| Total hip arthroplasty (THA) is an extremely effective procedure in relieving pain and dysfunction for patients with hip joint cartilage degeneration, femoral head necrosis and femoral neck fractures [1]. Periprosthetic femoral fractures (PFF) can occur following THA and are expected to increase because of the escalating number of hip joint replacements in treating bone disease and fractures [2].First described by Duncan and Masri, the Vancouver classification has been widely used to classify PFF according to the location of the fracture, the stability of the implant,and the quality of the remaining bone [3].It has been considered as gold standard in evaluating PFF on the femoral side. Unified classification system (UCS) has been recently proposed and has been applied in the femoral fractures after total hip arthroplasty (THA) between group and within group study. Its reliability has been proved by related research in North America and Europe [4-5]. Conventional Vancouver type B1 fractures are those occurring at the tip of the THA stem in which the hip implant is stable. In the UCS system, the characteristics of B1 PFF is as same as Vancouver B1 fractures. Management of these fractures remains a surgical challenge due to the presence of the underlying prosthesis.Current treatment algorithms generally recommend open reduction and internal fixation (ORIF) for this type of fractures [6]. Available fixations include single cerclage wire or screw, double circle cable or titanium cerclage cable, single column locking plate, plate-cable system, and allogeneic cortical bone plate [7]. However, there is no gold standard in treating Vancouver type B1 fractures despite various randomized controlled clinical trials with regard to different internal fixations. Besides, it has not been extensively investigated on the characteristics of type B1 PFF, the stress distribution and mechanical stability of different internal fixations.As an effective and accurate numerical method in studying irregular objects, finite element analysis (FEA) provides orthopedics or other specialists with suggestions on clinical treatment through computational models. These models based on the finite element (EE) method make it possible to assess the full pattern of strain and stress distribution. Such investigation can lead towards the optimum biomechanical management of PFF [8].In this study, FE models of PFF with various fixations were used to analysis the biomechanical performance of three internal fixations for Vancouver B1 PFF in normal and osteoporotic model. Single cerclage wire and screw were not investigated in this study because of their demonstrated high failure rate in treating type B1 fracture [7].And the cable-plate system will not be discussed here due to the biomechanical condition limits. Suggested by a latest biomechanical research, tangential bicortical screw fixationmay offer more optimal stability than cable-plate systems when using a plate applied laterally on the femur [9]. Double circle cable fixation and traditional locking titanium plate system are most commonly used methods in type B1 fracture [10].Therefore, the included fixations were double circle cable, traditional titanium locking compression plate and bicortical periprosthetic plate designed by the author and tutor.The quality of the remaining bone is closely related to the success of PFF treatment and osteoporosis is a demonstrated predisposing factor in PFF [11]. Therefore it is essential to carry out a comparative study between normal bone and osteoporosis on different internal fixations.The stress distribution, stiffness, maximum stress and relative displacement were compared under the same vertical and rotational loading using FEA. This study was aimed to provide new internal fixation device for Vancouver type Bl fracture by analyzing biomechanical characteristics of different internal fixations.Based on biomechanics study of finite element analysis in PFF model, with the support of Tianjin Zhengtian medical instructment Co,Ltd. and Beijing Naton Research Institute, my tutor and I developed and produced a novel bicortical titanium periprosthetic plate model and gradually improved the plate in order to achieve better anatomic and biomechanical stability. Finally, a novel periprosthetic plate was developed and produced in the factory. Solid models of sawbones were made and were carried on solid biomechanics experiment study.Objective:1. A three-dimensional finite element model of Vancouver B1 PFF was founded by digital medical technology.2. Three models with various fixations, included double circle cable, classic titanium locking compression plate, bicortical periprosthetic plate, were drawed by three-dimensional graphics software. Three FE models with various internal fixations were established and a comparative study of normal and osteoporosis model was performed.3. Design and mechanical experiment of a novel PFF titanium plate:Abaqus software was used to compare the stress distribution, maximum stress, stiffness and relative displacement on three models with various internal fixations. A comparative study of normal and osteoporotic model was performed. By loading the same vertical and rotation force, we performed the biomechanical study of three models with various internal fixations and provided a theoretical basis for clinical application. Finally we developed and producted a novel periprosthetic plate. With the support of Tianjin Zhengtian medical instructment Co,Ltd. and Beijing Naton Research Institute, my tutor and I developed and produced a novel bicortical titanium periprosthetic plate model and gradually improved the plate in order to achieve better anatomic and biomechanical stability. Finally, a novel periprosthetic plate was developed and produced in the factory. Solid models of sawbones were made and were carried on biomechanics experiment study.Method1. Three-dimensional FE models of femoral Periprosthetic fractures(PFF) were reconstructed:X-ray examination was performed to exclude hip fracture,cancer and bone destruction. Philips Brilliance iCT of 256 rows was used to scan femur for model reconstruction. Scan parameters:Tube voltage 120kV, tube current of 100mA,thickness 0.625mm. We got CT images of the femur and saved as.DICOM data format. The CT images were inported to Mimics 15.0 software. After thresholding and other operations, the three-dimensional model of the femur was reconstructed. We performed osteotomy and assembled prosthesis LCU (Waldemar Link, Hamburg, Germany) and simulated B1 PFF by cut tools. STL format data were exported.After surface modifying, we saved data as Iges format.2. According to various valuation of normal and osteoporosis