TTU Home COE Home Mechanical Engineering Orthopaedic Biomechanics Laboratory Projects Analysis of Fixation Devices for Distal Femoral Fractures

Analysis of fixation devices for distal femoral fractures

Background

The human femur is the longest bone in the human body and is capable of bearing loads of considerable magnitude. When the structural integrity of the femur is compromised by a fracture of either high or low energy, it can pose a significant surgical challenge to treat. Not only are these fractures ones of an articular nature – occurring in close proximity to the knee joint – but they are often very complicated breaks, resulting in many fragmented segments of bone that serve zero structural support to the femoral construction. The standard operating procedure for orthopaedic surgeons treating said fractures is to select – depending on the circumstances of the situation – one of several internal fixation devices which regularly include 1) intramedullary nails, 2) compression plates, and 3) locked plating systems.

Experimental Component

Student and faculty researchers have been in collaboration with orthopaedic surgeons of the Texas Tech University Health Sciences Center to analyze the mechanical performance of internal fixation devices for fractures of the distal femur. Currently under evaluation are the following three devices: 1) the 95° angled blade plate (ABP), 2) the locking compression plate (LCP), and 3) the less invasive stabilization system (LISS). These metallic implants are illustrated on synthetic Sawbone® femurs below in Figures 1, 2, and 3, respectively.

All Sawbone® femurs have had a 1 cm osteotomy applied in the region superior to the femoral condyles in order to simulate fracture. Every construct will be experimentally evaluated in an Instron® materials testing system. Our experimental setup is shown below in Figure 4.

Also included in Figure 4 are close-up views of the superior and inferior testing fixtures: one is used to simulate the acetabulum of the hip while the other replicates the femoral condyles. Cerrobend®, a low-melting-point alloy, was used to create the condylar mold shown in the lower right-hand corner of Figure 4. Strain gauges have also been placed at various locations on both the implant and the synthetic bone itself. Experimental testing modes have included static compression/bending and cyclic compression/bending.

Computational Component

Computational evaluation of the constructs is also being conducted, in part to co-validate the experimental results and also to potentially serve as a starting point for the development of a standardized set of loads and boundary conditions which can be used in the future to reduce the time-to-market for this type of implant. 

The computational evaluation is being conducted using ABAQUS, a finite element analysis package. The femur model which is being used in the computational evaluation was obtained from the repository of the Biomechanics European Laboratory CAD and was originally produced using CT scans of the same type of Sawbone® femur which is being used in the experimental evaluation.

The loading conditions for the computational evaluation are being modeled as accurately as possible to mimic the experimental setup. Just as in the experimental evaluation, the maximum loading for static compression/bending analyses is approximately 50% of body weight for an 80-kg individual. Strain data from the experimental evaluation will be compared to the computational evaluation in order to validate the analyses. Once the constructs have been experimentally tested to failure, the failure load will be used in simulation to allow for comparison of failure modes in the two separate evaluations.

The material properties of the bone have been approximated using an isotropic, elastic definition and simulations have been performed using material constants for both real human cortical bone and the e-glass filled epoxy which is used on the Sawbone® to simulate cortical bone. Recently, cancellous bone has been added to the femur model to increase accuracy. When under load, the screws in the LISS and LCP plates and in the blade on the ABP distribute loading on cortical and cancellous bone, thus the inclusion of the cancellous bone has been a necessary step in refining the analyses. Figure 5 shows the result of a preliminary static analysis on the LISS construct (femur not shown).