Preclinical wear evaluation is extremely important in hip replacements, wear being one of the main causes of failure. Experimental tests are attractive but highly cost demanding; thus predictive models have been proposed in the literature, mainly based on finite element simulations. In such simulations, the effect of friction is usually disregarded, as it is considered not to affect the contact pressure distribution. However, a frictional contact could also result in a shift of the location of the nominal contact area, which can thus modify the wear maps. The aim of this study is to investigate this effect in wear prediction for metal-on-metal implants. Wear assessment was based on a purpose-developed mathematical model, extension of a previous one proposed by the same authors for metal-on-plastic implants. The innovative aspect of the present study consists in the implementation of a modified location of the nominal contact point due to friction, which takes advantage of the analytical formulation of the wear model. Simulations were carried out aimed at comparing total and resurfacing hip replacements under several gait conditions. The results highlighted that the adoption of a frictional contact yields lower linear wear rates and wider worn areas, while for the adopted friction coefficient ($f=0.2$), the total wear volume remains almost unchanged. The comparison between total and resurfacing replacements showed higher scaled wear volumes (wear volume divided by wear factor) for the latter, in agreement with the literature. The effect of the boundary conditions (in vivo versus in vitro) was also investigated remarking their influence on implant wear and the need to apply more physiological-like conditions in hip simulators. In conclusion although friction is usually neglected in numerical wear predictions, as it does not affect markedly the contact pressure distribution, its effect in the location of the theoretical contact point was observed to influence wear maps. This achievement could be useful for increasing the correlation between numerical and experimental simulations, usually based on the total wear volume. In order to improve the model reliability, future studies will be devoted to implement the geometry update by combining the present model to finite element analyses. On the other hand, further experimental investigations are required to get out from the wide dispersion of wear factors reported in the literature.

Introduction: The use of cyanoacrylate glue as sealant on the exit site of peripherally inserted central catheters (PICCs) may offer some important clinical advantages. However, concerns exist about the potential interaction between cyanoacrylate and the material of the catheter itself. The aim of this study was to investigate the possibility of damage to the catheter secondary to a long-term contact with a two-component skin glue (N-butyl + octyl cyanoacrylate). Methods: Twelve PICCs of different brands and types were selected (11 made of polyurethane and one made of silicon). PICCs were glued onto artificial skin pads, slightly wetted with Earle solution. The pads were kept in an egg incubator at 34°C and 60% humidity, for up to twelve weeks. Possible signs of degradation were monitored by surface analyses and mechanical tests. Scanning electron microscopy observations, surface roughness measurements, pressure strength and uniaxial tests were performed. Results: Samples were analyzed after 4, 8 and 12 weeks of contact with the glue. No chemical reaction between the glue and the material of the catheters was evident. The mechanical strength of PICCs was consistently within the ranges of ISO standards. An expected increase in the stiffness of the samples covered with glue was observed in uniaxial tests. The silicon catheter was weaker than the polyurethane catheters and was damaged while trying to remove it from the pad for tests. Conclusions: The long-term use of N-butyl + octyl cyanoacrylate glue on polyurethane PICCs is not expected to be associated to any damage to the catheter.

A 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

Pin-on-plate and pin-on-disk wear tests are typically used for assessing the wear behavior of a given material coupling and estimating its wear coefficient using the Archard wear law. This study investigates differences in the Archard law for pin-on-plate and pin-on-disk cases, particularly for flat-ended pins. Both analytical and finite element models of the two tests were developed, assuming a 21 N normal load and a 50π mm sliding distance. In pin-on-disk simulations three different distances between pin and disk axes were considered, i.e., 1.25–2.5–5 times the pin radius (5 mm). For the results, wear volumes, pressure and wear depth maps were compared. Some interesting aspects arose: (i) the rotational effect in pin-on-disk tests causes higher wear volumes (up to 13%) with respect to pin-on-plate tests: the nearer the pin to the disk axis, the higher the wear volume; (ii) a simple quadratic formula is defined to correct the wear volume estimation for pin-on-disk tests; (iii) pressure redistribution occurs with higher values closer to disk axis, opposite to the wear depth trend. Due to the high computational costs, only the running-in phase of wear tests was considered. Numerical strategies are currently under investigation to extend this study to the steady state phase.