Thermal (Traditional, Conventional, or Continuous) Radiofrequency

Thermal RF was first introduced in the 1930s. The thermal lesion is created by the introduction of an insulated electrode with an active tip of (2 to 10 mm), a dispersal plate, and a power source to complete the circuit (Fig. 15-1).

Fig. 15-1 The radiofrequency circuit.

High-frequency electrical current is then applied adjacent to the structure of the nerve that is intended to ablate; such current leads to ionic oscillation and frictional dissipation of the ions and electrolytes, which produce heat. The heat produced is directly related to the amplitude of the applied current and electrode size and indirectly related to distance from the electrode. The tip of the electrode measures the tissue temperature. Larger lesions are created by increased temperature, size of the electrode, and duration of applied current. Monopolar lesioning is performed when one electrode is used. Bipolar lesioning occurs when two electrodes are used in close proximity to one another. Laboratory evidence suggests that cellular damage occurs at temperatures of 60° to 65°C. As suggested by the aforementioned isotherm mapping from the electrode tip, 80° to 85°C is required at the needle tip, where the desired 60° to 65°C is achieved in the surrounding tissue, producing a prolate ovoid-shaped lesion (Fig. 15-2). Therefore, because the lesion created does not extend distal to the electrode, proper electrode position is parallel to the target nerve.

Fig. 15-2 Traditional thermal radiofrequency lesion.

(Courtesy of Dr. Nagy Mekhail, MD, PhD, Cleveland Clinic.)

The neurotomy created by the thermal technique is dependent on proximity to the targeted nerve, size of electrode, the temperature (or amplitude of current) applied, and the duration of lesioning. When applied immediately adjacent to the dorsal root ganglion, temperatures of 45°, 55°, 65°, 75°, and 85°C produce complete destruction of unmyelinated and near complete destruction of myelinated fibers.⁸

Pulsed Radiofrequency

Pulsed RF was first introduced in 1998, and the aforementioned circuit is used. Interestingly, there is no creation of a histologic lesion, and no nerve degeneration occurs (no Wallerian degeneration). The treatment is produced by providing current of 50,000 Hz in 20-msec pulses at a frequency of 2 per second. The temperature is maintained to be below 42°C or 45 V (Fig. 15-3). The greatest current density delivered is at the tip of the electrode; so ideal placement is perpendicular to the target nerve.

Fig. 15-3 Pulsed radiofrequency current parameters.

The mechanism of the therapeutic treatment is less clear compared with thermal RF and is surrounded by controversy. The mechanism appears to be independent of temperature and is neuromodulatory in nature,^9–12 and one resounding conclusion can be made: There is no histologically detected cellular destruction.

Cooled Radiofrequency

Cooled RF is a misnomer. The neurotomy produced is secondary to thermal destruction; however, the cooled RF uses a lower temperature than traditional thermal RF lesioning (Fig. 15-4). The traditional RF lesion size is limited by the charring of tissues around the electrode as the temperature of tissues rises. On the other hand, cooled RF maintains the temperature adjacent to the electrode at a desired lower level to prevent tissue charring, hence allowing the lesion to expand and increase in size.¹³

Fig. 15-4 Temperature distance comparison for cooled and thermal radiofrequency.

The resultant lesion is spherical in shape (Fig. 15-5). The radius of the spherical lesion equals the length of the active tip of the electrode. Simply, the tip of the electrode is the center of the spherical lesion. Therefore, the cooled RF lesion is eight times by volume the thermal RF lesion for the same length electrode tip.¹³ Whereas the spherical lesion projects distally from the electrode, conventional thermal RF produces a prolate ovoid parallel to the active tip of the needle (see Fig. 15-2) with little or no projection distally. Consequently, unlike thermal RF, which requires parallel electrode placement adjacent to the target nerve, cooled RF lesions are independent of electrode orientation.¹⁴

Fig. 15-5 Cooled radiofrequency lesion.

(Courtesy of Dr. Nagy Mekhail, MD, PhD, Cleveland Clinic.)

Summarily, and as described, important differences regarding lesion size, electrode placement, and neural destruction need to be noted (Table 15-2).

Postprocedure Pain and Neuritis

There is sparse literature describing the complications associated with RF treatments (pulsed, conventional, or cooled) for facetogenic pain.^7,20–29 No cooled RF of the lumbar median branches has been formally reported, although some advocate its use. Kornick et al³⁰ performed a retrospective review over a 5-year period at the Mayo Clinic in Jacksonville, Florida, in a total of 92 patients receiving 616 RF lesions (80°C for 90 seconds) during 116 separate procedures. Of the 616 lesions, six complications were noted—three cases of localized pain at the RF sites lasting more than 2 weeks and three cases of neuritic pain lasting less than 2 weeks (Table 15-3). No motor, sensory, vascular, or infectious complications were noted.³⁰ A case report of neuroma formation after multiple RF ablation procedures has been described. It was successfully treated with an open minimally invasive medial branch neurectomy.³¹

Frequently, steroids are injected after denervation, which seems counterintuitive because the goal in thermal rhizotomy is to create a histologically detectable lesion, and although anecdotal evidence suggests postoperative neuritis reduction, studies are lacking. No studies to date have compared efficacy with long-term follow-up of steroid application after conventional or cooled neurotomy.

Dobrogowsi et al³² investigated strategies to reduce the inflammatory pain associated with the lesioning. In a randomized prospective trial, patients were randomized to intraoperative methylprednisolone (10 mg), pentoxifylline (10 mg), or saline (1 mL). No “severe local tenderness” was reported in either the methylprednisolone group or pentoxifylline group, with minor tenderness resolving within 1 month, compared with the saline group, which had four of 15 patients with severe pain at 1 week and one patient for longer than 1 month. Other authors³³ report that 3-day dosage of diclofenac to be effective in reducing procedural pain after conventional RF neurotomy of lumbar median branches (Fig. 15-6).

Fig. 15-6 Lumbar median branches and dorsal ramus of L5.

Burn Injuries

Burn injuries have been reported early in literature and were the result of equipment malfunction, insulation breaks within the electrodes, a unipolar electrosurgical unit return plate, or unknown causes.^34–36 Whereas operating room power supplies are ungrounded systems, office buildings are typically grounded systems. This difference dramatically impacts the chance of burn injury (or macroshock) to the patient because isolated systems are more difficult to deliver aberrant current. Furthermore, burn severity is related to current density, either greater the current or the smaller the area applied.

Infectious Complications

There very few reported infectious complications. Barnsley³⁷ reported a superficial infection after cervical conventional RF that resolved after enteral antibiotics were administered.³⁸ Furthermore, some contend that the inherent heating of the tissue during the thermal lesioning may provide some means of bacteriocidal effect.⁶

Needle Placement

Proper needle placement during RF denervation is essential, as described in Chapter 7. Improper placement can lead to thermal destruction of nontargeted tissues, including the ventral rami. Although a serious concern, with proper motor testing before lesioning with an increase of stimulation to at least 2 V at 2 Hz, this complication should be avoided.