Mechanistic Investigation and Performance Evaluation of Ultrasonic Bone Drilling under Longitudinal-Torsional Coupled Vibration

Bone drilling is widely used in orthopedic and trauma surgery. However, conventional drilling generates high cutting forces and excessive heat, which may cause mechanical damage and increase the risk of thermal damage, thereby compromising hole quality. To address these limitations, a kinematic model of longitudinal-torsional ultrasonic vibration drilling (LTUVD) of cortical bone was developed. The model elucidates how periodic tool-bone contact and separation, micro-impacts, reverse shearing, dynamic changes in the effective rake angle, and complex cutting-edge trajectories collectively govern force generation and heat accumulation. Based on this model, three ultrasonic toolholders with different longitudinal-torsional coupling conditions were developed, and their effects on drilling force, temperature, and hole-wall surface quality were systematically evaluated. Compared with conventional drilling, LTUVD reduced drilling thrust by up to 55.9%, peak temperature by up to 17.8%, and hole-wall surface roughness by up to 42.4%. Among the tested longitudinal-to-torsional amplitude ratios (T/L), T/L = 0.425 produced the lowest drilling thrust and peak temperature. Surface roughness, however, varied non-monotonically and reached a minimum at T/L = 0.3, which also produced the most uniform hole-wall morphology. These findings indicate that the preferred T/L ratio depends on the performance criterion considered. The results provide a theoretical and experimental basis for selecting longitudinal-torsional vibration conditions and designing orthopedic drilling instruments with the potential to reduce bone damage.