Effect of drilling direction and depth on thermal necrosis during tibia drilling: An in vitro study

Abstract

Thermal necrosis is one of the main concerns in bone drillings. This study has been designed with the aim of improving the surgeons’ knowledge on how to reduce thermal necrosis in tibia drilling with various depths and directions. A drilling machine was developed, which made the direct transfer of gas coolants into the drilling site during drilling possible. Results indicated that 2000 r/min is the most proper rotational speed for minimizing thermal necrosis. Changing the drilling direction from radial to longitudinal raised the temperature at drilling site. Increasing the drilling depth from 8 to 50 mm raised the temperature by at least 22.5%. Increasing the drilling depth up to 50 mm raised the drilling site temperature above the threshold temperature of tibia thermal necrosis as well as the temperature durability at the drilling site. However, in contrast to conventional cooling modes, using gas coolants, especially CO ${}_{2}$ , brought the temperature to a level less than the threshold temperature of tibia thermal necrosis and reduced the durability of temperature at the drilling site by at least 1 minute. Using the drilling machine developed in this study and CO ${}_{2}$ coolant, orthopedic surgeons can perform tibia drilling in various directions up to the depth of 50 mm without the risk of thermal necrosis since the internal gas coolants, due to their direct contact with the drilling site and the rapid discharge of the chips, reduce the temperature increase in tibia caused by changing the drilling depth and the drilling direction from radial to longitudinal, greatly.

Keywords

CO2 gas N2 gas coolant non-radial drilling longitudinal drilling temperature durability

1. Background

Tibia drilling is mainly required during ankle replacement, anterior cruciate ligament (ACL) arthroscopy and ankle fusion as well as in many orthopedic surgeries on the area of tibia, in which the placement of plates and implants necessitates inserting screws through pilot holes. It is sometimes necessary to drill tibia during temporary bony fixation and surface preparation for joint fusion. In cases of drilling for plate or implant insertion, the improper drilling and the consequent failure of the screwed site lead to improper bone healing, fatigue increase and postoperative problems [1]. Generally in all cases of bone drilling, the thermal damage (necrosis) is one of the main concerns and can lead to improper drilling process if it is not controlled. The thermal conductivity of human bone is between 0.38 and 2.3 W/mk [2]. Due to the low value of this parameter, heat remains and dissipates at the drilling site and leads to temporary or permanent loss of blood supplied to the bone [2]. This also changes the level of alkaline phosphatase in bone and prepares the conditions for osteonecrosis, disorder in bone healing process and reduction of mechanical strength of bone at drilling site [3, 4]. Therefore, temperature at drilling site should be less than the threshold level of thermal necrosis for human bone. The human bone experiences necrosis if it is exposed to a temperature of 47 ${}^{\circ}$ C for one minute [3, 4]. The duration that bone endures the temperature decreases exponentially with each degree of temperature increase so that the temperature endurance period decreases to 30 s at 48 ${}^{\circ}$ C and to a fraction of second at 53 ${}^{\circ}$ C, at which bone cells experience instantaneously big damages and the full recovery of tissue may last several weeks [3]. Investigating the phenomenon of thermal necrosis during bone drilling is hence of great importance. Many studies have investigated the effective parameters in cell death due to temperature increase during bone drilling in orthopedic surgeries. These studies can be divided in three main groups.

In the first group of studies, the effect of geometrical characteristics and the type of drill bit used for drilling on reduction of thermal necrosis in bone during drilling was investigated. Augustin et al. investigated the effect of diameter and point angle of drill bit in bovine femur drilling and showed that change in drill bit point angle doesn’t affect the temperature increase [5]. Udiljak et al. and Scarno et al. suggested step drill bits with two different diameters and conical drill bits for reducing the temperature during drilling [6, 7]. Pandey and Panda investigated the effects of point angle and helix angle on reduction of thermal necrosis in bone [8].

In the second group of studies, the drilling process and the effect of parameters such as rotational speed or feed rate on reduction of thermal necrosis in bone were investigated. Karaca et al. showed that increase in feed rate leads to temperature decrease at drilling site [9]. Shakouri et al. suggested the most proper rotational speed for high speed drilling of femur [10]. The other studies in this group investigated more details and complexities to reach the optimal parameters for drilling process [11, 12, 13].

