# Orthopedics-Related Applications of Ultrafast Laser and Its Recent Advances
## 3 Ultrafast Laser in Orthopedic Surgery
### Ablation Parameters
also compared bone ablation efficiency in different wavelength regimes (IR--1030 nm, visible--515 nm, UV--343 nm), average powers (IR--6.27 W, visible--6.27 W, UV--3.9 W), PRRs (250, 500, 1000 kHz) and scanning speeds (1000, 2500, 400 mm/s) with the conclusion that visible regime, the lowest PRR and the highest scanning speed provided the best ablation rate without thermal tissue disturbance because bone chromophores responded differently to the three wavelength regimes [44]. Moreover, the ablation rate is sometimes affected by bone debris accumulation depending on the amount generated, and thus can be further upscaled by immediate debris removal after each pulse using a cooling system [42,48]. It was demonstrated that compressed air flow and water flow could reduce bone debris by 64% and 76%, respectively; however, significant laser energy loss was observed under water cooling conditions leading to the slowest ablation rate [42]. It has also been noted that sample conditions, specifically ex vivo vs. in vivo, dried vs. fresh and storage conditions, as well as bone surface processing such as sanding, polishing or unaltered can have a significant influence on the ablation performance [49, 50, 51]; thus, it is crucial to simulate a real clinical situation for accurate and consistent assessments. Table 2 summarizes the ablation rates from ultrafast laser bone ablation studies of different bone samples using different laser parameters. Characterization of the ablation threshold and the incubation effect likewise plays a major part in optimizing ablation performance on a particular bone type. Ablation threshold, which represents the minimal laser fluence needed to initiate material removal from a surface [39,50], can be measured by exposing the surface to ultrafast laser pulses of decreasing energy or beam radius until no material removal occurs [39,52] or using the D\({}^{2}\)-technique calculation based on the correlation between the diameters of ablated craters and different pulse energy levels [49, 50, 52, 54, 55, 56, 57]. Bone ablation is usually optimized when the pulse energy is sufficiently higher than the threshold to ensure pulse-to-pulse consistency, but not exceeding a limit that could induce collateral thermal damage [58]. The balance between ablation rate and fluence therefore needs to be well characterized for the bone tissue, otherwise negating the most unique advantage of minimizing thermal effects [59, 60, 61]. \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Author/Year** & **Bone Type** & **Ablation Rate (mm3/s)** & **Laser System Parameters** & **Potential Application** \\ \hline [PERSON] et al., & & & A CaF\({}_{2}\) objective; & A miniaturized surgical probe for robotic microsurgery such as spine \\  
* Reported as depth removal per pulse.  
\end{table} 
Table 2: Summary of recent studies on the use of ultrafast lasers for orthopedic applications. On the other hand, incubation effect refers to the phenomenon where a reducing ablation fluence threshold is accompanied by an increasing number of incident laser pulses in a power law relationship [62], and is typically caused by sufficient energy deposit from the few initial pulses for subsequent pulses permitting lower than single-pulse ablation threshold [41, 50, 57, 63, 64]. An incubation coefficient value of 1 indicates no incubation effect. Incubation effect is the most profound at lower pulse numbers where ablation threshold is rapidly reduced as the pulse numbers increase until a saturation point [50, 65], and has been shown to modify tissue structures significantly enough, especially at higher PRR, to create beam distortion, shadowing and substantial light scattering due to debris shielding, thus leading to considerable thermal disturbance and decrease in ablation rate [44, 60]. With that being said, a recent study by [PERSON] et al. concluded that there was none to very little incubation effect found for two types of cortical bones (bovine and ovine) by showing the same incubation coefficient (1.02 \(\pm\) 0.05) [49].