Design And Test A Kind Of Unilateral External Fixation With Continuous Display Of Pressure Between Fractured Fragments

Fracture of tublar bones of limbs familiar occured in traffic or other unexpected accidents. The treatments of the fracture are various. Many apparatus included external fixation can be used in surgical treatment. The treatment of external fixation is a half-invasive method which is different from surgery treatment. Although internal fixation is widely used in now days, applying external fixation is a simple and effective method with the benefit of short operation time, less damage, firm fixation, without second operation, and providing continued pressure to the fractured fragments. It can provide traction, diaplasis and early active exercise also. Using external fixation is still a effective method in fracture treatment of long tublar bones. The theory of external fixation is to establish a steady dimensional system by the knighthead and the taxation nail which is penetrate through the osseous. The added force is transmit to the bone by the length change of the knighthead, and the fracture is riveted or the bone is prolonged. But the added pressure has no determinate vale,. It is usually controlled by the experience and estimate of the doctors. The added pressure obviously reduced after operation and reached its minimal point 5 days after operation. Much more pressure to the fractured fragment was needed. In the upper time of the fracture healing, the added pressure should be step down to eliminate thestress countercheck and accelerate the shaping and rebuilding of the bone. The external fixation easily becomes flexible because of having many joints. This will influence the healing of fracture, or lead the shift of the fragments. It is well recognized that appropriate pressure between fractured fragments is benefit for the healing of fracture. It has great meaning to realize the pressure momentarily, adjust the pressure in time and control the added pressure in appropriate range. So we designed a kind of unilateral external fixation with continuous display of pressure between fractured fragments. This device consists of force sensor, press apparatus, display instrument and fixation. In mechanical test and sample experiment, the precision and measure range of this device can fulfil the practical demand in clinical applying.. The pressure of fractured fragments can be shown continuously and the added pressure can be adjusted to a proper level to provide an appropriate mechanical environment for the healing of fracture. The capability of the external fixation can be observed by the display instrument also. The value of the added pressure was discussed by finite element analyse method with the three-dimensional model of human thighbone which was made from digital virtual human database. Objective:1. Design a kind of force measure device applying to external fixation. The device included press apparatus, suited force sensor and display instrument. The added pressure of the press apparatus can be dynamic display by the display instrument. The press state of the fractured fragments can be known indirect.2. Test the capability of the external fixation, include the mechanical test and the error analyse of the sensor.3. Mounted this external fixation to transverse fracture models, which were made from human dry bones. Compare the added pressure and the pressure between fractured fragments. Test the practicability of the external fixation. Find out therelation between the added pressure and the pressure of the fractured fragments. 4. Reconstruct three-dimensional finite element model of human thigh bone byapplying virtual human image database. Analyse the influence of fracture lineangle to the pressure of fractured fragments and provide reference gist to thevalue of added pressure. Methods:1. The sensor of the force measure apparatus was made of high strength aluminium alloy, which was machining to hollow columniform figure. Four strain gauges, normal resistance were 500 Q, were affixed to the outer surface. The strain gauges were linked with null-type fridge circuit, and fetch out to link temperature compensating resistance, accuracy compensating resistance and balance compensating resistance. The gauges connected display instrument with a bnc connector plugs. The press apparatus and fixation were dividual. One end of the press apparatus was mounted to fixation, the other end was linked to nut and bolt. The force sensor was added to one end of the bolt to induce the added pressure. The display instrument applied AT89C2051-24PI CMOS chip, with LED screen, battery inside to ensure the register of initiatory value of the force sensor without power supply. The main power supply was provided by external 10V DC. The circuit includes filter, amplifier and linearization modulating.2. The sensor was tested on MTS858 material test system(model:858 Mini Bionix, USA), clamp down reduplicatly six times, 20Kg per unit. The value and result of the course and backhaul were registered by the computer.3. Four human dry long bones of transverse fracture model at middle part were made in sample test. Mounted external fixation to the fractured model. Add force sensors to press apparatus and fractured fragments. The press apparatus added pressure per 5Kg in the range of 0~50Kg. Registered the pressure of fracturedfragments. The result was import to SPSS 10.0 software (SPSS Ltd, USA). Analyse the data and compare the two pressure value.4. Applying virtual human image database to reconstruct three-dimensional model of human thigh bone. The three-dimensional model was export in STL format, and import to Freeform carve system for surface modifying. The modified model was import to Ansys7.0 (Ansys Ltd. USA) with IGS format and compartmentalized reseau. Analyse the influence of different fracture line angle to the pressure of fractured fragments.Results:1. This external fixation can provide firm fixation to fractures of tublar long bones of limbs. The added pressure can be obtained momentarily from the sensor and the press apparatus while the press apparatus changing its pressure.2. The mechanical capability of the force sensor was test on MTS858 material test system. In the range of 0~100Kg, its linearity and repeatability better than 1% , the accuracy is 0.5Kg, the delicacy is O.OlOlmv/V, non-linear error is -0.9732%, delay error is -0.6455%, repeatability error is 0.1986%.3. The added pressure has well correlation to the pressure between fractured fragments in the range of 0~50Kg. We can estimate the pressure value between the fractured fragments by the added pressure shown by the display instrument. When the added pressure is above 50Kg, the pressure between fractured fragments increase slowly.4. The established three-dimentional finite element analysis model has 95621 landmarks which divided into 63692 tetrahedral elements. The model was defined as homogeneous material. Evaluate modulus is 3000MPa, Poisson's ration is 0.25. Applying finite element analyse method, the vertical pressure is about 78% of the added pressure when the fracture line is 15°, the vertical pressure is about 60% of the addedpressure when the fracture line is 30°, the vertical pressure is about 50% of the added pressure when the fracture line is 45° . Conclusions:1. The press apparatus were separate from force sensor of fixation. This design enable to dynamic display the added pressure in the period of bed rest and disconnect the display instrument while the patient doing active exercise. The added pressure should be examined termly by reconnecting the display instrument to the force sensor. Though this design provide more practicality to this new fixation.2. The measure range of the force sensor is lOOKg, and the accuracy is 0.5Kg, Normally, the appropriate pressure between the fractured fragments is about quarter to half of the patient's weight if the fracture is transverse and about quarter of the patient's weight if the fracture is oblique. Though the measure range is sufficient in clinical applying demand.3. The pressure between fractured fragments can be estimated by the display instrument because the two pressures have correlation. Refer to the pressure and adjust the added pressure to an appropriate level to accelerate the fracture healing. The appropriate pressure range to the healing of fracture is relative wide. The more importance we focus on is the variety of the pressure between fractured fragments. It is the attenuation of the pressure between the fractured fragments or the diminution caused by flexible fixation.4. The pressure of the fractured fragments increased slowly when the added pressure is above 50Kg. If the fracture is not transverse, much more pressure should be added to ensure the appropriate pressure of the fractured fragments if the fracture is not transverse. But too much more pressure will cause the instability of the fracture. So we must make all things considered when choosing the addedpressure.