The liquid-assisted infrared laser ablation of hard biological tissue in clinical surgery is superior to the use of mechanical tools in terms of accuracy and compatibility. Owing to the pressure gradient inside and outside the bubble, the laser-induced bubbles expand, contract, and eventually collapse in the liquid environment. As the bubble collapses, the released energy causes the surface of the biological hard tissue to ablate. Moreover, the dynamic behavior of laser-induced bubbles is affected by boundary constraints. During pulsation, the nearby boundary can significantly affect the shape of the bubble and make it deviate from the original spherical shape, which results in the generation of a high-speed microjet when the bubble collapses. Therefore, the existence and shape of the boundary directly affect the ablation of the bubble microjet on the material. Bone tissue has a natural curvature and often imposes requirements associated with the drilling and cutting of curved surfaces during orthopedic surgery. Moreover, studying effects of curved surfaces of natural bones on the evolution behavior of nearby bubbles and generated microjets can improve the accuracy and rate of bone ablation and contributes to the practical application of laser ablation in orthopedic surgery. Therefore, this paper presents research on the effect of the curved boundary of natural bones on the bubble evolution behavior and generation of microjets and relative mechanisms, which is significant for guiding the improvement of laser water-assisted bone ablation.
The experimental system used in this study is shown in Fig. 1. Laser pulses with a wavelength of approximately 2 μm are transmitted from a fiber to a bone target immersed in distilled water to induce a bubble, and a high-speed camera is used to capture the transient features of the bubble. Fresh pork bones are used as the target material; the curved boundary of the joint (curvature radius r≈10 mm) is used as the target position, and distilled water is used as the liquid environment. Moreover, the bone target is fixed on the jig to ensure the target position is immersed in distilled water at a depth of 10 mm, and the distance between the end of the fiber and target surface is adjusted to obtain bubbles at different distances from the boundary. To study and compare the evolution characteristics of bubbles with different Rmax, the dimensionless distance
The shape of the bubble is asymmetric during stage Ⅰ owing to the extrusion of the bubble by the boundary of the bone surface ( column 2 in Fig. 2). During stage II, the bubble deviates from nearly spherical ( column 5 in Fig. 2) and becomes attached to the curved surface of the bone with a disc shape ( column 6 in Fig. 2) until its disappearance at the end of stage IV. During stage II, the bubble appears to shift to one side. As shown in Table 1, the time period of stages I and II of the bubble decreases with a decrease in the value of γ, and the time period of stage II is shorter than that of stage I. The time period of stages III and IV first decreases and then increases with the value of γ, reaching a minimum when γ=0.4. During stage II, the centroid of the bubble significantly shifts toward the higher side of the natural bone curved boundary. By analyzing the change in the position of the bubble centroid, the fiber preset offset for the positioning error compensation of the microjet effect is obtained. The numerical simulation results indicate that the pressure gradient at the bubble wall near the higher side of the surface boundary is smaller owing to the asymmetry of the natural bone curved boundary, and it is larger on the lower side of the surface boundary. This results in different moving speeds on the different sides of the bubble wall, causing the centroid of the bubble to shift to the higher side of the surface boundary. The direction of deflection of the micro-jet generated by the bubble is also similar to that of the displacement of the bubble centroid in late stage II of bubble formation. The micro-jet velocity generated by the bubble near the natural bone surface with 0.2≤γ≤0.6 is obtained, and the velocity of the micro-jet generated by the bubble increases as γ decreases.
In this study, the evolutionary behavior and mechanism of a 2 μm wavelength laser-induced bubble with 0.2≤γ≤0.6 near the boundary of a natural bone curved surface are investigated. Based on the results, the time period of stages I and II of bubble pulsation near the boundary of the curved surface of the natural bone decreases with a decrease in γ, the time period of stages III and IV first decreases and then increases with γ, and the periods of the bubble expansion and contraction phases are different. Owing to the irregular shape of the natural bone curved boundary, the shape and evolutionary behavior of the bubble may appear asymmetrical. When the bubble contracts, the centroid of the bubble is shifted to the higher side of the natural bone curved boundary, and the microjet generated by the bubble deflects in the same direction. The numerical simulation results show that the asymmetry of the natural bone curved boundary causes a pressure gradient and different moving speeds near the two sides of the bubble wall when the bubble shrinks, causing the centroid of the bubble and bubble wall to shift to the higher side of the natural bone curved boundary. Owing to the deviation in the microjet direction, the ablation accuracy in certain applications such as laser water-assisted bone ablation is affected. Moreover, the fine error compensation is performed in this study for related operations by presetting the offsets of the fiber at a natural bone surface position and using different γ values. Laser water-assisted bone ablation can actually be applied in surgery when the micro-jet velocity increases with a decrease in γ, and the ablation rate of bone tissue can be improved by appropriately reducing γ.
