1.1 Inclusion and Exclusion Criteria
Included in the standard:(1) Patients with severe osteoporotic intertrochanteric fracture; (2) According to the Evans classification of intertrochanteric fractures, type ⅱ; (3) Age > 65 years.
Exclusion criteria :(1) the patient had a history of previous surgery; (2) metabolic bone disease and congenital bone variation; (3) Tumor history; (4) Taking drugs affecting bone metabolism in recent years.
1.2 General Information
A prospective study. One patient with intertrochanteric fracture of the left femur was admitted to the Orthopaedic Center of the First Affiliated Hospital of Xinjiang Medical University on October 5,2020. The patient was a 73-year-old female with a height of 160 cm,weight 56 kg, and BMI of 22 kg/m2. The time from fall injury to admission was 3 hours, and the patient chief complained with with left hip pain and limited activity. X-ray examination on admission revealed a fracture of the left intertrochanteric femur. Bone mineral density test: T-4.3, Z-2.0, indicating severe osteoporosis. The patient had no underlying medical diseases such as hypertension, diabetes, and cardiovascular disease .
This study was approved by the Ethics Committee of the First Affiliated Hospital of Xinjiang Medical University (20201106-11). The patient understood the purpose of the trial and signed the informed consent form.
1.3 CT Examination Equipment and Methods
GE 64 - slice spiral CT machine was used. The patient was placed in the supine position and the scan range was bilateral from the hip to the upper leg. Scanning parameters: tube voltage 120 kV, tube current 250 mA, layer thickness 2 mm, layer distance 5 mm, matrix 512×512.
1.4 Data Processing
Import CT data into Lenovo P52 graphics workstation;Mimics 20 software (Materialise, Belgium);Geomagic Wrap 2017 software (Geomagic, USA);Solidworks 2017 software (Dassault, USA) were used Systemes);3-Matic Medical 11.0 Software (Materialise, Belgium);Maya 2011 software (AOtudesk, USA);ZBrush software (Pixologic, USA);ANSYS2019 software (ANYSYS, USA);Industrial design UG software (Siemens, Germany) for data processing.
1.5 Right Femur Model was Established
The CT data of the volunteers were imported into MIMics 20 software, and the "Bone" threshold was selected to identify the femur. The 3D model of the right femur was obtained after the steps of editing mask → layer filling →3D extraction → rounded processing, and the model was stored in STL format and imported into Geomagic Wrap 2017 software. After optimizing the topology surface through network doctor, storing STL file, importing 3-Matic Medical 11.0 software, The Adaptive Remesh tool under Remesh software was used to re-optimize the topological model of the femur model to achieve a triangular structure with equal area of polygons that could be matched by the finite element. Use the Create Volume Mshh parameter "Eement Type (Tet4) Maximum edge length (3.0)" in Remesh to Create the Mshh. In the FEA Mesh window, select the Material Assignment tool, and use the grayscale values (Material Assignmet mehod: Gray value based) is used as a reference, and the material attribute data of the inner reference finite element model corresponds to the parameters filled in.
1.6 Simulated Bone Cement Modeling
The STL file was exported from the 3D model of the right femur built in Mimics 2.0, and the OBJ file was exported into Geomagic Wrap 2017 software. The Polygons model was created by using Maya 2011 software to find the location of bone cement in the 3D model of the right femur. Use the face selection tool to randomly select the surface of the spherical model, use the extrusion tool to randomly fill in the height parameters and export the edited Polygons model, import the right femur model into ZBrush, and use the Dynamesh tool to grid edit the spherical model in the assembly editor. Brush tools Clay and Move were used to gradually adjust the shape and the length range of bone cement extension, and then the deformation tool was used to align and make the surface rough by noise processing. Then the OBJ model is exported and imported into Geomagic Wrap 2017 software to convert STL data into Mimics 20 to view the relative position.
1.7 Establishment of PFNA Model
The PFNA screw was modeled by Solidworks 2017 software, and the PFNA internal fixation model was obtained by comparing the standard AO proximal femoral anti-rotation intramedullary nail with the surgical assembly standard using Autodesk MAYA software. Main parameters of the PFNA spiral blade model: nail length is 105 mm, diameter is 10mm. Main parameters of PFNA nail: length 170 mm, main parameters of PFNA nail and spiral blade assembly model: Angle of 130°, valgus Angle of 5°.
1.8 Establish the Reinforced Screw Model
UG modeling module was used to insert the sketch in the plane to draw the two-dimensional dimension of the screw. The radius of the lower end of the screw was 8.9mm, the chamfer number was 105.7°, the chamfer length was 20mm, the total length of the lower end was 143mm, the radius of the upper end was 12.6mm, and the chamfer Angle was 102.3°. The Angle between the lower end and the middle line of the upper end is 5°, and the screw thread is evenly opened to complete the sketch, as shown in Figure 1. Then, the screw contour was obtained by rotating the sketch with the rotation command, and the corresponding hole was made by using the hole command at the upper height of 69.1mm from the lower end, and the conventional hole diameter was 6mm. After the completion, the STL format could be exported.
