1,721,036 research outputs found
Biotribology of artificial hip joints
Full text linkHip arthroplasty can be considered one of the major successes of orthopedic surgery, with more than 350000 replacements performed every year in the United States with a constantly increasing rate. The main limitations to the lifespan of these devices are due to tribological aspects, in particular the wear of mating surfaces, which implies a loss of matter and modification of surface geometry. However, wear is a complex phenomenon, also involving lubrication and friction. The present paper deals with the tribological performance of hip implants and is organized in to three main sections. Firstly, the basic elements of tribology are presented, from contact mechanics of ball-in-socket joints to ultra high molecular weight polyethylene wear laws. Some fundamental equations are also reported, with the aim of providing the reader with some simple tools for tribological investigations. In the second section, the focus moves to artificial hip joints, defining materials and geometrical properties and discussing their friction, lubrication and wear characteristics. In particular, the features of different couplings, from metal-on-plastic to metal-on-metal and ceramic-on-ceramic, are discussed as well as the role of the head radius and clearance. How friction, lubrication and wear are interconnected and most of all how they are specific for each loading and kinematic condition is highlighted. Thus, the significant differences in patients and their lifestyles account for the high dispersion of clinical data. Furthermore, such consideration has raised a new discussion on the most suitable in vitro tests for hip implants as simplified gait cycles can be too far from effective implant working conditions. In the third section, the trends of hip implants in the years from 2003 to 2012 provided by the National Joint Registry of England, Wales and Northern Ireland are summarized and commented on in a discussion
Influence of the wear partition factor on wear evolution modelling of sliding surfaces
Full text linkWear of engineering components is crucial to assess their performance during all their service life. Numerical wear models are a promising tool, cheaper and quicker than experimental tests, both to investigate wear effects and to compare design solutions. However, frequently, numerical models assume that only one body gets worn or both elements undergo the same volume loss.
This study proposes a generalization of the Archard wear law, introducing the concept of wear partition factor to take into account a different wear behaviour of the rubbing elements of a coupling. The proposed approach is applied to the case of a cylinder sliding over a plane with different stroke amplitudes st. A numerical wear model has been developed in Abaqus®, exploiting the UMESHMOTION routine. Implementation procedures are described and discussed along with the model convergence. Twenty combinations of and st were simulated covering the cases both of unilateral/bilateral wear and fretting/sliding wear. Results provide important indications on the evolution of wear volumes, wear profiles and contact variables with travelled distance, revealing the remarkable role of .
The present study aims to an improved understanding and modelling of sliding wear evolution thus clarifying some critical issues slightly discussed by the literature
FE analyses of edge loading in ceramic-on-ceramic hip replacements with different rim design
No full textThis study presents a comprehensive numerical investigation on contact mechanics of ceramic-on-ceramic hip replacements under edge-loading condition. In particular, the combined effect of implant positioning and design on the contact pressure is analysed for different load levels. The final aim is to identify design/configuration solutions which minimize the risks associated
to edge-loading
A novel approach to the estimation and application of the wear coefficient of metal-on-metal hip implants
Full text linkA novel approach is proposed to estimate and model the wear of metal-on-metal hip implants. The approach is based on two distinct wear coefficients for the head and cup, derived from separate measurements on the two components. This is in contrast to the usual assumption that a single wear coefficient (k) is valid for both bodies. Actually, the head and cup do not wear equally; thus, assuming equal wear leads to predictive errors. Additionally, in most papers, k is chosen considering only implant materials while neglecting geometry and testing conditions. It is suggested that experimental procedures designed for hip implants should measure the head and cup volume losses separately and that wear maps should be provided to validate numerical models
Simple analytical description of contact pressure and wear evolution in non-conformal contacts
Full text linkThe present study proposes an analytical approach to describe contact pressure and wear evolution in line and point contacts. Starting from the unworn condition, described by Hertz theory, modifications of geometry and pressure distribution due to wear are included. Under the basic assumption of parabolic wear and pressure profiles, derived from Finite Element simulations, simple equations governing the phenomenon are derived, where the maximum wear depth evolution is described by a first order differential equation which can be easily solved. Interestingly, the maximum pressure remains dependent on the radius of curvature at the nominal contact point according to Hertz theory, but pressure is not null at the extremes of the contact region. The contact width can be derived from equilibrium conditions and wear law. The reliability of the procedure is proved by the perfect agreement with Finite Element simulations
A comparative study of wear laws for soft-on-hard hip implants using a mathematical wear model
Full text linkWear of UHMWPE acetabular components is the most serious threat to the long-term success of hip replacements. Consequently numerical and experimental wear simulations are of great interest. The present study proposes a mathematical wear model and compares the most recent wear laws, based on the so-called cross-shear (CS) effect. Simulations highlighted the variability of wear predictions with the wear factors/laws. Moreover a sensitivity analysis underlined that the kinematic conditions affect volumetric/linear wear more than the loading ones. This study confirms the importance of the CS in wear predictions even if some critical issues are still open, requiring further investigations
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