THERMAL MODE RADIOFREQUENCY IN LOW BACK PAIN

Lumbar Facet Joint Pain

Pain originating from the facet or zygapophyseal joints is responsible for about 15% or less of all low back pain complaints. The facet joints are true synovial joints and pain can be precipitated by various causes such as facet joint degeneration, intervertebral disc degeneration, postural abnormalities such as lumbar scoliosis, and problems arising from the bony structures such as collapse (due to osteoporosis or pathological fractures) or defects (e.g., spondylolisthesis). It can also come about as the result of repeated minor trauma.
These conditions result in arthritic changes in the facet joints, which in turn leads to inflammation and swelling. This stretches the joint capsule and creates pain.
Clinically, the patient presents with axial low back pain which is ill-defined and poorly localized with frequent vague (i.e., nonsegmental) radiation into the groin or thigh.
It tends to be posture-related and is usually worse at rest (sitting/standing) but helped by mobility. The pain can be quite bad at night and is frequently accompanied by early morning pain and/or stiffness.
On examination, the patient exhibits pain on extension, rotation, and lateral flexion of the lumbar spine; there is frequently tenderness over the affected facet joint(s) although this may be difficult to elicit in a well-built muscular patient. Sometimes there is hypersensitivity to light touch over the painful area.
In the absence of any concomitant pathology such as a prolapsed intervertebral disc, there are usually no abnormal neurological findings or specific changes on the MRI scan.
Degenerative changes of the joints themselves may or may not be seen on imaging, but there is no correlation between the degree of any degeneration and the pain.

Fig.1 Medial branch, lumbar posterior primary ramus

The diagnosis is made on the basis of the history, examination, and a diagnostic
The diagnostic block can be either an intra-articular block or, preferably, a medial branch block, using a short-acting local anesthetic such as 2 % lidocaine.
A positive diagnostic block is essential for reaching a diagnosis. Radiofrequency (RF) facet denervation is currently considered the standard treatment of facet-mediated persistent pain.

Lumbar Facet Joint Denervation

There are various techniques used to carry out lumbar facet joint denervation.

Anatomy

The treating doctor must be very familiar with the medial branch of the posterior primary ramus, this is the target (Fig.1 ). For facet denervation, target the medial branches at the levels you want to treat together with the medial branch to the level above.

Position of Patient

The patient should be lying prone on a radiolucent table; stand on the left side of the patient if you are right-handed and vice versa if you are left-handed.
With the C-arm image intensifier in the posteroanterior axis, obtain a clear view of the lumbar vertebrae; if necessary, adjust the position of the image intensifier so as to obliterate any double end plates. It is done by angling the image intensifier, which is in the posteroanterior axis, very slightly caudally. This maneuver results in the lower border becoming a single line on X-ray screening. Occasionally, the double end plate is removed by moving the axis of the C-arm image intensifier very slightly cranially.
The best place to start trying to locate the medial branch of the posterior primary ramus is the point where it enters the groove on the back of the vertebral lamina. For this you need to move the image intensifier from its initial posteroanterior axis (corrected for “double end plates”) obliquely away from the patient so as to obtain a good view of the so-called Scottie dog. Your preliminary target is the “eye of the dog”; this point overlies the medial branch.

Technique

Use a 25# needle to infiltrate the superficial tissues only; do not go down as far as bone, as you will anesthetize the medial branch and be unable to locate it by stimulation.
Insert a 22#, 100.5 mm (5 mm exposed tip) RF needle along the angle of the X-ray beam so as to hit the “eye of the Scottie dog” in tunnel vision.
Replace the RF needle stilette with the thermocouple electrode and try to locate the medial branch by sensory stimulation, using the following parameters on your machine:
Frequency: 50 Hz
Pulse width: 1 ms
Voltage: up to 0.5 V
NB! If you only manage to locate the nerve at a voltage greater than 0.5 V, keep looking! You are unlikely to produce an effective lesion here.
If you cannot locate the nerve on bone, then slip forward off bone and into the groove close to the intervertebral foramen and try again. If you still cannot locate the nerve, advance deeper and very slowly checking the position of your needle in the lateral axis. The tip of your needle must never lie anterior to an imaginary line passing through the posterior margin of the intervertebral foramen. If you lesion anterior to this point , you run the double risk of causing neuritis and of damaging the motor root . As you gain experience in the technique, you may decide to slip forward into the groove from the “eye of the Scottie dog” as a matter of routine. After identification of the nerve in the groove means that you are using the shaft of the needle as opposed to its tip, and many workers consider it to be a better way of obtaining a permanent lesion.
Once you have achieved localization by sensory stimulation, test for motor stimulation using the following parameters on your machine:
Frequency: 2 Hz
Pulse width: 1 ms
Voltage: double the sensory threshold but at least 1 V
NB! It is very common to see localized contractions around the needle area (due to stimulation of the multifidus muscle by the motor component of the medial branch); these can safely be ignored. You are on the lookout for rhythmical contractions in the lower limb. Should these appear, reposition the needle.
You are now ready to carry out a lesion.
Preset the timer to 60 s.
Preset the temperature maximum to 85 °C.
Remove the thermocouple electrode and inject 1 ml of 2 % lidocaine through the needle.
Replace the electrode.
Switch your machine to lesion mode and gradually increase the power, which will in turn cause a temperature rise. When the temperature reaches 80 °C, switch the timer on, in order to create the lesion. When the lesion has been performed, remove the electrode and inject 1 ml of a mixture of 0.5 % bupivacaine plus a depot steroid preparation in order to reduce postprocedure discomfort.

