Microscopic asperity contact and deformation of ultrahigh molecular weight polyethylene bearing surfaces

A closed-form formulation for the conformal articulation of metal-on-polyethylene hip prostheses: Contact mechanics and sliding distance

Using Hertz contact law results in inaccurate outcomes when applied to the soft conformal hip implants. The finite element method also involves huge computational time and power. In addition, the sliding distance computed using the Euler rotation method does not incorporate tribology of bearing surfaces, contact mechanics and inertia forces. This study, therefore, aimed to develop a nonlinear dynamic model based on the multibody dynamic methodology to predict contact pressure and sliding distance of metal-on-polyethylene hip prosthesis, simultaneously, under normal walking condition. A closed-form formulation of the contact stresses distributed over the articulating surfaces was derived based upon the elastic foundation model, which reduced computational time and cost significantly. Three-dimensional physiological loading and motions, inertia forces due to hip motion and energy loss during contact were incorporated to obtain contact properties and sliding distance. Comparing the outcomes with that available in the literature and a finite element analysis allowed for the validation of our approach. Contours of contact stresses and accumulated sliding distances at different instants of the walking gait cycle were investigated and discussed. It was shown that the contact point at each instant was located within the zone with the corresponding highest accumulated sliding distance. In addition, the maximum contact pressure and area took place at the stance phase with a single support. The stress distribution onto the cup surface also conformed to the contact point trajectory and the physiological loading.

Numerical and experimental investigations for the evaluation of the wear coefficient of reverse total shoulder prostheses

In the present study, numerical and experimental wear investigations on reverse total shoulder arthroplasties (RTSAs) were combined in order to estimate specific wear coefficients, currently not available in the literature. A wear model previously developed by the authors for metal-on-plastic hip implants was adapted to RTSAs and applied in a double direction: firstly, to evaluate specific wear coefficients for RTSAs from experimental results and secondly, to predict wear distribution. In both cases, the Archard wear law (AR) and the wear law of UHMWPE (PE) were considered, assuming four different k functions. The results indicated that both the wear laws predict higher wear coefficients for RTSA with respect to hip implants, particularly the AR law, with k values higher than twofold the hip ones. Such differences can significantly affect predictive wear model results for RTSA, when non-specific wear coefficients are used. Moreover, the wear maps simulated with the two laws are markedly different, although providing the same wear volume. A higher wear depth (+51%) is obtained with the AR law, located at the dome of the cup, while with the PE law the most worn region is close to the edge. Taking advantage of the linear trend of experimental volume losses, the wear coefficients obtained with the AR law should be valid despite having neglected the geometry update in the model.

A New Formulation for the Prediction of Polyethylene Wear in Artificial Hip Joints

Artificial joints employing ultra-high molecular weight polyethylene (UHMWPE) are widely used to treat joint diseases and trauma. Wear of the polymer bearing surface largely limits the use of these joints in younger and more active patients. Previous studies have shown the wear factor used in Archard's law for the conventional polyethylene to be highly dependent on contact pressure and this has produced variability in experimental data and has constrained the reliability and applicability of previous computational predictions. A new wear law is proposed, based on wear volume being dependent on, and proportional to, the product of the sliding distance and contact area. The dimensionless proportional constant, wear coefficient, which was independent of contact pressure, was determined from a multi-directional pin on plate study. This was used in computational predictions of the wear of the conventional UHMWPE hip joints. The wear of the polyethylene cup was independently experimentally determined in physiological full hip joint simulator studies. The predicted wear rate from the new computational model was generally increased, with an improved agreement with the experimental measurement compared with the previous computational model. It was shown that wear in the UHMWPE hip joints increased as head size and contact area increased. This resulted in a much larger increase in the wear rate as the head size increased, compared with the previous computational model, and is consistent with clinical observations. This new understanding of the wear mechanism in artificial joints using the UHMWPE bearing surfaces, and the improved ability to predict wear independently and to address previously described discrepancies offer new opportunities to optimize design parameters.

Automated In-Vitro Testing of Orthopaedic Implants: A Case Study in Shoulder Joint Replacement

This investigation presents the design and preliminary validation of a single station simulator with biaxial motion and loading designed to mimic the kinematics of the glenohumeral joint during arm abduction in the scapular plane. Although the design of the glenoid holder allows the glenoid component to translate in all three axes, it is primarily loaded axially, which brings it into contact with the oscillating humeral head, but is also loaded superiorly to simulate common subluxation of the humeral head. Simulating arm abduction in the scapular plane simplifies component alignment and removes the need for anterior—posterior loading, thereby creating a stable joint without the need to simulate capsular constraints. In this more physiologically accurate simulator design, the load and motion profiles influence the contact kinematics, but the wear path is ultimately determined by the conformity and constraint designed into the bearing couple. The wear data are determined and correlated with clinically retrieved glenoid components, as well as previously reported in-vitro studies, thus verifying use of the simulator in testing alternative materials and designs. The key design features, as well as the improvements proposed through this study, can be incorporated into the design of test fixtures for any other orthopaedic implant such as the hip, knee, spine, elbow, and finger.

