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Drilling is a common technique used in orthopedic surgery procedures but causes increases in temperature that can lead to cell damage and death. The extent of this depends largely on the magnitude of the increase in temperature. The commonly accepted limit to prevent osteonecrosis is less than 47°C for 60s. There is controversy when it comes to the optimal drilling parameters that limit temperature increases and cell death. In addition to this, less research has been done on the drilling effects in the osteochondral area of joints. Osteochondral tissue damage can interfere with the daily lives of patients and if severe enough will need to be treated. We hypothesize that increasing tool speed and drill bit size will increase temperature that could be above the osteonecrosis limit.
Ex-vivo experiments were conducted on porcine shoulder joints that tested the thermal effects of different tool speeds and drill bit sizes. A thermal camera was used to record and measure real time temperature changes while drilling. Three drill bit sizes and five tool speeds were used. Statistical analyses includes Welch's ANOVA with Games-Howell Post Hoc analyses, multivariate linear regression, and surface response regression were used to explore the association of tool speeds and drill bit size on temperature.
Results and Conclusions
All the tool speed and drill bit size combinations lead to an increase in temperature that were under the commonly accepted limit. The highest temperature reached was 44°C with a tool speed of 1150 RPM and 3070 RPM and drill bit size 5.159mm. It was found that increasing the tool speed increased the temperature change and increasing the drill bit size increased the temperature change.
Many orthopedic surgeries involve cutting, drilling, or the transformation of bone. During these processes there is an increase in temperature. [1,2] Surgeons have to be cognizant of the temperature increases as they can lead to complications and postoperative failures.  Heat can affect bone in many ways, causing denaturation of proteins, decreased osteoclastic and osteoblastic activity, and apoptosis.  A common consequence of increased temperature is osteonecrosis, which is characterized by irreversible damage or death of bone cells.Immediate effects of osteonecrosis due to exposure to increased temperatures include cellular membrane disruption and impaired blood flow, as well as later onset apoptosis.  The extent of osteonecrosis seen depends largely on the magnitude of increase in temperature and duration of temperature exposure.  The commonly accepted limit for this is less than 47 °Celsius for 60s. , ,  Osteonecrosis with respect to drilling is a complex phenomenon, not only does drilling cause an increase in temperature, but it also creates microfractures which leads to the death of osteocytes.  The control of drilling parameters is vital to prevent a large increase in temperature and subsequent osteonecrosis. 
Currently there is some controversy when it comes to the optimal drilling parameters that should be used to limit temperature increase and osteonecrosis.  There are many parameters to consider when drilling including drill speed, feed rate, cooling/irrigation, drilling depth, and predrilling. With regards to drilling speed and force, the majority recommend a high drill speed with a larger force because it increases the rate of temperature production, but decreases the time of drilling leading to a decrease in the amount of total heat generated. [2,5] With an increase in drill feed rate there is shorter drilling time, but an increase in bone friction.  There is insufficient data from the literature, but there is a general consensus that higher feed rate is significant enough to reduce drill time and decrease heat production.  The use of coolant during drilling was found in  to significantly decrease the increase in temperature. They also found that the drilling depth influences the temperature increase during bone drilling. It was found that increasing the drilling depth leads to an increase rise in temperature. This was thought to occur because there is an increase contact time with increasing depth. Predrilling is known as a multistep technique where a smaller drill diameter is used and then it is gradually increased. Researchers found that predrilling can dramatically affect the rise in temperature. Predrilling gradually removes material from the drill site decreasing the friction and resulting in better heat dissipation. It also allows more time to cool when changing the drill bits, leading to a decrease in the overall rise in temperature. 