The Medial Branch of the L5 Posterior Primary Ramus

Your target here is slightly different. With the image intensifier in the posteroanterior axis, visualize the sacrum; your target is the junction between the superior articular process and the upper surface of the lateral part of the sacrum (Fig.2); very often you can locate the medial branch here without needing to move the image
intensifier off the posteroanterior axis; instead, you may find it useful to angle your needle, departing from strict “tunnel vision.” Hunt for the nerve as already outlined above.

Fig-2: Right L5 root projection.

If you cannot locate the nerve on bone, then slip forward off bone and into the groove close to the intervertebral foramen and try again. If you still cannot locate the nerve, advance deeper and very slowly checking the position of your needle in the lateral axis.
The tip of your needle must never lie anterior to an imaginary line passing through the posterior margin of the L5 intervertebral foramen. If you lesion anterior to this point, you run the double risk of causing neuritis and of damaging the motor root.
As you gain experience in the technique, you may decide to slip forward into the groove from bone as a matter of routine. After identification of the nerve in the groove means that you are using the shaft of the needle as opposed to its tip, and many workers consider it to be a better way of obtaining a permanent lesion.

Branch from S1

This lies just lateral to the S1 foramen (Fig-3); you do not need a motor test at this point.

 

Aftercare

Warn the patient about temporary numbness and limb weakness due to the local anesthetic; do not discharge the patient until you are certain that he/she can walk unaided. Warn the patient about residual soreness, which may last for a couple of weeks; this usually readily responds to NSAID therapy.

Radiofrequency (RF) lesioning

Radiofrequency (RF) lesioning refers to the delivery of high-frequency electrical current in the RF range (≈500 kHz) to patient tissue via an RF electrode to induce a biological effect, such as the thermal destruction of nerves that carry painful impulses. RF methods used in pain management today can be subdivided by the following broad characteristics, each of which involves different physical and clinical considerations.
• Waveform / Set Temperature
– Thermal RF ( TRF ): The sustained tissue temperature exceeds 42 °C grossly.
A continuous RF (CRF) waveform and tissue temperatures in the range of 70–90 °C are typical. The clinical objective is gross thermal nerve ablation. This category includes “cooled RF” methods, where the electrode is internally cooled, but induced tissue temperatures are neurolytic.
– Pulsed RF ( PRF ): The tissue temperature is held at or below 42 °C on average.
RF is delivered in short high-intensity bursts so that the RF electric field strength is increased without gross heating. The clinical objective is neural modification by electric and thermal fields (Cosman and Cosman 2005), but the pain-relief mechanism remains under scientific investigation, as described later by Cahana et al.
• Electrode Polarity
– Monopolar RF : Current passes between a needle electrode and a large-area reference ground pad. RF current intensities are highest near the needle electrode’s uninsulated tip. In monopolar thermal RF, an ellipsoidal heat lesion is generated. With proper full adhesion of the ground pad to the skin, current densities are low over the pad’s large area, and thus nearby tissue is not typically elevated to lesion levels.
– Bipolar RF : Current passes between two needle-electrode tips, and the current density is high at both locations. Thus, in bipolar thermal RF, a heat lesion is generated near both tips. When parallel tips are brought close together, the electric field is focused between the tips and a large “strip” lesion is formed (Fig.4).

Spacing  10 mm	12mm	15mm
Fig.4: Bipolar lesion size for 20 gauge, 10 mm tip length, 90 Celsius, 3 min and increasing spacing.

