Thermal, mechanical, optical, and morphologic changes in bovine nucleus pulposus induced by Nd : YAG (lambda=1.32 mu m) laser irradiation

Background and Objective: To examine the biophysical effects of phototherma l heating on herniated intervertebral discs during laser decompression surg ery. Study Design/Materials and Methods: Ex vivo bovine nucleus pulposus specime ns were irradiated with a Nd: YAG laser (lambda = 1.32 mum, 100 seconds exp osure time, 9-31 W/cm(2), 4.8 mm spot diameter), whereas changes in tissue thermal, mechanical, and optical properties were monitored by using, respec tively, infrared radiometry, tissue tension measurements, and diffuse refle ctance from a HeNe probe laser. Morphologic changes and mass reduction were monitored by recording shape changes on video and weighing specimens befor e and after laser exposure. Results: At power densities below 20 W/cm(2), evaporation of water and spec imen volume reduction (shrinking) were consistently observed on video durin g irradiation. In contrast, above 20 W/cm(2), vapor bubbles formed within t he specimen matrix and subsequently ruptured (releasing heated vapors). Whe n radiometric surface temperature approaches approximately 60 to 70 degrees C (denaturation threshold for tissue), tissue tension begins to increase, w hich is consistent with observations of specimen length reduction. The onse t of this change in tissue tension is also reflected in characteristic alte rations in diffuse reflectance. With cessation of laser irradiation, a sust ained increase in tissue tension is observed, which is consistent with chan ges in specimen length and volume. Higher laser power results in a faster h eating rate and subsequently an accelerated tension change. Specimen mass r eduction increased with irradiance from 19 to 72% of the initial mass for 9 -31 W/cm(2), respectively. Irradiated specimens did not return to their ori ginal shape after immersion in saline (48 hours) in contrast to air-dried s pecimens (24 hours), which returned to their original shape and size. Conclusion: These observations suggest that photothermal heating results in irreversible matrix alteration causing shape change and volume reduction ( observed on video and evidenced by the increase in tissue tension) taking p lace at approximately 65 degreesC. Inasmuch as high laser power results in vapor bubble formation and specimen tearing, the heating process must be co ntrolled. Diffuse reflectance measurements provide a noncontact, highly sen sitive means to monitor dynamically changes in tension of nucleus purposus. (C) 2001 Wiley-Liss, Inc.