The third group of studies introduced new methods and processes for bone drilling with the aim of reducing thermal necrosis. Den Dunnen et al. suggested the most optimal diameter of water jet nozzle for bone drilling [14]. Some studies suggested the most optimal parameters for temperature reduction during bone drilling using ultrasonic vibrations [15, 16].

Generally, most of the previous studies have focused on femur and a very limited number of studies have investigated the thermal necrosis in drilling of tibia although the density and maternal properties of femur and tibia are different and extensive studies have demonstrated that the difference in these two parameters in femur and tibia have a significant effect on the drilling condition and thermal necrosis of these bone [17, 18]. On the other hand, drilling in surgeries on the area of tibia has various types and is done in various directions and depths depending on the technique applied in surgery. For example a non-radial, quasi-longitudinal or longitudinal drilling of tibia is necessary in oblique medial malleolar osteotomy, ACL arthroscopy, knee osteochondritis dissecans and ankle arthroplasty. The effect of drilling direction hasn’t been dealt with in previous studies. The effect of drilling depth has been examined in a very limited number of studies like the study by Sener et al. in a limited range of depths and for radial drilling of femur [19] while the drilling depth is an important parameter with regard to using various screws of different lengths especially in non-radial drilling. As the effect of gas coolants on reducing thermal necrosis in radial drilling of bovine femur has been investigated recently [20], the present study compared the changes regarding the thermal necrosis in various depths and directions during tibia drilling with gas coolants and conventional cooling modes.

2. Methods

Applying the coolant during drilling is one of the most appropriate methods of reducing temperature at the drilling site. Due to the infection risk of normal saline [10, 21, 22], the effect of biocompatible gas coolants on reduction of thermal necrosis during drilling has been investigated in the present study. Biocompatibility is one of the concerns of using gas coolants. Among gases that can be used as coolant, carbon dioxide (CO ${}_{2}$ ) and N ${}_{2}$ have been selected for this study since they have no risk of infection or biocompatibility problem according to previous studies [20, 23, 24]. It should be noted that the use of CO ${}_{2}$ and N ${}_{2}$ is very common in medical applications. The CO ${}_{2}$ gas is commonly used for laparoscopic operations and the N ${}_{2}$ gas is also used for decreasing pain during labor. Thus, these gases are available and applicable in medicine.

Figure 1.

(a) The schema of drilling machine: section A shows the pathway of gas coolant. (b) The developed drilling machine and drilling equipment.

The use of manual drilling is accompanied by many limitations and problems [25]. There is, however, no drilling machine that can transfer gas directly into the drilling site. A drilling machine with the ability of transferring the gas coolant directly through the drill bit into the drilling site was developed in the present study (Fig. 1). Furthermore, internal coolant drill bit was used in order to transfer gas directly through the internal hole of drill bit into the drilling site (Fig. 2a). Each drill bit was used according to standard for drilling 40 holes [26].

Figure 2.

(a) Internal coolant drill bit. (b) The two-channels Lutron Thermometer TM-925.

A digital tachometer (model: Victor DM6236P) was used for measuring the rotational speed of the drill bit. A two-channel thermometer (model: TM-925) with a measurement range between $-$ 50 $\sim$ 1300 ${}^{\circ}$ C and a resolution of 0.1 degree was used for measuring temperature at the drilling site (Fig. 2b). It should be noted that for temperature measurement in all tests, the thermocouple was placed based on available protocols at a distance of 0.5 mm from the hole contour and in the depth of 3 mm [3, 22]. The placement location of thermocouple was covered with a heat conducting pulp in order to minimize heat dissipation and raise the precision.

Figure 3.

Temperature at drilling sites with five different rotational speeds under four cooling modes: (a) radial drilling. (b) longitudinal drilling. Red line shows 47 ${}^{\circ}$ C.