Wear simulation of ultra-high molecular weight polyethylene hip implants by incorporating the effects of cross-shear and contact pressure

The effect of multi-directional cross-shear (CS) motion and contact pressure on ultra-high molecular weight polyethylene (UHMWPE) wear was investigated in this study, based on an integrated experimental and computational approach. The wear factor as a function of CS was determined experimentally from a multi-directional pin-on-plate wear tester under a nominal contact pressure of 1 MPa. A computational wear model was developed which included the effects of CS as well as the load and sliding distance imposed on the hip joint employing a UHMWPE cup against a metallic femoral head under both gait and Leeds ProSim hip joint simulator conditions. The CS ratios were quantified over the articular surface of the UHMWPE cup and the CS-dependent wear factors derived from multi-directional pin-on-plate studies were applied in the computational wear model. Outputs from the computational wear model were validated independently against an experimental hip simulator study. Comparisons of linear and volumetric wear were made between the computational wear model and the hip simulator testing for a nominal conventional (0 MRad) UHMWPE cup of 28 mm diameter and a highly cross-linked (10 MRad) UHMWPE cup. The difference between the computed and experimental volumetric wear was approximately 30 per cent for the 0 MRad UHMWPE, although the worn areas between the prediction and the measurement were similar. For the 10 MRad UHMWPE, the discrepancy was reduced to 16 per cent. In both cases, the computational model predicted a lower wear rate than the experimental simulator testing. The effect of using alternative wear factors under a different nominal contact pressure of 3 MPa was also considered. The input wear factor to the computational model, derived from a constant loaded pin-on-plate test configuration, may underestimate the dynamic effect due to the variation in the load in the hip joint simulator.

Simulation of polyethylene wear in ankle joint prostheses

The performance of total ankle replacements (TARs) have not been comparable to those of the other major joints of the lower extremity. The aim of this work was to develop a new simulator test to compare the wear of a new mobile bearing TAR (Mobility) with one with a good clinical history, the Buechel Pappas, using kinematic inputs derived from the literature. The wear rate for the Mobility components was lower than that for the Buechel-Pappas ankle joints at all time points. The wear rate for both sets of components increased with the inclusion of an anterior/posterior displacement in the kinematic inputs. This was expected as the components are subjected to higher kinematic demands and reproduces similar effects found in knee prostheses. This study has demonstrated that it is possible to study wear of TARs in a modified simulator originally designed for total knee replacements. It was also shown that the new Mobility ankle compares favorably with the Buechel Pappas ankle, which has a successful clinical history, under the simulator test conditions described.

A simple fully integrated contact-coupled wear prediction for ultra-high molecular weight polyethylene hip implants

A fully coupled contact and wear model was developed in the present study for hip implants employing an ultra-high molecular weight polyethylene (UHMWPE) cup in combination with a metallic or ceramic femoral head. A simple elasticity equation based on the concept of constrained column model was employed to solve the contact mechanics between the acetabular cup and the femoral head under the three-dimensional physiological loading condition. The wear model was based on the classical Archard-Lancaster equation in common with all other studies reported in the literature. The fully coupled contact and wear model was applied to both conventional and cross-linked UHMWPE cups under a wide range of design parameters such as the clearance and the femoral head radius. The predicted linear and volumetric wear as well as their rates for conventional UHMWPE cups were found to be in good agreement with those obtained from a similar analysis by Maxian but using the finite element method for the contact mechanics analysis. The predicted maximum contact pressure was found to decrease rapidly within the first 10(6) cycles, and below the limit to cause plastic deformation within the UHMWPE cup with a nominal radial clearance of 0.2 mm. The effect of the clearance between the head and the cup on the predicted wear was found to be negligible. For the cross-linked UHMWPE cup with relatively large diameters up to 48 mm and a fixed outside diameter of 50 mm, the predicted wear, which was found to increase with increasing femoral head radius, remained small owing to the small wear factor associated with these materials. Furthermore, if the head diameter increases beyond 42 mm, a rapid increase in the contact pressure was predicted, owing to the decrease in the wall thickness of the cross-linked UHMWPE cup.