Effect of stimulation intensity of a differential target multiplexed SCS program in an animal model of neuropathic pain.

BACKGROUND: Spinal cord stimulation (SCS) has been proven to be an effective treatment for patients suffering from intractable chronic neuropathic pain. Recent advances in the field include the utilization of programs that multiplex various signals to target different neural structures in the dorsal spinal cord associated with the painful area. Preclinical studies have been fundamental in understanding the mechanism by which this differential target multiplexed programming (DTMP) SCS approach works. Transcriptomic- and proteomic-based studies demonstrated that DTMP can modulate expression levels of genes and proteins involved in pain-related processes that have been affected by a neuropathic pain model. This work studied the effect of the intensity of DTMP signals on mechanical hypersensitivity and cell-specific transcriptomes. METHODS: The spared nerve injury model (SNI) of neuropathic pain was induced in 20 animals which were 1:1 randomized into two SCS groups in which the intensity of the DTMP was adjusted to either 70% or 40% of the motor threshold (MT). SCS was applied continuously for 48 h via a quadripolar lead implanted in the dorsal epidural space of animals. Controls, which included a group of implanted SNI animals that received no SCS and a group of animals naive to the SNI, were assessed in parallel to the SCS groups. Mechanical hypersensitivity was assessed before SNI, before SCS, and at 48 h of SCS. At the end of SCS, the stimulated segment of the dorsal spinal cord was dissected and subjected to RNA sequencing to quantify expression levels in all experimental groups. Differential effects were assessed via fold-change comparisons of SCS and naive groups versus the no-SCS group for transcriptomes specific to neurons and glial cells. Standard statistical analyses were employed to assess significance of the comparisons (p < 0.05). RESULTS: SCS treatments provided significant improvement in mechanical sensitivity relative to no SCS treatment. However, the change in the intensity did not provide a significant difference in the improvement of mechanical sensitivity. DTMP regulated expression levels back toward those found in the naive group in the cell-specific transcriptomes analyzed. There were no significant differences related to the intensity of the stimulation in terms of the percentage of genes in each transcriptome in which expression levels were reversed toward the naive state. CONCLUSIONS: DTMP when applied at either 40% MT or 70% MT provided similar reduction of pain-like behavior in rats and similar effects in neuron- and glia-specific transcriptomes.

Spinal Evoked Compound Action Potentials in Rats With Clinically Relevant Stimulation Modalities.

OBJECTIVES: Rats are commonly used for translational pain and spinal cord stimulation (SCS) research. Although many SCS parameters are configured identically between rats and humans, stimulation amplitudes in rats are often programmed relative to visual motor threshold (vMT). Alternatively, amplitudes may be programmed relative to evoked compound action potential (ECAP) thresholds (ECAPTs), a sensed measure of neural activation. The objective of this study was to characterize ECAPTs, evoked compound muscle action potential thresholds (ECMAPTs), and vMTs with clinically relevant SCS modalities. MATERIALS AND METHODS: We implanted ten anesthetized rats with two quadripolar epidural SCS leads: one for stimulating in the lumbar spine, and another for sensing ECAPs in the thoracic spine. We then delivered two SCS paradigms to the rats. The first used 50-Hz SCS with 50-, 100-, 150-, and 200-µs pulse widths (PWs), whereas the second used a 50-Hz, 150-µs PW low-rate program (LRP) multiplexed to a 1200-Hz, 50-µs PW high-rate program (HRP). We increased SCS amplitudes up to the vMT in the first paradigm, and in the second, we increased HRP amplitudes up to the HRP ECAPT with a fixed amplitude (70% of the vMT) LRP. For each test case, we captured ECAPTs, ECMAPTs, and vMTs from each rat. RESULTS: vMTs were 3.0 ± 0.7 times greater than ECAPTs, with vMTs marginally (3.0 ± 3.6%) greater than ECMAPTs (mean ± SD) across all PWs with the first paradigm. With the second paradigm, we noted a negligible increase (3.6 ± 6.2%) on the LRP ECAP as HRP amplitudes were increased. CONCLUSIONS: Our results demonstrate reasonable levels of neural activation in anesthetized rats with SCS amplitudes appropriately programmed relative to vMT or ECMAPT when using clinically relevant SCS modalities. Furthermore, we demonstrate the feasibility of ECAP recording in rats with multiplexed HRP SCS.

Modulation of microglial activation states by spinal cord stimulation in an animal model of neuropathic pain: Comparing high rate, low rate, and differential target multiplexed programming.

