Predicting UHMWPE wear: evidence for rapid decline in wear rates following a change in sliding direction

Extending the service life of total joint replacements by improving the wear resistance of UHMWPE remains a priority that is driven by the demands of a more active patient [1]. Extensive experimental testing has shown that wear rates depend on the details of the wear path: Unidirectional paths produce minimal wear while those with direction change produce high wear [2-4]. This implies that a linear wear path that undergoes a sudden change in direction will generate a high wear rate immediately following that direction change and then will transition to a near zero wear rate with continued sliding. To the authors’ knowledge, this variation in wear rate with sliding distance after a change in sliding direction has not been explored experimentally. The goal of this study was to investigate the incremental wear as the sliding distance is increased between step-changes in sliding direction. We hypothesized that small increases in sliding distance after a direction change would produce additional wear while no additional wear would be produced at longer sliding distances sufficient to reestablish ‘unidirectional’ sliding

INDUCED STRESSES AND SHELL/LINER MICROMOTIONS IN THA COMPONENTS DURING DISLOCATION. A FINITE ELEMENT STUDY.-asme/terms-of-use

INTRODUCTION During total hip replacement (THR) many supporting structures for the retention of the femoral head within the acetabulum are removed. Previous studies have determined that this lack of supporting structures leads to separation of the femoral head within the UHMWPE liner. During the relocation phase the femoral head impacts with the liner creating high stress conditions that may contribute to premature polyethylene wear and instabilities. The proposed study focuses on the femoral head, liner and acetabular shell when subjected to intermittent loading conditions due to dislocations of the femoral head. The objective of this paper is to address the effect of dislocation and the induced stresses during impact. The parameters studied include the dislocation or gap distance, the impact force duration and the friction between all the components involved. The research highlights the build up of residual stresses and their contribution to the cup/liner and acetabular bone instability. MATERIAL AND METHODS A three-dimensional 3595 element FE model ( A rigid material type was chosen for the titanium components, such as the acetabular shell and the femoral head, and a Linear Elastic Isotropic material for the UHMWPE liner. The material properties of the liner were based on the information obtained in the available literature (E=975 Mpa, µ=0.46) [1]. A linear contact between implant components was modeled using an automatic surface-to-surface contact (ASTS) with an experimental friction coefficient between titanium and UHMWPE of µ=0.038. The acetabular cup was fully constrained; the femoral head was constrained with respect to the rotational degrees of freedom. In order to simulate the fixation grooves of the liner and the acetabular shell, specific nodes placed in the external surface of the liner were fully constrained while the remainders of the liner's nodes were fully un-constrained. The Model was validated with recent literature [1]. Figure1: Fe model According to previous studies by Douglas A. Dennis et al [2], where the separation of the femoral head and the liner was measured using fluoroscopy techniques, the separation values ranged from ∆y=0 mm to ∆y=5.3 mm. Most of the data provided [1] highlighted the axial separation (∆y, STRESS FR

Predicting wear of UHMWPE: decreasing wear rate following a change in direction

Computational tools are emerging as design tools for the development of total joint replacement with improved wear performance. The current wear models of polyethylene assume that wear is linearly proportional to sliding distance; however, it is hypothesized that the wear rate varies and is higher near a change in direction, but diminishes with continued unidirectional sliding, which eventually exhibits negligible wear. Our goals were to (1) reveal the presence of a variable wear rate in polyethylene; (2) identify the sliding distance required to reestablish unidirectional sliding subsequent to a change in sliding direction. The wear of polyethylene was evaluated in pin-on-disk testing for several different sliding distances (0 mm, 1 mm, 2 mm, 5 mm, 10 mm, and 100 mm) after a 90° change in direction. The results indicate the wear rate immediately following the change in direction is high, but with continued linear sliding the wear rate appears to drop to near zero--returning to the low wearing condition of unidirectional sliding. Furthermore, this transition appears to occur nonlinearly below 5 mm from the change in direction. While more studies are required to explore other paths and uncover the underlying mechanisms, these results should aid the development of computational tools for the design of advanced joint replacement

Research Article Using a Herd Profile to Determine Age-Specific Prevalence of Bovine Leukemia Virus in Michigan Dairy Herds

License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. Enzootic bovine leukosis is a contagious disease of cattle caused by the retrovirus, bovine leukemia virus (BLV) and is the most common cause of malignant neoplasm in cattle. In order to facilitate surveillance of this disease in dairy herds, we developed a method to combine ELISA of milk collected during routine production testing with a prescribed sampling of cows that is independent of the proportion of cows within each lactation. In 113 Michigan dairy herds, milk samples from ten cows in each of the 1st, 2nd, 3rd, and ≥4th lactations were analyzed for anti-Bovine Leukemia Virus (BLV) antibodies by milk ELISA. For each herd, a BLV herd profile (BHP) was calculated as the simple average of the percent of BLV-positive cows within each of the four lactation groups. The mean BHP for all herds was 32.8%, with means of 18.5, 28.8, 39.2, and 44.8 % of 1st, 2nd, 3rd, and ≥4th lactation animals infected, respectively. In eight herds, we determined the correlation between the BHP, and true herd prevalence by testing the entire lactating herd (r = 0.988, P<0.0001). The BHP allows discrimination of lactation-specific BLV prevalence within a dairy herd, to help identify risk factors and management plans that may be important in transmission of BLV. 1