Radiofrequency (RF) ablation is the use of radio waves to create a current that results in heat. The name derives from the commonality in frequency of the alternating current used in RF with AM radio waves in the electromagnetic spectrum. In the context of chronic and cancer pain, RF currents pass through an electrode that is placed nearby a nociceptive sensory nerve or spinal pathway, to produce a thermal lesion. This electrical current, interrupts pain transmission and can therefore provide lasting relief in certain chronic pain conditions by targeting the contributing nerve. The RF energy causes damage to the biological tissues through protein denaturation, the extent of which is dependent on the temperature reached and the duration of time for which the heat is applied for.

Radiofrequency ablation for pain was first documented in the 1930s, with percutaneous cervical cordotomy being performed in the 1960s, before wider applications in the 1970s emerged. In the latter part of the 20th century, treatment started for the management of facet joint pain which still remains the most common type of pain treated by this procedure.1 In addition to changes in equipment and techniques, it has also evolved to provide relief for a plethora of conditions that extend beyond that of facet joint pain in modern day practice.

There are a number of different mechanisms used for the application of RF energy. These include: continuous, pulsed and cooled.

Continuous application is the conventional method of creating a thermal lesion. It can be delivered via a monopolar RF technique between the electrode and the patients’ tissue or bipolar RF that uses individual cathode and anode electrodes spatially placed closely together with a continuous RF current. Constant current is delivered at 500 kHz between the electrodes and the tissue to reach a target temperature of 80–90°C for 60–90 seconds. The temperature is detected by a thermocouple located in the tip. Of note, nerve damage occurs at 45°C and the lesion formation is dependent on both the temperature and duration.

Radiofrequency ablation yielding a heat lesion is not the only method in which this field has made an impact on chronic pain. Radiofrequency energy in short burst has been used as a non- or minimally neurodestructive technique that avoids the heat lesion. RF is delivered by either a monopolar or bipolar circuit similar to continuous, however the current is applied to the tissues in short cycles. The standard procedure is usually 90-150 seconds seconds in total, applying 500 kHz for 20 milliseconds with a 480 millisecond pause in-between. This allows any addition heat that is generated to dissipate resulting in minimal thermal injury and no resultant gross nerve damage, therefore providing a better risk profile than continuous application and a benefit to sensitive regions of the body. The active electrode temperature is maintained at 42°C and positioned perpendicular to the target.2 This process is thought to work by causing structural cellular damage, neuronal activation and gene expression alteration.

Bipolar techniques allow greater precision to the specific target and therefore reduce possible damage to surrounding structures.

When the lesion size required for analgesia is greater due to either multiple contributing sensory nerves in close proximity (e.g. sacroiliac joint denervation) or to ensure lesioning of a nerve with an anatomically variable path within a specific anatomical region, cooled also known as water cooled RF ablation (WCRF) can be used. This utilizes a multichannel electrode that is cooled by the continuous flow of water. The temperature is monitored by a thermistor and usually regulated to 60°C. WCRF allows for continuous RF and creates a greater lesion size in a controlled fashion without reaching higher temperatures that could subsequently damage the tissue. The resultant larger lesion, known as an isotherm, can be used in accessing deeper nerves or those with a more complex course.3

Regardless of type, the procedure is performed in a clinical area with trained professionals. Consent and consideration of monitoring in keeping with the Association of Anaesthetists standards are in place. Sedation is often not administered as it arguably improves the safety profile, allowing constant communication between patient and operator. This is particularly important when using sensory testing where patient feedback is required. Patients may require an intravenous cannula depending on the site of injection; or if there is a potential risk of complication for example, the possibility of a dural puncture. The procedure itself is carried out under aseptic conditions. Local anaesthetic at the surface of the skin anaesthetises the point of entry. A standard 16 to 23 gauge cannula or RF introducer is placed under fluoroscopy or ultrasound guidance. The tip should be parallel to the target nerve, then the needle or stylet is removed and replaced with the radiofrequency probe. The cannulae used are usually Teflon coated with an uninsulated Quincke tip. Sensory testing (if used) will then be carried out at 50–100 Hz whereby stimulation is increased in 0.1 V increments. The lower the sensory threshold, the closer the proximity of the electrode tip is to the target nerve. This in addition with ultrasound or fluoroscopic imaging ensures accurate needle placement. Following this and depending on site, motor stimulation at 2 Hz can be carried out at 1.5–2 V to ensure no motor nerve fibres will be affected by the lesion created.3 Local anaesthetic is often administered and allowed to disperse before the RF is applied.

As previously mentioned RF can be performed by either fluoroscopy or ultrasound guidance. Studies have not suggested one is superior in lesion formation or pain reduction. Ultrasound guidance gives additional visualization of the tendons and vessels, avoids radiation exposure, is portable and can be more cost effective. In addition it can provide real-time assessment but is user/technique dependent.4