Bone drilling is the most common surgical intervention used to install implants and treat fractures, and the microstructure of bone holes significantly affects fracture stability and the postoperative healing process. Traditional mechanical drilling can easily damage the microstructure of bone holes, produce cracks and bone chips, and seriously affect fracture stability. The contactless interaction of laser beams with bone has attracted increasing interest because it offers greater freedom and higher precision during drilling and provides a promising solution to improve the anti-instability performance of the drilled bone hole for clinical application. However, current studies on bone holes drilled by laser mainly focus on the analysis of macroscopic morphologies of bone cracks; moreover, the microstructure and anti-instability behavior of bone holes drilled by laser concerning heterogeneous structures are rarely investigated. To further promote the clinical application of the laser drilling of bone, this study systematically evaluates the microstructural characteristics and relationship between the microstructure and related anti-instability behavior of bone holes drilled by laser, which is helpful for optimizing the laser drilling strategy and providing guidance for the promotion of the laser drilling of bones for clinical application.

Freshly extracted sheep tibia are selected for in vitro laser bone drilling. A mid-infrared laser with a wavelength of ~2 μm is used to drill 3 mm-diameter bone holes on the sheep tibia. First, the atomic force microscopy (AFM) is used to characterize and compare the elastic modulus distributions of the tissues around the bone holes drilled by a laser with different power densities and by a mechanical drill bit (Fig. 1). The microstructure of the bone-hole tissue is then quantitatively analyzed and evaluated using fractal evaluation. Subsequently, a heterogeneous four-phase composite model is established by considering the heterogeneous bone structure and mechanical properties of the bone hole. The stress distributions of the bone holes drilled by the laser and by mechanical drill bit under the action of axial force are simulated to visualize the anti-instability behavior of the drilled bone holes.

The width of the elastic modulus affected zone around the bone hole decreases with decreasing laser power density (Fig. 2). Under the optimal laser power density of 5.70×10⁴ W/cm² and scanning speed of 4 mm/s, the width of the elastic modulus affected zone near the bone hole edge is only 200 μm, the width of the elastic modulus affected zone of the hole bone drilled by laser is 67% smaller than that of the bone hole drilled by mechanical drill bit, and the other areas near the bone hole maintain the intrinsic microstructures and mechanical properties of the original bone tissues (Fig. 3). The fractal dimension analysis result shows that the fractal dimension and porosity of the bone tissue near the bone hole drilled by laser are 1.88 and 0.17, respectively, both of which are smaller than those of the mechanically drilled bone hole (Fig. 4). The results indicate that the microstructure of the bone tissue around the laser-drilled bone hole is hardly affected, the intrinsic structure of the bone tissue is maintained, and no cracks are observed on the bone hole wall (Fig. 5). The simulation results show that the stress concentration tendency of the laser-drilled bone hole is lower than that of a mechanically drilled bone hole under the action of axial force, and the cracks are inhibited or deflected around the cement line (Fig. 6), which effectively improve the instability resistance of the bone hole.

In the present study, the microstructural characteristics and anti-instability behavior of laser-drilled bone holes are evaluated using the combination of experiments and simulations. It is found that the elastic modulus of the bone tissue around the bone hole tends to decrease with decreasing power density and increasing distance from the hole wall, and the effect of laser drilling on the elastic modulus distribution around the bone hole is relatively lower than that of mechanical drilling. A quantitative method for evaluating the microstructural morphology under laser action is established based on fractal evaluation, and the fractal dimension and porosity of bone tissue near the laser-drilled bone hole are merely 1.88 and 0.17, respectively, which are almost the same as the intrinsic structure parameters of bone tissue, indicating that laser drilling has a slight effect on the tissue around the bone hole, providing guidance for the optimization of laser drilling parameters. The optimized laser power density and scanning speed is 5.70×10⁴ W/cm² and 4 mm/s, respectively, for minimal microstructural damage. The width of the elastic modulus affected zone around the bone hole is only 200 μm, which is 67% smaller than that of the mechanical-drilled bone. There are no cracks but open Haversian canals on the bone hole wall. The stress distribution is obtained using the finite element method to visualize the anti-instability behavior of the bone hole. The observed phenomenon reveals that the maximum stress propagates in the interstitial matrix; however, the growth of cracks caused by stress concentration is quite different for the laser- and mechanically-drilled bone holes. The cracks in the laser-drilled bone hole cannot penetrate the cement line, but are inhibited or deflected around the cement line, which improves the instability resistance of the bone hole. Our study shows that laser bone drilling is a new strategy for clinical applications that minimizes microstructural damage and improves fracture stabilization.