Taper Wear in Large Head Metal-on-Metal Hips

Introduction: Large Head Metal on Metal hips were introduced with the promise of lower wear rates, reduced chance of dislocation and an increased range of motion compared to conventional MoM hips. However, the 2010 National Joint Registry reports a 5 year failure rate of LHMoM hips of 7.8% compared to 6.3% for hip resurfacings and comparisons of well functioning hips have shown the LHMoM have significantly increased metal ion levels compared to resurfacings. These differences are despite LHMoM hips and resurfacings having identical bearings surfaces; thus the difference in in-vivo performance must be related to the modular head / neck taper junction. This study aims to examine the taper junctions of explanted LHMoM to try and understand the increased failure and metal ion levels. Method: The internal taper inside the modular head and external taper on the femoral neck were examined using a Coordinate Measuring Machine (CMM), Roundness Machine and Optical Profilometer. The taper angle, cylindricity and wear were measured using the CMM and roundness machine, while the surface topography was measured using the Roundness machine (with diamond stylus) and optical profilometer. The surface topography was correlated with the wear measurements. Results and Discussion: There was a range of wear rates measured for the tapers ranging from the un-measureable to a 2 - 3 mm3, which is a similar magnitude to the volumetric wear from the bearing surface for typical hip joints. Examination of the surface topography revealed that in some cases the ridged surface texture of the neck had been worn into the surface of the inside taper of the head. Examination of the taper angles, revealed that the head was a more acute angle than the stem, suggesting that the contact will occur near the end of the neck, in the centre of the head. Conclusion: Taper wear of LHMoM hips is a significant problem in some cases and may contribute to the elevated failure rate. The examination of explanted tapers provides a valuable insight into the in vivo behaviour of taper junctions and the wear mechanisms

Electrochemical Transfer of S Between Molten Steel and Molten Slag

S transfer between molten steel and molten slag was investigated in view of the electrochemical character of S transfer. C-saturated molten steel containing S was allowed to react with CaO-SiO2-Al2O3-MgO slag at 1673 K (1400 degrees C) until the two phases arrive at a chemical equilibrium. The application of an electric field of constant current through graphite electrodes lowered the S content in the molten steel below its chemical equilibrium level, and the system arrived at a new equilibrium level (electrochemical equilibrium). However, subsequent shutting off of the electric field did not lead to the system reverting to the original chemical equilibrium: reversion of S was observed but to a limited extent. The application of an electric field of opposite direction or flowing of CO gas allowed significant reversion of S. Side reactions (decomposition of oxide components) were observed, and these were considered to be coupled to the transfer of S. An electrochemical reaction mechanism was proposed based on the experimental observations found in the present study. (C) The Minerals, Metals & Materials Society and ASM International 201811Nsciescopu

Quasi-chemical viscosity model for fully liquid slag in the Al2O3-CaO-MgO-SiO2 system. Part II: evaluation of slag viscosities

A model is presented that enables viscosities to be predicted reliably over the whole range of compositions and temperatures in the AlO -CaO-MgO-SiO slag system above liquidus in the temperature range from 1543 K to 2643 K (1270°C to 2370°C). Experimental procedures and data from the studies reported in the literature have been collected and critically reviewed with particular attention to the viscometry methods and possible contamination of slag samples to select reliable data points for further model development. Relevant revised formalism to describe the complex viscosity trends including charge-compensation effect of the Ca and Mg cations on the formation of tetrahedrally coordinated Al was introduced. Parameters of the quasi-chemical viscosity model have been optimized to reproduce within experimental uncertainties most of the selected experimental data in the AlO -CaO-MgO-SiO system and all subsystems. This study is part of the overall development of the self-consistent viscosity model of the AlO-CaO-FeO-FeO- MgO-Na O-SiO multicomponent slag system