Morphologic analysis of bipolar radiofrequency lesions: implications for treatment of the sacroiliac joint

Sacroiliac (SI) joint dysfunction is an important cause of mechanical low-back pain. Bipolar radiofrequency ablation has been proposed as a long-lasting treatment for pain in patients with SI dysfunction who report temporary pain relief with local-anesthetic injection into the joint. No data are available to guide the technical aspects of bipolar radiofrequency lesion creation. This study documents the optimal spacing of cannulae and time required to produce bipolar lesions by use of radiofrequency technology.

Two radiofrequency cannulae were secured in a parallel position 2, 4, 6, 8, and 10 mm apart and submerged in egg white for lesion production in a medium that would allow visualization of the size and shape of the lesions as they were created. Temperatures of the probes were raised from 40 degrees C to 90 degrees C at a constant rate and were held at 90 degrees C for 190 seconds. The progress of lesion formation was photographed every 10 seconds, and the resultant surface area of the lesions was measured from the digital images.

Contiguous strip lesions were produced when the cannulae were spaced 6 mm or less apart; unipolar lesions around each cannula resulted if they were spaced more than 6 mm apart. Ninety percent of the final lesion area was reached by 120 seconds, and the final lesion size was reached by 150 seconds, regardless of spacing.

Bipolar radiofrequency treatment creates continuous "strip" lesions proportional in size to the distance between the probes when the distance between cannulae is 6 mm or less. Spacing the cannulae 4 to 6 mm apart and treating at 90 degrees C for 120 to 150 seconds maximizes the surface area of the lesion.