| BackgroundThe intertrochanter fractures are commonly seen in hips in clinic, most of which are seen in old people with osteoporosis. Old people with intertrochanter fractures always combined with some medical diseases. Conversation treatment would induce many complications, high mortality and descending of life quality. With the development of internal fixation techniques, open reduction and internal fixation was thought to be the best treatment of intertrochanter fractures. Currently, conservative treatment already use less, early surgical treatment has become the basic principles of the treatment in intertrochanter fractures. The internal fixators are extramedullary and intermedullay implants. For internal fixations, the dynamical hip screws (DHS) are mostly used. For the later, various intramedullary nails are mostly used. Both of the implants have several biomechanical advantages respectively. However, the DHS was still thought as a more effective and safe implant in treating intertrochanter fractures. The DHS was wildly used since1955and thought to be the "Golden standard" in the treatment of intertrochanter fractures. The PCCP-a new minimally invasive internal fixation,was designed by Gotfried in1990s. Several clinical study have showed that PCCP is better than DHS in blood loss, the time of operation, postoperative infection and pain. But the study refer to the biomechanical properties of tow internal fixation is few.The experiment of traditional biomechanics was based on the animal and cadaver models. There are so many deficiencies and limits. The animal model is different from human, so the result of animal biomechanics cannot resolve the problem of human. The result in vivo is the most reliable, but due to the limited management, it is difficult to attain the data under the physiological state. The cadaver model gains an advantage in the geometry similarity, but it changes the characteristics of living tissue, and is difficult to attain the mechanic’s characteristics. The same cadaver model cannot use again; it decreases the comparability of compared research. The cost of the experiment is high. Meanwhile it is difficult to gain the cadaver now.With the development of the formation of imaging, improvement of the three dimension reconstruction software, and application of finite element in medical biomechanics, it is becoming much easier to establish a precise three dimensional finite element model. It is a common used method to establish a finite element model using CT images. The models are more precise, and the structure of the tissues is well represented. Meanwhile, the geometrical and biomechanical similarity are good also. The images can play the geometrical profile and provide the density of the bone tissue, which made it possible to distribute the physical material properties of them. The Mimics can switch different types of images. The images could be also segmented, and the three dimensional models are easy to be edited and simulated with the software so as to construct kinds of models. By the way, the material distribution module could be used to elevate the accuracy of models. The dominant advantage of finite element models was that it could provide the stress changes inside of the entities, which was difficult to traditional biomechanical study using cadavers. The methods of finite element analysis are also keeping improving. The material of the entities was expanded from linear to nonlinear, and the procession was improved from static to transient. Associated with three dimension reconstruction and design software, finite element models may be comprised with bones, ligaments, and the properties of every kind of tissues were applied to attain more convinced results.ObjectivesTo establish a three dimensional model with Mimics using the proximal femoral CT images of healthy adults, as well as improve the accuracy of the model. And then there dimensional model with Evans-Jensen Ⅱ type fractured mode which is fixed by various internal fixators were produced by osteotomy on the intact femurs. A finite element analysis was undertaken. We aimed to compare the difference between PCCP and DHS in von Mises distribution, the von Mises distribution of the femur and the fracture surface contact pressure.Methods1. Subjects and collection of imagesOne volunteer was selected which is without hip disorders by X-ray examination. Both hips were scanned with CT and the thickness of each slice was0.625mm. All the images were saved and one femur was randomly included.2. Establish three dimensional finite element modelAll the CT images were imported into the Mimics13.0and segmentation was undertaken. Three dimensional model of intact femur was established by setting thresholding and modifying the morphology. Outcome was saved in the format of STL. Then the STL data was imported to Geomagic studio10.0software to fix the surface. A Evans-Jensen Ⅱ type fractured mode was made by cutting technique, and the outcome was saved in the format of Iges. The model of internal fixators were established by the software of UG, and