| BackgroundThe intertrochanter fractures are commonly happened in hips, 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. The internal fixators are extramedullary and intermedullay implants. The former is comprised with plate and screws, and dynamical hip screws (DHS) are mostly used. Proximal femoral nails (PFN) are mostly used as an intramedullary implant. 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.It is always difficult to achieve anatomical reduction in the posteriomedial cortex of the proximal femur because of its characteristic anatomy. There are several reasons for the failure of internal fixation, such as the anatomical characteristics of the proximal femur, internal fixation techniques, types of fractures, and reduction of the fragments. The anatomical reduction of the fracture regions is most important to avoid a failure fixation. Biomechanical researchers found that the posteriomedial fragments affected the stability of the intertrochanter fractures, as well as the stability of the internal fixation. So the complication of cut-out of the screw and coax vara always happened and resulted in a failed operation. Whether fix the posteriomedial fragments was still under controversy because the biomechanical influence of different posteriomedial fragments for the proximal femur was not clear.With the development of the formation of imaging, improvement of the three dimension reconstruction software, and applicant of finite element in medical biomechanics, it is becoming much easier to establish a precise three dimensional finite element model. It is a common method to establish a finite element model using CT images. The models are more precise, and the structures of the tissues are well represented. Meanwhile, the geometrical and the biomechanical similarity are well also. The images can play the geometrical profile and provide the density of the bone tissue, which made it possible to assign 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 assignment 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 for traditional biomechanical test 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 posteriomedial cortex fractured were produced by osteotomy on the intact femurs. A finite element analysis was undertaken. We aimed to find the changes of stress distribution and magnitude after different types of posteriomedial cortex defected in different gaits, which could be guidelines for reduction of proximal femur and its rehabilitation.Methods1. Subjects and collection of imagesEight volunteers were selected. Four of them were male and the others were female. People with hip disorders were excluded by X-ray examination. Both hips were scanned with CT and the thickness of each slice was 0.625mm. All the images were saved and one femur of each adult was randomly included.2. Establish three dimensional finite element modelAll the CT images were imported into the Mimics 13.0 and segmentation was undertaken. Three dimensional model of intact femur was established by setting thresholding and modifying the morphology. The posteriomedial defected model was produced by osteotomy based on the intact femur. They are lesser trochanter, lesser trochanter with half of the medial cortex, and lesser trochanter with entire medial cortex. Models were remeshed, and the quantity of triangles was reduced, the quality was elevated also. Then material properties were assigned by Mimics, and all models were imported into Ansys 10.0 for finite element analysis.3. Boundary of the finite element model and extraction of dataAll models were taken into finite element analysis in static module. To simulate the gait of statically standing and walking slowly, half and 2.5 times of body weight were applied on the femoral head respectively with the direction of gravity. The distal femur was entirely constrained. The plot contour of stress was attained. The max Von Mises Stress above and below the femoral neck, and in the lateral and medial femur cortex was extracted from all intact and defected models.4. StatisticsData analysis was undertaken by the software of SPSS 13.0. The stress in defected models were compared with intact models of every regions using repeated measure analysis, the interaction of types of fracture and the regions of stress were analyzed also. An associated probability of 5%(P＜0.05) was considered statistical significant.Results1. Intact and posteriomeidal cortex defected proximal femur models were established using Mimics and Ansys. The number of nodes of the models was between 319 448 and 453 236 (mean 380 416), and the number of elements was between 211 034 and 303 860 (mean 252 537). Models were more accurate.2. In the intact femoral model, the value of stress above and below the femoral neck, in medial and lateral cortex was higher. However, the value of stress in femoral head, the anterior and posterior cortex was much lower. The max value was located in the femoral shaft below the lesser trochanter. The stress below the femoral neck was higher than that above the femoral neck and in the lateral cortex. All these results were similar with former researches.3. The posteriomedial cortex could disperse the stress of proximal femur. The max value of stress transferred from the femoral shaft to anteriomedial cortex in the defected models. The stress was concentrated in the fracture site. The stress above and below the femoral neck, as well as in the lateral and medial cortex did not changed significantly when standing. However, when half of the medial cortex involved, the stress significantly increased by 47％in medial cortex and increased by 12％in the lateral cortex. The stress in medial and lateral increased by 247％and 66% respectively when entire medial cortex was involved.When slowly walking, the stress of femoral neck, medial and lateral cortex did not significantly changed after the lesser trochanter fracture. The stress in medial and lateral cortex increased by 57％and 23％respectively when half of the medial cortex involved. But it was not significantly changed in the femoral neck. When entire medial cortex was involved, the stress increased by about 49％and 45％above and below the femoral neck respectively. At the same time, the stress in medial and lateral cortex elevated by 223％and 75%, respectively.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, such as bone trabecula and calcar femorale. Therefore, the accuracy of models was improved. Then we used the module of simulation of Mimics and several kinds of defected models were established by osteotomy. 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 values and distribution of stress were changed after posteriomedial cortex fracture. The max value of stress transferred from the femoral shaft to the anteriomedial cortex in the defected models. The stress was concentrated in the fracture site. When entire posteriomedial cortex fractured, the stress was delivered through the residual cortex, and with a tendency of extension to the anterior cortex. The stress of lateral and medial cortex did not change too much when the lesser trochanter defected only. The stress on the tension and pressure sides increased with the defection of medial cortex. So the residual cortex had to bare more stress when the posteriomedial fragments were larger. A continues medial cortex was helpful to disperse stress and decrease the stress of medial and lateral cortex. The less medial cortex was involved, the better to disperse the stress for medial cortex.3. Considering for biomechanical effect, we thought that the isolated fragment of lesser trochanter could not be fixed. If half of medial cortex was involved, the stability of the dynamic hip screws would be impacted. Meanwhile, it was influenced with the increasing of stress on femur. So, it had better to fix the fragments. What' more, post-operation exercise should be limited and patients should not bear full weight. But when entire posteriomedial cortex involved, it was necessary to reduce entire or part of the fragments to decrease the local stress in order to keep the plate-screw system stable. It was dangerous to bear weight after operation.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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