Thermal osteonecrosis and bone drilling parameters revisited
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- Augustin, G., Davila, S., Mihoci, K. et al. Arch Orthop Trauma Surg (2008) 128: 71. doi:10.1007/s00402-007-0427-3
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During the drilling of the bone, the temperature could increase above 47°C and cause irreversible osteonecrosis. The result is weakened contact of implants with bone and possible loss of rigid fixation. The aim of this study was to find an optimal condition where the increase in bone temperature during bone drilling process would be minimal.
Materials and methods
Influence of different drill parameters was evaluated on the increase of bone temperature. Drill diameters were 2.5, 3.2 and 4.5 mm; drill speed 188, 462, 1,140 and 1,820 rpm; feed-rate 24, 56, 84 and 196 mm/min; drill point angle 80°, 100° and 120° and external irrigation with water of 26°C.
Combinations of drill speed and drill diameter with the use of external irrigation produced temperatures far below critical. Without external irrigation, temperature values for the same combination of parameters ranged 31.4–55.5°C. Temperatures above critical were recorded using 4.5 mm drill with higher drill speeds (1,140 and 1,820 rpm). There was no statistical significance of different drill point angles on the increase or decrease of bone temperature. The higher the feed-rate the lower the increase of bone temperature.
The external irrigation is the most important cooling factor. With all combinations of parameters used, external irrigation maintained the bone temperature below 47°C. The increase in drill diameter and drill speed caused increase in bone temperature. The changes in drill point angle did not show significant influence in the increase of the bone temperature. With the increase in feed-rate, increase in bone temperature is lower.
KeywordsThermal osteonecrosisBone drilling
Many orthopaedic procedures depend on the screw fixation of implanted devices to the bone. The compression fixation demands a high degree of stability of the fixating screws. Consequently, the loss of bone at the drilling site could negate any beneficial effects of this type of device. The implant failure rate for lower leg osteosynthesis is 2.1–7.1% [1–4]. One of the causes, not most important but present is the increase in bone resorption around the implanted screws from thermal osteonecrosis caused by preparative drilling [5, 6]. This phenomenon is observed as ring sequestra on X-rays. The threshold for thermal osteonecrosis is 47°C lasting for 1 min . In this experiment, the maximum temperature elevations were measured in vitro, on porcine femoral cortices while changing the drill speed, feed-rate, drill diameter, drill point angles and the use of external irrigation. These parameters were used in previous studies where all these parameters were not analyzed within same experiment [8–10]. The leading idea was that the analysis of all parameters together should diminish inter-observer errors and thus, provide the real impact of every parameter used on increase of bone temperature. Another reason was that, even in the single study, broad range of temperature values developed by specific parameter was presented. The goal of this experiment is to obtain optimal method, in vitro, that would maximally decrease the increase of bone temperature during drilling with all mentioned parameters.
Materials and methods
This study is presented in two parts.
In part 1, influence of drill diameter, drill speed and external irrigation was analyzed. Drills of 2.5, 3.2 and 4.5 mm with drill point angle of 100° were used. Drill speed was 188, 462, 1,140 and 1,820 rpm. Feed-rate was set at 84 mm/min. This experiment was divided into two parts: measurements without external irrigation and with external irrigation with water of 26°C as in other published experiments [8, 13].
In part 2, influence of drill point angle, feed-rate and drill speed was analyzed. Three different drill point angles (80°, 100° and 120°), four feed-rates (24, 56, 84 and 196 mm/min) and two drill speeds (1,140 and 1,820 rpm) were used with 4.5 mm drill. Specific combinations of feed-rates were used (84 and 24 mm/min and 196 and 56 mm/min) because in both combinations of feed-rate values, higher feed-rate value was 3.5 times higher than the lower feed-rate value for easier comparation.
Statistical analysis included several methods. First it must be stressed that the most rigorous criteria of thermal osteonecrosis (47°C) was used. Thus, not average but maximum temperature values were used as definitive results. These temperatures are expressed as upper limits of the P = 0.05 confidence interval (95% probability). Duncan’s multiple range test was used for comparison of different combinations of parameters during drilling. Regression analysis was used to delineate the strength of relationship between specific parameters and the increase of bone temperature during drilling. Partial correlation (in the regression analysis) was used for determination of the correlation of two variables (drill parameters) while influencing of the third (bone temperature).
