Bone tissue ablation by industrial fs laser systems

Laser technology is currently gaining an increasing attention in many surgical fields where high precision and minimization of tissue damage are essential, thanks to the continuous development of more accessible laser systems which can be adapted to the constraints of clinical environment. In the frame of bone surgery, the unicity of the laser approach with respect to conventional mechanical techniques, lays in specific properties such as the absence of direct contact with the tissue, the possibility to cut complex geometries and a unique positioning precision. Because of the positions of the absorption peaks of water and hydroxyapatite around 3 µm, CW or QCW Er:YAG lasers are today the tool of choice for bone tissue ablation demonstrating competitive ablation rates [1] . Nevertheless, the strong thermal effects resulting from processing with CW/QCW laser sources are associated to high degrees of tissue carbonization which prevent the process of tissue regeneration necessary for an efficient post-surgery healing. Consequently, the use of ultra-short pulse laser sources has become increasingly interesting in order to minimize the thermal damage in the treated bone tissue and leave the largest part of healthy tissue intact. The ablation rates related to this kind of source remain still limited nonetheless [2] . The use of high average power sources might therefore be indicated in order to upscale and optimize this process in view of possible clinical applications. In this study, industrial femtosecond laser sources (Amplitude Systems) with a pulse duration of 350fs were employed to carry out a comparative study on ablation efficiency on porcine femurs at wavelengths of 1030nm and 515nm. Before laser treatment, the bones were cleaned with a scalpel and the soft tissue, together with the marrow and the fat were removed. Finally, the diaphyses were separated from the epiphyses by diamond-blade cutting and laser processed at different process parameters. An air-knife was employed during laser irradiation for a more efficient removal of the ablated particles as well as for systematic cooling of the bone tissue. Laser-processed diaphyses were firstly dried in ethanol in a multi-step approach at different ethanol/water concentrations and then analyzed by optical microscopy, SEM (Vega3 by Tescan) and EDX to evaluate the laser-induced modifications of the bone surface in terms of its topography and chemistry. Results show that an optimization of the process parameters is necessary to achieve an optimal quality of ablation without tissue carbonization. Regardless the laser wavelength employed for the ablation, higher ablation rates are obtained when the spatial overlap between successive laser pulses, which defines the thermal accumulation in the bone tissue, was the lowest. An optimal value of laser repetition rate was found at 500kHz and 250 kHz for processing at 1030nm and 515nm, respectively. These results demonstrate the possibility to exploit the full average power of the laser sources allowing for process upscaling. At the highest repetition rate the ablation rate dropped regardless the laser wavelength, highlighting the role of incubation in the optimization of the ablation efficiency. Regardless the processing parameters, an increase of process efficiency was observed in the case of processing at 515nm with a maximum measured ablation rate of about 0.7mm 3 /s. Nevertheless, a strong carbonization was observed on a larger parameter windows when processing at 515nm and high values of spatial overlap. This behavior may be connected to a strong absorption in the visible wavelengths range of other bone tissue components as for instance melatonin and hemoglobin which on the other hand suffer more easily from carbonization. In conclusion, for both wavelengths broad parameters windows were defined where carbonization-free ablation of porcine femur was achieved with ablation rates up to 0.7 mm 3 /s. Moreover, the possibility of upscaling the process was demonstrated up to hundreds of kHz depending on the laser wavelength. Although a few technical optimizations can be considered to further improve the ablation efficiency, fs laser processing appears nowadays as a competitive approach in the frame of bone surgery opening up the possibility of a complete process automatization by robot-guided systems.