Specimens were removed from the femurs of twenty cows at the routine post-mortem examination. This selection was based on the fact that swine and cow bones have the most similarity to human bones regarding the biomechanical properties [3]. Most of the specimens were tested less than 20 hours after death and some specimens were first stored according to the recommendation of Hillery et al. at $-$ 18 ${}^{\circ}$ C in Ringers solution and then were tested [3]. As the various techniques of orthopedic surgeries on tibia require drillings in various directions, the drilling of tibia in the present study was done in two extreme directions of longitudinal and radial. Further, drilling was performed at four depths of 8, 12, 30 and 50 mm for longitudinal direction and two depths of 8 and 12 mm for radial direction. Since the drilling was done merely in the cortical part of the bone, the depths of greater than 12 mm were not necessary in radial drilling for investigating the effect of drilling depth. All tests were designed in the following four cooling modes (A-D) in order to evaluate the effect of coolant type: without cooling (mode A), external cooling with normal saline (mode B), internal cooling with CO ${}_{2}$ gas (mode C) and internal cooling with N ${}_{2}$ gas (mode D). The cooling modes A and B are conventional cooling modes. It is noteworthy that each test was repeated 5 to 7 times and the mean value of the measured temperatures was registered after ensuring that the coefficient of variation is less than 0.014.

3. Results

First, drilling was done with rotational speeds of 1100, 1500, 2000, 2650 and 3000 r/min using all coolants to obtain the optimal rotational speed for minimizing the temperature at drilling site. According to Fig. 3a, the drilling site temperatures at the depth of 8 mm in radial drilling using normal saline as coolant under rotational speeds of 1100, 2650, 1500 and 3200 r/min were respectively 4.8%, 4.5%, 5.4% and 5.6% greater than temperature under rotational speed of 2000 r/min. The similar numbers were respectively 14.5%, 19.9%, 14.5% and 18.2% for CO ${}_{2}$ coolant and 13.1%, 15.4%, 12.4% and 10.3% for N ${}_{2}$ coolant. This tendency was also available in the longitudinal drilling (Fig. 3b) and even the temperature difference in other rotational speeds comparing to the 2000 r/min was in longitudinal drilling greater than that in the radial drilling.

Surface defects and temperature at drilling site were examined for selecting the optimal rotational speed. The samples were stained with eosin (Sigma-Aldrich) and haematoxylin (Sigma-Aldrich) after drilling and temperature measurement to better reveal the surface defects. The samples were placed then under stereoscopic zoom microscope (model SMZ 1500; Nikon, Melville, NY, USA) and were photographed by a Canon Powershot SX60 HS camera at various rotational speeds using cooling modes A-D. Interestingly in all cooling modes, the least cracks and the slightest surface defects at the drilling sites of the bone were observed at the rotational speed of 2000 r/min (Fig. 4).

Figure 4.

(a) shows drilling area of bone. (b), (c), (d), (e) and (f) Microscopic images of surface defects (60X) in the drilling area at rotational speeds of 1100, 1650, 2000, 2600, and 3200 (r/min), respectively under cooling mode C.

According to results of temperature measurement (Fig. 3) and observation of cracks and surface defects (Fig. 4), 2000 r/min is considered as the optimal rotational speed for drilling tibia and the effect of other parameters on thermal necrosis has been reported at this rotational speed.

Bone exhibits anisotropic behavior and its biomechanical properties vary in different directions because of the specific arrangement of collagen fibers [27]. Bone shows the strongest biomechanical properties in the longitudinal direction and the weakest biomechanical properties in the radial direction [27]. For this reason, drilling in the present study has been done in two extreme directions of longitudinal and radial in order to compare the extremum of thermal necrosis in these two directions.

Figure 5.

Shows comparison of temperature changes in the drilling site under two drilling directions and four cooling modes at depths of 8 mm. Red line shows 47 ${}^{\circ}$ C.

According to Fig. 5, the temperatures at drilling site in longitudinal drilling under cooling modes of A, B, C and D were 8.8%, 6.1%, 6.5% and 6.2% higher than those in radial drilling, respectively. The results showed that in all cooling modes, the drilling site temperature was higher in the longitudinal drilling than the radial drilling. Thus, it can be deduced that under the same conditions, the risk of damage due to bone thermal necrosis is in the longitudinal drilling higher.

