# Orthopedics-Related Applications of Ultrafast Laser and Its Recent Advances
## 3 Ultrafast Laser in Orthopedic Surgery
### Thermal Effect  
As discussed before, ultrafast laser ablation is overall characterized by minimal thermal damage and limited heat diffusion outside of the focal volume, because the dominant mechanism, namely multiphoton absorption of light and avalanche ionization or hydrodynamic plasma expansion [66], is not thermally mediated with only little heat deposition, making it an auspicious technique in the clinic [20, 67, 68, 69]. Still early studies demonstrated some carbonization, cracking and melting [53, 57, 60], which could be mitigated by employing cooling systems [42, 70]. Nevertheless, recent studies have shown more optimized performance [34, 71, 72]. In particular, [PERSON] et al. reported no observations of a heat-affected zone at the maximum laser fluence and pulse numbers by assessing any \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Author/Year** & **Bone Type** & **Ablation Threshold (J/cm\({}^{2}\))** & **Pulse Duration (fs)** & **Wavelength (nm)** \\ \hline [PERSON] et al., 2021 [41] & Bovine cortical bone (fresh; unaltered) & \(1.38\pm 0.18\) & & \\  & & (N = 25.83 *; multi-pulse & 600 & 1552 \\  & & threshold) & & \\ \hline [PERSON] et al., 2020 [49] & Bovine and ovine & 0.92 (bovine) & & \\  & cortical bone & 0.97 (ovine) & 140 & 800 \\  & (fresh; unaltered) & (N = 1000) & & \\ \hline [PERSON] et al., 2014 [46] & Porcine cortical bone & 1.5 & & \\  & (fresh; unaltered) & (N = Not Reported) & \(8\times 10^{3}\) & 1064 \\ \hline [PERSON] et al., 2012 [57] & Bovine cortical bone & 0.32 \(\pm\) 0.04 & 500 & 1030 \\ \hline [PERSON] et al., 2012 [60] & Bovine cortical bone & 0.23 & & \\  & (fresh; polished) & (N = 1000) & & \\ \hline [PERSON] et al., 2012 [50] & Porcine cortical bone & 1.75 \(\pm\) 0.55 & & \\  & (fresh; unaltered) & (N = 1000) & 170 & 800 \\ \hline [PERSON] et al., 2009 [63] & Bovine cortical bone & 1.22 \(\pm\) 0.29 (strong) & & \\  & (fresh; polished) & 0.79 \(\pm\) 0.18 (gentle) & 150 & 775 \\  & (N = 1000) & & \\ \hline [PERSON] et al., 2007 [39] & Porcine cortical bone & 0.69 \(\pm\) 0.08 (fresh; polished) & 200 & 775 \\ \hline \hline \end{tabular}  
* Estimated averaged N value predicted by simulation.  
\end{table} 
Table 3: Summary of ablation thresholds determined for different bone types. N = Number of pulses. forms of carbonization, discoloration and microcracking around the craters using light microscopy [49]. [PERSON] et al. evaluated thermal effects on fresh bovine femur using a nanosecond laser source (20 ns, 355 nm, 2-100 kHz) as compared to a picosecond laser source (12 ps, 1064 nm, 100-600 kHz), and found that the picosecond laser, although not as ideal as femtosecond lasers, resulted in significantly reduced heating compared to the nanosecond laser [71]. The study by [PERSON] et al. provided optimization strategies and described the observation of increasing thermal accumulation while decreasing scanning speed and increasing PRR individually in the IR and visible wavelength regimes [44]. When both the scanning speed and PRR were increased, not only thermal loads increased but expanding plasma plume was also produced, leading to reduced ablation efficiency and precision. However, the observation did not apply to the UV regime, where collagen and hemoglobin are the main absorbers. No specific behavior was detected with changing parameters; the thermal load was comparatively high enough at the lowest PRR to generate laser-induced bone calcination. In Figure 1, laser-irradiated damages such as the typical thermal-induced particle-like roughness and micro-cracks can be seen under SEM images at a higher PRR (Figure 1a), whereas native bone structures containing blood vessels and osteocytes were preserved at a lower PRR (Figure 1b,c) [44]. The interrelationship between scanning speed, PRR and wavelength choice therefore requires thorough investigation for an optimized laser ablation performance without collateral thermal damages on bone tissues. Similarly, [PERSON] et al.