| Summary: | 博士 === 國立陽明大學 === 醫學工程研究所 === 97 === Total hip arthroplasty is a successful surgery that fails at a rate of approximately 10% at ten years from surgery. Revision reasons varied according to the age, gender and diseases etc. Aseptic loosening of the femoral component is the main reason leading to femoral stem revised. Stovepipe canal shape which caused by aging is the main factor leading to aseptic loosening of cemented femoral component. Avascular necrosis of the femoral head is the main indication for total hip replacement in Asia. It decreased the density of cancellous bone and is considered as the main factor leading to aseptic loosening of cementless femoral component. There were many designs of femoral stem but can not avoid the aseptic loosening. The purpose of this study was, therefore, to find out the effects of femoral canal geometries, bone densities and stem shapes on the stresses of proximal femur and stability of femoral stem using finite element method. We expected that the results could provide suggestions for surgeons to select a femoral stem in patients with avascular necrosis of the femoral head or stovepipe canal before total hip arthroplasty.
Four finite element models of proximal femurs including normal and stovepipe femoral canal shapes were used to predict the failure of cement mantles for different stem shapes (Omnifit and Charnley cemented stem) and Young’s modulus of bone. Furthermore, two finite element models of proximal femur (normal canal shape) were used to evaluate the stability and stresses distribution of cementless femoral stem for different designs (VerSys and ABG) and Young’s modulus of cancellous bone. All models were simulated the loading of hip joint during heel strike of the gait cycle using the finite element analysis software (ANSYS 7.0).
The results on cemented stem analysis showed that high failure volume of cement mantle (8.2%) and tensile stress (73.5 MPa) occurred in the cement mantle of the stovepipe canal model with a low flare sem (Charnley) as compared with normal canal model (1.7% and 23.3 MPa). A high flare stem (Omnifit) reduced the peak stress and failure volume of the cement mantle not only in the stovepipe canal model but also in the normal canal model. Moreover, the peak stress and probability-of-failure of the cement mantle were sensitively with the decrease of bone density (Young’s modulus) in the stovepipe canal models. The results on cementless stem analysis showed that in the model with VerSys stem (Straight type), the peak micromotion between bone and stem was 135.7μm, and 0% of contact area at the bone/stem interface exceeded 150μm. In the model with ABG stem (Anatomic type), the peak micromotion between bone and stem was 199.25μm, and 2.2% of contact area at the bone/stem interface exceeded 150μm. The peak micromotion at the bone/stem interface was increased when the cancellous bone quality (Young’s modulus) was decreased whether implant with straight or anatomic stem. The peak micromotion was lower in the model with straight stem when decreased the modulus of cancellous bone as compared with the model with anatomic stem.
We concluded that the femoral canal shape and bone quality of patients should be considered before hip replacement surgery. A high flare, cemented femoral stem decreased the stress in the cement mantle around the stem tip. It should be considered as a better type of femoral prosthesis for patients with stovepipe canal shape or inferior bone quality. Because the cement mantle around the stem tip is thicker than proximal due to the wide isthmus of stovepipe canal after implanted a regular cemented stem. It decreased the stiffness of femur model at the region of stem tip and then increased the stress in distal cement mantle when loading occured. For young, non-trauma avascular necrosis of the femoral head patients, a cementless, straight stem which fill the isthmus is better than anatomic stem which fill the proximal part of the canal only. Because the weak cancellous bone can not resist the loading from the anatomic stem and increased the micromotion at stem/bone interface.
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