Further, the effect of drilling depth on thermal necrosis of tibia was investigated under different cooling modes. The results of longitudinal drilling in four depths of 8, 12, 30 and 50 mm have been shown in Fig. 6a. The results showed that the drilling site temperature increased with depth increase. The drilling site temperatures at the depth of 50 mm under cooling modes A, B and D were respectively 56.9%, 47.0%, and 9.2% higher than those in drilling with CO ${}_{2}$ coolant. The corresponding data were respectively 57.7%, 54.9%, and 19.5% at the depth of 30 mm, 49.8%, 38.3%, and 23.1% at the depth of 12 mm and 48.3%, 30.9%, and 23.2% at the depth of 8 mm (Fig. 6a) Therefore, the lowest temperature at the drilling site in all cooling modes and all depths occurred during drilling with CO ${}_{2}$ . It should be noted that in drilling with this gas, the maximum temperature occurred in the longitudinal drilling and at the depth of 50 mm. The temperature in this case was still 7.7% lower than the threshold level of bone thermal necrosis. The results of radial drilling also confirmed that increasing the drilling depth is accompanied by temperature rise and that the lowest temperature at the drilling site occurred while using the CO ${}_{2}$ coolant (Fig. 6b).

Figure 6.

(a) shows comparison of temperature changes in the drilling site under four drilling depths and four cooling modes under longitudinal drilling. (b) shows similar data under two drilling depths and radial drilling.

4. Discussion

4.1 Effect of drilling depth and direction in thermal necrosis of tibia

According to the results in Fig. 5, tibia drilling without cooling is not recommended in any direction because the drilling site temperatures under longitudinal and radial drilling increase to 18.3% and 8.7% above the threshold level of bone thermal necrosis even at the depth of 8 mm. On the other hand, the results of Fig. 6 showed that the increase in depth caused a sharp rise in temperature in all cooling modes and that using the conventional cooling modes (A and B) at four depths has caused the temperature to trespass the limit of 49.1 ${}^{\circ}$ C. It is noteworthy that osteoclasts begin to die when temperatures exceeds 47 ${}^{\circ}$ C. According to these results, the conventional cooling modes (A and B) are not recommended in any direction even for inserting the 8 mm-long screws while in many surgical procedures in the area of tibia and in many surgical techniques such as plate osteosynthesis, which require quasi-longitudinal or longitudinal tibia drilling, screws with a length of up to 50 mm are used. Figures 5 and 6 showed that the surgeons can use gas coolants, especially CO ${}_{2}$ , to perform drilling freely in any required direction and with any depth without the risk of thermal necrosis The main reason of temperature rise with depth increase, especially in conventional cooling modes, can be the greater chips accumulation with the increase in drilling depth. This can lead to the rise of the force causing the plastic deformation of chips, friction between drill bit and chips and the friction between chips and the hole walls. As a result and considering the low thermal conductivity of human bone, the heat remains in bone cells and this provides the conditions for temperature rise at drilling site and damage to bone cells.

The reason of chips accumulation in conventional cooling modes is that there is no pressure loading for chips removal in cooling modes A and B. Observations have shown that in drilling without coolant, chips stick together due to the lack of the coolant pressure at the end of the drilling hole. In cooling mode B, normal saline fluid is injected from outside into drilling site. The liquid cannot penetrate into the end of hole especially in the holes with a depth of 30 mm and 50 mm and this leads to improper cooling or improper chips removal. In tibia drilling with this cooling mode, the chips and the normal saline fluid encountered and ultimately to change of chips into a pulp form (Fig. 7a) and this causes disturbance in chips discharge.

Figure 7.

(a) and (b) show images of chip discharge from hole by normal saline and gas coolant, respectively.

In drilling with internal injection of gases, the chips are removed quickly in form of powder from the drill bit groove (Fig. 7b). In internal cooling, the gas coolant passes directly through the internal hole of drill bit into the drilling site and reaches easily to the drilling site and induces a high quality and quick cooling at drilling site. The pressure of exiting gas, which has also entered through the drill bit tip into the drilling site, pushes the chips out. Therefore in the internal gas cooling, the coolant reaches better to the drilling site, a more efficient cooling of the drilling site is achieved and the chips removal is done more quickly and efficiently.

Figure 8.

