Controlling the temperature rise during the implant site preparation represents one of the important factors influencing the development of osseointegration. Temperature above 47°C during the drilling into the bone could irreversibly cause bone necrosis.1–5 Over the past years, the effects of various factors related to implant drilling on the temperature rise have been explored. Some of those include drilling force and speed, withdrawing process, drill design, irrigation system, drill-bone contact surface, etc.6 Reduction in length of drilling might limit frictional heat, and the drills of the same length might induce lower changes in bone temperature when comparing with a pilot drill.7
However, only few studies investigated various osteotomy lengths during the implant site preparation and temperature rise on the crestal bone, determining the possible risk of osteonecrosis. A review done by Mishra et al8 assessed data about various factors influencing heat generation during rotary cutting of drills. The authors concluded that there is a lack of scientific knowledge and that the complexity of the multifactorial relationship between implant drilling osteotomy, and heat generation is not fully studied. Augustin et al9 also reviewed the current literature on cortical bone drilling and thermal osteonecrosis. They stated that there is a need for a more precise experimental setup and also highlighted some of the areas that need further investigation. Therefore, the objective of this research was to compare the temperature rise of the crestal bone during implant site preparation between different lengths of osteotomies.
Materials and Methods
To resemble a human bone and an implant drilling protocol, this study was conducted in laboratory conditions on animal bone specimens. Experiments were performed in a closed system, with no external influences on the final results. Bovine ribs were used to simulate the cortical bone of the human mandible, as was described by Abouzgia and James10 and Benington et al.11 Bones were obtained from animals that were killed, and the same day, they were transported in the saline solution. Specimens were cut in approximately 12-cm long parts with an electric saw. The ribs that were similar in size and thickness were chosen for experiment, and only middle sections of the ribs were used for drilling. The ribs were then taken out of the solution, and the lower part was soaked into a custom-made water bath heated to approximately 36.6°C to get as closer as possible to clinical conditions (Fig. 1). The starting temperature of bone samples before drilling did not differ and was approximately 30.3°C for all samples, which is lower than the realistic temperature in the human mouth.
For the purpose of experimental analyses, a measurement system was built (Fig. 2).12 Drilling was performed in such a manner that the implant bed was prepared with twist drill, and all other drills were inserted through the same osteotomy (Fig. 3). All systems were used on the same rib, subsequently performing 2 sets of drillings for all 3 systems on 1 rib. Between the drilling, a 30-second time interval was used to switch the drilling bits for second drilling. A total of 10 ribs were used, which also included the ribs that were used for test drillings.
The measurement system allowed us to set the final length of the osteotomy. The rotational movement was created using a DC motor. The location of the drilling bit was measured with a linear variable differential transformer displacement sensor. The load cell mounted under the test specimen measured the drilling force, which was transmitted from the drill bit to the test specimen. The drilling speed was measured with a tachogenerator. Thermocouple type J (1.8 mm) that was custom made for this experiment was placed 2 mm deep, so it is still in the level of crestal bone and as close as possible to the osteotomy site (1 mm). The correct placement of the thermocouple and the distance to the osteotomy site were controlled by x-ray and by the measurement system. Before submerging the thermocouple, the osteotomy site was filled with a thermally conductive paste. While drilling the osteotomy site for thermocouple, external water cooling was used to prevent bone damage.
Drilling force, drilling speed, drilling length, and temperature were recorded with data acquisition devices and were saved continuously on a personal computer during the drilling.
Drilling bits from 3 different implant systems were used—Astra Tech, Ankylos, and XiVE. The drilling protocol set by the manufacturer for each system was followed. The osteotomy for the Astra 3.5-mm implant, for the Ankylos 3.5-mm implant, and for the XiVE 3.4-mm implant was prepared. Astra protocol: a pilot hole was drilled with a twist drill bit with diameter D = 2.0 mm to define the direction of the implant. After that, a twist drill bit D = 3.2 mm was used as a final drill. Ankylos protocol: a pilot hole was drilled with a twist drill bit D = 2.0 mm to define the direction of the implant. For the final osteotomy, the Tri-Spade drill bit A D = 3.4 mm was used. XiVE protocol: a pilot hole was drilled with the XiVE D = 2.0-mm twist drill bit to define the direction of the implant. After that, osteotomy with D = 3.0-mm twist drill bit was made. Finally, twist drill bit D = 3.4 mm was used for the final osteotomy.
