Three Dimensional Finite-element Analysis Of Different Types Of Pauwels Femoral Neck Fracture Fixed With Five Kinds Internal Fixation

BackgroundFemur plays an important role in movement and load-bearing, and has becoming a research hotspot of sports medicine and bone science. Because of the multiple and diversity of femoral neck fracture as well as more complications, the golden treatment has not defined yet. As a result, it is still a problem on the clinical treatment.Femoral neck fracture is account for50%to60%of hip fracture, still with a high incidence in elderly patients, and more with a severe osteoporosis, women are more vulnerable than men. The choice of treatment can be affected by a variety of factors, including age, type of fracture, mental factors etc. The main treatment includes internal fixation, the hemiarthroplasty and total hip arthroplasty. The THA was the terminal choice for treatment, for most patients, it is advised to take the internal fixation as the first choice, to give a chance of a recovery to the femoral head. It is well convinced that, patients less than60years old, without a clear avascular signs, preferred internal fixation; hip arthroplasty for displaced fractures in patients over the age of80; Treatment of patients between the ages of60to80, according to the patient’s injury, fracture classification, psychological factors, and whether combined with other systemic diseases to be comprehensive selection. The optional includes two cannulated screw, three cannulated screw, DHS, proximal femur locking plates, etc.Pauwels type femoral neck fracture was based on the angle of the fracture line, the Pauwels type Ⅰ fracture line less than30°, Pauwels Ⅱ fractures fracture line at30°to50°Pauwels type Ⅲ fracture fracture line is greater than50°. Because as the Fracture line clip angle increased, the more vertical fracture line, the greater the shear-stress force, fracture are more unstable, as a result, fracture nonunion rate increases. Surgeons did not pay more attention to this fracture classification, but the situation changed with the research on the nonunion and avascular necrosis.FEM has a powerful modeling and simulation with complex geometry, material parameters and objects under different loading conditions in the static and dynamic state, it has to be applied to a growing number of human biomechanics.Objectives:1. Use of digital technology, the establishment of a virtual simulation of three-dimensional digital model of femoral neck fractures and finite element model, and t to verify the validity of the finite element model by cadaver;2. With the help of three-dimensional graphics software, five different types of femoral neck model were established, respectively, for the2low-angle screws,3low-angle lag screws (typically135°) in an inverted triangle position,3low-angle complete thread screws (typically135°) in an inverted triangle position, femoral neck locking plate and InterTan CHS model, the finite elements model were established as well.3. To compare the stress distribution, displacement distribution and life circle between these models with the help of Ansys. Therefore, to make deep comprehensive evaluation of biomechanical feature on each kinds of fixation and provide a theoretical basis for clinical applications. Methods:1. Construction and verification of digital three-dimensional finite element model of femur:The including criteria is the hip without fracture, deformity, tumor, bone destruction confirmed by X-ray examination. The CT scan of femur was taken under Philips/Brilliance64-slice spiral CT. Scanning parameters:Tube voltage120kV, tube current of100mA over the greater trochanter, thickness0.625mm. About489slices has been obtained of a two-dimensional CT images. Data was saved with the. DICOM format to save, then input into the Mimics,10.01software, after automatic or manual threshold segmentation, three-dimensional reconstruction of a complete femur was established, and then export the STL format data import Geomagic Studio10.0software, based on the polygon stage extraction feature set and edit patches are automatically fitted NURBS surfaces, save as the iges format and then output. At the same time, the establishment of the femoral neck Pauwels-Ⅱ type fracture model, fracture line of50°and fixed with3low-angle lag screws (typically135°) in an inverted triangle position. According to the material properties of the bones and internal fixation, without taking into account the friction between the joints and cartilage, simplify the processing the muscle and tendon. Set the situation as that the fracture surface completely broken and is in the contact state, the friction coefficient of0.2. The DOF constraints and external condyle of the femur three-dimensional finite element model of the lower edge of all the nodes to0as the