1.9 Construct PFNA model of Intertrochanteric Fracture of Femur
The 3D model of right femur was osteotomized according to the standard intertrochanteric EvansII fracture in 3-Matic software to obtain the intertrochanteric EvansII fracture model, which was assembled and combined with the PFNA internal fixation model according to the standard surgical techniques in Solidworks 2017 software. Standard Tip Apex distance (TAD) ≤25 mm was set and stored in STL format. The grid was divided by network Doctor in Geomagic Wrap 2017 software to check its good performance. In order to reduce computation, the distal femur was deleted, and the friction between each part was simplified as the friction between the surface and the surface, and the corresponding friction coefficients were set: 0.46 between bone and bone, 0.30 between bone and internal fixation, and 0.23 between screw and main screw, which were stored as model A [19].
1.10 Material, Boundary and other Parameters
Model A was set according to the finite element analysis of contralateral cortical locking screw in the treatment of osteoporotic femoral fractures by Nie SB, Zhao YP , Li JT et al. [20]. Cancellous bone, cortical bone and other materials were set according to the CT data of volunteers with severe osteoporosis. The model was loaded according to the biomechanics and body weight of the volunteer patients. A force of 560N was applied linearly, the load direction was 10° with the long axis of the femoral shaft and 90° with the spherical surface of the femoral head, and the unit model of the farthest part of the bone was bound to ensure stability during operation [20-21]. In corresponding model A, part of cancellous bone around the spiral blade of common PFNA screws was set as bone cement components and stored as model B, as shown in Figure 2. The apparent density, elastic modulus, Poisson's ratio and yield stress parameters of model B's bone cement material are shown in Table 1 [22-28].
1.11 Construction of Enhanced Screw Model for Intertrochanteric Fracture of Femur
In 3-MATIC software, the 3D model of right femur was osteotomized according to the standard intertrochanteric EvansII fracture to obtain the intertrochanteric EvansII fracture model, which was assembled and combined with the enhanced screw internal fixation model according to PFNA standard surgical techniques in Solidworks 2017 software. Standard Tip Apex distance (TAD) ≤25 mm was set and stored in STL format. The grid was divided by network Doctor in Geomagic Wrap 2017 software to check its good performance. In order to reduce computation, the distal femur was deleted, and the friction between each part was simplified as the friction between the surface and the surface, and the corresponding friction coefficients were set: 0.46 between bone and bone, 0.30 between bone and internal fixation, and 0.23 between screw and main screw, which were stored as the C model [19]. Similarly, the model was loaded according to biomechanics and the body weight of the volunteer patients. The load direction was 10° with the long axis of the femoral shaft and 90° with the spherical surface of the femoral head. The unit model of the farthest part of the bone was bound to ensure stability during operation. In model C, the cancellous bone around the proximal end of the enhanced screw was set as the bone cement component, as shown in Figure 2 [23]. All parameters of model C's bone cement materials remain unchanged, as shown in Table 1 for reference.
Table 1. Material attribute parameters of each part of the model
Materials
|
Density
|
Dodulus of Elasticity (MPa)
|
Yield Stress(MPa)
|
Poisson's Ratio
|
Tangent Modulus(MPa)
|
The Hardeni-ng Paramet-er β
|
Strain Rate Parameter
|
Failure Strain Rates
(%)
|
g/cm3
|
C
|
P
|
Cortical Bone
|
1.8
|
15000
|
109
|
0.37
|
1500
|
0.10
|
2.5
|
7
|
0.9
|
Spongy Bone
|
0.2
|
105.20
|
0.24
|
0.40
|
10.52
|
0.10
|
2.5
|
7
|
0.7
|
PFNA
|
4.43
|
110000
|
850
|
0.35
|
1590
|
0.12
|
80000
|
1.1
|
22
|
bone cement
|
1.18
|
220000
|
100
|
0.2
|
一
|
一
|
一
|
一
|
一
|
Strengt-hen the Screw
|
4.43
|
110000
|
850
|
0.35
|
1590
|
0.12
|
80000
|
1.1
|
22
|
1.12 Operation and Comparison Index
The Explicit Dynamics module in ansys2019 software was used to analyze the K files respectively. The main observation indicators were as follows: (1) by comparing the amount of deformation, collapse, and disappearance of cancellous bone element mesh before and after the femoral head by the screw bone cement surface, the degree of cutting of cancellous bone at the proximal femoral head by the bone cement surface of the enhanced screw model was compared; (2) The femoral neck varus Angle was calculated according to the contact area between the femoral head and neck bone block and the femoral lesser trochanter bone block and the disappearance of the cortical bone element grid in the part of the lower trochanter bone under compression. (3) With the distal femoral shaft as the reference, the femoral neck internal rotation Angle was calculated according to the overlap, collapse, and disappearance of the medial cortical bone units at the proximal femoral neck under afterloading. (4) The surface stress distribution of proximal femoral screws in different screw models was observed under afterloading. (5) The distance difference between the proximal apex of the femoral head and the femoral shaft before and after the afterload was measured, which was the displacement of the proximal femoral head.