INTRODUCTION: Radiofrequency ablation (RFA) is a minimally invasive thermal ablation procedure commonly used to ablate multiple target sites, including the genicular nerve of patients suffering from knee osteoarthritis-induced chronic pain1. In standard radiofrequency (SRF) ablation, thermal lesions are created where the temperature drops from 80°C as the radius from the tip increases2. Cooled radiofrequency (CRF) ablation overcomes the lesion size limitations inherent to SRF by circulating fluid around a 60°C probe tip to act as a heat sink by removing heat from tissue adjacent to the electrode2. This unique thermal profile allows for CRF ablations to have larger lesion volume, for angle independence in the procedure, and a larger distal projection2, all of which in turn increases the likelihood of the physician ablating the target nerve. CRF ablations also provide longer pain relief that lasts up to twelve months3. While the immediate physical effects of RFA-mediated denervation have been evaluated previously in ex vivo tissue models, primarily by lesion size analysis, a thorough characterization of the physiological changes elicited in response to RFA is lacking. Furthermore, previous studies have shown an upregulation in inflammatory markers in the nerve post SRF ablation, however, there are no studies that have assessed differences in the immune response between SRF or CRF treatments4.  Here, we present the first in vivo longitudinal analysis of changes that occur along the ablated nerve in response to CRF and SRF procedures.  Given that CRF generators deliver a significantly higher amount of energy, we hypothesize that CRF produces larger lesions (in both volume and length) in vivo, which are present longer when compared to SRF because of delayed immune repair. 
METHODS:  The rodent sciatic nerve was selected as the ablation target, as it is of comparable size to human nerves of interest and provides both sensory and motor innervation. Moreover, the rodent allows for the feasible evaluation of changes that are suspected to occur over months and years in the human, since 2 weeks in the lifespan of a rodent is approximately equivalent to 1 human year.  One group of Lewis rat sciatic nerves were treated to recommended clinical parameters of SRF (90s, 80°C, 22 gauge, 5mm active tip probe) or CRF (150s, 60°C, 17 gauge, 2mm active tip probe), and nerves were harvested immediately post-ablation for histological analysis. Following fixation in 10% formalin, sciatic nerves were embedded in paraffin blocks, and longitudinal sections were stained in H&E. Another group of Lewis rat sciatic nerves were exposed to optimized levels of SRF (50s, 80°C, 22 gauge, 5mm active tip probe) or CRF (80s, 60°C, 17 gauge, 2mm active tip probe), to ensure long-term survival without severe adverse events. This group of survived rats were tracked and assessed for structural modifications at the lesion zone via histological analysis (H&E), immunohistochemistry (CD11b, macrophage staining), lesion progression via magnetic resonance imaging (MRI - using a 9.4T Bruker scanner), and lesion visibility at necropsy (caliper measurements). Volumetric analysis of MR scans was performed using Image J to quantify the size of the lesions created by both SRF and CRF.
RESULTS: Using H&E staining to assess the structure of lesions created on the sciatic nerve by clinical RFA parameters, we observed that CRF ablations produced lesions with an average lesion length of 4.75 ± 1.07mm, which is approximately 1.5x longer than SRF lesions (Fig 1). We also observed within the ablation zone a region of lesser damage (pink) flanked by two regions of more severe damage (purple), which parallels the thermal profile of cooled RF lesions observed ex vivo. Preliminary volumetric analysis from MRI scans of rats that were treated with clinical RFA parameters (Fig 2) also suggest that CRF ablations have a larger volume compared to SRF ablations (1311.8±101.8mm3 in CRF vs. 1054.7mm3 in SRF, n=2). Caliper measurements at the time of ablation (t=0) also demonstrate that CRF ablations produce significantly larger lesions (9.83±0.28mm) when compared to SRF lesions (4.33±0.28mm). Additionally, by 14 days post-ablation or 1 human year, the visible SRF lesion had resolved (0±0mm), whereas the CRF lesions persisted (5.83±0.76 mm). Finally, preliminary staining of nerves 7 days post ablation with CD11b markers suggests that SRF treated animals have increased macrophage infiltration compared with CRF treated animals at the same time point.
DISCUSSION: This study provides evidence for the first time in vivo that CRF generates a larger lesion as measured by both histology and MRI. Furthermore, CRF lesions appear to persist for longer at the ablation site, as indicated by caliper measurements. Collectively, these results have begun to identify the time frame in which nerves repair following RF ablation, and provide preliminary structural evidence as to the underlying mechanistic differences associated with different RF techniques.
CLINICAL RELEVANCE: The observation of lesions at day 14 (1 human year) only in CRF treated animals and not in SRF treated animals, provides a preliminary explanation as to why patients experience a return of pain sooner following treatment with SRF, and not with CRF. Furthermore, an increased presence of CD11b macrophage staining at day 7 in the nerves ablated with SRF suggests that these nerves undergo immune repair faster, providing another explanation for why patients treated with SRF return to pain sooner. Additional studies and analyses are necessary to further explore the underlying mechanisms of longer lasting pain relief provided by CRF as compared to SRF.
REFERENCES: 
1.	Davis et al. Prospective, Multicenter, Randomized, Crossover Clinical Trial Comparing the Safety and Effectiveness of Cooled Radiofrequency Ablation with Corticosteroid Injection in the Management of Knee Pain from Osteoarthritis (Pubmed Link)
2.	What is cooled radiofrequency? The physics of RF ablation: (Link)
3.	Halyard Health Announces New Data Supporting COOLIEF* Cooled RF for the Treatment of Chronic Osteoarthritis Knee Pain (Article Link)
4.	S. Choi et al. Internal-specific Morphological Analysis of Sciatic Nerve Fibers in a RF-induced Animal Neuropathic Pain Model. PLOS One. 2013

1836 - Cooled Radiofrequency Ablation Produces Enhanced Thermal Lesions In Vivo