(a), (b), (c) and (d) are temperature-time diagrams of drilling site in cooling modes A, B, C and D under longitudinal drilling and two drilling depth.

4.2 Temperature durability in the drilling site

Figure 8 illustrates the process of temperature drop over the time with four cooling modes at two depths of 30 and 50 mm. In all cooling modes, the temperature reaches its maximum value at the depth of 50 mm about 5 seconds later than the depth of 30 mm. This delay is acceptable due to higher value of maximum temperature at the depth of 50 mm. Eriksson et al. demonstrated the destructive effect of temperature durability above 47 ${}^{\circ}$ C on the microscopic structure of the bone [28]. Therefore, both a temperature of higher than 47 ${}^{\circ}$ C and the durability of temperature are significant factors in the thermal necrosis of bone. The durability of temperature over the threshold level of bone necrosis ( $t_{2}-t_{1}$ in Fig. 9a and b) in the cooling modes A, B, C and D at the depth of 30 mm are 82, 67, 0 and 4 s, respectively (Fig. 9c). The similar data for the depth of 50 are 137, 84, 0 and 6 s mm respectively (Fig. 9c). Therefore, using the gas coolant in longitudinal drilling of tibia at the depths of 30 and 50 mm reduces the temperature durability at least one minute. By the way, temperature durability is longer at the depth of 50 mm than the depth of 30 mm, which causes more severe and destructive damages to the bone cells at drilling area and to the surrounding bone tissue. Thus, using gas coolants not only has led to a reduction in maximum temperature compared to conventional cooling modes, but also has resulted in significant reduction in temperature durability. This plays a very effective role in reducing the likelihood of thermal damage to bone cells.

Figure 9.

(a) The durability of temperature over the threshold level of bone necrosis ( $t_{2}-t_{1}=\Delta t$ ). (b) The range of time for shows the durability of temperature in the four cooling modes at two depths of 30 and 50 mm under longitudinal drilling. (c) The comparison of the durability of temperature in the four cooling modes at two depths under longitudinal drilling.

4.3 Limitations and future work

Despite the biocompatibility of N ${}_{2}$ and CO ${}_{2}$ [23, 24] and the fact that these gases are commonly used for reducing labor pain and laparoscopic operations, it is suggested to investigate the effects of gas coolants on the cellular environment of bone during bone drilling in future studies before using these gases as coolant during tibia drilling in operation rooms.

5. Conclusions

The present study is designed to improve the knowledge of orthopedic surgeons on how to reduce thermal necrosis in tibia drilling with different depths and directions and also to develop an appropriate drilling machine and to suggest a proper coolant for keeping the drilling site temperature under the threshold level of bone necrosis. Examining surface defects in borehole and temperature at drilling site in both longitudinal and radial drilling revealed 2000 r/min as the optimal rotational speed. By changing the drilling direction from radial to longitudinal, even at the depth of 8 mm, the temperature may increase up to 9% and only gas coolants, especially CO ${}_{2}$ , can keep the drilling site temperature in any direction under the threshold temperature of thermal necrosis. By increasing the drilling depth from 8 to 50 mm, the temperature increases by at least 22.5%. However, in contrast to conventional cooling modes, using gas coolants, especially CO ${}_{2}$ , keeps the temperature under the threshold level of thermal necrosis up to the depth of 50 mm. Internal gas cooling prepares better conditions than external cooling for reduction of thermal bone necrosis by making the direct contact of coolant with drilling site possibleand quickening the chips removal. Increasing the drilling depth increases the temperature durability at the drilling site, but the use of gas cooling in the longitudinal drilling of tibia at depths of 30 and 50 mm leads to a significant reduction in maximum temperature and reduces the temperature durability at the drilling site at least 1 minute comparing to conventional cooling modes. Nowadays, using non-radial, longitudinal and quasi-longitudinal drillings even up to a depth of 50 mm would be necessary in many surgical procedures. Therefore based on these results, the maximum temperature at drilling site can be kept under the threshold level of thermal necrosis and the temperature durability can be controlled to prevent damages to bone tissue using the CO ${}_{2}$ coolant and the drilling machine proposed in the present study.

Footnotes

Conflict of interest

None to report.

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