While numerous studies and patient experiences have demonstrated the efficacy of spinal cord stimulation as a treatment for chronic neuropathic pain, the exact mechanism underlying this therapy is still uncertain. Recent studies highlighting the importance of microglial cells in chronic pain and characterizing microglial activation transcriptomes have created a focus on microglia in pain research. Our group has investigated the modulation of gene expression in neurons and glial cells after spinal cord stimulation (SCS), specifically focusing on transcriptomic changes induced by varying SCS stimulation parameters. Previous work showed that, in rodents subjected to the spared nerve injury (SNI) model of neuropathic pain, a differential target multiplexed programming (DTMP) approach provided significantly better relief of pain-like behavior compared to high rate (HRP) and low rate programming (LRP). While these studies demonstrated the importance of transcriptomic changes in SCS mechanism of action, they did not specifically address the role of SCS in microglial activation. The data presented herein utilizes microglia-specific activation transcriptomes to further understand how an SNI model of chronic pain and subsequent continuous SCS treatment with either DTMP, HRP, or LRP affects microglial activation. Genes for each activation transcriptome were identified within our dataset and gene expression levels were compared with that of healthy animals, naïve to injury and interventional procedures. Pearson correlations indicated that DTMP yields the highest significant correlations to expression levels found in the healthy animals across all microglial activation transcriptomes. In contrast, HRP or LRP yielded weak or very weak correlations for these transcriptomes. This work demonstrates that chronic pain and subsequent SCS treatments can modulate microglial activation transcriptomes, supporting previous research on microglia in chronic pain. Furthermore, this study provides evidence that DTMP is more effective than HRP and LRP at modulating microglial transcriptomes, offering potential insight into the therapeutic efficacy of DTMP.

Proteomic Modulation in the Dorsal Spinal Cord Following Spinal Cord Stimulation Therapy in an In Vivo Neuropathic Pain Model.

OBJECTIVES: Spinal cord stimulation (SCS) provides relief for patients suffering from chronic neuropathic pain although its mechanism may not be as dependent on electrical interference as classically considered. Recent evidence has been growing regarding molecular changes that are induced by SCS as being a key player in reversing the pain process. Here, we observed the effect of SCS on altering protein expression in spinal cord tissue using a proteomic analysis approach. METHODS: A microlead was epidurally implanted following induction of an animal neuropathic pain model. After the model was established, stimulation was applied for 72 hours continuously followed by tissue collection and proteomic analysis via tandem mass spectroscopy. Identified proteins were run through online data bases for protein identification and classification of biological processes. RESULTS: A significant improvement in mechanical sensitivity was observed following 48 hours of SCS therapy. Proteomic analysis identified 5840 proteins, of which 155 were significantly affected by SCS. Gene ontology data bases indicated that a significant number of proteins were associated to stress response, oxidation/reduction, or extracellular matrix pathways. Additionally, many of the proteins identified also play a role in neuron-glial interactions and are involved in nociception. CONCLUSIONS: The development of an injury unbalances the proteome of the local neural tissue, neurons, and glial cells, and shifts the proteomic profile to a pain producing state. This study demonstrates the reversal of the injury-induced proteomic state by applying conventional SCS therapy. Additional studies looking at variations in electrical parameters are needed to optimize SCS.

Modulation of Glia-Mediated Processes by Spinal Cord Stimulation in Animal Models of Neuropathic Pain.

Glial cells play an essential role in maintaining the proper functioning of the nervous system. They are more abundant than neurons in most neural tissues and provide metabolic and catabolic regulation, maintaining the homeostatic balance at the synapse. Chronic pain is generated and sustained by the disruption of glia-mediated processes in the central nervous system resulting in unbalanced neuron-glial interactions. Animal models of neuropathic pain have been used to demonstrate that changes in immune and neuroinflammatory processes occur in the course of pain chronification. Spinal cord stimulation (SCS) is an electrical neuromodulation therapy proven safe and effective for treating intractable chronic pain. Traditional SCS therapies were developed based on the gate control theory of pain and rely on stimulating large Aß neurons to induce paresthesia in the painful dermatome intended to mask nociceptive input carried out by small sensory neurons. A paradigm shift was introduced with SCS treatments that do not require paresthesia to provide effective pain relief. Efforts to understand the mechanism of action of SCS have considered the role of glial cells and the effect of electrical parameters on neuron-glial interactions. Recent work has provided evidence that SCS affects expression levels of glia-related genes and proteins. This inspired the development of a differential target multiplexed programming (DTMP) approach using electrical signals that can rebalance neuroglial interactions by targeting neurons and glial cells differentially. Our group pioneered the utilization of transcriptomic and proteomic analyses to identify the mechanism of action by which SCS works, emphasizing the DTMP approach. This is an account of evidence demonstrating the effect of SCS on glia-mediated processes using neuropathic pain models, emphasizing studies that rely on the evaluation of large sets of genes and proteins. We show that SCS using a DTMP approach strongly affects the expression of neuron and glia-specific transcriptomes while modulating them toward expression levels of healthy animals. The ability of DTMP to modulate key genes and proteins involved in glia-mediated processes affected by pain toward levels found in uninjured animals demonstrates a shift in the neuron-glial environment promoting analgesia.