Monopolar thermal RF is the most common and basic form of RF treatment and has been used widely in pain management and neurosurgery since the earliest RF generators were built by B. J. Cosman, S. Aranow, and O. A. Wyss in the early 1950s (Sweet and Mark 1953; Cosman and Cosman 1974, 1984). In the 1990s, monopolar pulsed RF was introduced by Sluijter, Cosman, Rittman, and van Kleef (1998) and is used where conventional thermal RF is contraindicated (e.g., neuropathic pain) or could be potentially hazardous (e.g., DRG lesioning).

Bipolar thermal RF between parallel electrodes has been used in pain management for the last decade (Ferrante et al. 2001; Burnham et al. 2007), but only recently has the large size of bipolar RF lesions been fully appreciated (Cosman and Gonzalez 2011). A pioneering application of bipolar pulsed RF has been reported, and this was in the
treatment of carpel tunnel syndrome pain (Ruiz-Lopez 2008).
In one author’s clinical experience (CAG), there are some basic rules which should be followed in RF lesioning. Thermal RF should be used only for treatment of nociceptive pain. RF should not be used in patients with marked psychological overlay and/or drug dependency. RF should not be used in patients with total body pain.
You should ensure that the patient has realistic expectations since the total abolition of pain may not be possible. You should exhaust all other nondestructive forms of treatment first and achieve unequivocal benefit from preliminary prognostic blocks.

Monopolar Thermal RF

Using standard equipment, the steps for monopolar RF lesioning in the spine typically include the following steps:
1. Place the ground pad on the skin near the treatment site.
2. Place the RF cannula percutaneously near the target nerve.
3. Stimulate: The RF electrode delivers sensory and motor nerve stimulation to ensure that the cannula’s tip is near the target nerve and distant from nontarget nerves.
4. Inject anesthetic through the cannula to prevent pain during lesioning.
5. Lesion: The electrode delivers RF current to the cannula’s tip and the nearby nerve(s) are lesioned with temperature control.

The RF cannula is typically a hollow 22G, 21G, 20G, 18G, or 16G needle that is fully insulated except at the tip. The cannula’s hollow interior accepts either (a) a stilette to make the cannula solid for insertion, (b) injected fluid anesthetics and steroids, or (c) a 28G thermocouple (TC) electrode for tip temperature measurement and delivery of stimulation and RF currents. In some applications, such as cordotomy, DREZ, brain, and even spinal lesioning, the electrode and cannulae are integrated into a single device. X-ray guidance is typically used to position the cannula nearby the target nerve by reference to bony landmarks. Once positioned, the cannula’s stilette is removed and is replaced by the electrode. The operator then seeks the nerve by sensory stimulation, which are low-voltage electrical pulses delivered at 50 Hz (pulses per second). A stronger sensory response at a lower voltage indicates the cannula’s tip is closer to the nerve. In the clinical experience of one author (CAG), the cannula needs to be within 3 mm of the nerve in order to create an adequate heat lesion, and a stimulation level of at most 0.6 V is indicative of this.
The operator should always ensure that the cannula/electrode is not dangerously close to any motor nerve in the vicinity of the sensory nerve he/she is trying to lesion. To accomplish this, low-frequency motor stimulation pulses are delivered at 2 Hz. In the clinical experience of one author (CAG), if no muscle twitch in the territory of the nerve is noted at twice the voltage strength necessary to achieve sensory stimulation, it can be safely assumed that there are no motor paths within 3 mm of the needle and that, consequently, there is no risk of damage to any motor nerve.
When working on spinal nerves, e.g., medial branches of posterior primary rami, one should not worry about localized contractions close to the area of needle insertion; one is concerned with motor twitches at more distant sites, e.g., the arm orthe leg.
When the operator is satisfied that the needle is safely in position, RF current is delivered to the electrode and cannula. Frictional heating occurs near the cannula’s uninsulated tip due to tissue electrolytes being pulled to and fro by the RF current alternating at approximately 500 kHz (500,000 cycles per second). While heating occurs only in the tissue and not within the electrode, within a few seconds of sustained RF heating, the temperature measured in the electrode/cannula’s tip registers the maximum tissue temperature (Cosman and Cosman 2003; Cosman 2010) (Fig.5).

Fig-5: Monopolar thermal RF lesion zone and the 45 Celsius isotherm.