A hole was drilled with external water cooling and drilling speeds of 1200 rpm for Astra and 800 rpm for Ankylos and XiVE. Irrigation was achieved by the NSK Surgic pro physio dispenser. Flow was set to 10 mL/min. To imitate the actual conditions during the drilling into the bone, the drilling was done manually in a 3-time withdrawal manner. The burrs and holes were positioned every time exactly at the same distance, which was controlled by the fixation of the burrs and the stoppers on the machine. Osteotomy lengths of 8, 12, and 16 mm were investigated.
Statistical analyses were performed using IBM SPSS Statistics 21 software (IBM, Armonk, NY). The temperature before drilling and its increase during and after the first and second drillings represented dependent variables. Differences between temperature before and after the drilling have been compared with the paired t test. Level of significance α < 0.05 was evaluated as a statistically significant difference. For independent variables, the length of osteotomy and 3 different systems for drilling in the crestal bone (Astra, Ankylos, and XiVE) were determined. The temperature of the bone was measured before, during, and after the first and second drillings. The results were recorded in °C. Temperature was estimated as the dependent variable and was statistically analyzed by series of t tests through 2-way analysis of variance analysis and then continued with the post hoc analysis. The results were statistically significant if the P-value was less than 0.05.
Average Temperatures of the Crestal Bone Before and After Drilling
After drillings, the temperatures of bone were recorded lower than before the drilling (P < 0.001). In addition, second drillings showed even lower temperatures on the crestal bone with statistically significant differences (P < 0.001) for all comparisons (Table 1).
Average Temperatures of the Crestal Bone After First Drilling
Statistically significant differences were observed only between osteotomy lengths (P = 0.022), which contributed to approximately 24% of the variation in temperature (eta square = 0.238). When comparing different drilling systems, no statistical significance in the rise of temperature was observed (P = 0.224). Average drilling temperatures are presented in Table 2.
When comparing the maximum temperature during the first drilling, it was proven that the drilling length showed F = 4.418 and P = 0.021, which represented a statistically significant difference between different osteotomy lengths. Eta-squared value was determined and showed a score of 0.240, which meant that different drilling lengths contributed to approximately 24% of the variation in temperature. Drilling systems showed F = 5.055, P = 0.013, and eta square = 0.265 and lead to significant differences in temperature in 27%. The interaction between the drilling lengths and drilling systems showed F = 1.774 and P = 0.162, which was not a statistically significant difference.
Clinically significant differences between Astra and XiVE (P = 0.042) were calculated, which indicated that XiVE had statistically lower temperatures. Another difference between Ankylos and XiVE (P = 0.004) was also observed. XiVE showed lower temperatures (Table 3).
Average Temperatures of the Crestal Bone After Second Drilling
The difference between osteotomy lengths was observed when compared 8 to 12 mm and 8 to 16 mm. An 8-mm length showed the lowest temperatures, which is to be expected when drilling into the bone, as generally the shorter drilling time produces less heat (Table 4).
Temperatures after second drilling showed F = 9.033 and P = 0.001, which was a statistically significant difference between osteotomy lengths. An eta-squared value of 0.376 was determined, which meant that different drilling lengths contributed to approximately 38% of the variation in temperature. When comparing different drilling systems, F = 0.476, P = 0.626, and eta square = 0.031 were demonstrated, which was not statistically significant. The interaction between the osteotomy lengths and the drilling systems also showed no statistical significance.
Maximum temperatures during the second drilling for all osteotomy lengths showed F = 4.925 and P = 0.014, which represented statistically significant differences. The eta-squared value was 0.247, which meant that different osteotomy lengths contributed to approximately 25% of the variation in temperature. When comparing different drilling systems, F = 7.258, P = 0.003, and eta square = 0.326 were demonstrated. Different systems led to 33% of significant differences in temperature. The interaction between the drilling lengths and the drilling systems showed F = 2.115 and P = 0.104 and was not statistically significant.
