Optical fibers for endoscopic high-power Er:YAG laserosteotomy

Abstract.

Significance: The highest absorption peaks of the main components of bone are in the mid-infrared region, making Er:YAG and ${\mathrm{CO}}_2$ lasers the most efficient lasers for cutting bone. Yet, studies of deep bone ablation in minimally invasive settings are very limited, as finding suitable materials for coupling high-power laser light with low attenuation beyond $2\mu \mathrm{m}$ is not trivial.

Aim: The first aim of this study was to compare the performance of different optical fibers in terms of transmitting Er:YAG laser light with a $2.94\text{-}\mu \mathrm{m}$ wavelength at high pulse energy close to 1 J. The second aim was to achieve deep bone ablation using the best-performing fiber, as determined by our experiments.

Approach: In our study, various optical fibers with low attenuation ( $\lambda =2.94\mu \mathrm{m}$ ) were used to couple the Er:YAG laser. The fibers were made of germanium oxide, sapphire, zirconium fluoride, and hollow-core silica, respectively. We compared the fibers in terms of transmission efficiency, resistance to high Er:YAG laser energy, and bending flexibility. The best-performing fiber was used to achieve deep bone ablation in a minimally invasive setting. To do this, we adapted the optimal settings for free-space deep bone ablation with an Er:YAG laser found in a previous study.

Results: Three of the fibers endured energy per pulse as high as 820 mJ at a repetition rate of 10 Hz. The best-performing fiber, made of germanium oxide, provided higher transmission efficiency and greater bending flexibility than the other fibers. With an output energy of 370 mJ per pulse at 10 Hz repetition rate, we reached a cutting depth of $6.82\pm 0.99\mathrm{mm}$ in sheep bone. Histology image analysis was performed on the bone tissue adjacent to the laser ablation crater; the images did not show any structural damage.

Conclusions: The findings suggest that our prototype could be used in future generations of endoscopic devices for minimally invasive laserosteotomy.