Effect of pulse targeting strategy on Er:YAG laser ablation rate and efficiency in ex vivo cancellous bone

Lasers are an attractive alternative to mechanical tools for bone preparation, but clinical adoption is limited by low removal rates. This study investigated whether spatiotemporal sequencing of Er:YAG pulses can improve thermomechanical bone ablation, and how pulse separation and delivery order affect ablation rate, efficiency, and heating.

A prototype Er:YAG system, with galvanometer scanning, ablated cancellous bone in eight ex vivo lamb femurs. Arm 1 varied the separation between consecutive pulses (0.175 to 0.275 mm). Arm 2 compared four targeting patterns within a fixed pulse grid, maintaining constant fluence and total energy for three of the four patterns. Thermal imaging and pre/post confocal scans quantified heating and volumetric removal.

Ablation rate was consistent across pulse separations (521 to 552mm³/min), while layer depth scaled linearly with deposited energy (R² = 0.94), decoupling depth control from throughput. Targeting strategy significantly affected ablation efficiency, ranging from 0.19-0.32mm³/J (p < 0.001). Hatching was fastest and most efficient, sustaining consistently high peak surface temperatures (200-250 °C) but lower mean surface temperatures than either scatter strategy.

Ablation efficiency varied with pulse order under controlled conditions, indicating a dependence beyond single pulse dynamics. We propose inter-pulse thermal coupling, whereby residual heat from a preceding pulse lowers the energy required by the next. Optimising the targeting strategy achieved the fastest reported removal rate for cancellous bone, identifying pulse sequencing as a lever for more efficient ablation.