ADCOM Applied Computational Mechanics

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			Hadassah University Hospital

		Nonlinear Finite Element Analysis of Dynamic Hip Screw within a fractured femur

The study was carried out by the Orthopedic & Medical Engineering Departments

Analysis Goal:
Dynamic hip screws are widely used by surgeons as a treatment (internal fixation) for proximal femur fracture. The DHS comprises a plate fixed to the lateral aspect of the proximal femur (usually by four screws), incorporating a barrel to accommodate a sliding leg screw which inserts into the femoral head. Figure 1 shows an x-ray of a typical DHS implant. The goal of the current study is to compare two types of DHS, shown in Figure 2, with respect to the probability of bone fracture. The first DHS has a long plate and four cortical bone screws piercing the cortex at eight points. The second DHS has a shorter plate and only two cortical bone screws, oriented at 45 degrees with respect to the first DHS, piercing the cortex at four points. The motivation to use the second DHS is to reduce trauma to the patient (minimize blood loss, surgery time etc.).

Figure 1

Figure 2


Simulation Details: The femur geometry was built from a serious of CT images that were converted to lines representing the inner and outer boundaries of the femur. This is shown in figure 3. In order to generate three dimensional mesh, areas were defined from lines and used to define meshable volumes. See figure 4 for illustration (volumes are colored for clarity). Since the focus of this study is the level of stress developing in the region where the bone screws pierce the cortex, the FE model contains the femur geometry only up to the fracture site. The fracture line was defined at 70 degrees with respect to the transverse axis of the femoral shaft. The DHS plate geometry was defined by the manufacturer and revised to meet meshing requirements. Two FE models were built to match the cases shown in figure 2. The models differ in the number of screws, their orientation and the DHS plate length. The models are shown in figure 5 and 6.	Figure 3 Figure 4

The bone, plate and screws are meshed with 3D brick elements. Figure 7 shows a region of the bone and screw mesh. The load transfer between the DHS plate and bone is realized by defining contact surfaces between the cortical bone screws and the cortex holes. The cortical bone material is modeled as isotropic nonlinear with a bilinear constitutive law. The material follows a linear stress-strain relationship until it yields and then a reduced linear stress-strain relationship thereafter.

Figure 7

Figures 8 and 9 show the stress field obtained at the four screw and 2 screw models respectively.

Figure 8

Figure 9

Results and Benefits
The FEA study was proven to be a valuable tool in predicting a possible bone fracture resulting from a specific DHS device configuration. This information is based on the stress and strain field obtained from the FEA.