In addition to drilling parameters, drill specifications can affect the temperature increase. These include drill design, drill diameter, and drill wear. The drill design includes the rake angle, the clearance angle, and the flute and helix angle as seen in Fig. 1, Fig. 2. The rake angle is the angle between the cutting edge and the plane perpendicular to the workpiece. There is a general consensus that as the rake angle increases the bone cutting forces decrease for a single edge cutting tool.  The clearance angle is the angle by which the flank of the drill clears the material during drilling. A suggested clearance angle of 15 ° was recommended by many studies. Another suggestion to reduce heat generation is the addition of a split point, or an increase in clearance angle halfway along the surface.  The flute is a deep groove that twist around the drill, allowing material waste to exit. A study on a 2 versus 3 fluted drill found an increase in cutting efficiency with the 3 fluted drill, but did not find a decrease in heat generation or improvement in bone healing.  A helix angle is the angle formed by the edge of the flute with the line parallel to the drill center line. Helix angle and rake angle are interrelated and a larger helix angle leads to a larger rake angle. [3,5] The recommended helix angle is 25 to 35 ° or use of a drill but with a parabolic flute.  This conclusion was made because temperature was found to decrease uniformly with increasing helix angle and a parabolic flute enables great clearance of waste material decreasing the friction and therefore heat generation.  It is found that temperature increases with increasing drill diameter. [3,5] Surgeons have to make sure not to use too small of a drill diameter because drill bending can occur with drill diameters less than 3.2mm.  Drill wear is another important aspect to consider to minimize heat generation. Repeated use of drills leads to the cutting edges to wear out. The wear increases the surface roughness leading to an increase in temperature.  There is no consensus on the number of times a drill can be used before it produces a significant increase in temperature, but worn drills can be visualized with scanning electron microscopy or by light microscope. 
In order to safely drill in bone, optimal drilling parameters need to be determined. These results can help to diminish the current controversy in the literature. In addition, this study will help to provide information to determine optimal drilling parameters for drilling into bone that can be used in the creation of new devices. This study focuses on temperature changes measured in the osteochondral surface of porcine bones during drilling while changing the tool speeds and drill bit sizes while taking into consideration the current recommendations for the other parameters in the literature. We hypothesize that increasing the tool speed and drill bit size will cause an increase in temperature, which can be above the osteonecrosis limit.
The aim of this study is to determine tool speeds and drill bit sizes that minimize the temperature increase in the osteochondral areas to safely drill in these areas during orthopedic procedures.
It is a non-randomized experimental study in which the osteochondral surface of porcine shoulder bones were drilled with varying tool speeds and drill bit sizes into the osteochondral surface. The temperature changes were be measured with a thermal camera.
The bone samples were obtained from a food
Temperature changes were measured with varying tool speeds and drill bit sizes. In Fig.4 the fastest speed, 3070 RPM had the biggest temperature change. The medians for the temperature change and tool speeds, seen in Fig.4, increased from 760 RPM to 1150 RPM, then decreased from 1150 RPM to 1630 RPM, then increased again from 1630 RPM to 2180 RPM, and then increased from 2180 RPM to 3070RPM. The Welch's ANOVA found statistically significant mean temperature change differences between tools
Drilling is commonly done in orthopedic procedures and can lead to post-surgical complications when done incorrectly.  Drilling into bone can lead to osteonecrosis and cell damage. [1,2] One of the main mechanisms of this is by the increase in temperature.  Research has been conducted on drilling into bone, but there are no set or optimal parameters.  The drilling of osteochondral regions is common in fracture treatment, intra-articular soft tissue repair, and arthroplasty. In order to
Source of funding
University of Central Florida College of Medicine provided funding for student research.
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
- T.A. Howard et al.Damage control articular surgery: maintaining chondrocyte health and minimising iatrogenic injury
- D.A. Houston et al.Chondrocyte death after drilling and articular screw insertion in a bovine model
- N. Bertollo et al.A comparison of the thermal properties of 2- and 3-fluted drills and the effects on bone cell viability and screw pull-out strength in an ovine model
Clin Biomech (Bristol, Avon)
- R.K. Pandey et al.Drilling of bone: a comprehensive review
J Clin Orthop Trauma
- J. Lee et al.Parameters affecting mechanical and thermal responses in bone drilling: a review
- G. Augustin et al.Cortical bone drilling and thermal osteonecrosis
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Revista Brasileira de Reumatologia, Volume 55, Issue 6, 2015, pp. 501-511
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