| BackgroundsThe fractures in the modern department of orthopedics is the most common disease in the treatment of fracture, many viewpoints, that strong and effective internal fixation, early functional exercise. So strong, effective internal fixation implant is an indispensable tool in modern trauma department of orthopedics, the most commonly used clinical at present is titanium alloy material, its biggest drawback is not self-degradation, long-term stay in the body or take out two times operation, and the elastic modulus of the bone is high, easy to cause long-term fixation stress shielding, skeletal strength weakened around. Although at present has developed a kind of degradable polymer material, but its strength is low, not enough to maintain the stabilization of fracture, mainly used for various types of non-load-bearing parts and parts of bearing fixed and orthopedic fracture operation. So there is an urgent need to produce stress shielding of the implant material to develop a kind of high strength, biodegradable and avoid stress shielding occurs. Magnesium alloy is the research hotspot in the Department of orthopedics implant materials, because of its good biocompatibility, biodegradation, and the elastic modulus and the skeleton is close, avoid stress shielding occurs, people pay more and more attention to. At the same time, China’s magnesium is rich in resources, and the magnesium and titanium metal compared to low price, therefore the development of a new type of magnesium alloy biomaterials, especially bone surgery and orthopedic internal fixation or implant materials is very optimistic about the prospects for. Good internal fixation material should have the following conditions:(a) the mechanical strength and strength enough to maintain the time, the elastic modulus and the bone near, strength gradually decay at the fracture site to get enough stress stimulation;(b) a good biocompatibility, non-toxic, no antigenicity and induced cancer;(c) biological degradation of appropriate, degradation products can be cleared of physiological system;(d) as much as possible to reduce complications. Magnesium alloy has all of the above conditions, known as "the metallic biomaterials revolutionary".Now the research of magnesium alloy are mostly in vitro experiments such as extraction experiment, cell toxicity test, the maximum sensitization test and animal in vivo implantation to observe its biological and degradation properties, a series of experiments proved that magnesium alloy material has good biocompatibility, and animal implantation in5weeks began to degrade,20weeks faster degradation. But the reports about the biomechanical properties of magnesium alloy are less.Odontoid fracture is the most common upper cervical fracture fracture according to Anderson classification, type Ⅱ is the most common type, and the anterior hollow screw fixation is the treatment of a kind of high efficient, safe operation of fractures of odontoid type Ⅱ. Because of the complexity of the upper cervical spine structure, the use of titanium alloy hollow screw fixation in clinic generally only operation to remove screws, long-term stay in the human body, will inevitably electrolytic release inflammatory substances harmful to human body. Moreover, in the long-term activities of the cervical vertebra, screw deformation caused by metal fatigue, Lead screw loosening, nail withdrawal etc, if the screw compression of the spinal cord, and even lead to life-threatening. The above problem is the use of titanium alloy screws can not solve the problem, so the treatment of odontoid process fracture of II type is urgent need a sufficient strength, can degrade, no stress shielding, a new implant materials contribute to bone properties. The magnesium alloy has all the conditions of good internal fixation materials, can be used to manufacture hollow screw fixation of odontoid process fracture assumption, basic can solve the above problems, and, the magnesium alloy has contributed to the bone, also can reduce the odontoid fracture hollow screw fixation after not healing rate, greatly improve the success rate of treatment.This research through magnesium alloy processing into dens hollow screw, once again verify the biocompatibility and the evaluation of its mechanical properties, which can provide basis for clinical use.ObjectivesStudy measurements of normal axis and odontoid process fracture through the image, making magnesium alloy hollow screw, and simulated in cadaver bank operation, instant stability of feasibility and observation of screw screw design verification of fracture fixation.Evaluate preliminary biological of the screw by magnesium alloy hollow screw extracts of cultured mouse fibroblasts (L929) and osteoblasts (MG63).Evaluate mechanical properties of magnesium alloy hollow screw by three dimensional finite element analysis, computer simulation of odontoid fracture of II type hollow screw fixation model, were given magnesium alloy, titanium alloy metal properties, comparison of two kinds of screw maximum stress of maximum stress and maximum displacement, odontoid process.Evaluate mechanical properties of magnesium alloy hollow screw through the human body specimen making dens, odontoid fracture models of IIA, respectively, using magnesium alloy hollow screw, hollow screw fixation of titanium alloy, fracture of axis measurement model hollow screw fixation of type II odontoid process on the biomechanical machine, comparing the load-displacement relation and the maximum load screwsMethodsProduction and operation simulation of magnesium alloy hollow screw:normal CT imaging data of150, image data type II odontoid fracture of43, the CT image on the scale, direct measurement of height, coronal axis odontoid process on the computer, the length of odontoid base diameter and sagittal diameter, gear process axis angle, bone broken line to the odontoid tip distance of fixed length, type II odontoid fracture. Reference measurement data, the design of Mg-Al hollow screw sketch, the data into computer machinery processing software, connection machine, through the drilled