Is ALT control really necessary for routine ART monitoring in resource poor settings?

2006 AIDS Conference in Toront

Biotribology of artificial hip joints

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

Influence of the wear partition factor on wear evolution modelling of sliding surfaces

Wear 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

A novel approach to the estimation and application of the wear coefficient of metal-on-metal hip implants

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

Extended pelvic lymph node dissection at the time of robot-assisted radical prostatectomy: Impact of surgical volume on efficacy and complications in a single-surgeon series

INTRODUCTION: We assessed the impact of surgical volume on perioperative outcomes and complications of robotic extended pelvic lymph node dissection (ePLND). METHODS: From November 2008 to October 2012, a total of 233 consecutive patients with intermediate- or high-risk clinically localized prostate cancer underwent robot-assisted radical prostatectomy (RARP) and ePLND by a single, experienced open and laparoscopic surgeon. Data were prospectively collected. Complications were classified according to the Modified Clavien System. Complications potentially related to ePLND were documented. The minimum follow-up was 3 months. To evaluate the impact of surgical volume on the results, 4 patient subgroups (subgroup 1: cases 1-59; 2: 60-117; 3: 118-175; 4: 176-233) were compared using the Chi-squared and Kruskal-Wallis tests. RESULTS: The mean (range) operative time for ePLND was 79 minutes (range: 48-144), with a steady performance over time (p = 0.784). The count of resected lymph nodes plateaued after 60 procedures (mean [range]: 13 [range: 6-32], 15 [range: 7-34], 17 [range: 8-41], 16 [range: 8-42] in Groups 1 to 4, respectively, p = 0.001). Tumour lymph node involvement was 12% in Groups 1 and 2, 7% in Group 3 and 9% in Group 4 (p = 0.075). Overall, 115 complications were reported in 98/233 patients (42%), with a significant decrease after 175 cases (p = 0.028). In Group 4, 3 patients reported an ePLND-related bleeding requiring open revision. Lymphoceles were detected in 10/233 patients (4.2%) and 1 patient (1.7%) in each of the Groups 2 to 4 required a percutaneous drainage. CONCLUSIONS: A surgeon with extensive experience is expected to achieve a safe learning curve for ePLND during RARP. A learning curve of 60 cases is suggested for optimal lymph node yield

A comparative study of wear laws for soft-on-hard hip implants using a mathematical wear model

Wear 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

Estimation of Wear Factors of MoM Hip Implants from Simulator Tests

Nowadays wear is recognized as one of the main concern of metal-on-metal (MoM) hip implants, causing osteolysis and the release of dangerous metallic ions. Numerical wear simulations of hip implants are an attractive tool to investigate and predict long-term wear at low cost. A few wear models have been proposed in the literature for MoM bearings [1-3], all based on the Archard wear law, as adhesion and abrasion are considered the main wear mechanisms. The reliability of such models mainly depends on a dimensional wear factor k whose evaluation/choice is actually a critical issue. Indeed k depends on many factors, such as lubrication regime, bearing materials and geometry, loading and kinematic conditions and thus can vary during a wear test/implant lifetime. This complex scenario is simplified in wear simulations which typically assume two constant values of k - equal for head and cup - one higher for the initial running-in phase (kri) and the other lower for the steady state phase (kss), according to experimental observations (Fig. 1). Such k values are generally estimated by matching numerical and experimental wear volumes obtained by hip joint simulator tests [2-3], sometimes even simulating conditions different from the experimental ones [3]. For these reasons and because of a complete neglect of the comparison between numerical and experimental wear maps, the validity of the wear models can be disputable

A Simple Modular Test Rig for Measuring Static and Dynamic Friction

In this paper an apparatus for investigations on both static and dynamic friction is described. The apparatus is simple but very modular and can be used in two different configurations: (i) controlling the displacement of an upper specimen connected by an elastic wire to a moving load cell; (ii) controlling the displacement of a lower specimen connected to a moving plate. The capabilities of the apparatus are shown through some preliminary tests that also highlight the several factors that can affect the friction coefficient estimation

A new digital image correlation software for displacements field measurement in structural applications

Recently, there has been a growing interest in studying non-contact techniques for strain and displacement measurement. Within photogrammetry, Digital Image Correlation (DIC) has received particular attention thanks to the recent advances in the field of low-cost, high resolution digital cameras, computer power and memory storage. DIC is indeed an optical technique able to measure full field displacements and strain by comparing digital images of the surface of a material sample at different stages of deformation and thus can play a major role in structural monitoring applications. For all these reasons, a free and open source 2D DIC software, named py2DIC, was developed at the Geodesy and Geomatics Division of DICEA, University of Rome "La Sapienza". Completely written in python, the software is based on the template matching method and computes the displacement and strain fields. The potentialities of Py2DIC were evaluated by processing the images captured during a tensile test performed in the Lab of Structural Engineering, where three different Glass Fiber Reinforced Polymer samples were subjected to a controlled tension by means of a universal testing machine. The results, compared with the values independently measured by several strain gauges fixed on the samples, demonstrate the possibility to successfully characterize the deformation mechanism of the investigated material. Py2DIC is indeed able to highlight displacements at few microns level, in reasonable agreement with the reference, both in terms of displacements (again, at few microns in the average) and Poisson's module

Estimating accruals models in Europe: industry-based approaches versus a data-driven approach

Accruals models have been estimated using a variety of approaches, but the industry-based cross-sectional approach currently seems to be the standard method. This estimation approach cannot be easily used in the vast majority of European countries where several industry groups do not have sufficient yearly observations. Using data from France, Germany, Italy and the UK, we artificially induce earnings manipulations to investigate how the ability to detect those manipulations through accruals models is affected by the use of different industry classifications. Moreover, we propose an alternative estimation approach based on a data-driven statistical procedure that provides an optimal choice of estimation samples. Our analyses show that enlarging the industry classification and/or pooling observations across years reduces the probability of discovering earnings manipulations but allows for the estimation of abnormal accruals (AA) for more firms. The data-driven approach, however, in most cases outperforms the industry-based estimation approaches without sample attrition. This result suggests that there is still ample room for improving the accruals model estimation process for capital markets of European countries. Furthermore, the analysis documents which accruals model outperforms the others in each of the four countries and the probabilities to detect earning management in a high variety of circumstances