Holmium:YAG laser ablation of human intervertebral disc: Preliminary evaluation

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Abstract
Percutaneous discetomy has become a viable alternative in the treatment of herniated intervertebral disc. This study determined the effectiveness of holmium:YAG laser for ablation of human disc tissue. Human cadaveric intervertebral disc was harvested and stored in cold saline-soaked gauze for evaluation within 24 hr of removal. Using a specially designed apparatus, a 600 μm diameter fiber was advanced perpendicular through the annulus fibrosis at a controlled force of 0.098 Newtons (10 g). Samples were lased in air (n = 17) and in room temperature saline (n = 32). The laser energy was delivered at 5 Hz, 250 μsec pulsewidth, and from 50 mJ/mm2 to 1,100 mJ/mm2 fluence. Three to six holes were lased using identical parameters in each tissue specimen and were evaluated histologically and by morphometric analysis. The maximum zone of thermal necrosis and thermal denaturation occurred at 700–1,100 mJ/mm2; 140 μm and 590μm in air and 80 μm and 730μm in saline, respectively. At fluences between 200 and 700 mJ/mm2, the thermal necrosis ranged from 20 to 60 μm in air and from 10 to 50μm in saline, the zone of denaturation also being less. The holes created with the 600μm fiber were circular in shape, with a mean diameter of 500 μm (n = 3). The etch rates (penetration/pulse) appeared to increase with increasing fluences. In saline, the etch rate ranged from 7 to 53 μm/pulse (r = 0.57, P ≤ 0.10), and, in air, the values ranged from 7 to 65 μm/pulse (r = 0.79, P ≤ 0.03). No tissue ablation occurred at 50 mJ/mm2, suggesting that the ablation threshold lies between 50 and 200 mJ/mm2. This study demonstrates that controlled ablation of human disc tissue can be accomplished with the hol-mium:YAG laser, the degree of thermal injury being related to the amount of energy delivered.