# Orthopedics-Related Applications of Ultrafast Laser and Its Recent Advances
## 3 Ultrafast Laser in Orthopedic Surgery  
### Ablation Parameters  
Early research of ultrafast laser ablation of bone tissues was predominantly focused on modification and microsurgery of teeth, ear [36] and spine, with very few publications in orthopedic surgery of the peripheral extremities (long bones of arm and leg). Despite all the advantages, the main issue that limits its clinical translation and acceptance, namely the low ablation removal rate, has remained the same even to date [35,37,38,39]. For instance, two studies published in 2007 discussed the use of ultrafast laser for bone ablation. [PERSON] et al. conducted a comparison study of laser osteotomy on bovine bone tissues using a femtosecond (fs) Yb:glass laser (pulse duration = 330 fs; \(\lambda\) = 1040 nm; pulse repetition rate (PRR) = 1 kHz; max. average pulse energy = 130 \(\upmu\)J) and a conventional Er, Cr:YSGG laser system (pulse duration = 53 \(\upmu\)s; \(\lambda\) = 2780 nm; PRR = 20 Hz; max. average pulse energy = 300 mJ) [35]. The ablation rate of the femtosecond laser (\(\sim\) 4 \(\times\) 10\({}^{-4}\) mm\({}^{3}\)/s) was found to be approximately 400-fold slower than the Erbium laser (\(\sim\)0.15 mm\({}^{3}\)/s) given a power density of \(\sim\)1000 W/cm\({}^{2}\), but could be further optimized by increasing the PRR while maintaining the favorable cut surface morphology. The ablation thresholds were calculated to be 0.82, 0.78 and 0.54 J/cm\({}^{2}\) for spongiosa bone, compacta bone and cartilage, respectively, with an average of 72.4 laser pulses overlapped. Moreover, an ultrafast laser system consisting of a diode-pumped Yb:glass laser seed source and a Yb:KYW thin disk laser head (pulse duration = 900 fs; \(\lambda\) = 1030 nm; PRR = 45 KHz; max. average pulse energy = 100 \(\upmu\)J) was used by [PERSON] et al. to achieve a maximum ablation rate of 0.15 mm\({}^{3}\)/s in porcine femora at the given power output, which was acceptable clinically for knee arthroplasty [38]. Additionally, in precision surgery such as stapedotomy, ultrafast laser has also been proven ideal by offering minimal thermal and acoustic damage [36,40]. Encouragingly, in the past decade, several studies have discovered more novel practices using ultrafast lasers and attempted to tackle the obstacles in the clinic. Notably, [PERSON] et al. have developed a lightweight, miniaturized surgical ultrafast laser probe that offers clinically acceptable ablation speed in orthopedic surgery, enabling the potential of robotic integration [41]. Another group also established an optical real-time monitoring of ultrafast laser bone drilling utilizing plasma emission spectroscopy, which allowed for differentiation between bone and bone marrow [42]. The feasibility of drilling large-sized, deep holes on cortical bones has also been demonstrated [42,43]. Furthermore, the fastest ablation rate on cortical bone tissues to date is 0.99 mm\({}^{3}\)/s in the literature, which was performed on fresh ex vivo sheepshank bone under a cooling condition by [PERSON] et al. in 2020 [42], followed by 0.66 mm\({}^{3}\)/s on dried ex vivo defrost and dried porcine femurs under a non-cooling condition reported by [PERSON] et al. in 2021, using industrially available femtosecond laser sources [44], in comparison to mechanical tools that enable drilling and cutting speeds of up to 5 mm/s [45]. The upscaling of ablation rate is fundamentally constrained by the average powers combined with considerations of repetition rate optimization as well as thermal effect on the tissue [39,46]. The phenomenon arises from the fact that, at the given laser average power and wavelength, an increase in single pulse energy resulting from a decrease in PRR improves the ablation rate until the saturation point where laser energy begins to spread outside the penetration volume [47]. [PERSON] et al.