material, we can establish FE model accurately. The friction between the joints was excluded and cartilage was ignored, muscle and tendon stress was simplified. The model was assumed to be completely broken and the fracture surface was in contact. The distal part of FE model of femur was constrained and the x, y, z-axis position of model is zero. With the bottom restrained we added 500 N vertical force(on the top of the femoral head)and 7N.m rotation force on three FE models with various internal fixations. Stress distribution of posthesis and internal fixation was measured to evaluate two models of normal and osteoporosis by loading the same vertical and rotational force.3. Draw three-dimensional model with internal fixations:The study involved three internal fixations,which included double circle cable, classic titanium locking plate and bicortical periprosthetic plate designed by author. The three finite element models of B1 PFF with different fixations were established. Three models with various internal fixations were assembled by pre-processing software-Hypermesh. After nodes and elements were generated the models were imported to finite element analysis software-abaqus. Those following tests were performed to evaluate the biomechanical properties of three models. (①Stress distribution and positions of internal fixations and the peak stress;②stress distribution and stress positions of femur;③ relative displacement; ④stiffness test)4. With the support of Tianjin Zhengtian medical instructment Co,Ltd. and Beijing Naton Research Institute, my tutor and I designed and produced a novel bicortical titanium periprosthetic plate model and gradually improved the plate in order to achieve better anatomic and biomechanical stability. Finally, a new type of periprosthetic plate was developed and produced in the factory. We eventually used imported sawbones from USA to create solid model. Considering titanium locking compression plate and femoral anatomic locking compression plate from Tianjin Zhengtian medical instrument company as the control group, the novel plate of the same material as experimental group,each group 3 cases, all models were carried on solid biomechanical trial.Result1. Finite element analysis(FEA) is an important component of the theoretical biomechanics study. The human bone and other tissues can be simulated by FEA according to various biomechanical properties. So it can be used to analysis,verify and complete the experimental biomechanical study. It can be used to develop novel prosthesis and fixations. In this study, with the help of Mimics, Hypermesh and other software, CT images of normal human femur was used to construct the finite element model of B1 PFF. Based on precise data from iCT scanner, the software of Mimics, Geomagic Studio were used to reconstruct 3D digital virtual simulation model of B1 periprosthetic femoral fractures. This way of modeling is feasible,fast, effective and it has not harm to the human body. So we can use FE model of B1 PFF to accurately simulate B1 periprosthetic femoral fractures for biomechanical analysis by Abaqus.2. Three internal fixations models were drawed by three-dimensional graphic software, and three FE models of B1 PFF with various internal fixations were reconstructed. Abaqus software was used to perform finite element analysis. With the bottoms of PFF model constrained, we gave 500N vertical load and 7Nm rotation force on the study. Finally the study revealed that:the stress force peak of three models all located near to the fracture postion. Compared with double circle cable and classic locking titanium plate, the stress distribution of bicortical periprosthetic plate model was more uniformed. Higher fixation stiffness, smaller relative displacement and smaller maximum stress of prosthesis and fixation were founded in the model of bicortical periprosthetic plate.3. Both normal bone and osteoporosis model of B1 PFF were performed the finite element analysis. By loading the same vertical and rotational force, compared with a double circle cable and classic locking titanium plate of single-row screws, Higher stiffness, more uniform of stress distribution and better stability were detected in the FE model of bicortical periprosthetic plate; Smaller relative displacement and smaller maximum stress of prosthesis and fixation were also founded in the model of bicortical periprosthetic plate,which means it is more stable and stronger than other internal fixations for B1 periposthetic fractures.4. Compared with other two femoral locking compression plate of the same material which were all produced by the same factory, our final biomechanical study of novel bicortical periprosthetic plate confirmed that the bicortical locking plate had a highest stiffness and best stability by loading the same vertical,bending and rotational force. The maximum stress on the novel plate was lowest. The relative displacement,flexibility and torsion angle of novel plate under the same loading were also lowest. The incidence of the internal fixation breakdown and refracture would be lowest.ConclusionBecause of the prosthesis stability of Vancouver B1 PFF, the prosthesis was usually retained and the surgery of open reduction and internal fixation was performed to treat B1 PFF. But it is difficult to confirm which one is a better solution for the Bl PFF. By systemtic modeling and finite element analysis,we prove that FEA is a feasible, effective, highly qualified way to simulate and reconstruct a 3D digital virtual model of B1 periprosthetic femoral fractures. And the further study on normal bone and osteoporotic FE models confirmed that with the same loads of vertical and rotational force, compared with a double circle cable and classic locking titanium plate, higher stiffness, more uniform of stress distribution and better stability were detected in the FE model of bicortical periprosthetic plate; Smaller relative displacement and smaller maximum stress of prosthesis and fixation were founded in the model of bicortical periprosthetic plate,which means it is more stable and stronger than other internal fixations for B1 periposthetic fractures.. After the completion of production of the design plate, the final biomechanical study confirmed that the designed bicortical locking plate had a highest stiffness and best stability by loading the same vertical,bending and rotational force. The maximum stress on the novel plate was lowest. The relative displacement,flexibility and torsion angle of novel plate under the same loading were also lowest. The incidence of the internal fixation breakdown and refracture would be lowest. |
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