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1、 Artificial hip joint1. IntroductionIt has been recognised by a good number of researchers that the computation of the pressure distribution and contact area of artificial hip joints during daily activities can play a key role in predicting prosthetic implant wear 1, 2, 3 and 4. The Hertzian contact
2、 theory has been considered to evaluate the contact parameters, namely the maximum contact pressure and contact area by using the finite element method 1 and 2. Mak and his co-workers 1 studied the contact mechanics in ceramic-on-ceramic (CoC) hip implants subjected to micro-separation and it was sh
3、own that contact stress increased due to edge loading and it was mainly dependent on the magnitude of cup-liner separation, the radial clearance and the cup inclination angle 3 and 4. In fact, Hertzian contact theory can captured slope and curvature trends associated with contact patch geometry subj
4、ected to the applied load to predict the contact dimensions accurately in edge-loaded ceramic-on-ceramic hips 5. Although the finite element analysis is a popular approach for investigating contact mechanics, discrete element technique has also been employed to predict contact pressure in hip joints
5、 6. As computational instability can occur when the contact nodes move near the edges of the contact elements, a contact smoothing approach by applying Gregory patches was suggested 7. Moreover, the contributions of individual muscles and the effect of different gait patterns on hip contact forces a
6、re of interest, which can be determined by using optimisation techniques and inverse dynamic analyses 8 and 9. In addition, contact stress and local temperature at the contact region of dry-sliding couples during wear tests of CoC femoral heads can experimentally be assessed by applying fluorescence
7、 microprobe spectroscopy 10. The contact pressure distribution on the joint bearing surfaces can be used to determine the heat generated by friction and the volumetric wear of artificial hip joints 11 and 12. Artificial hip joint moment due to friction and the kinetics of hip implant components may
8、cause prosthetic implant components to loosen, which is one of the main causes of failure of hip replacements. Knee and hip joints moment values during stair up and sit-to-stand motions can be evaluated computationally 13. The effect of both body-weight-support level and walking speed was investigat
9、ed on mean peak internal joint moments at ankle, knee and hip 14. However, in-vivo study of the friction moments acting on the hip demands more research in order to assess whether those findings could be generalised was carried out 15.The hypothesis of the present study is that friction-induced vibr
10、ation and stick/slip friction could affect maximum contact pressure and moment of artificial hip joints. This desideratum is achieved by developing a multibody dynamic model that is able to cope with the usual difficulties of available models due to the presence of muscles, tendons and ligaments, pr
11、oposing a simple dynamic body diagram of hip implant. For this purpose, a cross section through the interface of ball, stem and lateral soft and stiff tissues is considered to provide the free body diagram of the hip joint. In this approach, the ball is moving, while the cup is considered to be stat
12、ionary. Furthermore, the multibody dynamic motion of the ball is formulated, taking the friction-induced vibration and the contact forces developed during the interaction with cup surface. In this study, the model utilises available information of forces acting at the ball centre, as well as angular
13、 rotation of the ball as functions of time during a normal walking cycle. Since the rotation angle of the femoral head and their first and second derivatives are known, the equation of angular momentum could be solved to compute external joint moment acting at the ball centre. The nonlinear governin
14、g equations of motion are solved by employing the adaptive RungeKuttaFehlberg method, which allows for the discretisation of the time interval of interest. The influence of initial position of ball with respect to cup centre on both maximum contact pressure and the corresponding ball trajectory of h
15、ip implants during a normal walking cycle are investigated. Moreover, the effects of clearance size, initial conditions and friction on the system dynamic response are analysed and discussed throughout this work.2. Multibody dynamic model of the artificial hip jointThe multibody dynamic model origin
16、aly proposed by Askari et al. 16 has been considered here to address the problem of evaluating the contact pressure and moment of hip implants. A cross section A-A of a generic configuration of a hip joint is depicted in the diagram of in Fig. 1, which represents a total hip replacement. Fig. 1 also shows the head and cup placed inside of the pelvis and separated from stem and neck. The forces developed along the interface of the ball