Spinal cord stimulation using differential target multiplexed programming modulates neural cell-specific transcriptomes in an animal model of neuropathic pain.

Spinal cord stimulation is a proven effective therapy for treating chronic neuropathic pain. Previous work in our laboratory demonstrated that spinal cord stimulation based on a differential target multiplexed programming approach provided significant relief of pain-like behavior in rodents subjected to the spared nerve injury model of neuropathic pain. The relief was significantly better than obtained using high rate and low rate programming. Furthermore, transcriptomics-based results implied that differential target multiplexed programming modulates neuronal-glial interactions that have been perturbed by the pain process. Although differential target multiplexed programming was developed to differentially target neurons and glial cells, our previous work did not address this. This work presents transcriptomes, specific to each of the main neural cell populations (neurons, microglia, astrocytes, and oligodendrocytes), obtained from spinal cord subjected to continuous spinal cord stimulation treatment with differential target multiplexed programming, high rate programming, or low rate programming compared with no spinal cord stimulation treatment, using the spared nerve injury model. To assess the effect of each spinal cord stimulation treatment on these cell-specific transcriptomes, gene expression levels were compared with that of healthy animals, naïve to injury and interventional procedures. Pearson correlations and cell population analysis indicate that differential target multiplexed programming yielded strong and significant correlations to expression levels found in the healthy animals across every evaluated cell-specific transcriptome. In contrast, high rate programming only yielded a strong correlation for the microglia-specific transcriptome, while low rate programming did not yield strong correlations with any cell types. This work provides evidence that differential target multiplexed programming distinctively targeted and modulated the expression of cell-specific genes in the direction of the healthy state thus supporting its previously established action on regulating neuronal-glial interaction processes in a pain model.

Effects of Phase Polarity and Charge Balance Spinal Cord Stimulation on Behavior and Gene Expression in a Rat Model of Neuropathic Pain.

OBJECTIVE: To investigate the effect of phase polarity and charge balance of spinal cord stimulation (SCS) waveforms on pain behavior and gene expression in a neuropathic pain rodent model. We hypothesized that differing waveforms will result in diverse behavioral and transcriptomics expression due to unique mechanisms of action. MATERIALS AND METHODS: Rats were implanted with a four-contact cylindrical mini-lead and randomly assigned to two control (no-pain and pain model) and five test groups featuring monophasic, as well as charge-unbalanced and charge-balanced biphasic SCS waveforms. Mechanical and cold allodynia were assessed to measure efficacy. The ipsilateral dorsal quadrant of spinal cord adjacent to the lead was harvested post-stimulation and processed to determine gene expression via real-time reverse-transcriptase polymerase chain reaction (RT-PCR). Gene expression, SCS intensity (mA), and behavioral score as percent of baseline (BSPB) were statistically analyzed and used to generate correlograms using R-Studio. Statistical analysis was performed using SPSS22.0, and p < 0.05 was considered significant. RESULTS: As expected, BSPB was significantly lower for the pain model group compared to the no-pain group. BSPB was significantly improved post-stim compared to pre-stim using cathodic, anodic, symmetric biphasic, or asymmetric biphasic 1:2 waveforms; however, BSPB was not restored to Sham levels. RT-PCR analysis showed that eight genes demonstrated a significant difference between the pain model and SCS waveforms and between waveforms. Correlograms reveal a linear correlation between regulation of expression of a given gene in relation to mA, BSPB, or other genes. CONCLUSIONS: Our results exhibit that specific SCS waveforms differentially modulate several key transcriptional pathways that are relevant in chronic pain conditions. These results have significant implications for SCS: whether to move beyond traditional paradigm of neuronal activation to focus also on modulating immune-driven processes.

Electrical Stimulation of C6 Glia-Precursor Cells In Vitro Differentially Modulates Gene Expression Related to Chronic Pain Pathways.

