FACTORS THAT AFFECT RADIOFREQUENCY HEAT LESION SIZE

Authors: Cosman E. Jr, Dolensky JR, and Hoffman RA.
Study type: Feasibility – Prospective, single center
Publication: Pain Medicine 2014; 15(12): 2020-36 (Link to PubMed)
Key Words: Sacroiliac Joint - Bipolar RF vs Cooled RF - Lesion Geometry

Figures are reproduced with permission from Copyright Clearance Center

STUDY GOAL

  • To compare RF heat lesion size across a broad range of active tip diameters, active tip lengths, set temperatures, set times, and modalities available for interventional pain management.
  • To evaluate typical cannula and generator configurations, configurations that maximize lesion size, the RRE “Ray” electrode, cooled RF, and bipolar RF under controlled conditions.

METHODOLOGY

Monopolar RF lesions were generated in bovine liver, using sharp cannulae with varying tip diameters (22-, 23-, 20-, 16-, and, 18-Gauge), tip lengths (5, 6, 10 and, 15 mm), set temperatures (60°, 70°, 80° and, 90°C) and set times (1, 1.5, 2, 3, 5, and, 10 minutes). For lesion size comparison, the following cannulas were used:

RF Cannulae table
RF Cannulae
RF Cannulae. From top to bottom: 1. Curved, sharp, bevel-tip; 2. Straight, sharp, bevel-tip with stylet; 3. RF thermocouple electrode(TE) with nitinol shaft; 4. RF-TE within cannula’s inner lumen; 5. Trocar-tip “Ray” RRE electrode.
RF heat lesion size and influencing factors image left
RF heat lesion size and influencing factors image right
RF heat lesion size and influencing factors. A. Monopolar lesions are egg-shaped. W and D are similar, due to the active tip’s predominant radial symmetry. Lesion size depends on d, l, T, t. B. Bipolar lesions are influenced by s. The electric field and current density are more intense between closer tips. As s increases, the lesion expands in W and narrows in both L and D at the midline. For nearby parallel tips, bipolar lesions have a rounded brick shape. At large distances, bipolar lesions have a monopolar shape. Length L, Lesion Width W, depth D, tip diameter/gauge d, tip length l, tip temperature T, lesion time t

RESULTS

  • All the factors (cannula diameters, active tip lengths, set temperatures, and set times) analyzed in the study, were found to significantly affect RF heat lesion size.
  • Increasing temperature and/or time enables a thinner cannula to generate lesion dimensions similar to those produced by a thicker cannula at lower temperatures or shorter times.
  • With proper selection of generator settings; monopolar RF using a standard 18-gauge or 16-gauge cannulae produces heat lesions similar to those generated by cooled RF for the treatment of SIJ pain
  • Bipolar RF between parallel cannulae produces a rounded brick-shaped lesion of comparable shape to three sequential monopolar lesions generated using the same cannulae and generator settings.
Average size of RF lesions for tested conditions image
Average size of RF lesions for tested conditions. A. Cannula tip length; B. Cannula diameter/gauge; C. Tip size effect; D. Comparison with RRE electrode (Trocar-tip “Ray”); E. Temperature; E. Time; F. Higher temperature and/or longer lesion size compensate for smaller cannula diameter, and G. Bipolar RF lesion size depends on tip spacing (s), tip length, diameter, temperature and time.
Average size of large RF lesions – Monopolar vs Bipolar RF. A image
Average size of large RF lesions – Monopolar vs Bipolar RF. A. Monopolar heat lesions, including cooled RF. B. Bipolar lesions. C. Bipolar lesions compared to monopolar lesions at the minimal temperature achieving 10 mm average width.

AUTHOR’S CONCLUSIONS

Tip gauge, tip length, temperature, and time substantially affect RF lesion size.

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