Among the drilling systems, a statistically significant difference was detected between the Ankylos and XiVE (P = 0.001). The XiVE system showed the lowest temperatures. The results indicated that the XiVE system was effective for drilling into the bone because its maximum temperature did not cause strong heating of bone tissue (Table 5).
The purpose of this study was to compare the temperature rise of the crestal bone during implant site preparation between different lengths of osteotomies and different drilling systems. Moreover, it was investigated whether the temperature rise of a crestal bone is associated with the length of osteotomy (8, 12, and 16 mm) or with different implant systems that are commonly used. Strbac et al13 demonstrated that heat generation during osteotomies is a multifactorial scenario, mainly influenced by the osteotomy depth and mode of irrigation. External cooling can maintain the temperature within the safe range, whereas drilling without irrigation can lead to temperatures that exceed the acceptable limit.14 Controlling the temperature rise during rotary drilling is a very important factor that can influence on the development of osseointegration. Overheating up to 60°C on the osteotomy site induces significant crestal bone loss during healing.15 Temperature above 47°C during the drilling into the bone could irreversibly cause bone necrosis.1–5,16 Various lengths of osteotomy during the implant site preparation have an influence on the temperature rise on the crestal bone, determining the possible risk of osteonecrosis.
Bovine ribs were used as a model for the cortical bone of the human mandible and compared 3 different implant systems—Astra Tech, Ankylos, and XiVE. All implant systems used twist drilling bits. Statistically significant differences in temperature before and after drillings were observed, and eventually, the maximum temperature rise on the crestal bone was evaluated. Second drilling showed a lower temperature of the bone in all cases compared with first bone drilling. On average, differences between drillings were not more than 1°C. Water cooling (irrigation) during drilling with implant systems that were used in this experiment effectively contributed to lowering the temperature of the drilled bone. The average bone temperature before first drilling was approximately the same 30.3°C for all bone samples, and after the drilling, the temperature dropped because of water cooling to 29.19°C. These bone temperatures are actually lower than realistic temperatures in the human body. The maximum temperature during drilling did not exceed 30.39°C, so we can easily say that all systems run effectively and with a rise of temperature of only 1°C cannot cause bone necrosis. Although the starting bone temperature was 30°C, the rise of crestal bone temperature was only approximately 1°C, which is still well within the safe zone. A similar result was observed by Lucchiari et al,16 where bone heating during the osteotomy never exceeded 2°C and was clinically irrelevant, as thermal damage to bone has been reported only for temperatures beyond 47°C.16 After the second drilling, the measured temperature was even lower, 28.76°C, with maximal values during drilling of only 29.80°C. It is believed that a good irrigation system can effectively contribute to the prevention of damaging bone with enormous generation of heat. Isler et al17 tested irrigation with 25°C and 4°C saline for new bone formation, where osteoblasts were seen more active in group 4°C than group 25°C. However, they confirmed that there was no disadvantage to use 25°C, but it may be better to use 4°C for rapid cooling. At this point, it must be said that the thermocouple in our research was placed 1 mm from the osteotomy site, which leads us to conclude that the actual temperature on the crestal bone must have been a bit higher because the 1-mm bone wall acted as an isolator.
After the first drilling, statistically significant differences between different lengths of osteotomy were found (P = 0.022). Shortest osteotomy of 8 mm showed the lowest temperature of the bone after drilling. Meanwhile, the longest osteotomy showed the highest bone temperature. The results are as expected, because of the longer time of drilling and consequently caused more friction between the bone and drill bit, and are in concordance with the study by Sannino et al.7 They proved that reduction in the length of drilling had lower frictional heat, and the drills of lower length induced lower changes in bone temperature. When comparing different drilling systems, no statistical significance in the rise of temperature was observed (P = 0.224) after the first drilling because the drill bit (D = 2.00 mm) for all 3 systems was the same.