Drill diameter, drill speed and external irrigation
Descriptive statistics for variable maximum bone temperature in °C (with and without external irrigation) with different combinations of parameters
Drill diameter (mm)
Drill speed (rpm)
M ± SD
49.2 ± 3.1
46.2 ± 2.4
39.4 ± 1.8
33.8 ± 1.5
P = 0.05
M ± SD
40.8 ± 2.0
37.6 ± 1.9
34.5 ± 1.2
31.6 ± 0.9
P = 0.05
M ± SD
38.3 ± 1.6
37.8 ± 1.7
33.4 ± 0.9
29.5 ± 1.0
P = 0.05
M ± SD
32.0 ± 1.0
31.0 ± 0.8
30.2 ± 0.9
28.9 ± 0.7
P = 0.05
M ± SD
30.1 ± 0.7
29.8 ± 0.8
29.3 ± 0.8
29.3 ± 0.8
P = 0.05
M ± SD
29.6 ± 0.8
28.8 ± 0.8
28.7 ± 0.7
28.7 ± 0.6
P = 0.05
The most rigorous criteria was used to avoid any possibility of thermal osteonecrosis which means that the temperature threshold for thermal osteonecrosis is at 47°C. As the cut-off point for thermal osteonecrosis is defined, all the values of bone temperature, not their mean values must be below that temperature.
For every combination of drill speed and drill diameter during drilling, maximum temperature was far below critical (47°C) with the use of external irrigation (range 29.9–33.9°C). Without external irrigation, temperature values for the same combination of parameters ranged 31.4–55.5°C.
Use of drill diameter (2.5, 3.2 and 4.5 mm) was analyzed with constant drill speed set at 188, 462, 1,140 and 1,820 rpm. Without external irrigation, temperature ranges were: 31.4–41.5°C for 2.5 mm drill; 33.4–44.8°C for 3.2 mm drill and 36.9–55.5°C for 4.5 mm drill. Temperature ranges with external irrigation were: 29.9–31.1°C for 2.5 mm drill, 30.9–31.4°C for 3.2 mm drill and 30.2–33.9°C for 4.5 mm drill (Table 1). The measured temperatures were far below critical for all combinations of parameters with external irrigation. The temperature values without external irrigation are higher and values using 4.5 mm drill with higher drill speed (1,140 and 1,820 rpm) were above critical level (50.9 and 55.5°C, respectively).
Drill speeds analyzed were 188, 462, 1,140 and 1,820 rpm with constant drill diameter (2.5, 3.2 and 4.5 mm). The temperature range without external irrigation at 188 rpm was 31.4–36.9°C; at 462 rpm was 35.2–43.0°C; at 1,140 rpm was 41.2–50.9°C and at 1,820 rpm was 41.5–55.5°C. Higher drill speeds (1,140 and 1,820 rpm) with 4.5 mm drill developed temperatures above critical. Using external irrigation at 188 rpm, temperature range was 29.9–30.2°C; at 462 rpm was 30.1–31.9°C; at 1,140 was 30.4–32.7°C and at 1,820 was 31.1–33.9°C. All measured temperatures for all drill speeds used, using external irrigation, were below critical level.
Multiple variants of regression analysis of influence on bone temperature using Statistica 6.0
Without external irrigation
With external irrigation
R = 0.9120
R = 0.7134
Drill point angle and feed-rate
Descriptive statistics for variable bone temperature in °C (without external irrigation) with different combinations of parameters
Drill point angle
M ± SD
P = 0.05*
48.8 ± 3.8
45.6 ± 2.2
55.0 ± 2.1
50.3 ± 2.5
53.0 ± 2.7
49.0 ± 2.5
53.0 ± 2.7
52.0 ± 2.5
49.6 ± 2.5
43.3 ± 3.5
54.7 ± 3.2
46.3 ± 2.9
For combination of drill speed set at 1,140 rpm and different drill point angles (80°, 100° and 120°), feed-rates at 24 and 84 mm/min (Table 3), the measured temperatures were: 56.2 and 49.8°C (drill point angle 80°); 58.4 and 53.8°C (drill point angle 100°); 54.4 and 50.3°C (drill point angle 120°). Another combination of parameters included drill speed set at 1,820 rpm and feed-rates at 56 and 196 mm/min. The obtained values were 59.0 and 55.2°C (drill point angle 80°); 58.4 and 56.9°C (drill point angle 100°); 60.9 and 52.0°C (drill point angle 120°).
The absolute values of bone temperatures were 1.5–9°C lower for higher feed-rates used. The influence of feed-rate on the level of increase in bone temperature exists and is significant (P < 0.05) for given combination of parameters, but still all temperatures were above critical (49.8–60.9°C).
