[Study on parameters of robot-assisted ultrasonic drilling on bovine vertebral body]

Objective: To investigate the effect of ultrasonic parameter settings on maximum temperatures in the drilling site and penetration time and determine the most suitable parameters for efficient and safe robot-based ultrasonic bone drilling in spinal surgery. Methods: Five adult bovine thoracic and lumbar vertebrae specimens (T₁₀-L₆) were cut into 10 mm thick slices. A total of 50 slices were obtained. Among them, 30 and 20 slices were used for cancellous bone experiments and cortical bone experiments, respectively. In the cancellous bone experiment, the slices were randomly divided into three groups, corresponding to different feed rates of 0.8 mm/s, 1.6 mm/s, and 2.4 mm/s, respectively, with 10 slices in each group. The cancellous part of each slice was drilled 9 times with different output powers from 20% (48 W) to 100% (120 W). In the cortical bone experiment, the slices were randomly assigned into two groups, corresponding to a different feed rate (0.8 mm/s and 1.6 mm/s). Drilling was performed on the cortical part of each slice 4 times with different output power, which increased from 70% (84W) to 100% (120 W). All experiments were conducted at room temperature of 25 ℃. Maximum temperature and penetration time were recorded. The maximum grinding temperature and penetration time of cancellous bone and cortical bone under different output power and feed rate were compared. Results: At the same feed rate, the maximum temperature of the cancellous bone decreased as output power increased. There were statistically significant differences in the maximum temperature between the output powers of 120 W and 24 W under different feed rates(61.2 ℃±9.4 ℃ vs 70.9 ℃±5.7 ℃, 59.2 ℃±7.1 ℃ vs 69.5 ℃±10.7 ℃, 55.5 ℃±5.5 ℃ vs 69.2 ℃±9.3 ℃, all P<0.05). At the premise of the same output power, there was no significant difference in the maximum temperature among different feed rates (all P>0.05). At the feed rate of 0.8 mm/s, the maximum temperature of cortical bone decreased as the output power increased. The maximum temperature at the output power of 120 W was significantly lower than that of 84 W (P=0.048). However, at the feed rate of 1.6 mm/s, the maximum temperature could not be significantly lowered by the increase in output power (P>0.05). Under the same output power, the maximum temperature at the feed rate of 1.6 mm/s were all significantly lower than those of 0.8 mm/s (all P<0.05). The penetration time of cancellous bone did not decrease significantly with the increase in the output power (all P>0.05) while it decreased significantly as the feed rate increased (all P<0.05). Regarding cortical bone at the feed rate of 0.8 mm/s, the increase in output power could not shorten the penetration time (P>0.05); at the feed rate of 1.6 mm/s, the penetration time at the output power of 120 W was significantly shorter than that of 96 W (P=0.008). With the same output power, the penetration time at the feed rate of 1.6 mm/s were significantly shorter than those at 0.8 mm/s (all P<0.05). There was no statistical difference in the penetration failure rate among different feed rates with the same output power (all P>0.05). The penetration failure rate was 0 when the output power of cancellous bone was 48 W and above and the output power of cortical bone were 108 W and 120 W. Conclusions: The maximum temperature of vertebral cancellous bone and the cortical bone is primarily influenced by the output power and the feed rate, respectively; the penetration time of cancellous bone and the cortical bone is affected by the feed rate and both of feed rate and output power, respectively. The most suitable parameters are output power of 120 W and feed rate of 2.4 mm/s for cancellous bone and output power of 120 W and feed rate of 1.6 mm/s for cortical bone.