Glial cells comprise the majority of cells in the central nervous system and exhibit diverse functions including the development of persistent neuropathic pain. While earlier theories have proposed that the applied electric field specifically affects neurons, it has been demonstrated that electrical stimulation (ES) of neural tissue modulates gene expression of the glial cells. This study examines the effect of ES on the expression of eight genes related to oxidative stress and neuroprotection in cultured rodent glioma cells. Concentric bipolar electrodes under seven different ES types were used to stimulate cells for 30 min in the presence and absence of extracellular glutamate. ES consisted of rectangular pulses at 50 Hz in varying proportions of anodic and cathodic phases. Real-time reverse-transcribed quantitative polymerase chain reaction was used to determine gene expression using the ∆∆Cq method. The results demonstrate that glutamate has a significant effect on gene expression in both stimulated and non-stimulated groups. Furthermore, stimulation parameters have differential effects on gene expression, both in the presence and absence of glutamate. ES has an effect on glial cell gene expression that is dependent on waveform composition. Optimization of ES therapy for chronic pain applications can be enhanced by this understanding.

Comparisons of Lesion Volumes and Shapes Produced by a Radiofrequency System with a Cooled, a Protruding, or a Monopolar Probe.

BACKGROUND: Radiofrequency (RF) ablation for denervation has been utilized for decades in chronic pain management. This relies on the proper targeting of the affected nerve which may be obtained by creating an ablation lesion with a shape and volume that optimizes targeting. Various systems designed to improve lesion size are available. These include cooling the active tip (cooled-RF) and protruding the RF electrode outside the active tip (PERF). OBJECTIVES: This study compares lesion volumes of 3 commercially available RF systems: cooled-RF, "V" shaped active cannula and protruding electrode (18 g and 20 g), and monopolar RF (MRF; 16 g, 18 g, and 20 g). STUDY DESIGN: Ex vivo study using clinically relevant conditions. SETTING: Biophysical laboratory in an academic institution. METHODS: RF ablation lesions were generated in additive-free chicken breast specimens (n = 10) with the RF probes fully inserted in them. For cooled RF, a 17 g probe (4 mm active tip) was used. RF was applied for 150 seconds at 60°C. PERF was applied using 18 g or 20 g introducers (10 mm active tip) for either 90 or 150 seconds at 80°C. For MRF ablation, introducers diameter were 16 g, 18 g, or 20 g (10 mm active tip), while RF was applied for 90 seconds at 80°C. Tissues were dissected through the midpoint of the lesion, and measurements of the longitudinal, transversal, and depth lengths were taken and used to calculate the lesion volume. Measurements from the distal edge in the transverse and longitudinal directions were also recorded. One-way ANOVA was used to determine statistical significance between volume means (P < 0.05). RESULTS: Mean lesion volume with cooled RF (595 mm3) is significantly larger than any other mean volume measured. The second largest volume is produced with MRF using a 16 g introducer (360 mm3), which is significantly larger than those obtained with 18 g or 20 g. This is also significantly larger than the one obtained with PERF using an 18 g introducer. Mean lesion volume produced with PERF (80°C for 90 seconds) and an 18 g diameter tip (215 mm3) is significantly larger than the respective one produced with MRF (169 mm3). Increasing lesioning time to 150 seconds significantly increases the volume (283 mm3). Using a 20 g tip produces the smallest lesions at 80°C for 90 seconds with either PERF or MRF, although a lesioning time of 150 seconds makes it significantly larger (207 mm3). LIMITATIONS: The study is ex vivo and therefore does not account for the dynamic effects of the anatomy and physiology of a living organism. CONCLUSIONS: The results indicate that the lesion produced with a cooled-RF system (17 g, 4 mm tip) is significantly larger than that produced with either of the other systems trialed (18 g or 20 g, 10 mm active tip protruding electrode or 16 g, 18 g, or 20 g monopolar electrode). Interestingly, a 16 g, 10 mm active tip monopolar electrode produced a larger lesion than the one produced with the 18 g protruding electrode. Key words: Radiofrequency, ablation, lesion shape, lesion size, cooled-RF, protruding electrode RF, monopolar RF.

Changes in Dorsal Root Ganglion Gene Expression in Response to Spinal Cord Stimulation.