During drilling, the resistance of compact cortical bone causes increase of bone temperature. The main cause for increase in bone temperature is frictional heat [10, 16, 17], which can result in thermal bone necrosis. Determining the specific thermal damage threshold for living bone tissue is a rather complex problem. The cellular death caused by heat is immediately evident with temperatures above 70°C [7, 18]. Others found that 50°C caused irreversible cortical bone necrosis. Lundskog conducted one of the few thorough studies on thermal damage to living bone tissue. He performed biomechanical, histochemic and morphologic studies on rabbit tissue. He demonstrated that the threshold for irreversible enzymatic disturbance to cortical bone is 50°C during 30 s . Bonfield and Li  reported irreversible bond weakening of the bone–collagen hydroxyapatite complex at 50°C. On the other hand, Eriksson and Albrektsson found that the minimum critical temperature for delayed death of osteocytes, not seen until 3 weeks or more after the injury, is much less and around 47°C. The exposure to temperature of 47°C for 1 min causes bone resorption and subsequent replacement and also disturbs the middle- and long-term anchorage of implants . They constructed optical thermal chambers for in situ microscopy. The thermal chamber method makes it possible to follow the “true” tissue reactions after a defined heat trauma by repeated light microscopic observations of the same bone specimen for an indefinite follow-up period [19, 20].
The most rigorous criteria to avoid any possibility of thermal osteonecrosis with temperature threshold for thermal osteonecrosis of 47°C was used in this study. Newer surgical instruments have integrated sensors to sense actual temperature. Thus, the MicroAire console automatically shuts off its power as the temperature increases to 43°C . Apart from the thermal damage, additional problems arise in bone drilling. The difficulties in maintaining a freehand control of the drill even when using a drill guide in attaining geometrical accuracy in hole size and location were observed. When the drill bit begins to enter the bone surface, it tends to “walk” or slip on the bone surface [23, 24]. The goal of this study and that of every orthopaedic surgeon is to build a robotic system that will control these parameters. The procedure would then minimize the technical errors despite the clinical condition of a patient and the type of the fracture .
In this experimental study, the influences of drill diameter, drill speed, external irrigation, drill point angle and feed-rate on the increase of bone temperature were analyzed. The goal was to find optimal combination of these parameters in order to minimize the increase of bone temperature during drilling with standard Synthes (AO/ASIF) drills.
Since the critical temperature for thermal osteonecrosis is the most rigorously defined to be 47°C, all the values of bone temperature, not their mean values, must be below that critical level. That is the reason why all the temperature values are expressed as upper limits of the P = 0.05 confidence interval (95% probability).
Influence of drill diameter, drill speed and external irrigation
From the data obtained, external irrigation (water at 26°C) is the single, most important factor in decreasing the increase in bone temperature during drilling. For every combination of drill speed and drill diameter during drilling, maximum temperature was far below critical. There are three mechanisms that contribute to the lowering of bone temperature during drilling: (1) irrigant (water) directly lowers bone temperature by conduction; (2) irrigant eliminates bone chips, which contribute to increased friction and (3) it lubricates drills and thus lowers friction [13, 15, 26, 27]. Influence of internal irrigation was also investigated , but recent experiments in dentistry show no difference between internal and external irrigation .
The influence of drill diameter was analyzed at a constant drill speed. The results show that the increase in bone temperature increases with the increase in drill speed with and without external irrigation. The bone temperatures obtained during drilling with 2.5 and 3.2 mm drills, using all drill speeds, were found to be below critical level. Therefore, these smaller drills could be used without external irrigation. Higher drill speeds (1,140 and 1,820 rpm), using 4.5 mm drill without external irrigation developed temperatures above critical level. And therefore, external irrigation using 4.5 mm drill is mandatory. With external irrigation, the increase in drill diameter also causes the increase in bone temperature, but temperature levels were much lower. Absolute values are 10–22°C lower than without external irrigation (Fig. 2). All measured temperatures for all drill diameters were far below critical. The problem in this setting is that drill diameter cannot be changed because the use of drill diameter depends upon the magnitude of forces that act upon a specific bone. To eliminate this problem the process of pre-drilling with smaller drills can be introduced . Clinical observation is that this process significantly increases the operative time and that sometimes it is impossible to make larger holes with subsequent drilling in the same direction. This can also result in subsequent loosening of the fixating screws.