the outcome was saved in the format of Iges. Both of the data was imported to Geomagic studio10.0software to fit together. Then the models were assembled, meshed and given material parameter. Corresponding to the quantity and location of screws, tow different finite element models of surgical fixation methods were developed. Finally, the finite element models were imported into the FE package ANSYS for post-processing.3. Boundary of the finite element model and extraction of dataThe model was assumed that the bone was completely break, the contact element was targe170and conta174; the position between the internal fixators was moveless, and the strength of the internal fixators was strong enough to avoid rupture; the displacement of the proximal femur was zero in X, Y and Z axis. The load of700N was applied on the femoral head.Results1. The three dimensional model with Evans-Jensen Ⅱ type fracture was produced by Mimics and Ansys software. The number of nodes of the DHS models was26883, and the number of elements was111314. The number of nodes of the PCCP models was29734, and the number of elements was121786.2. The von Mises stress of the internal fixator in the DHS model distribute equally. The max stress was393MPa and stress concentration was seen in the junction between the lag screw and the sleeve. The von Mises stress of the internal fixator in the PCCP model also distribute equally. The max stress was295MPa and stress concentration was seen in the sleeve.3. The von Mises stress of the proximal femur in the DHS model distribute unequally. The total stress of the distal femur was higher than the proximal femur. The stress concentration was seen in the fracture line which is above the femoral calcar. The max stress was134MPa. The von Mises stress of the proximal femur in the PCCP model distribute equally. The stress transmit equally from the proximal femur to the distal femur. The stress concentration was not obvious. The max stress was134MPa which is seen in the fracture line above the femoral calcar.4. Contact pressure of the fracture surface in the DHS model was lower than the PCCP model. The stress increased gradually from inside to outside of the fracture surface in DHS model. The max stress was45MPa which is seen above the femoral calcar. The stress distribute equally in the PCCP model. The gradual stress increasing was not detected, and the max stress was57MPa which is seen above the femoral calcar.Conclusions1. In our research, we established finite element models of proximal femurs using CT images. The models had good contour and the geometrical similarity were well. The physical characterize was as good as real samples. What’s more, the models were effective and accuracy. They were easy to reproduce also. We remeshed the models to improve the quality and quantity of triangles. Meanwhile, we assigned material properties of the tissues to simulate the special anatomical characters of proximal femur. Therefore, the accuracy of models was improved. We use cutting technique to establish Evans-Jensen II type fractured mode, at the meanwhile we establish the internal fixator by UG software. These models were effective and suitable for our study. The results of finite element analysis were output as stress contours, which was easy to observe the stress changes.2. The stress concentration was seen in both internal fixators. The stress concentration of the DHS model was seen in the junction between the lag screw and the sleeve. The stress concentration of the PCCP model was seen in the proximal screw. The max stress was393MPa in DHS model and295MPa in PCCP model. It showed that PCCP is better than DHS in biomechanics. It is usually500-600MPa when fatigue failure happens. But in our study the max stress of PCCP and DHS is lower than that figure. So we conclude that to stably intertrochanter fracture, both PCCP and DHS are good in biomechanics; to biomechanics only, PCCP is better than DHS.3. Contact pressure of the fracture surface in the DHS model was lower than the PCCP model. The stress increased gradually from inside to outside of the fracture surface in DHS model. The max stress was seen above the femoral calcar. The reason as follow:1. The proximal load is still mainly on the femoral calcar in stably intertrochanter fracture;2.The function of DHS is like a tension band, it can not distribute the stress equally and lead to stress concentration inside the proximal femur. Thus, the stress concentration is happened inside the fracture surface. So PCCP distribute the stress more equal than DHS and it can promote union of the fracture.4. There were some limitations in our study. We applied vertical force on the femoral head only, but the traction of muscles around hips was ignored. Secondly, we supposed that the bone was a material of homogeny, continuous and isotropy, which was different from the fact. The method of finite element and biomaterial test of specimen had their own advantages. The two methods should be complementary to attain more scientific results. |
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