boundary conditions, i.e. the distal end of each node x, y, and z-axis displacement of0. Using simplified models, that the force on the femoral head from the pelvic acetabular fossa was only taken into consideration, take only the greater trochanter near abductor (gluteus medius muscle and piriformis) and vastus lateralis muscle force, external load force abductor as finite element analysis, be axial600N stress, average acting on the femoral head and acetabular contact surface axially down, then set mesh density generated grid and set an example attribute operator.2. Finite element model validation:Take the body of an adult cadaver of the femoral neck, remove the soft tissue around, save the complete bone structure, X-ray examination to exclude tumors, fractures and other lesions, bone mineral density test value of1.17g/cm2.-80℃freezer spare sealed in double plastic bags, remove the natural thawing at room temperature24h before the test, fracture simulation osteotomy of the femoral neck Pauwels-II type fracture with the fracture line of50°, and fixed with cannulated screw. Femoral neck cannulated screw:cannulated screws total length of90mm, and diameter of6.5mm, the threaded portion of was20mm in length, hollow part of the diameter of2.5mm. The lag screws in an inverted triangle position and separated from each other. Specimen model was fixed in the the electronic universal dynamic static material testing machine, the distal femur bicondylar connection with the horizontal parallel proceeds the femur axis with the sagittal line showed a23°angle, piriform fossa and greater trochanter fixed by nylon thread (tensile elastic modulus of9.7N/mm2and0.15N/mm2), respectively to be fixed, the fixing strength of10MPa. Subsequently,600N force perpendicular to the direction of femoral head, record the stress on eight points around the femoral neck and three times each, and compared with finite element analysis results.3. Three-dimensional model of the internal fixation:This study involved a total of four kinds of fixed structures, respectively, cannulated screws, fully threaded cannulated screws, femoral neck locking plate, InterTan CHS. The cannulated screw90mm in length and6.5mm in diameter, threaded in the screw distal threaded portion is20mm in length, cannulated screws hollow part of diameter2.5mm; fully threaded cannulated screws, cannulated screw90mm in length and diameter of7.3mm, the proximal and distalend threaded cancellous bone threaded proximal from screw end of the beginning to about35mm, remote from the screw part of a long25mm,25mm in the central screw cortical bone screw, cannulated screw hollow section diameter of2.5mm. The femoral neck locking plate, the structure including an inverted triangle anatomical plate and three locking screws; two universal locking hole of the steel plate above, a locking hole on the bottom, the locking hole with the slope140°;3locking screw nails, two of them were partially threaded cancellous lag screws, with diameter6mm, length85to90mm, distal threaded length of20mm, hollow part of a diameter of2.0mm; another one is fully threaded screws, length of90～100mm, diameter of6.5mm, the diameter of the hollow part of2.5mm; The parameter of InterTan CHS (Smith&Nephew, UK) provided by the company and reconstructed with proE4.0(PTC, USA).4. Finite element modeling and analysis:Stimulated three types of Pauwels fractures to establish femoral neck fracture finite element model, while the establishment of five kinds of internal fixation model. Including:a:two cannulated screws model;:b:3low-angle lag screws in an inverted triangle position;c:3low-angle complete thread screws in an inverted triangle position, d:femoral neck locking plate; and d:InterTan CHS model. The femur and internal fixation model were import into Hypermesh10.0for assemble, and generate nodes and elements in finite element analysis software Ansys13.0for FEA, all the models are made of solid185unit. By the analysis of the five kinds of indicators on the mechanical properties of the five kinds of internal fixation model (a-e):①The stress distribution and peak stress on internal fixation;②the stress distribution location and peak stress on femur;③The displacement distribution of peak displacement on internal fixation;④The displacement distribution of and the peak displacement on the femur;⑤the life circle of bone and internal fixation:0.5Hz frequency,600N perpendicular to the direction of stress line fatigue test, calculated to reach the bone and internal fixation of the number of yield pressure.Results:1. Based on CT scan data using Mimics, Geomagic Studio, UG software,3D digital simulation to establish a femoral neck fracture model, this method is feasible and effective modeling faster, and no harm to the human body. The three-dimensional finite element method is a theoretical