Controlled fiber laser bone ablation using inline coherent imaging
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Laser material processing is becoming increasingly popular in various disciplines of science and industry due to its clean and quiet operation, absence of tool wear and high positioning accuracy. In particular, laser osteotomy ("bone cutting") offers new opportunities with its well-identified advantages over conventional methods utilizing mechanical saws/drills. However, lack of depth control is a long-standing barrier to wide clinical implementation. In-process monitoring and real-time feedback is highly desirable for depth-sensitive surgical operations where cutting is performed above critical tissues (e.g., brain surgery). In this work, a low-coherence imaging technique known as inline coherent imaging (ICI) is utilized to monitor bone ablation processes and provide depth information about the sample with on-the-fly signal processing. ICI is analogous to spectral domain optical coherence tomography with its sample arm built "inline" with the ablation laser beam path. ICI is capable of an imaging speed up to 240 kHz and a high dynamic range of over 60 dB. The laser system used for bone ablation is a 1070 nm ytterbium-doped fiber laser. Its fiber delivery allows flexible cutting angles to access hard-to-reach areas in surgery. Laser bone ablation by the fiber laser is characterized by percussion drilling experiments and ICI depth tracking of the sample. The ablation parameters are optimized for high ablation efficiency and minimal thermal damage. Closed-loop feedback based on in situ ICI measurement is developed to achieve controlled ablation with complexity and high precision. Designed features, including 1D holes, 2D trenches and 3D morphologies, are ablated with accuracy verified by an ex situ stylus profiler. The thermal damage to the surrounding bone tissue is assessed by histology. The heat-affected zone is quantified as 5-10 um for 3D features created on both fresh and dry bone samples. This proof-of-principle study shows promising potential for ICI-guided laser osteotomy.