Sarah Green

XPS characterisation of surface modified Ni-Ti shape memory alloy

Ni-Ti shape memory alloy has been surface amorphised by N+ ion implantation and by controlled shot peening in order to improve surface mechanical properties. X-ray photoelectron spectroscopy (XPS) and surface wetting measurements have been used to characterise the surface modified Ni-Ti to provide a fuller understanding of the biomaterial potential of such a material. The results of this study show that both the modified and unmodified Ni-Ti surfaces were predominantly covered with TiO2 and the underlying substrate crystallography determined both the affinity for surface OH−/chemisorbed water and ultimately the wetting behaviour of distilled water. Additionally, N+ ion implanted Ni-Ti contained a TiN phase within the surface which reduced wetting, demonstrating a reduced interfacial energy. The surface concentrations of Ni were unaffected by the surface modifications, with all samples containing less than 3 at.% Ni. This study has shown that the surface TiO2 oxide layer was maintained despite the surface amorphisation treatments. It can be assumed that the TiO2 layer is almost identical in unimplanted and implanted surfaces and they will display the same biocompatibility.

The wear of metal-on-metal total hip prostheses measured in a hip simulator

Abstract New generation metal-on-metal prostheses have been introduced to try and overcome the problem of osteolysis often attributed to the wear particles of the polyethylene component of conventional metal-on-ultra-high molecular weight polyethylene (UHMWPE) joints. The wear rates of four metal-on-metal joints (two different clearances) were assessed along with that of a conventional metal-on-UHMWPE joint. Friction measurements of the metal-on-metal joints were taken before and after the wear test and compared. Two distinct wear phases were discernible for all the metal-on-metal joints: an initial wear phase up to 0.5 × 106 cycles and then a lower steady state wear phase. The steady state wear rate of the 22 μUm radial clearance metal-on-metal joint was lower than that for the 40 μUm radial clearance joint, although this difference was not found to be significant (p > 0.15). The wear rates for all the joints tested were consistent with other simulator studies. The friction factors produced by each joint were found to decrease significantly after wear testing (p < 0.05).

The surface performance of shot peened and ion implanted NiTi shape memory alloy

Equiatomic NiTi shape memory alloy in austenitic and martensitic forms has been modified by shot peening with glass media and by N+ ion implantation. The effect upon microstructure, surface hardness and coefficient of friction was measured, along with the effect of the two treatments upon the shape memory behaviour. Examination using transmission electron microscopy (TEM) showed amorphous regions within the surfaces treated by both peening and ion implantation. In addition, peened and ion implanted samples showed similar increases in surface hardness and reduced surface friction coefficient. Differential scanning calorimetry showed that both surface treatments had no effect upon the bulk shape memory phase transformation behaviour.

Statistical wear analysis of PA-6/UHMWPE alloy, UHMWPE and PA-6

This paper, based on an orthogonal experimental design and analysis method, reports the wear of a blend of polyamide-6/ultrahigh molecular weight polyethylene (PA-6/UHMWPE), using a pin-on-disc test, and rubbing against a stainless steel counterface. The wear behavior of PA-6 and UHMWPE was also investigated for the purpose of comparison. The main purpose was to study the influence of the parameters: sliding distance, contact pressure and sliding speed on the wear performance of the investigated materials. Statistical analysis was carried out to develop an equation in which the wear volume of the specimen was expressed in terms of sliding distance, contact pressure and sliding speed. It was observed that the wear rate was lower for PA-6 than the other two materials. UHMWPE exhibited the lowest wear resistance. Contact pressure was found to be the most important factor in the wear of materials, followed by sliding distance and sliding speed. Sliding distance had the highest relative effect on the wear of PA-6/UHMWPE. Sliding speed seemed to have the least effect on the wear volume of the investigated materials. The results show that the wear behavior of these materials, and the effects of factors on the wear, depend on the physical and mechanical properties of the materials.

Effects of operating parameters on the lubricated wear behavior of a PA-6/UHMWPE blend: a statistical analysis

In this paper, based on an orthogonal test design and analysis method, the lubricated wear performance of a ultra-high molecular weight polyethylene and polyamide (PA-6/UHMWPE) alloys were studied using a pin-on-disc method. The effects of several parameters on the wear of the PA-6/UHMWPE alloy, rubbing against a stainless steel counterface, are reported. The main purpose was to study the influence of parameters such as sliding distance, counterface surface roughness, load and sliding speed, as well as their interactions on the wear performance. Statistical analysis was carried out to develop an equation, in which the wear volume of the polymeric specimen was expressed in terms of the investigated parameters. It was observed that the pressure and surface roughness are the two important and controlling factors; sliding distance and sliding speed have a minor effects on the wear of the specimens. Although the two-factor and three-factor interactions have little effect, the four-factor interaction has a strong effect on the wear of specimens. The results give a comprehensive insight into the wear of the PA-6/UHMWPE alloy.

