Study Design.
The effects of intradiscal electrothermal therapy (IDET) on intervertebral discs in sheep were studied experimentally.

Objectives.
Posterolateral annular lesions were experimentally induced and allowed to mature for 12 weeks in the intervertebral discs of sheep. IDET was performed in an attempt to denervate and repair the annular lesion. The histologic and immunohistochemical effects of IDET were studied.

Summary of Background Data.
IDET continues to be used as a minimally invasive treatment for chronic discogenic low back pain, with success rates reported in up to 70% of cases. The mechanism of action by which IDET exerts its effect is poorly understood. Proposed mechanisms include the contraction of collagen and the coagulation of annular nociceptors. An ovine model was used firstly to induce a posterolateral annular lesion, secondly to assess the innervation of such a lesion, and thirdly to assess the effect of IDET on this innervation.

Materials and Methods.
Posterolateral annular incisions were made in 40 lumbar discs of 20 sheep. Twelve weeks were allowed for each annular lesion to mature. IDET was then performed in the disc with the posterolateral annular tear and in another control level. IDET was performed using a modified intradiscal catheter (Spine-CATH; Oratec Interventions, Menlo Park, CA). Temperatures were recorded in the nucleus (TN) and the posterior annulus (TPA). The spines were harvested at predetermined intervals up to 18 months. Histologic sections of the discs were graded for disc morphology to assess degeneration and immunohistochemical staining to assess potential denervation.

Results.
Vascular granulation tissue consistent with a healing response was observed in the posterior annular tear of all incised discs from 12 weeks. Protein gene product 9.5 (PGP 9.5) positive nerve fibers were clearly identified in the adjacent periannular tissue and the outer few lamellae of the posterior annulus. During the IDET procedure, the mean maximum TPA was 63.6C and the mean maximum TN was 67.C.At sacrifice, the number of nerve fibers identified in the posterior annular tear was the same for those specimens that had undergone IDET and those that had not. From 6 weeks after IDET, there was evidence of thermal necrosisin the inner annulus and adjacent nucleus but sparing the periphery of the disc.

Conclusions.
Vascular granulation tissue and posterior annular neo-innervation was observed in the experimentally induced posterolateral annular lesion. IDET delivered at 90C in the sheep consistently heated the posterior annulus and the nucleus to a temperature normally associated with coagulation of nociceptors and collagen contraction. IDET did not denervate the posterior annular lesion. Thermal necrosis was observed within the inner annulus and adjacent nucleus from 6 weeks after IDET. The reported benefits from IDET appear to be related to factors other than denervation and repair.