ABAQUS 6.9-1
ABAQUS has become an important tool in the assessment of patient-specific rupture-risk of abdominal aortic aneurysms (AAAs) here at the CABER. Patient-specific 3D reconstructions from medical datasets are imported into ABAQUS where realistic boundary conditions are applied to the 3D geometry. ABAQUS can then approximate the wall stress that the AAA would experience in vivo. This capability has allowed the CABER to develop tools capable of determining the actual likelihood of AAA rupture. The work performed on AAAs at the CABER has revealed that the maximum diameter criterion, which is the primary factor currently used to assess the severity of the AAA, may be unreliable. AAAs should therefore be analysed on a patient-specific basis. Figure 1 shows the wall stress computed in ABAQUS on an extremely diseased aorta.
Figure 1: 3D reconstruction of an aneurysmal aorta with a thoracic abdominal aorta aneurysm (TAA), abdominal aortic aneurysm (AAA) and iliac aorta aneurysm (IAA) generated from CT images. This reconstruction leads to the generation of a finite element mesh in ABAQUS and then the computed wall stress in the vessel. Shown also is the post-operative 3D reconstruction of the aorta with the implanted stent-graft in the thoracic abdominal aorta (TAA). (Doyle et al., Irish Journal of Medical Science 2009;178:321-328).
FEARI
A Finite Element Analysis Rupture Index (FEARI) was developed at the CABER and relies on the use of FEA to determine the rupture potential of patient-specific AAAs. The tool uses FEA to determine the patient-specific wall stress and couples this quantity with previously published data on the ultimate tensile strength of AAA tissue to return a simple parameter of rupture-risk. This index works whereby, 0 = low rupture-risk and 1 = high rupture-risk. It was shown that patient-specific rupture-risk is not accurately defined by the maximum diameter criterion. The new rupture-risk index is easy to interpret and can be readily incorporated into the clinical decision to surgically repair the AAA or continue with monitoring the growth of the AAA.
Figure 2: FEARI. Peak stress is computed using ABAQUS. Location of peak stress is coupled with a location-specific tensile strength (from literature) and returns a simple rupture-risk index (Doyle et al., Vascular Disease Prevention 2009;6:114-121).
MANDIBLE BIOMECHANICS
FEA is used to assess the biomechanics of the jaw bone in CABER. A 3D model is constructed from CT scan data using Mimics. In conjunction with ABAQUS, boundary conditions and realistic material properties are assigned to the model where appropriate. Various loading conditions of biting could therefore be examined such as static biting, uniform and left-side biting. Areas of fracture and mini-plate fixation are also examined. Resulting compressive/tensile stress and displacement of the model can then be assessed using ABAQUS. Thus far, the angle region, and symphysis region have been assessed biomechanically with current work involving the condyle of the mandible within the joint itself. A series of studies is thus produced which gives in depth analysis in the biomechanics of the jaw. This can be used further by clinicians to compile data pre-surgically.
Figure 3: Peak displacement of the mandible during static biting in the presence of an angle fracture (left) and a symphysis fracture (right). (Kavanagh et al., 2009)
HIP-REPLACEMENT STUDIES
CABER uses FEA to analyses both cemented and cementless hip prostheses. This is achieved using both static and dynamic realistic loading conditions from motion analysis studies carried out within CABER. These numerical models allow both the performance of the hip prostheses to be examined and also the effect of the realistic loading conditions on the femoral bone and cement mantle. Fatigue loading is also been used to study the life span of hip replacements and the long term effects certain everyday movements may have on promoting aseptic loosening and reducing the life of the hip replacement.
Figure 4: Peak stresses in an implanted hip prosthesis model. The model includes bone, cement and stainless steel prosthesis (O’Reilly, 2009).