Analysis Of Finite Element Modeling On Reconstruction Of Acetabulum With Bone Defect

Recently, with the development of the artificial prosthesis, materials and improvement of surgical technique, the number of patient had total hip arthroplasty (THA) and total hip revision (THR) increased urgently. These procedures solved the patients’disease well, while also followed a series challenge and complex problems need surgeons to defeat. And the acetabulum reconstruction with bone defect is one of the challenges during THA and THR. There were many methods to deal with acetabular bone defect. The treatment and the outcome varied from the classification of the bone defect and the master of the surgeons. Finite element analysis (FEA) through established the bone and joint three-dimensional finite element model to simulate the normal bone and joint mechanical characters, analysis the innovation and failure of surgery, had many advantages compared with traditional biomechanical research methods, therefore became one of the popular mechanical research in orthopedics. As to lots of methods to treat acetabular bone defect, we chose two common methods:high hip center technique and reinforcement ring with hook (Ganz ring) to investigate anatomical data of the ilium above the acetabular dome by three-dimensional method, and the biomechanics of high hip center and reinforcement ring with hook by FEA methods, providing theoretical support of these two methods.The first section was to evaluate the thickness and width of the human ilium and related acetabular cup coverage for guiding acetabular component placement in high hip center technique.120normal hips in60 cases of adult patients from lower extremities CTA DICOM data was chosen for the study. After importing the data to the mimics software, we chose the cross sections every5-mm increments from the rotational center of the hip to the cephalic of the ilium according the body sagittal axis, then we measured the thickness and width of the ilium for each cross section in axial plane, calculated the cup coverage at each chosen section. At the acetabular dome, the mean thickness and width of the ilium was49.71mm (±4.88mm) and38.92mm (±3.67mm) respectively, whereas at1cm above the dome, decreased to41.35mm (±5.13mm) and31.13mm (±3.37mm) respectively, and2cm above the dome, decreased to31.25mm (±4.04mm) and26.65mm (±3.43mm) respectively. Acetabular cup averaged coverage for40-,50-, and60-mm hemispheric shells, were100%,89%, and44%at the acetabular dome,100%,43.7%, and27.5%for1cm above the dome, and37.5%,21.9%, and14.2%for2cm above the dome, and14.9%,9.0%and6.1%for3cm above the dome. High hip center reconstructions within1cm above the acetabular dome will be acceptable and smaller diameter prosthesis would be better.The second section was to investigate the mechanical change of acetabular cup varied from different distance of high hip center technique using the finite element method, providing theoretical support for clinical decision of HHC technique. The standard hip of Sawbone CT DICOM data was chosen for the study. After importing the data to the mimics software, vector processed, and import to MSC/PATRAN, formatted the finite element model of the pelvis. Established different model with every5mm elevated high hip center from acetabular dome. The inclination and antevertion of acetabular component is designed for45°and10°in all the five models. The displacement and Von Mises stress of acetabular component and the acetabulum increased significantly compared with native acetabular reconstruction, but the value were not bigger, therefore, we can choose high hip center technique during THA in suitable cases.The third section was to determine the effect of the severity of acetabular dysplasia, number and positions of screws and type of bone graft material used on the acetabular reinforcement ring with hook (Ganz ring) by the finite element method. The standard hip of Sawbone CT DICOM data was chosen for the study. After importing the data to the mimics software, vector processed, and import to MSC Marc, formatted the finite element model of the pelvis. Ganz ring was fixed in acetabulum with cancellous bone screw. An acetabular cup was mounted on the Ganz ring with a lateral opening angle of45degrees and an anteversion angle of15degrees, assuming cement fixation to be1mm in thickness. The femoral head was modeled as a hemisphere and bonded with the cup. The results show that the stability of the cup decreased as the severity of acetabular dysplasia increased while Von Mises stress increased and tended to increase as the number of screws increased, but varied according to screw placement position. Increased strength of the bone graft material led to increased cup stability and decreased Von Mises stress. Biomechanically, the Ganz ring can be placed securely using3screws in patients with Crowe I dysplasia. However, in patients with Crowe Ⅱ or higher dysplasia, it is necessary to spread at least4screws across an area of good host bone.According to the three investigations we recommended that as long as the correct indications and the delicate surgical technique, HHC technique was a good solution of acetabular bone defect, as to the anatomical basis and biomechanics are reasonable. The Ganz ring and also offered an acceptable initial stability with suitable screws fixation and bone graft material.