The gel box - a testing device for the characterization of cryo- and radiofrequency lesions employed in interventional pain therapy.

Background: Radiofrequency (RF) and cryolesioning are established methods for the therapeutic interruption of sensory nerve supply to facet joints and other painful musculoskeletal structures. The varying clinical success rates of these treatments have - among other technical issues - been attributed to the small size of these lesions combined with the limited precision in placing them. Since there are 2 different physical methods for lesioning and a wide range of probes and lesion generators available, it is likely that the lesions generated by them may be of different size.

Objectives: We sought to devise an experimental setup that would allow for the reproducible and comparable evaluation of the size of cryo and RF lesions as they are being used in interventional pain therapy.

Methods: A wide range of potential media was evaluated for this purpose. Based on technical specifications, as well as on preliminary testing, a specific agar agar gel with a gel point of between 32 degrees C and 35 degrees C and a melting point of between 80 degrees C and 85 degrees C was selected for these experiments. Two different testing containers were constructed from transparent acrylic: one with a volume of 1,500 mL and the other with a volume of 12 mL. Each of them allows for the introduction of a cryo or a RF probe and 2 bundles of thermoelements into the gel volume. A water bath was used to maintain the gels at 37 degrees C and bundled, ultrafine NiCr-Ni thermoelements type K were used for measuring the isotherms. A series of RF and cryolesions were performed within these experimental setups to evaluate their suitability for the comparative testing of cryo and RF probes and generators.

Results: Both testing setups generated reproducible results and proved to be suitable for measuring RF as well as cryolesions. Visual observation of the lesions was better with the small testing container and rewarming / recooling after performing a cryo / RF lesion was more rapid with the smaller gel volume.

Limitations: Our setup allows for the comparative measurement of RF and cryolesions, but it cannot simulate the realities within living tissue. While convection as a confounding factor was excluded by use of a gel, capillary perfusion and the specific characteristics of different tissues cannot be simulated.

Conclusions: The testing setup described in this manuscript can serve for the comparative and reproducible study of RF and cryolesions that are commonly used in interventional pain therapy.