Stryker Osteonics: Prosthetic Knee Joint

We examine, within a simple bearing model of a knee joint that only consideres pure sliding, the effect of the presence of a small vertical hole in the load area on the fluid film properties. The calculations indicate that fluid is entrapped in such a hole, which, for constant load, causes a smaller minimal film separation of the two surfaces. This will lower the horizontal friction, but may also bring about surface contact in high load situations

A porohyperelastic lubrication model for articular cartilage in the natural synovial joint

This work focuses on the proposed mechanisms for the lubrication of synovial joints and applies them to an idealised bearing geometry considering a porohyperelastic material (cartilage) rotating against a stationary rigid impermeable surface. The model captures the behaviour of all lubrication regimes including fluid film formation and boundary contact as the load capacity is increased, representing a major advancement in modelling cartilage mechanics. Transient responses in the fluid phase are shown to be faster than those in the solid phase with the former decaying over time as fluid is exuded from the material. The complex behaviour of fluid migrating to and from the lubricating film is captured which leads to a better understanding of the hydration and friction mechanisms observed

Tribology and Dowson

It is with great sadness that we note the passing of Professor Duncan Dowson on 6th January 2020. Duncan was an esteemed member of the Editorial Board of this journal. He will be remembered as one of the founding fathers of tribology and as a true gentleman. He was the last living member of the Jost Committee, set up by the UK Government (1964–1966) to investigate the state of lubrication education and research, and to establish the requirements of industry in this regard [1]. This committee coined the term “tribology”.Duncan contributed to many areas of tribological research and established many of them, including elastohydrodynamic theory and biotribology

Rough porous circular plates lubricated with couple stress fluid and pressure dependent viscosity

In this paper, the effect of PDV on the couple stress squeeze film lubrication between rough porous circular plates is presented. Keeping the base of Christensen’s stochastic theory, modified Reynolds equation is derived. Fluid film pressure, squeeze film time and load carrying capacity are solved using standard perturbation technique. The results are presented graphically for selected physical parameters and found that the squeeze effect is depleted in a porous bearing compared to its non-porous and increasing permeability has an adverse effect on the pressure, load carrying capacity and time of approach.Publisher's Versio

Tribological Investigation of Articular Cartilage Substitution in the Medial Compartmental Knee

In the development of any cartilage substitution device, an understanding of the tribological response of the natural joint, as well as the treated joint is of upmost importance. Many previous studies have investigated the tribology of potential cartilage substitution therapies, using small scale pin on plate experiments. The aim of the current study was to develop an anatomical and physiological simulation of the medial compartmental bovine knee joint and to use this simulation to investigate a number of cartilage substitution therapies for damaged or diseased knee joints. A pendulum friction simulator was used to apply physiological loading and motion to medial compartmental knees. The wear of the cartilage was characterised following the experiments using three different techniques: surface topography analysis, volumetric wear quantification using µMRI scans and histological assessment. Three main interventions were investigated using this novel anatomical simulation - the tribological response of meniscectomy, the effect of conformity of tibial hemiarthroplasty designs, and finally, a number of potential cartilage substitution materials were investigated as osteochondral repair devices in the femoral condyle. In the first two studies, the removal of the meniscus and a decrease of tibial hemiarthroplasty conformity, resulted in an elevation of the coefficient of friction, contact stress, friction shear stress and subsequently the wear and degradation of cartilage. In the defect repair study, biphasic defect repair materials (hydrogels) had a superior tribological performance to non-compliant single phase materials (stainless steel). Across the three studies, the use of non-biphasic materials and/or the loss of joint congruity resulted in a more rapid decrease of cartilage interstitial fluid pressurisation and biphasic fluid load support, resulting in increased cartilage solid-solid contact and increased wear. This tribological simulation can be used to investigate many potential knee joint intervention, from uni- or hemi-arthroplasty, cartilage defect repair, meniscus substitutes or tissue engineered substitutes. This simulation can be used to further our understanding of the tribological characteristics of more satisfactory and conservative therapies for damaged and diseased synovial joints