The quantitative assessment of UHMWPE wear debris produced in hip simulator testing: the influence of head material and roughness, motion and loading

Abstract Biomaterial wear particles are known to provoke a foreign-body reaction when released from total joint prostheses. Considerable effort is being invested in the search for materials which, by wearing less, will release fewer particles. In this research the role of joint wear simulators is paramount. The wear debris from 26 simulator tests of hip prostheses was extracted from the bovine serum lubricant and sized using a laser diffraction technique. The influence on particle size of a broad range of parameters was examined. The parameters considered included the bedding-in phenomenon, the femoral head material and roughness and physiological versus simplified load cycles and wear paths. The physiological wear simulators produced a similar size distribution of wear particles to that produced by implanted joints. Head material had no effect on this observation. Simplifying the loading cycle or wear path independently made no impact on this finding. This remained the case when simplified load and wear path were combined in one test. The effect of head roughness was pronounced with an increase in minimum particle size with increasing roughness. Joint simulators remain the optimal method for assessing new joint materials and designs. However, their use to characterise joints in terms of wear rate solely should be guarded against. Instead focus should be concentrated on a combination of the size and amount of the wear debris created. In this way the “loosening hazard” of a joint can be distilled.

Some failure modes of four clinical bone cements

Abstract The fracture or failure behaviours of four commercial acrylic-based bone cements have been examined in tensile, bending and compression modes, and their mechanical properties are reviewed. It was found that Palacos R-40 bone cement had high radiopaque agent concentration, with high surface hardness. It exhibited a much lower bending strength and bending modulus compared with the other three bone cements (CMW1, CMW2000 and Simplex P). The textures of tensile fracture surfaces produced were similar for the four bone cements studied. The fracture surface was fragmented by crevices, which developed through the matrix and around large undissolved polyme-thylmethacrylate (PMMA) beads. Three bands with different features existed on the bending fracture surfaces, with an abrupt transition between them. It appears that the agglomerates of zirconium dioxide particles are implicated in Palacos R-40 bone cement fracture surface. The examination of compressive failed specimens revealed that a ‘yielded crack band’ existed across the transverse section. Plastic deformation resulted in the PMMA beads being squashed in the longitudinal direction and dilated in the transverse direction.

Pre-heating of components in cemented total hip arthroplasty

Fatigue fractures which originate at stress-concentrating voids located at the implant-cement interface are a potential cause of septic loosening of cemented femoral components. Heating of the component to 44 degrees C is known to reduce the porosity of the cement-prosthesis interface. The temperature of the cement-bone interface was recorded intra-operatively as 32.3 degrees C. A simulated femoral model was devised to study the effect of heating of the component on the implant-cement interface. Heating of the implant and vacuum mixing have a synergistic effect on the porosity of the implant-cement interface, and heating also reverses the gradients of microhardness in the mantle. Heating of the implant also reduces porosity at the interface depending on the temperature. A minimum difference in temperature between the implant and the bone of 3 degrees C was required to produce this effect. The optimal difference was 7 degrees C, representing a balance between maximal reduction of porosity and an increased risk of thermal injury. Using contemporary cementing techniques, heating the implant to 40 degrees C is recommended to produce an optimum effect.

Co-deposition of titanium/polytetrafluoroethylene films by unbalanced magnetron sputtering

Abstract Graded nanocomposite coatings, consisting of a polytetrafluoroethylene (PTFE)-rich surface layer and functionally graded titanium–titanium carbide–PTFE mixed sublayer, were deposited onto stainless steel substrates using a radio frequence (RF) unbalanced magnetron sputter system with the purpose of improving the tribological performance of the substrate and endowing the surface with hydrophobicity. The results show that decomposition of PTFE during RF plasma sputtering results mainly in the evolution of fluoropolymer species. High incident power results in low F/C ratio in the resulting films, and low incident power results in high F/C ratio films. The tribological performance of the coatings depends on the fluoropolymer content in the film bulk, as well as on the film growth process. The films with high fluoropolymer content, with a gradient multilayer structure, possess good tribological performance especially at the initial stage of testing. During the co-deposition process, the segments were inlaid into the titanium matrix and became strongly mechanically bonded. It is speculated that some of the carbon atoms may react with titanium to form titanium carbide in the coatings. Multilayer structure attributed to the decrease in the stress developed between layers. All these factors attributed to the improvement of the tribological performance of the stainless steel substrate.

Wear analysis of failed acetabular polyethylene: A COMPARISON OF ANALYTICAL METHODS

There are many methods for analysing wear volume in failed polyethylene acetabular components. We compared a radiological technique with three recognised ex vivo methods of measurement. We tested 18 ultra-high-molecular-weight polyethylene acetabular components revised for wear and aseptic loosening, of which 13 had pre-revision radiographs, from which the wear volume was calculated based upon the linear wear. We used a shadowgraph technique on silicone casts of all of the retrievals and a coordinate measuring method on the components directly. For these techniques, the wear vector was calculated for each component and the wear volume extrapolated using mathematical equations. The volumetric wear was also measured directly using a fluid-displacement method. The results of each technique were compared. The series had high wear volumes (mean 1385 mm(3); 730 to 1850) and high wear rates (mean 205 mm(3)/year; 92 to 363). There were wide variations in the measurements of wear volume between the radiological and the other techniques. Radiograph-derived wear volume correlated poorly with that of the fluid-displacement method, co-ordinate measuring method and shadowgraph methods, becoming less accurate as the wear increased. The mean overestimation in radiological wear volume was 47.7% of the fluid-displacement method wear volume. Fluid-displacement method, coordinate measuring method and shadowgraph determinations of wear volume were all better than that of the radiograph-derived linear measurements since they took into account the direction of wear. However, only radiological techniques can be used in vivo and remain useful for monitoring linear wear in the clinical setting. Interpretation of radiological measurements of acetabular wear must be done judiciously in the clinical setting. In vitro laboratory techniques, in particular the fluid-displacement method, remain the most accurate and reliable methods of assessing the wear of acetabular polyethylene.