| Objective: The Finite Element Analysis is the method in which a continuous system is divided into limited unites or departments and the approximate solution to each unit is found, then, according to some standard criterion , all of the unites are assembled together to make a whole system that assimilates to the original system. When design a new hip prosthesis, one must consider the stress condition of it. And the stress condition has been shown to have a great effect on the behavior and life of a hip prosthesis. The first part of this experiment will analyze the effect of head-neck angle on the hip prosthesis stress. Fretting is described as the relative micromotion between two materials (low scale oscillate sliding), and at least one of the two materials is subjected to cyclic mechanical loading. The head-neck junction of the femoral prosthesis has been proved to have this damage of fretting, and it is the origin of a series of damage to the prosthesis. Fretting causes a local disruption of the passivation surface layer exposing new metal to the aggressive body environment. The most important consequence is the release of metal ions and small particles. Fretting often leads to other corrosion conditions, for example, crevice corrosion, pitting or galling. Fretting can also reduce the fatigue life of the metal. In spite of the small surface area involved in a taper joint, as compared to the huge articulating surfaces of afretting on the long-term clinical outcome of total hip arthroplasty. In spite of evidence that fretting plays a role in the corrosion of taper joints in modular hip implants, the statistics of the retrieval studies is too poor for a quantitative assessment. Moreover, the complexity of interacting mechanical loading, surface damage and corrosion leads to appreciable difficulties in the experimental description of the phenomenon and the terminology used . According to these, in the second part of this experiment , we studied the corrosion caused by the relative motion at the head-neck junction under the load of the hip, emphasis on the effect of four variables (taper angle, head diameter, head material and neck material) , using the finite element analysis method. And we have got at some conclusion that will be helpful in prosthesis -designing. Methods: In the first part of this experiment , the three-dimensional model of a femoral prosthesis was drawn using the CAD software(Unigraphic NX3. 0).Then the model was imported into a finite element analysis software(ANSYS9.0) and the relationship between the angle of neck-stem and the stress on the prosthesis was analyzed. In the second part of this experiment, the three-dimensional model of the head-neck junction in a modular hip prosthesis was drawn using a CAD software (Unigraphic NX3. 0). Then the model was imported into a finite element analysis software (ANSYS 9. 0), and in this analysis software, the model was automatically meshed; the loading condition, the material of the head and of the neck was defined. Using the numerous calculating ability of this software, we analyzed the fretting of the head-neck junction under different conditions, and compare the relative micromotion distance of thehead-neck junction in different taper angle, head size and head/neck material. The results were compared with the clinical practice. Results: The results of experiment one show that there is stress concentration at the neck and the proximal stem of the prostheses, and when the neck-stem angle is about 136 degree, the stress distribution will be most beneficial. The result of experiment two shows that: for any given head size and head/neck material , fretting will increase with the increase of the taper angle; for any combination of head material and neck material, fretting motion decreases with increase in head size ; for a given combination of head material and head size, the neck material exerts a strong effect on the fretting motion, which is to say as the modulus of elasticity of the neck material increases, fretting motion decreases; for a given combination of neck material and head size, the modulus of elasticity of the head material also has significant effect on fretting motion, that as the modulus of elasticity of head material increase, fretting motion decrease. This is compatible with the clinical results. Conclusions: The results of my study show that it is very practicable and reliable using the finite element analysis in evaluating the stress condition of hip prosthesis and the fretting at the taper of modular hip. There is stress concentration at the medial side of the neck and proximal stem of the prostheses. And it is reasonable for hip-prosthesis designer to achieve a prosthesis that can avoid stress shielding and strengthen the neck. The study shows that when the neck-stem angle is about 136 degree, the distribution will be most beneficial. And we draw the conclusion that a smaller amount of fretting motion at the... |
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