Computational and theoretical contact modelling of hip implant devices with the application of wear simulations

Contact mechanics, wear and tribology of hip implant devices have been studied since early implantations and the performance of the devices are becoming increasingly important. Wear and surface damage of these bearing surfaces occur through normal gait loading conditions. However, in addition to this, stripe wear patterns are observed on patient implant retrievals and following hip simulator studies. Novel computational and theoretical methods were used and developed based on advanced computer aided engineering techniques and the finite element method. Hip joint modelling and numerical methodologies of mechanical wear simulations were studied through a newly proposed scripting method. Shakedown theory and maps were referred to for studying the biotribology and contact mechanics of hip resurfacing devices under cyclic normal, severe and edge loading conditions. Through implicit and explicit finite element modelling lateral displacement and laxity based microseparation models were developed. The contact pressure under edge loading conditions was at least a factor of 2 larger than under normal loading conditions. The wear rates of both the femoral head and acetabular cup during the bedding-in period were between 1-3 mm3/mc (million cycles) and 80-110 mm3/mc based on a steady-state wear coefficient. Results showed that modelling and verifying the contact and stress results under edge loading conditions required more careful computational modelling than for normal loading conditions. The high contact pressures observed during simulations of microseparation models were consistent with the high level of wear and surface damage observed in experimental simulator studies and from patient retrievals. These methods can therefore be used as a technique to simulate wear of hip implant devices. Shakedown assessments showed that under normal, as well as edge loading and severe loading conditions the hip device remained below the elastic shakedown region of a rolling and sliding shakedown map, which is ideal for minimal surface contact and subsurface damage

Stress in the cartilage of the human hip joint

Thesis (Sc.D.)--Massachusetts Institute of Technology, Dept. of Mechanical Engineering, 1983.MICROFICHE COPY AVAILABLE IN ARCHIVES AND ENGINEERING.Bibliography: p. 181-195.by Thomas Macirowski.Sc.D

Numerical and Experimental Characterisation of Articular Cartilage – A Study on Biomechanics and Biotribology, Osteoarthritis and Tissue Engineering Solutions

Articular Cartilage (AC) is a soft tissue covering the articulating surface of human and animal joints. The tissue has remarkable and highly complex mechanical and wear properties allowing the joint to undergo complex kinematics and function correctly for several decades. However, trauma and degenerative joint diseases such as osteoarthritis (OA) can cause damage and excessive wear of the tissue and due to its limited regenerative capabilities, can severely compromise joint movement and impair the quality of life. OA is the most common type of degenerative joint disease and the primary cause of joint replacement surgery leading to high associated healthcare costs. Although the exact cause of this pathology remains unknown, it is thought to be mechanically induced via excessive and abnormal stresses and strains in AC which cause altered biochemical properties and a gradual decrease in the mechanical quality of the tissue. There is currently no available cure for OA and the disease is currently being diagnosed only via imaging techniques which are based upon morphological changes of the tissue, when the pathology is already in its advanced stages and has caused irreversible changes to the AC. In this respect, one of the greatest challenges to now remains the early diagnosis of OA, potentially by assessing biochemical and mechanical changes, allowing early treatments and prevention of disability thus improving the patient’s life. Hence, there is a need to apply fundamental engineering principles to the medical world in order to shed light on the pathogenesis and progression of OA. Furthermore, the need for artificial substitutes of AC has called for a deep understanding of the mechanical behaviour of the tissue in order to design and mimic the response of the real tissue in the most accurate manner. In this research a combination of numerical (finite element) and experimental techniques involving mechanical and tribological tests were used to fully characterise the mechanical behaviour of the tissue. Selective degradation of the AC constituents was then induced to simulate OA (OA-like AC) and the effect of different stages of degradation on the mechanical and tribological response as well as the wear properties of the tissue was investigated. The mechanical properties of osteoarthritic AC were then evaluated and compared to the OA-like AC in order to correlate similarities in the variations to the structure and the mechanical response as a result of degradation. Quantifying the mechanical response of the tissue at different stages of OA and different levels of degradation was done to ensure both a thorough understanding of the effect of the pathology’s progression on AC as well as to provide a potential map of mechanical quality and degradation, contributing to the potential future diagnosis of OA via mechanical parameters rather than morphological alone. Having investigated structural and mechanical variation in early OA, a promising solution to treat localised early OA and AC defects was also investigated as part of this research. In particular, novel micro fibrous tissue engineered scaffolds have been mechanically and tribologically assessed and compared to AC demonstrating the strong potential of matrix-assisted autologous chondrocyte implantation (MACI). Finally, the numerical models developed to characterise the AC using numerical – experimental methods, namely advanced biphasic models incorporating fine material descriptions such as intrinsic viscoelasticity as well as transverse isotropy, were applied to a patient specific 3D menisectomised tibio-femoral joint contact model in order to demonstrate the implications that the implementation of different AC models have for the prediction of the joint response to repeated walking cycles. The results obtained from the models were then used to predict the most likely location for the origin of mechanical damage and OA