A similar result was also proved during the first drilling into the bone. The shortest osteotomy showed the lowest maximum temperature and caused the least temperature rise on the crestal bone during drilling. Meanwhile, the longest osteotomy did not cause the highest temperature of the bone. The highest temperature (T = 30.90°C) was measured for osteotomy of 12 mm. The reason for such results may lay in the fact that the longer osteotomy site is also cooled longer by external water irrigation. Different drilling lengths contributed to approximately only 24% of the variation in temperature. During the first drilling also, the 3 implant systems (P = 0.013) showed differences. The Ankylos system showed the highest temperature and the XiVE system showed the lowest maximal temperature during the first drilling. Two clinically significant differences between the systems, between Astra and XiVE (P = 0.042), were evaluated, which indicated that XiVE had statistically lower temperatures. Another difference between Ankylos and XiVE (P = 0.004) was also observed. The results indicated that the implant system XiVE was effective in drilling into the bone because its maximum temperature did not cause overheat of the bone. The drilling protocol set by the manufacturer for each system was followed, which was the same for the first drilling. For all 3 systems, a pilot hole was drilled with a twist drill bit with diameter D = 2.0 mm to define the direction of the osteotomy. Therefore, bone temperature was affected by drilling time and depth. A lower drilling speed with higher pressure is necessary for better bone regeneration. The optimal drilling speed was estimated at 230 rpm by Karaca et al.18 It is an assumption that lower drilling speeds of 800 rpm for XiVE could cause a lesser temperature rise in the drilled bone. However, Ankylos also used 800 rpm and showed the highest temperature. For the Astra system, 1200 rpm was used. Drilling speed did not play a role in this case, but it would be interesting to test the Astra system with a speed of 800 rpm and compare it with the other 2. Berardini et al19 in their review observed that there is no significant difference in bone resorption and afterward implant failure between implants inserted with high or low insertion torque, but Marković et al20 showed differently, resulting in low torque drilling into preparation sites might be advantageous for generated heat on the bone. Other factors that can contribute to differences in temperature between systems are especially drill bit design and drill bit material. In addition, despite the highest speed, the Astra system showed the lowest temperature for 12-mm osteotomy. A similar result was found in other studies in which Scarano et al21 and Chacon et al22 evaluated that the drill bit design seems to be an important factor for heat generation during implant site preparation. Drilling protocols could also play the role here because the XiVE protocol uses 1 extra bur before the final osteotomy.
When comparing temperatures after second drilling, it was proven that the drilling lengths showed a statistically significant difference (P = 0.001) between different lengths and contributed to approximately 38% of the variation in temperature. The shortest osteotomy of 8 mm showed the lowest temperature of the bone (T = 28.40°C). The highest temperature was recorded for the longest osteotomy of 16 mm (T = 29.16°C). The shorter drilling time for sure produced less heat because of the less time of fraction between the bone and a drilling bit. Despite using different drill bits, no significant difference was found in comparison of drilling systems.. The Astra protocol calls for the twist drill bit D = 3.2 mm, Ankylos used the Tri-Spade drill bit A D = 3.4 mm for the final osteotomy, and for the XiVE implant system, a twist drill D = 3.4 mm was used for the final osteotomy. The protocol for the XiVE implant system consisted of 3 drillings. XiVE and Astra showed the lowest temperature at 8 mm and highest at 16 mm of osteotomy, respectively, but Ankylos showed the lowest temperature at 12 mm of osteotomy. Those results were unexpected and showed inconsistency for the Ankylos system. The difference can be explained by different designs of drill bits,7,23,24 but not drilling techniques, as was recently observed by El-Kholey25 or drill wear because of bone resistance.26
Similar results were observed during the second drilling for both, various lengths of osteotomy (P = 0.014) and for different implant systems of drilling (P = 0.003). Second drillings showed lower temperatures of crestal bone, with statistically significant differences on all measurements (P < 0.001). When comparing maximum temperatures during the second drilling, it was proven that the drilling lengths of osteotomy showed statistically significant differences between each other. The eta-squared value was 0.247, which meant that different drilling lengths contributed to approximately 25% of the variation in temperature. When comparing different drilling systems, P = 0.003 was calculated and eta square = 0.326. So, different implant systems lead to significant differences in temperature, in almost 33%. Osteotomy of 8 mm showed the lowest temperature (T = 29.41°C), 12-mm osteotomy showed T = 29.99°C, and osteotomy of 16 mm showed the highest temperature (T = 30.06°C). No difference between 12 and 16 mm (P = 0.804) was observed. The XiVE implant system once again showed on average the lowest temperature (T = 29.36°C), the Astra system was second (T = 29.84°C), and the Ankylos system was the last (T = 30.26°C). Among the systems, a statistically significant difference between Ankylos and XiVE (P = 0.001) was detected. The results indicated that the XiVE system was the most effective for drilling into the bone compared with other 2 because its maximum temperature did not cause strong heating of bone tissue. Astra Tech and Ankylos implant systems showed similar results, but it must be mentioned that the Astra protocol uses a higher drilling speed than others.