With the increase in drill speed (with constant drill diameter), there is a statistically significant difference in the increase of bone temperature during drilling as in other studies [13, 30]. Comparatively, drill speed has higher influence on increase of bone temperature than drill diameter. At lower drill speeds (188, 462 rpm), bone temperature did not attain critical temperature even without external irrigation. Use of higher drill speeds (1,140 and 1,820 rpm) with 4.5 mm drill, without external irrigation, caused increase in bone temperature above critical level. Therefore, as previously stated, it is mandatory to use external irrigation when higher drill speeds are used with larger drills.
High degree of correlation (R = 0.91) between increase in drill speed and drill diameter and increase in bone temperature during drilling was documented with high partial correlations: R(s)part = 0.87, than drill diameter R(d)part = 0.77. Results obtained with external irrigation showed lower degrees of correlation and partial correlation (R = 0.71, R(s)part = R(d)part = 0.57). Correlations indicate high influence of drill diameter and drill speed on increase in bone temperature, higher without external irrigation.
Some relevant studies came up with different conclusions with optimum drill speed of 750–1,250 rpm when the bone temperature was below threshold level of tissue damage [15, 31]. Another problem that has not been stressed sufficiently is the difference between free-running speed and drill speed during drilling (operating speed). At very high-speed rates of 20,000–100,000 rpm operating speed may be as much as 50% below the free-running speed . This 50% reduction is comparable to the 40% reduction reported by Sorenson et al.  for an air-turbine hand-piece. Machine drill press ALG-100 that we used, maintains operating speed at the same level as the free-running speed. This topic was not addressed in previous experiments and could be one of the explanations for the reporting of different results.
Influence of drill point angle and feed-rate
The ideal point angle depends on the material being drilled and the ideal point angle for bone is yet to be determined. The drill used was 4.5 mm because the results of the first part showed that 4.5 mm drill at higher drill speeds (1,140 and 1,820 rpm) without external irrigation caused increase in bone temperature above critical.
Results using drill point angle of 80°, 100° and 120° did not show significant difference on the increase in bone temperature during drilling. The more important observation other than statistical significance was that all measured temperatures were far above critical. Drill point angle as isolated parameter could not lower the increase of bone temperature below critical using 4.5 mm drill. (Fig. 3; Table 3). The same conclusion was recently published . In contrast, Wiggins and Malkin  found the ideal drill point angle to be 118°.
In our preliminary experiment, the drill point angle of 60° was also used. The problem was “walking” on the bone surface of the 60° drill and it was found that the drill hole was ellipsoidal and not circular. After drilling ellipsoidal hole, the screw could not be inserted in the bone and the drill point angle of 60° was excluded from the experiment. Wiggins and Malkin analyzed the influence of drill point angle on bone temperature. They did not notice this phenomenon of drill walking during the experiment with 60° point angle .
Influence of feed-rate on the increase in bone temperature was also studied. The absolute temperature values point out that the increase in feed-rate causes a significantly lesser increase in the bone temperature (1.5–9°C), the results also obtained by Toews . The second important observation is that for 4.5 mm drill, all feed-rates used caused increase in bone temperature above critical. From the data obtained, higher feed-rate causes lower increase in the bone temperature but for the defined combination of parameters it is not clinically applicable as an isolated parameter. Feed-rate must be used with other factors that lower bone temperature such as external irrigation or lower drill speed. Also, it must be pointed out that the higher feed-rate causes shorter drilling time and subsequently shorter period of increased bone temperature .
The present analysis has several limitations. None of the animal models is similar to the human situation for all examined parameters. Some animals, however, more closely resemble humans than others. In particular, with regard to bone density and quality parameters of the lumbar spine, humans appear to be very different from the other species examined. Based on a combination of all the parameters the characteristics of human bone are best approximated by the properties of dog and pig bone .
It should also be kept in mind that these experimental results were obtained in vitro, under conditions that differ from clinical situations: the bone specimens were porcine and there was no blood flow and therefore, no heat transfer by convection to cool and hydrate the specimens . These results could be somewhat different with the same combinations of parameters during drilling in human traumatology and orthopaedics.
In summary, many parameters have influence on the increase in bone temperature during drilling. External irrigation is the single, most important factor in decreasing the increase in bone temperature during drilling and must be used for bone drilling. Simply put, the optimal method for decreasing the increase in bone temperature during drilling is: use smaller drill diameters (if possible) with lower drill speed and higher feed-rates. External irrigation is MANDATORY!
This experiment is a part of the scientific project: Biomechanics of fractures and fracture healing and is supported by The Ministry of Science, Education and Sports.