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Internal immobilization of fractured parts of bone depends on the drilling of fracture site and screw fixation of implanted devices to the bone. During drilling, the temperature may rise allowable temperature of 47°C and causes irreversible thermal necrosis. This study is concerned with methods of drilling to determine the best processing condition to minimize the osteonecrosis. Drilling tests were performed with two drilling techniques: conventional, and high speed drilling on the bovine femur and increase of temperature in drilling site, duration of temperature raise and thrust force were measured. The result for conventional drilling shows that in all processing conditions, the generated heat is over the allowable limit, which makes thermal necrosis inevitable. On other hand, it was found that increase of cutting speed of drill bit to 7000 rpm, leads to considerable decrease of thrust force and increase of heat dissipation with chips, simultaneously and leads to decrease of local temperature raise in drilling site. But with more increase of rotational speed of drill bit, because of not sensible change in drilling force and considerable increase of friction between chips, drill bit body and drilled hole, amount of temperature elevation is increased.

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The problem of bone fracture in medicine due to an accident, aging or diseases, has existed from times when humans started to work and activity. The process of bone drilling is an essential part of internal immobilization in orthopaedic and trauma surgery. The force required to chip formation in drilling process, resulting in heat generation in drill site that leads to the occurrence of thermal necrosis. This research experimentally investigates the effect of ultrasonic vibration on thrust force in drilling of bovine femur bone. This method induces high-frequency and law-amplitude vibration in the feed direction during cutting, and has the potential to spread tiny cracks in bone and decrease friction leading to reduce of cutting forces and also increase the speed of chip disposal leading to reduction of machining forces, totally. Experimental results demonstrate that ultrasonic assisted drilling of bone produces fewer thrust force than conventional drilling and rotational speed of 1000 rpm is the optimal speed to achieve at minimum thrust force for all feed rates. Moreover, this method is due to the force independence of the feed rate in the rotational speed of 1000 rpm, is applicable in orthopedic surgery.

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Bone fracture occurs as a result of accident, old age and disease. Generally bone fracture treatment consists of stabilizing the fractured bone in the right position. In complex fractures, stabilizing internal and external tools and equipment is used to stabilize the fractured bone in position. Bone drilling is required in order to connect fixating devices. The forces required for chip formation, increase the temperature during bone drilling. The phenomenon of thermal necrosis of the bone occurs if the temperature exceeds 47 degrees Celsius. Thermal necrosis inhibit bone fixation and causes the wrong bone healing. The current study has been trying to examine the effect of the cooling gas on the reduction of temperature rise on drilling site as well as statistical analysis of the process. Tests have been carried out using direct injection of nitrogen gas using internal coolant drill bits. Using cooling gas reduced the increase in drilling temperature to 15 degrees Celsius and prevented the thermal necrosis. The maximum increase in temperature in conventional drilling was 56 degrees Celsius, while using cooling gas the increase in temperature of 43 degrees Celsius was achieved. This reduces the risk of thermal necrosis. Statistical analysis also indicates that in the drilling with direct cooling with nitrogen gas the temperature changes are almost independent of the rotational speed.

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In orthopedic surgery, the drilling process is used to internally fix the fracture zone. During bone drilling, if the temperature exceeds the limit of 47 °C, it results in altered bone alkaline phosphatase nature, occurrence of thermal necrosis, non-fixation and inadequate bone fusion In order to investigate the effective parameters of the drilling process, after three-dimensional modeling of the femur bone in Mimics software and determination of bone coefficients based on the Johnson-Cook model, numerical simulation of the cortical and trabecular bone oblique drilling process have been performed. The drilling process was performed in both normal and high speed modes based on reverse heat transfer theory using DEFORM-3D software. The results of numerical simulation after validation with experimental results showed that this theory is capable of estimating the temperature and heat flux in this process and the occurrence of necrosis in both processes (normal and high speed) is imminent. The temperature in the drilling area of the trabecular bone is higher than the cortical bone at all speeds and feed rates and the axial force of the trabecular bone is less than the cortical bone at all speeds and feed rates. The optimum point leading to the minimum temperature in normal drilling of trabecular and cortical bone is the feed rate of 150 mm/min and the rotational speed of 2000 rpm. This optimum point is obtained in the high-speed drilling of trabecular and cortical bone at the feed rate of 150 mm/min and rotational speed of 4,000 rpm and 7,000 rpm.