Search

Wear has been considered as a major issue for ball and socket artificial discs. This paper studies the effects of ball radius and radial clearance of the artificial disc on the von Mises stresses. Different material combinations, used in artificial discs, are compared. FEA simulation using Solidworks has been conducted for different disk geometries. The highest von Mises stress was 714MPa for 10mm radius metal against metal design. The lowest von Mises stress was 14.8MPa for 16mm radius of the UHMWPE/CoCr material combination, which exhibited the lowest von Mises stresses for all the radii of the ball and socket articulation. Considering radial clearance, the lowest von Mises stress was 14.8MPa for 0.015mm clearance of the UHMWPE/CoCr combination. The highest von Mises stress of 100.8MPa with a radial clearance of 0.25mm was recorded for the same combination. There is a strong relation between the von Mises stress and the geometry of the ball and socket of the artificial disc.

Background The large-scale clinical problem caused by unacceptable tribological behaviour of certain large diameter metal-on-metal prosthetic hips has directed attention to adverse condition testing. High metal-on-metal wear is connected with adverse reaction to metal debris. Friction is important because high friction may be associated with high wear, risk the fixation of the cup, and cause detrimental heating of periprosthetic tissues. Methods A friction measurement system was added to a multidirectional, established hip joint wear simulator, and its functionality was evaluated. In preliminary tests, a 50 mm diameter metal-on-metal prosthesis was tested in an optimal acetabular cup inclination angle (48°) and in a steep angle (70°) using a normal peak load (2 kN) and an increased peak load (3 kN). The test length was 100 h. Long-term adverse condition tests of 3 million cycles were run for three 52 mm metal-on-metal prostheses. The lubricant was diluted calf serum at 37 °C. Findings In the 100 h tests, metal-on-metal frictional torque was not highly sensitive to the angle, load and serum degradation, and it was close to that of a conventional 28 mm prosthesis with a polyethylene cup, mostly below 5 Nm. However, a manyfold higher frictional torque (10 to 20 Nm) was observed in long-term metal-on-metal tests with substantial wear. Interpretation To obtain a realistic prediction of the frictional behaviour of a hip design, long-term, multidirectional wear tests are necessary. The friction should preferably be measured during the wear test. In addition to normal conditions, adverse condition testing is strongly recommended.

The relationship between the wear behaviour of an ultra-high-molecular-weight polyethylene (GUR1050) pin and surface profile of a Co-28Cr-6Mo alloy (ASTM F-75) disc was investigated. Tribological tests have been performed by pin-on-disc wear testing machine with multi-directional pathways, obtaining the influence of micro slurry-jet erosion (MSE) processed Co–Cr–Mo alloy. The specific wear and the wear particles have been investigated. It was elucidated that the Co–Cr–Mo alloy surface processed by MSE influenced the specific wear rate of polyethylene; however, the morphological aspect of polyethylene wear particles was not drastically changed. The wear particles isolated from the lubricating liquid were added to a culture medium, and human monocyte-derived macrophages were incubated by using an upright/inverted method. The production of TNF-α and IL-6 seemed to have a correlation with the amount of wear particles added, however, the influence of particle size on the production of TNF-α and IL-6 was not obvious. This means that the improvements for the incubation method, i.e. upright/inverted method, need further investigation for the accurate analysis.

Article The paper deals with pressure distribution measurement in knee arthroplasty, which is an artificial replacement of human knee joint. The scope of the article is to verify the accuracy of a mathematical model by real measurements. The calculated pressure values basing on the mathematical model are compared with actually measured pressure values in the contact area of the joint. Hereby maximal load the in the contact area, the distribution of the pressure and any potentially dangerous pressure deviations during the walk cycle are checked. To enable accurate pressure distribution measurement without interfering into human’s body, a sophisticated measuring setup was created: the contact area of the joint was equipped with several pressure sensors and a machine simulating the human walk cycle was used. The measured pressure data are finally compared with those from the mathematical model and with the strength limit of the used material, to verify the accuracy of the mathematical model experimentally.

Since 1960, when the first hip prosthesis was introduced, up to now, several implant typologies have been proposed trying to meet the increasing clinical demands of more and more active and young patients. A substantial evolution of implant design has been occurring, both in terms of materials and geometry, basically driven by their tribological performances. Indeed, the main concern of hip implants consists in the release of wear debris, which can lead to implant loosening and failure. Thus, many studies on wear and lubrication of hip prostheses have been published in the last 15 years, mainly focused on experimental researches but also on numerical/modeling approaches. The aim of this work is to review the history of hip implants from a tribological point of view with a focus on ceramic-on-ceramic replacements, which represent the most advanced solution in terms of wear strength and chemical inertness. The main drawbacks of these implants, as the brittleness and the squeaking, are discussed and novel solutions examined.

As a key dynamic feature of the rock mass, joint results in wave attenuation when a stress wave propagates across it. An experimental study has obtained the relation between the transmission coefficient and the contact area ratio of joints with different thicknesses (Chen et al. (doi:)). However, the spatial geometry of the contact surface also determines the dynamic behavior of the discontinuous masses. Thus, the present study is focused on the effects of distribution and dimension of the sawn notches on the contact surface of the joint, which lead to the different spatial geometry. The joint matching coefficient (JMC) was used in this study to define the contact condition. Using a modified SHPB apparatus, all the bars and specimens were aluminum, and the artificial joint formed a rough surface of the specimen contacted to the output bar. Based on the wave separation method, the incident, reflected and transmitted waves across the joint were acquired from the records of the strain gauges on pressure bars. Then the transmission coefficient and specific stiffness of the joint were obtained. Comparisons of them were made on different JMCs, spatial geometries and thicknesses of the joint.