BACKGROUND AND OBJECTIVES: Spinal cord stimulation (SCS) has been shown to influence pain-related genes in the spinal cord directly under the stimulating electrodes. There is limited information regarding changes occurring at the dorsal root ganglion (DRG). This study evaluates gene expression in the DRG in response to SCS therapy. METHODS: Rats were randomized into experimental or control groups (n = 6 per group). Experimental animals underwent spared-nerve injury, implantation of lead, and continuous SCS (72 hours). Behavioral assessment for mechanical hyperalgesia was conducted to compare responses after injury and treatment. Ipsilateral DRG tissue was collected, and gene expression quantified for interleukin 1b (IL-1b), interleukin 6 (IL-6), tumor necrosis factor α (TNF-α), GABA B receptor 1 (GABAbr1), substance P (subP), Integrin alpha M (ITGAM), sodium/potassium ATP-ase (Na/K ATPase), fos proto-oncogene (cFOS), serotonin receptor 3A (5HT3r), galanin (Gal), vasoactive intestinal peptide (VIP), neuropeptide Y (NpY), glial fibrillary acidic protein (GFAP), and brain derived neurotropic factor (BDNF) via quantitative polymerase chain reaction. Statistical significance was established using analysis of variance (ANOVA), independent t tests, and Pearson correlation tests. RESULTS: Expression of IL-1b and IL-6 was reversed following SCS therapy relative to the increase caused by the injury model. Both GABAbr1 and Na/K ATPase were significantly up-regulated upon implantation of the lead, and SCS therapy reversed their expression to within control levels. Pearson correlation analyses reveal that GABAbr1 and Na/K ATPase expression was dependent on the stimulating current intensity. CONCLUSIONS: Spinal cord stimulation modulates expression of key pain-related genes in the DRG. Specifically, SCS led to reversal of IL-1b and IL-6 expression induced by injury. Interleukin 6 expression was still significantly larger than in sham animals, which may correlate to residual sensitivity following continuous SCS treatment. In addition, expression of GABAbr1 and Na/K ATPase was down-regulated to within control levels following SCS and correlates with applied current.

Spinal Cord Stimulation Modulates Gene Expression in the Spinal Cord of an Animal Model of Peripheral Nerve Injury.

BACKGROUND AND OBJECTIVES: Previously, we found that application of pulsed radiofrequency to a peripheral nerve injury induces changes in key genes regulating nociception concurrent with alleviation of paw sensitivity in an animal model. In the current study, we evaluated such genes after applying spinal cord stimulation (SCS) therapy. METHODS: Male Sprague-Dawley rats (n = 6 per group) were randomized into test and control groups. The spared nerve injury model was used to simulate a neuropathic pain state. A 4-contact microelectrode was implanted at the L1 vertebral level and SCS was applied continuously for 72 hours. Mechanical hyperalgesia was tested. Spinal cord tissues were collected and analyzed using real-time polymerase chain reaction to quantify levels of IL1ß, GABAbr1, subP, Na/K ATPase, cFos, 5HT3ra, TNFα, Gal, VIP, NpY, IL6, GFAP, ITGAM, and BDNF. RESULTS: Paw withdrawal thresholds significantly decreased in spared nerve injury animals and stimulation attenuated sensitivity within 24 hours (P = 0.049), remaining significant through 72 hours (P = 0.003). Nerve injury caused up-regulation of TNFα, GFAP, ITGAM, and cFOS as well as down-regulation of Na/K ATPase. Spinal cord stimulation therapy modulated the expression of 5HT3ra, cFOS, and GABAbr1. Strong inverse relationships in gene expression relative to the amount of applied current were observed for GABAbr1 (R = -0.65) and Na/K ATPase (R = -0.58), and a positive linear correlations between 5HT3r (R = 0.80) and VIP (R = 0.50) were observed. CONCLUSIONS: Continuously applied SCS modulates expression of key genes involved in the regulation of neuronal membrane potential.

Genomics of the Effect of Spinal Cord Stimulation on an Animal Model of Neuropathic Pain.

