in pain at a 6-month follow-up after undergoing cooled RFA. Pulsed RFA is another potential treatment for chronic knee pain. Despite limited research and the small scale of existing studies on genicular nerve pulsed RFA, the initial results are encouraging (10). Further research is necessary to ascertain the relative effectiveness comparing continuous, cooled, and pulsed RFA against one another for treating chronic knee pain.

Hip joints

Similar to the knee joint, the hip joint is another frequent site of osteoarthritis, which often results in chronic pain. For patients who do not respond to conservative therapy, are unsuitable for hip replacement surgery, or continue to experience pain post-hip replacement, hip joint RFA can be considered. The procedure targets the articular branches of the femoral and obturator nerves (13). Performed under fluoroscopic guidance in the supine position, the optimal target for the femoral articular branch is the superior aspect of the acetabulum at the midline of the acetabulum. For the obturator articular branch, two locations are identified. The first is adjacent to the “teardrop” shape, which is formed at the “junction of the ischium and pubis inferiorly” (13,14). The second site is slightly caudal to the bottom of this teardrop shape (13).

As with the previously mentioned RFA procedures, the process begins by positioning the RFA needles, followed by the injection of local anesthetic, such as 1 mL of 2% lidocaine (14). Motor stimulation is then tested at 2 Hz to ensure there is no contraction of the lower extremity muscles (13). Typically, cooled RFA is employed, using a temperature of 60 °C for 120 seconds at each needle site (13,14). As an alternative, continuous RFA can be performed at 80 ˚C for 90 seconds (13).

Unique targets

Recent research examined the efficacy of RFA and pulsed RF for several new targets to treat conditions as headache (15), trigeminal neuralgia (16), occipital neuralgia (17), intercostal neuralgia (18), meralgia paresthetica (19), groin pain (20), superior cluneal neuralgia/chronic low back pain (21), dorsal root entry zone (22), greater trochanteric pain syndrome (23), foot pain (24), and shoulder pain (25).

Conclusion

The efficacy of RFA, whether continuous, cooled, or pulsed, is demonstrated in its ability to provide targeted pain management, especially for patients who have limited options due to the ineffectiveness of conservative treatments or surgical constraints. Understanding all parameters on the machine, nerve types and function, surrounding structures, needle and probe types and sizes are all important to be able to perform RFA or pulsed RF safely and effectively to treat different pain conditions.

Acknowledgments

Funding: None.

Footnote

Provenance and Peer Review: This article was commissioned by the editorial office, Annals of Palliative Medicine, for the series “Advances in Radiofrequency Ablation”. The article did not undergo external peer review.

Conflicts of Interest: Both authors have completed the ICMJE uniform disclosure form (available at https://apm.amegroups.com/article/view/10.21037/apm-24-3/coif). The series “Advances in Radiofrequency Ablation” was commissioned by the editorial office without any funding or sponsorship. A.A.E. served as the unpaid Guest Editor of the series and serves as an unpaid editorial board member of Annals of Palliative Medicine from June 2022 to May 2024. He also serves as a consultant for Avanos. The authors have no other conflicts of interest to declare.

Ethical Statement: The authors are accountable for all aspects of the work in ensuring that questions related to the accuracy or integrity of any part of the work are appropriately investigated and resolved.

Open Access Statement: This is an Open Access article distributed in accordance with the Creative Commons Attribution-NonCommercial-NoDerivs 4.0 International License (CC BY-NC-ND 4.0), which permits the non-commercial replication and distribution of the article with the strict proviso that no changes or edits are made and the original work is properly cited (including links to both the formal publication through the relevant DOI and the license). See: https://creativecommons.org/licenses/by-nc-nd/4.0/.

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Alaa Abd-Elsayed