Fluorescence guided excimer laser ablation of intervertebral discs in vitro

Laser induced fluorescence of intervertebral discs was investigated in this study for the first time in order to develop a fluorescence guided percutaneous excimer laser discectomy. For this purpose 35 human cadaveric intervetebral discs from level L1 to L5 were irradiated with a 308 nm XeCl excimer laser with a 60 nsec pulse width at 10, 20 and 30 mj/mm2. Laser light was transmitted over a 1000 microns core optical fiber. Fluorescence emitted from the irradiated tissue was transmitted back over the same fiber over a semireflective mirror to another optical fiber, coupled into a spectrograph and O-SMA optical simultaneous multichannel spectral analyzer system. The spectral lineshape of both the annulus fibrosus and the nucleus pulposus was characterized by a broadband emission between 320 nm and 616 nm with 3 peak emissions at 358, 423 and 457 nm. Relative intensities were calculated by forming two ratios of the peak intensities of the three peaks (R1 = I 423/I 358; R2 = I 423/I 457). Statistical analysis of both ratios revealed a highly significant discrimination between annulus fibrosus and nucleus pulposus (P less than 0.001). Under penetrating conditions multiple recorded spectra showed the spectral profile of subsequently ablated nucleus and annulus. The results of this study demonstrate that discrimination between annulus fibrosus and nucleus pulposus by excimer laser-induced fluorescence is feasible under laboratory conditions. Fluorescence spectroscopy could possibly be developed into a valuable guiding system for percutaneous laser discectomy.