Effective laser ablation of bone based on the absorption characteristics of water and proteins

Background: Collateral thermal damage and residual char formation have seve rely limited the use of conventional lasers in the surgical preparation of bony tissue. Thermal damage from lasers can be minimized by selecting a wav elength that is strongly absorbed and by reducing the laser pulse duration. In contrast to the fixed wavelengths and microsecond pulse intervals of co nventional lasers, the Vanderbilt free electron laser (FEL) can be set at w avelengths ranging from 2.1 to 9.8 mu m, and the pulse duration can be redu ced to a series of 1 to 2 picosecond (ps) micropulses delivered in successi on over intervals of 4 microsecond macropulses. The purpose of this study w as to compare the morphologic and chemical changes induced in the near-surf ace region of bone following exposure to the FEL at 3.0, 6.1, and 6.45 mu m wavelengths. The selected wavelengths coincide with the vibrational modes of proteins and water within bone. Methods: Under general anesthesia, laser incisions were made in the tibias of 14 skeletally mature rabbits. Laser parameters included 22.5 +/- 2.5 mJ/ pulse delivered in individual 4 microsecond macropulses at a repetition rat e of 30 Hz, focused to 200 mu m and 500 mu m spot sizes. Laser incisions we re made using a computer-assisted surgical program, and control incisions w ere created with a bone saw. Rabbits were euthanized after the final incisi on, tibias recovered, and non-decalcified specimens processed for light mic roscopy. Separate samples were prepared for FTIR (Fourier transform infrare d) photoacoustic spectroscopic analysis. Results: The light microscopy sections of the ablation defects created at t he differing wavelengths showed similar features, i.e., 2 zones of collater al damage, a zone generally <10mm of extensive thermal damage, and a wider zone of empty lacunae. In comparing treated and untreated surfaces, the spe ctral differences were limited to a relative decrease in intensity of the a mide II and III absorption peaks in all laser-treated surfaces. Conclusions: Spectroscopic and histologic results indicated minimal thermal damage to bone ablated at 3.0, 6.1, and 6.45 mu m wavelengths using the FE L (Fourier transform infrared) at the specified parameters. The FTIR photoa coustic spectroscopic results suggest that the char layer is limited to an area less than approximately 6 mu m from the surface.