This occurs due to coherent heat diffusion into the electrode tip from all sides. This maximum temperature can be directly controlled by the operator. It must be cautioned that for cooled RF, where the electrode is cooled by internally circulating water, the electrode does not measure the maximum tissue temperature; rather, the maximum tissue temperature occurs at a variable location remote of the electrode and can far exceed the temperature measured within or nearby the electrode (Wright 2007). As the current is applied at the destructive levels typical of thermal RF, a well circumscribed heat lesion appears. It will grow until a steady state is reached; at this point, the passage of current only maintains the temperature. Little further spread takes place at the edge of the lesion, since (a) the electric field and rate of heating decrease with distance from the electrode and (b) the rate of RF heating within the lesion volume is roughly balanced by the rate of heat diffusion into the surrounding tissue, heat diffusion into the electrode shaft, and blood-flow cooling.

37                     44                   51              58                 66  Celsius

Fig-6: Monopolar thermal RF: Electric field (above), steady state tissue temperature (below), and the heat lesion boundaries (black)

The heat lesion is shaped like a match head (Fig-6) and is commonly defined as the tissue regions for which the temperature exceeds 45–50 °C for at least 20 s (Brodkey 1964; Dieckmann 1965; Smith 1981; Cosman and Cosman 1974, 1984).
Though permanent neurological damage occurs when tissue is exposed to temperatures exceeding 42 °C over longer durations (Cosman et al. 2009), for practical purposes, when we talk about lesion size, we mean the volume of tissue within the 45 °C isotherm (Fig-5). According to Abou-Sherif et al. (2003), thermal RF produces the following effects in the rat sciatic nerve at 6–8 weeks: Wallerian degeneration in all nerve fibers, physical disruption of the basal laminae, focal disruption of the perineurium, degranulation of mast cells, recruitment of exogenous macrophages, local muscle necrosis, delayed axonal regeneration, and prolonged
changes in the microvascular bed (vascular stasis) with extravasation of erythrocytes, this latter resembling the ischemic changes of reperfusion injury.
The heat lesion extends maximally around the shaft of the cannula, with a diameter that ranges from 2 to 10 mm depending on the cannula’s diameter/gauge, the tip temperature, and lesion time. The lesion extends 1–2 mm both ahead of the tip and up the shaft, yielding a total length 2–3 mm longer than the tip length
(Cosman and Cosman 1984). Because of this geometry, many physicians prefer “parallel”/“side-on” cannula placement for monopolar thermal RF lesioning so that the nerve is positioned at the side of the cannula tip where the lesion extends maximally.
In the alternative “perpendicular”/“point-on” approach, the nerve is placed directly ahead of the cannula tip, thus exposing a smaller volume of the nerve to neurolytic temperatures.

For a given electrode/cannula tip temperature, if lesion size is plotted against exposure time, it will be observed that the size increase is relatively linear over the early part of the curve, but then begins to slow as the steady state is approached. For electrode/cannula of the sizes used in pain management, the steady state lesion size is not reached until 30–90 s after the tip temperature reaches its set value. Thus, the tip should be held at the desired temperature for this duration of time to ensure that the lesion has reached its full spread for that temperature. The steady state lesion size is strongly influenced by the tip temperature and electrode/ cannula diameter. All other things being equal, a larger heat lesion will be produced by a larger electrode tip and a higher tip temperature (assuming that boiling does not shut down RF current flow). Additionally, several factors can affect lesion size and dynamics, including variations in tissue densities, proximity to bone,
and proximity to CSF (especially in trigeminal lesions), blood vessels, etc.
It is advisable to keep the tissue temperature below boiling (100 °C). Boiling can lead to uncontrolled gas discharges, burning steam that travels up the electrode’s shaft to the skin, irregular lesion geometry, and charring at the electrode tip. In one author’s clinical practice (CAG), the lesion temperature is held below 85 °C to give a broad temperature margin relative to 100 °C.
The resistance to the flow of electrical current from the tip of the cannula, the impedance, can be measured and should be observed by the operator. A very high impedance, or open circuit, can indicate that the electrode or ground pad is not in proper contact with the patient or that the cables are disconnected. A rising, high
impedance can also indicate that the tissue is boiling at the cannula’s tip, since electrical current cannot easily traverse boiling gas bubble; this is an important safety check in case the temperature sensor is broken or misplaced outside the cannula’s tip (Cosman 2010). A very low impedance, or short circuit, can indicate a failure of the RF equipment or direct contact between the electrode and the ground pad or contact with a large metallic implant. Impedance can also be of use in certain procedures since it can indicate the tissue type in which the cannula’s tip is positioned.
For example, during a percutaneous cordotomy, the impedance will be 400 Ω when the tip is in the extradural tissues, fall to 200 Ω as the needle tip enters the CSF, and then rise to over 800 Ω as the needle tip enters the spinal cord. When working in the intervertebral disc, the impedance is usually very high in the outer annulus, falling to less than 200 Ω in the nucleus pulposus.
For facet denervations, some physicians use “pole needles.” These are nontemperature-monitoring, tissue-piercing electrodes with integrated, flexible, fluid injection lines. They are used when it is felt that the electrode position must not be perturbed through stimulation, injection, and lesioning. Typically, 20 V is applied with the expectation of producing an 80 °C heat lesion. However, in vivo clinical experiment shows that the tip temperature is not consistently 80 °C but rather can range from values less than 80 °C to those exceeding boiling (Buijs et al. 2004; Gultuna et al. 2011). As such, when pole needles are used, one should halt RF delivery if an impedance rise is observed that indicates tissue boiling; and when precise lesion control is required, one should use temperature-monitoring injection electrodes.
There are several radiofrequency lesion generators in common use around the world. We use Inomed N50 and MultiGen Stryker.

