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

Collateral thermal damage and residual char formation have severely limited the use of conventional lasers in the surgical preparation of bony tissue. Thermal damage from lasers can be minimized by selecting a wavelength that is strongly absorbed and by reducing the laser pulse duration. In contrast to the fixed wavelengths and microsecond pulse intervals of conventional lasers, the Vanderbilt free electron laser (FEL) can be set at wavelengths ranging from 2.1 to 9.8 microm, and the pulse duration can be reduced to a series of 1 to 2 picosecond (ps) micropulses delivered in succession over intervals of 4 microsecond macropulses. The purpose of this study was to compare the morphologic and chemical changes induced in the near-surface region of bone following exposure to the FEL at 3.0, 6.1, and 6.45 microm wavelengths. The selected wavelengths coincide with the vibrational modes of proteins and water within bone.

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 rate of 30 Hz, focused to 200 microm and 500 microm spot sizes. Laser incisions were made using a computer-assisted surgical program, and control incisions were created with a bone saw. Rabbits were euthanized after the final incision, tibias recovered, and non-decalcified specimens processed for light microscopy. Separate samples were prepared for FTIR (Fourier transform infrared) photoacoustic spectroscopic analysis.

The light microscopy sections of the ablation defects created at the differing wavelengths showed similar features, i.e., 2 zones of collateral damage, a zone generally < 10 mm of extensive thermal damage, and a wider zone of empty lacunae. In comparing treated and untreated surfaces, the spectral differences were limited to a relative decrease in intensity of the amide II and III absorption peaks in all laser-treated surfaces.

Spectroscopic and histologic results indicated minimal thermal damage to bone ablated at 3.0, 6.1, and 6.45 microm wavelengths using the FEL (Fourier transform infrared) at the specified parameters. The FTIR photoacoustic spectroscopic results suggest that the char layer is limited to an area less than approximately 6 microm from the surface.