As it was demonstrated significant differences in the osteotomy lengths and differences for different implant systems of drilling, respectively, it could not be confirmed which of these parameters have contributed to those differences. The Astra system showed inconsistency in results during second drilling because temperature in osteotomy of 16 mm was the lowest (T = 29.58°C). The XiVE system showed significant differences between the length of 8 and 12 mm (P = 0.022) and between 12 and 16 mm (P = 0.007). The longer drilling length of osteotomy increased the temperature of the drilled bone. The Ankylos implant system also showed the difference between the 8 and 16 mm (P = 0.042), which also indicated that more length and longer drilling time into the bone generate more heat and thus a higher temperature of the bone tissue. Overall, the XiVE implant system showed greater performance and temperature differences between its various lengths of osteotomy compared with other 2 systems, which could be explained by different drilling protocols like it was mentioned before.
Hochscheidt et al27 and Gehrke et al28 showed that temperature variation during osteotomy with different dental implant drills was influenced by irrigation, number of drill uses, drilling depth, drill diameter, geometry, and material. Different drilling lengths contribute to the variations in temperature regardless of the implant system. All second drillings showed lower temperatures. Longer drills and longer osteotomies induced higher temperatures on the crestal bone. The maximum temperature difference between the shortest and the longest osteotomies was under 1°C. Differences that were observed in this study were prone to the implant systems used. Regarding the system, the following factors could contribute to the rise of bone temperature: the drilling length, drill bit design or geometry, drill-bone, and contact area. Irrigation system, drilling force and speed, or bone samples could not influence on the results, because of the excellent and unified study design of comparing different implant systems, because these factors were the same for all osteotomy lengths and implant systems. Furthermore, drilling series were only repeated twice, so the repeated drillings and drill wear could not contribute to heat generation, as was observed in the study by Koo et al.24 In our study, the second drillings showed lower temperatures of the crestal bone, which was expected because of the external irrigation and the lower resistance from the cortical bone. Another important observation is that with all systems, the critical temperatures above 47°C were not reached; although the starting bone temperature was 30°C, the rise of crestal bone temperature was only approximately 1°C, which shows that if the starting temperature was in fact 37°C, it would still be well within the safe limits even with the longest drill bit. So within the limitations of this study, the tested implant systems showed good performance and can be efficiently used in clinical use.
There is a lack of scientific knowledge about the influence of different implant drilling systems on heat generation on the bone. Despite the limits of this study, it was observed significant temperature differences on the crestal bone in both the various lengths of osteotomy and 3 implant systems. That means that different drilling lengths contribute to the variation of temperature, as well as different implant systems. Shorter drilling lengths of osteotomy produced less heat to the bone because of lesser fraction and shorter drill bit—bone contact time. Only one implant system (XiVE) always showed statistically significant differences between various lengths of osteotomy, which could contribute to drill design and geometry and different drilling protocols. XiVE showed best performance with the lowest temperatures during and after drilling into the bone compared with Astra or Ankylos.
The authors claim to have no financial interest, either directly or indirectly, in the products or information listed in the article.
Roles/Contributions by Authors
Z. Katic: Performed the experimental part of the study and wrote the manuscript. T. Jukic: Gathered and helped in analyses of the data. D. Stubljar: Performed statistical analyses, wrote the manuscript, gave constructive comments in the manuscript design, and gave final approval for the manuscript.
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