biomechanical research methods, geometric model can simulate a variety of structures, given the biological material properties of the various organizations, can reflect the overall trend of the biomechanical properties, which can be used as a specimen experiment, is a good supplement method to biomechanics research. In this study, the use of a human femur CT data, with Mimics, Geomagic Studio, Hypermesh, Anasys software, established the finite element model of normal human femoral neck fractures and has good geometric similarity. To discuss the model validation through the cadaver and demonstrated that the model has a good physical similarity, more accurate and complete simulation of its mechanical characteristics of femoral neck fracture, conducive to their biomechanical analysis.2. Pauwels type-1femoral neck fractures, five models peak stress are concentrated in the internal fixation around the fracture line at the bottom of which model b, c, e, are more evenly distributed, model a, d distribution is more concentrated. Peak stress:a:136.90MPa; b:52.47MPa; c:27.16MPa:d:118.69MPa; e:50.40MPa. Displacement of all of the models are centralized in the femoral head at its peak:a:1.49mm; B:1.62mm; C:1.33mm:d:1.40mm; e:0.89mm. On fatigue testing within a fixed yield did not occur in1e6, the life cycle of the ends of the bones were:a:1.5367e5; b:1.8578e5times; c:1.9458e5times; d:1.2111e5times; e:1.769e5.3. Pauwels type-2femoral neck fractures, stress concentration region differentiate from each models, a, d is more concentrated distribution, stress concentration areas mainly located near the fracture line and the internal fixation; model b, c, stress distribution is more even; Model e stress mainly fixed to the top of screw. Peak stress:a:188.07MPa; b:88.96MPa; c:57.43MPa:d:167.92MPa; e:53.63MPa. Displacement of all of the models are centralized in the femoral head at its peak:a:1.63mm; b:1.27mm; c:1.06mm:d:1.52mm; e:0.89mm. On fatigue testing within a fixed yield did not occur in Ie6, the life cycle of the ends of the bones were:a:1.5089e5次, b:1.1689e5次; c:1.849e5次; d:1.6932e5次; e:1.7964e5.4. Pauwels type-3femoral neck fractures, fracture force as a shearing force in the vertical direction, five models stress concentration region is different, the stress distribution of model a, d are more concentrated located near the fracture line; model b, c stress more evenly distributed; model e stress is mainly concentrated in the internal fixation at the top of the screw. Peak stress:a:210.91MPa; b:116.49MPa; c:62.56MPa:d:142.49MPa; e:65.72MPa. Displacement of all of the models are centralized in the femoral head at its peak:a:1.59mm; b:1.61mm; c:1.33mm: d:1.48mm; e:0.91mm. On fatigue testing within a fixed yield did not occur in Ie6, the life cycle of the ends of the bones were:a:1.4647e5次; b:1.3438e5次; c:1.9156e5次; d:1e6次; e:1.6254e5.Conclusion:The biomechanics features is different with different types of Pauwels femoral neck fracture. The stress between fracure fragments is more high with Pauwels type-1fracture, fracture line can be not perpendicular to the screws, prevention of femoral neck shortening is the main therapeutic purposes. All internal fixation can be used for the treatment of this type of fractures, use with3low-angle complete thread screws in an inverted triangle position to achieve a more stable biomechanical stability, they can effectively prevent short femoral neck shortening, and have the advantages of minimally invasive, as recommended treatment. The Pauwels type-Ⅱ fractures, shear force is greater than in the vertical direction than Pauwels type-1fractures, the screw direction and the direction of the fracture line is closely to vertical, the most prone to the femoral neck axial pressure, has a strong dynamic compression effect and can help to achieve high fracture healing rates. Two screws has poor biomechanical stability, does not recommended,3lag screws in an inverted triangle position is easy caused femoral neck shortening, for complete threshed screw model, the screw appears more easy to penetrate into the acetabular joint. As a result, the femoral neck lock plates and InterTan CHS as the preferred fixed. Comprehensive analysis, the3lag screws in an inverted triangle position can be used in patients with good BMD, but for osteoporosis patient, proximal femoral screw and plate system are recommended. Pauwels type-3fractures, main force comes from the shearing force in the vertical direction, the principle of fixation is to maintain the stability of the fracture, two lag screws was not recommend in treating with these patient. On contrast, the femoral neck locking plate and InterTan CHS can be the recommended for treatment, but if patient with sever fractures fragmentation, in order to avoid avascular necrosis, femoral neck locking plate perhaps the only choice.