Dear Editor, With evidence for clinical efficacy [1] and effectivenss [2,3], as well as recent Federal Drug Adminitration approval [4], genicular nerve radiofrequency ablation (RFA) is increasingly offered as a pain management option for patients with chronic knee pain due to osteoarthritis (OA). Patients who do not respond to conservative and injection-based treatment, yet are poor candidates for total knee arthroplasty (TKA) due to medical comorbidity, body mass index, or patient preference, are candidates for this treatment. To date, no significant complications of genicular nerve RFA have been described in the literature [1–3]. However, we report a case of a third-degree skin burn at the site of the inferiomedial genicular nerve following a conventional thermal RFA procedure. A 50-year-old man (BMI 24.2 kg/m2) with five years of right knee pain and radiographic evidence of moderate to severe tri-compartmental OA, who experienced no meaningful relief from conservative management and corticosteroid injections, underwent superiomedial, superiolateral, and inferiomedial genicular nerve RFA using an accepted protocol [5,6]. Conscious sedation (midazolam 2 mg IV, fentanyl 50 mcg IV) was used. The skin and soft tissues were anesthetized with 2 mL of 1% lidocaine at each site prior to cannula placement. Appropriate electrode placement was confirmed by fluoroscopy in anterior-posterior (Figure 1A) and lateral (Figure 1B) views. Motor nerve stimulation was ruled out with 2 V testing at 2 Hz, and then each genicular nerve was anesthetized with 1 mL of 2% lidocaine. Lesioning with an 18-gauge conventional thermal RFA electrode with a 10-mm active tip (Cosman Medical Inc., Burlington, MA, USA) was performed at 80°C for 90 seconds. During tissue lesioning, fluoroscopic views were rechecked in order to confirm lack of active tip migration. The patient was conversant during the entire procedure and tolerated lesioning without excessive discomfort. Following electrode and cannula removal, the skin overlying the inferiomedial genicular nerve RFA site (a circular region with an approximately 7–8 mm diameter) appeared erythematous, yet without evidence of skin blanching or breakdown. Each cannula puncture site was covered with a Band-Aid, and the patient was instructed to keep the sites clean and dry and to contact us if further skin changes occurred. Upon re-evaluation at five weeks post-RFA (the patient missed his three-week follow-up appointment and rescheduled), he reported 90% reduction in right knee pain, increased walking tolerance, and ability to perform a standing squat, which had not been possible for more than three years due to pain and knee joint stiffness. Inspection of the knee revealed an 8-mm diameter circular lesion with a central eschar surrounded by a rim of granulation tissue (Figure 2A). Query revealed that the patient had noticed this lesion three to four days after RFA, but that it was not painful or bothersome, so he kept it covered with a Band-Aid and did not seek medical attention. He was instructed to keep the lesion clean and dry and to follow-up in two weeks. At this time the lesion had healed, with only residual skin discoloration (Figure 2B). Anterior/posterior (A) and lateral (B) fluoroscopic images of the final needle positions during diagnostic block genicular nerve radiofrequency ablation. The inferiomedial site is identified at the confluence of the medial tibial shaft and the tibial flare in the A-P plane and the midpoint of the tibial diameter in the lateral plane. Appearance of skin burn at five weeks (A) and seven weeks (B) after radiofrequency ablation. Based on this case, we speculate that RFA lesion extension to the dermis with subsequent superficial necrosis is possible at the inferiomedial genicular nerve site in patients with minimal subcutaneous tissue overlying the medial tibial flare. Based on ex vivo studies, the RFA parameters used in this case would result in a lesion 13.4 mm in length and 9.9 mm in width [7], yet lesion size may be larger in vivo and when local anesthetic is used prior to lesioning [8]. Indeed, third-degree skin burns have been reported in the literature in patients with slender body habitus in association with RFA of relatively superficial targets including a thoracic medial branch nerve [9] and a thyroid nodule [10]. In order to prevent this potential complication, careful individualized selection of RFA electrode size and length, lesioning time, and temperature must be considered, specifically in relation to patient body habitus and depth of the lesion target. For example, at a fixed lesion time and temperature, use of a large-gauge RFA electrode with a short active tip increases the skin-to-lesion distance without compromising the lesion diameter [7]. While speculative, use of a 16-gauge RFA electrode with a 6-mm active tip may have decreased the severity of skin burn or prevented this complication in the case presented. In addition, RFA cannula insulation should be inspected for defects, and cannulae should not be reused. Serial fluoroscopic images should be obtained prior to and during the RFA lesioning period, and vigilance for electrode migration should be maintained. If there is concern for active tip migration, the RFA cannula should be manually secured for the duration of lesioning. Given the expanding use of genicular RFA for management of chronic knee pain associated with OA, it is important for clinicians to remain cognizant of the possibility of skin burn at the inferiomedial genicular nerve site, specifically in patients with low BMI. Conflicts of interest: Dr. McCormick serves as a board member of the Spine Intervention Society.
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McCormick et al. (2017) studied this question.