Influence of Drill Diameter on Temperature and Time during Slow Drilling Protocol - In Vitro Stud

Introduction: Bone drilling causes an increase in bone temperature, and a temperature above 47°C is critical because it causes thermal bone necrosis. Recently the concept of slow-speed drilling (50 rpm) was suggested as an alternative to the conventional implant-bed preparation. Some advantages for slow drilling over high speed drilling like better control, recovery of vital bone, lower heat generation and patient management are claimed. It is critical for the success of dental implants that minimal heat is generated in the bone during the drilling of the implant sites. Bone tissues are very susceptible to thermal injury, and the temperature threshold for tissue survival during osteotomy is 47 °C when drilling is maintained for more than 1 min. The relationship between heat generated and implant drilling osteotomy is multifactorial in nature and its complexity has not been fully studied. Lack of scientific knowledge regarding this issue still exists. In this in-vitro experiment, the effect of drill diameter on temperature changes and time taken for drilling different bone qualities were evaluated. Materials and Methods: Tapered drills with same design and different diameters (3.2 mm, 3.6 mm, 4.5 mm and 4.8 mm) were used for drilling in Type D2, Type D4 synthetic bone block and fresh porcine rib bone at 10 mm depth. Three experiments were performed in bone with different bone qualities. Experiment 1 in Dry artificial bone blocks, Experiment 2 in Wet artificial bone blocks and Experiment 3 in porcine rib bone. Test groups were numbered from 1--4 respectively for all 3- individual experiment. A 1.9 mm pilot drill was used as control. Slow drilling at constant speed of 50 rpm with no irrigation was used by a calibrated operator for performing the osteotomy. Twenty preparations on both Type D2 and Type D4 bone blocks and 10 preparations on porcine rib bone were performed by each drill. Temperature and time were recorded using a thermocouple system. A thermocouple was mounted in a prepared hole, approximately 1--1.5 mm from the osteotomy walls for all osteotomies performed. The Labchart software was connected to the thermocouple instrument for recording data. Statistical analysis was performed using non-parametric ANOVA test with p value set at 0.05. Mann-Whitney test was used to compare the data collected for the same drill between different bone blocks and models used. Results: Maximum temperature (Tmax), differential temperature (DeltaT) and time required to drill 10 mm implant beds were obtained and compared for all the test groups. No significant difference was observed for DeltaT (net difference in temperature) between all the four drills for Type D2 and Type D4 Dry and Wet synthetic blocks as well as for porcine rib bone (p<0.05). A longer time was required for the narrow diameter (3.2 & 3.6 mm) drills compared to the wider diameter (4.5 & 4.8 mm) drills (p<0.05) while drilling Type 4 bone. The increase in Temperature was significantly higher in Dry model than Wet model (p<0.05) for both types of bone blocks. The increase in temperature was significantly higher in both Dry and Wet synthetic bone model (Type D4 and Type D2) than in porcine rib bone. The Longest time was required to drill porcine rib bone. Conclusion: Within the limitations of this experimental in vitro study on Type D4 and Type D2 synthetic bone block and fresh slaughtered porcine rib bone it can be concluded that the net difference (DeltaT) in temperature is insignificant between all the four different drill diameters with the same design for all the three models at drilling at low speed of 50 rpm. Increase in temperature at slow drilling speed of 50 rpm for drills with diameters from 1.9 mm to 4.8 mm did not exceed 28°C. Wet models conduce to less amount of heat generation compared to Dry models. The Type of bone has more impact on heat generation than the diameter of the Implant drill when slow drilling speed of 50 rpm was used. Drilling time is influenced by the characteristics of the bone and the drill diameter when drilling at slow speed of 50 rpm. Denser bone requires more drilling time compared to soft bone.