Bipolar Thermal RF

Whereas a monopolar configuration drives RF current between an electrode’s exposed tip and a distant ground pad, a bipolar configuration drives RF current between two nearby electrode tips. As bipolar electrode tips are brought closer together, the resulting thermal lesion shape transitions from that of two volumes surrounding each tip separately to that of a single volume connecting the tips (Fig-4). The connected geometry and larger total lesion volume are strongly influenced by a focusing of the electric and current density fields between closely spaced electrode tips. Bipolar electrodes can be arranged collinearly or in parallel, but parallel arrangements produce the largest lesion size increases (Cosman et al.1984). Important features of parallel bipolar heat lesions include:
• Large : Bipolar RF lesions are larger than cooled RF lesions as used in pain management. The size of one bipolar RF lesion is roughly that of three conventional monopolar RF lesions placed side by side (Fig-7).

Fig-7 Comparison of thermal bipolar RF lesion size with that of cooled and conventional monopolar RF.

• Conformal: Bipolar RF applied to closely spaced electrode tips produces heat lesions shaped like a rounded brick, also known as a “strip lesion.” To conform to anatomical constraints, the width and length of the strip can be adjusted nearly independently of each other and the lesion depth. As such, a large lesion can be produced without unnecessary damage to healthy tissue and with reduced risk to sensitive structures. This is not possible for monopolar lesions around a cylindrical electrode since the lesion width and depth are the same.
• Connected strip lesions : By leapfrogging electrodes (Ferrante et al. 2001), bricklike strip lesions can be placed side by side without gaps to produce an elongated lesion zone that has consistent height and thickness (Figs-7).
This is not possible for cooled and conventional monopolar RF without positioning electrodes very close together.
• Robust : Strip lesions can be generated reliably for parallel tip spacings of 10 mm, tip temperature 90 °C, and lesion time 3 min. Perturbations of these geometric and RF parameters do not substantially affect lesion size (Cosman and Gonzalez As an example, all these features are illustrated by the RF palisade approach to sacroiliac joint (SIJ) denervation (Fig-8).

Fig-8: Palisade sacroiliac joint denervation.

In this approach, four to five large bipolar RF lesions are placed side by side like bricks in wall to traverse the region between the dorsal sacral foramina and SIJ line in which sacral lateral branch nerves form the SIJ’s dorsal innervation. While each lesion is large in the inferior-superior direction, its depth is constrained in the left-right direction, thus reducing the risk of damage to the sacral nerve roots. Because lesion size is robust to variations in tip spacing and because adjacent lesions overlap, the total lesion zone has a consistent thickness and height from the sacral surface.

Bipolar RF lesions of the sizes shown in Fig-4 have been used successfully in pain management (Ferrante et al. 2001; Burnham et al. 2007; Cosman and Gonzalez 2011). Ex vivo experiments by Cosman and Gonzalez (2011) document further flexibility in the size and shape of bipolar lesions. Indeed, bipolar lesions with dimensions exceeding 2 cm can be readily created with standard RF equipment. As for all RF lesioning, before the clinical use of novel bipolar configurations, a physician must consult lesion-size studies to determine whether that configuration is appropriate for the target anatomy. The proximity of target nerves to nontarget nerves, blood vessels, skin surface, and other sensitive structures imposes an upper bound on the safe size of any heat lesion, especially in the spine.