Ultrafast pulsed Bessel beams for enhanced laser ablation of bone tissue for applications in LASSOS

Using a femtosecond pulsed laser system (pulse width = 100fs, repetition rate = 500 Hz, λ=800nm), a zero-order Bessel beam was generated using a LCOS-Spatial light modulator (LCOS-SLM) with an effective cone angle of 4.56°. Ablation threshold studies of fresh bovine and ovine load bearing cortical bone was identified using the method of least damage and found to be identical at φth = 0.15 ± 0.03 J cm -2 , irrespective of the target species. The ablation threshold is significantly reduced compared to the ablation threshold determined for Gaussian beams in bovine and ovine cortical bone (Load Bearing: φth = 0.91 ± 0.03 J cm -2 , Skull: φth = 1.19 ± 0.06 J cm -2 ). Incubation effects were investigated and the incubation coefficient was determined to be ζ = 0.93 ± 0.06, indicating no incubation effects are present. The relationship between tissue removal and the number of pulses applied was explored. By altering the translation rate of the sample under the Bessel region of the incident laser, the number of pulses applied at each point along the linear ablation features was varied. Cross sections of ablation features were measured using scanning electron microscopy (SEM) and maximum depths of the ablation features measured. The ablation rate of bovine and ovine cortical was found to be 2.69 – 13.21 ± 0.05 μm pulse -1 and 2.49 – 12.79 ± 0.03 μm pulse -1 respectively for fluence values ranging from 25.0 – 2.5 Jcm -2 , significantly higher than those of Gaussian beams. Structural analysis of the ablation features using SEM and optical microscopy showed no signs of heat affected zone (HAZ) in the form of thermal shockwave cracking, molten debris deposition or charring of the tissue.