Peer-review started: December 29, 2013
First decision: February 13, 2014
Revised: March 25, 2014
Accepted: May 14, 2014
Article in press: May 14, 2014
Published online: January 18, 2015
Hip 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.
Core tip: In this paper, the biotribology of hip implants is described at different levels, from the more general definitions of friction, lubrication and wear and from some basic equations, to clinical data of different implants. The topic is presented both qualitatively and quantitatively, which we believe is an original approach for a review of this kind. Some simple mathematical tools are provided, which can also be useful for non-specialists to better understand the matter and to deal with simple tribological problems. Finally, the trends of artificial hip joints over the last ten years are discussed on the basis of tribological concepts.