BACKGROUND: Few studies have evaluated single-gene changes modulated by spinal cord stimulation (SCS), providing a narrow understanding of molecular changes. Genomics allows for a robust analysis of holistic gene changes in response to stimulation. METHODS: Rats were randomized into six groups to determine the effect of continuous SCS in uninjured and spared-nerve injury (SNI) animals. After behavioral assessment, tissues from the dorsal quadrant of the spinal cord (SC) and dorsal root ganglion (DRG) underwent full-genome microarray analyses. Weighted Gene Correlation Network Analysis (WGCNA), and Gene Ontology (GO) analysis identified similar expression patterns, molecular functions and biological processes for significant genes. RESULTS: Microarray analyses reported 20,985 gene probes in SC and 19,104 in DRG. WGCNA sorted 7449 SC and 4275 DRG gene probes into 29 and 9 modules, respectively. WGCNA provided significant modules from paired comparisons of experimental groups. GO analyses reported significant biological processes influenced by injury, as well as the presence of an electric field. The genes Tlr2, Cxcl16, and Cd68 were used to further validate the microarray based on significant response to SCS in SNI animals. They were up-regulated in the SC while both Tlr2 and Cd68 were up-regulated in the DRG. CONCLUSIONS: The process described provides highly significant interconnected genes and pathways responsive to injury and/or electric field in the SC and DRG. Genes in the SC respond significantly to the SCS in both injured and uninjured animals, while those in the DRG significantly responded to injury, and SCS in injured animals.

A continuous spinal cord stimulation model attenuates pain-related behavior in vivo following induction of a peripheral nerve injury.

OBJECTIVES: Models that simulate clinical conditions are needed to gain an understanding of the mechanism involved during spinal cord stimulation (SCS) treatment of chronic neuropathic pain. An animal model has been developed for continuous SCS in which animals that have been injured to develop neuropathic pain behavior were allowed to carry on with regular daily activities while being stimulated for 72 hours. MATERIAL AND METHODS: Sprague-Dawley rats were randomized into each of six different groups (N = 10-13). Three groups included animals in which the spared nerve injury (SNI) was induced. Animals in two of these groups were implanted with a four-contact electrode in the epidural space. Animals in one of these groups received stimulation for 72 hours continuously. Three corresponding sham groups (no SNI) were included. Mechanical and cold-thermal allodynia were evaluated using von Frey filaments and acetone drops, respectively. Mean withdrawal thresholds were compared. Statistical significance was established using one-way ANOVAs followed by Holm-Sidak post hoc analysis. RESULTS: Continuous SCS attenuates mechanical allodynia in animals with neuropathic pain behavior. Mechanical withdrawal threshold increases significantly in SNI animals after 24 and 72 hours stimulation vs. SNI no stimulation (p = 0.007 and p < 0.001, respectively). SCS for 24 and 72 hours provides significant increase in mechanical withdrawal thresholds relative to values before stimulation (p = 0.001 and p < 0.001, respectively). Stimulation did not provide recovery to baseline values. SCS did not seem to attenuate cold-thermal allodynia. CONCLUSION: A continuous SCS model has been developed. Animals with neuropathic pain behavior that were continuously stimulated showed significant increase in withdrawal thresholds proportional to stimulation time.

An ex vivo comparison of cooled-radiofrequency and bipolar-radiofrequency lesion size and the effect of injected fluids.

BACKGROUND AND OBJECTIVES: Radiofrequency (RF) neuroablation is a common therapy for alleviating chronic pain. Larger lesion volumes lead to higher chance of ablating small sensory nerves; therefore, bipolar-RF and cooled-RF are improved alternatives to conventional monopolar-RF. This work provides an ex vivo comparison of bipolar-RF to cooled-RF lesioning in the presence of bone structure using some conventional temperature and time programs and in conjunction with injection of a variety of clinically used substances. METHODS: Studies were performed using chicken muscle near a bone structure. Cooled-RF was applied using standard parameters at 60°C for 150 seconds and perpendicular to the bone. Bipolar-RF was applied using interelectrode distances (IEDs) of 5, 10, or 15 mm at 80°C for 90 or 150 seconds with the electrodes positioned either paralleled between the bone and muscle or perpendicular to the bone. The effect of injection of various fluids (sterile water, 0.9% saline, 7.3% saline, 2% lidocaine, 0.25% bupivacaine, lidocaine/methylprednisolone (Depo-Medrol), or lidocaine/betamethasone (Celestone) on lesion size was compared with no fluid injected in the muscle. Temperature profiles of lesioning were also obtained using an infrared camera. RESULTS: The volume of bipolar-RF lesions is dependent on IED, being more favorable at IED equals 10 mm. The injection of some fluids induces significant (P < 0.05) changes in bipolar-RF lesion volume, although the changes are dependent on IED. Cooled-RF induces larger lesions than bipolar-RF, with no changes in volume induced by injecting fluids. CONCLUSIONS: Cooled-RF yields larger lesions than bipolar-RF under the conditions used in this study. The spherical shape of cooled-RF lesions provides larger volume coverage than lesions obtained with bipolar-RF at IED equals 5, 10, or 15 mm under similar electrode tip temperature and lesioning time.