a b s t r a c tThermal response of the human cutaneous thermoreceptors depends statically upon temperature and dynamically on the temperature change rate at the depth of the thermoreceptors. Therefore, it is very important to estimate the time-dependent thermoreceptors temperature with a good accuracy. On the other hand, the temperature distribution in skin tissue may be significantly affected by thermoregulatory mechanisms such as shivering, regulatory sweating and vasomotion. In the present study, a new simplified thermoregulatory bioheat model is proposed to describe heat transfer in skin tissue. The new model is constructed by combining the well-known Pennes equations with Gagge's two-node model. In this model, the human skin is subdivided into three layers (epidermis, dermis and subcutaneous) and the time-dependent temperature of skin tissue is obtained by solving the bioheat equations taking into account thermoregulatory mechanisms. The model has been verified by extensive comparisons with the published analytical and experimental results where a good agreement was found. Therefore, the present model can estimate the skin temperature under transient environments with a very good accuracy.
Bone loss due to thermo necrosis may weaken the purchase of surgically placed screws and pins, causing them to loosen postoperatively. The heat generated during the bone drilling is proportional to cutting speed and force and may be partially dissipated by the blood and tissue fluids, and somehow carried away by the chips formed. Increasing cutting speed will reduce cutting force and machining time. Therefore, it is of interest to study the effects of the increasing cutting speed on bone drilling characteristics. In this article, the effects of the increasing cutting speed ranging from 500 up to 18,000 r/min on the thrust force and the temperature rise are studied for bovine femur bone. The results of this study reveal that the high-speed drilling of 6000-7000 r/min may effectively reduce the two parameters of maximum cortical temperature and duration of exposure at temperatures above the allowable levels, which in turn reduce the probability of thermal necrosis in the drill site. This is due to the reduction of the cutting force and the increase in the chip disposal speed. However, more increases in the drill bit rotational speed result in an increase in the amount of temperature elevation, not because of sensible change in drilling force but a considerable increase in friction among the chips, drill bit and the hole walls.
In case of human bone fracture, the best way to better and faster knitting is when a traumatologist fixes the fractured bone ends by drilling and setting the immobilization plates by screws. Heat generation during bone drilling may result in thermal injury due to exposure to elevated temperatures, with potentially devastating effect on the outcome of orthopedic surgery. A recent and promising method for reducing temperature in bone drilling is the use of ultrasonic assistance, where high-frequency and low-amplitude vibrations are added in feed direction during cutting process. In this research, experimental tests are carried out in five cutting speeds and three feed rates. The results demonstrate that ultrasonic-assisted drilling offered lower thrust forces and lower process temperatures as compared to conventional drilling at 1000 r/min. In addition, it is obvious that at 2000 r/min, since the values of temperature rise and thermal injury are independent from the feed rate, this method can be applied in the orthopedic surgery.
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