hole, turning, milling, fine shape, plum, milling, threading tapping groove fitter deburring, polishing the outer circle process, production of magnesium alloy hollow screw. In6human cervical specimens, made of odontoid type IIA and IIB fractures model, fixed with magnesium alloy hollow screw, validate rationality of screw design, comparison of odontoid fracture displacement and magnesium alloy screw fixation before fixation, real-time observation of the stability of screw fixation of odontoid process fracture.observe the cytotoxicity and bony of magnesium alloy hollow screw: experimental method of reference fifth national standards of biological evaluation of medical devices of the people’s Republic of China:the in vitro cytotoxicity test. According to the ISO10993-1standard (sample surface area/immersion medium=3cm2sample/mL) preparation of magnesium alloy hollow screw extract, cultivating two kinds of cells in L929and MG63, at37℃,5%CO2culture box, respectively at2,4,7d under the microscope observation of live cells form and photography and measuring the absorbance value (OD value). Two kinds of cells compared to magnesium alloy hollow screw extracts of cultured cell growth pattern and its relative growth rate.Three-dimensional finite element analysis of magnesium alloy hollow screw: The use of German Siemens64rows of dual-source spiral CT scan of a normal adult male volunteers cervical, images obtained by on-line workstation DICOM format data file. DICOM format data, import the3D reconstruction software Mimics10.01, using threshold segmentation and region growing technique, separating axis model, grinding, filling, denoising, smoothing treatment on the model, the appearance is smooth, close to the axis structure, then to point cloud file format into Solidworks2010, model shaping, obtain a smooth, no broken axis solid, hollow screw fixation of odontoid process simulation of IIA type fracture model in Solidworks, the guide into the finite element software Ansys13.0to simulate the mechanics analysis, comparison of Mg-Al maximum stress of hollow screw and titanium alloy cannulated screw and the maximum stress and maximum displacement of vertebral body force, odontoid.Biomechanics of Mg-Al alloy hollow screw fixation of odontoid process fracture of type II:20bodies of the accidental death of young people fresh frozen cadaveric specimens were made into models of IIA axis, odontoid fracture, respectively using magnesium alloy hollow screw, hollow screw fixation of titanium alloy, fracture of axis measurement model hollow screw fixation of type II odontoid process on the biomechanical machine, comparing the load-displacement relation and the maximum load screws, mechanical properties evaluation of magnesium alloy hollow screw. ResultsAxis height (35.11±4.15), odontoid length(14.99±1.26), coronary odontoid base diameter of (8.94±1.26), sagittal diameter of (10.20±1.02), fracture line from the odontoid tip distance of (10.49±1.75), needle after the dip (17.56±5.29). Magnesium alloy hollow screw operation convenient operation is similar to the clinical use of AO odontoid titanium alloy hollow screw, odontoid fracture end immediate stability.Morphological observation showed that L929attachment, cell death, cell nuclear fragmentation dissolved, MG63growth of adherent cells, morphologically normal, no obvious dissolution. MTT magnesium alloy AZ31B leaching toxicity of liquid on L929cells of1-2, on MG63cell toxicity grade0, no obvious cytotoxicity, there was statistically significant difference between the two kinds of cells OD (P<0.05).(1) the axis and odontoid process fracture with hollow screw fixation of IIA finite element model of appearance, good geometric similarity, normal axis consists of29103elements,50386nodes; odontoid fracture; hollow screw fixation of IIA finite element model includes32929tetrahedron elements,59265nodes, can reflect normal axis and hollow screw fixation model structure is true.(2) head extension, flexion, titanium alloy screws maximum stress were42.3MPa,62.8MPa, the maximum displacement of fracture ends are respectively0.028mm,0.040mm, magnesium alloy screw; the maximum stress were33.6MPa,52.1MPa, the maximum displacement of fracture ends are respectively0.036mm,0.056mm.The maximum load average fracture need are respectively560N and600N when magnesium alloy, titanium alloy screw fixation model refracture again,but the difference was not significant (P>0.05). When the loading<400N, no significant displacement difference between the two groups (P>0.05), when the displacement difference load.>500N fracture has statistical significance (P<0.05). Magnesium alloy screw pullout force of483N titanium alloy screws, average maximum pullout force496N, the difference was statistically significant (P>0.05).ConclusionsPeople with type Ⅱ odontoid fracture fixation screw length38cm, diameter3.5mm, length of1cm is suitable for remote thread. Design of biodegradable AZ31B magnesium alloy hollow screw appearance, consistent, and clinical use of titanium alloy cannulated screw shape, convenient operation, visual fracture end immediate stability, provide the basis for the next foot biomechanical testing.Magnesium alloy has low cytotoxicity, met the clinical criteria, can promote bone cell growth, cell toxicity test of magnesium alloy using L929cell line detection is more sensitive, more accurate, and inspection to magnesium alloy bone can choose MG63, but with the cytotoxicity test accuracy of MG63magnesium alloy is easy to appear the deviation.Application of CT scans to obtain Axis axis spatial structure information model established can be used for biomechanical experiment, the yield strength of two kinds of material screw fixation of odontoid fractures of type Ⅱ, and the maximum stress is less than the material, magnesium alloy hollow screw to maintain the stability of fracture.From the in vitro biomechanical test show that magnesium alloy hollow screw can play a role of fixing is similar with the titanium alloy cannulated screw fixation of odontoid process fracture in the model. |
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