Temperature and thermal dose distributions during intradiscal electrothermal therapy in the cadaveric lumbar spine.

Abstract

Human cadaveric lumbar spines were used to assess the temperature and thermal dose distribution during intradiscal electrothermal therapy in vitro. To determine whether intradiscal electrothermal therapy produces adequate tissue temperatures to denature annular collagen or ablate nerve cells. Several hypothesized mechanisms for the effect of intradiscal electrothermal therapy have been suggested and include: 1) shrinkage of the nucleus and/or the annulus fibrosus by contraction of collagen fibers; and 2) thermal ablation of sensitive nerve fibers in the outer annulus. Intradiscal electrothermal therapy was performed using the standard clinical protocol on 12 lumbar specimens in a 37.0 degrees C water bath using the SpineCath by Oratec. Temperatures were recorded simultaneously at 40 different locations in the disc. Thermal dose (Equivalent Minutes 43.0 degrees C) was calculated at each temperature point. The highest temperature measured (out of 520 points) was 64.0 degrees C and was within 1 mm of the heating coil. Temperatures in excess of 60 degrees C were all within 1 to 2 mm of the intradiscal electrothermal therapy catheter surface, the 50 to 60 degrees C range extended approximately 6 mm, above 48 degrees C extended approximately 7 mm, and above 45 degrees C extended to approximately 10 mm. Less than 2% of points achieved temperatures sufficient for collagen denaturation (>60 degrees C). On average, 42.5% of points accumulated >250 Equivalent Minutes 43.0 degrees C, a conservative common dose threshold for thermal necrosis of cells. The time history of thermal measurements demonstrated that the disc temperature had not reached steady state by the end of the heating protocol (16.5 minutes). Except for a very limited margin (1-2 mm) around the catheter, the temperature necessary to induce collagen shrinkage was not observed within the disc. Temperatures sufficient to ablate nerves were developed in some areas but were not reliably produced in clinically relevant regions, such as the posterior annulus. These results suggest that beneficial clinical outcomes may be critically dependent on probe placement or other factors unknown.