Biotribology of the Natural Ankle Joint

The ankle joint is a stable and congruent joint that helps to protect the joint surfaces from high impact forces. However, possible trauma to the joint such as severe ankle sprain or fracture can cause cartilage breakdown and eventually lead to cartilage degeneration, resulting in arthritis. Ankle arthritis is considered to be a major cause of morbidity and disability. Although the ankle joint is least affected by arthritis compared to knee and hip joints, the pain and lack of mobility of end stage ankle osteoarthritis (OA) are equally debilitating and tend to be overlooked compared to hip and knee OA. Differences in the incidence rates of osteoarthritis (OA) across the joints could be partly attributed to the biomechanical properties of the articular cartilage. The aim of the thesis was to improve the understanding of mechanical characteristics of the human ankle cartilage through developing and refining methodologies (i.e. indentation and thickness methods) on immature porcine ankle tissues. As porcine ankle joint seems to closely represent the human ankle with comparable anatomical features, cartilage deformation, cartilage thickness, coefficient of friction, surface roughness, contact mechanics and biological properties were also determined. Comparisons of mechanical characteristics between porcine and human tissues were reported. A methodology was developed to identify the most suitable type of specimen (osteochondral samples versus whole joints) for mechanical characterisation as specimen preparation via pin extraction was hypothesised to have an effect on the tissue quality and thus on biomechanical properties. Specimen preparation of osteochondral pins had no impact on properties as cartilage deformation and thickness measurements of pins were comparable to whole joints. Therefore, for mechanical characterisation of human ankle cartilage, osteochondral pins were studied. Porcine talar cartilage was found to be thicker, with higher surface roughness, increased water content, increased contact pressures and lower glycosaminoglycan (GAG) content compared to porcine tibial cartilage. Based on such results, the talar cartilage in the young porcine tissue (3 to 6 months) appeared more susceptible to deterioration over time when compared to tibial cartilage as these properties were considered as unfavourable potentially affecting joint function and quality of tissue during high impact forces. Overall, there were significant differences in thickness, deformation and roughness measurements (ANOVA, p<0.05 for all comparators) across the porcine and human tissues. These differences between animal and human tissues can be attributed to many factors such as age, gait, lifestyle and mechanical properties. The immature porcine cartilage was considered to be a poor representative model for tribological studies. On the human ankle joint, cartilage thicknesses, deformation and surface roughness measurements were all in a comparable range between talar and tibial joint surfaces (ANOVA, p<0.05 for all comparators). Although ankle lesions were reported to be commonly found in the talar surface rather than the tibial surface, and it was assumed to result in unfavourable properties, this was not reported in the current study as no significance was observed between both joint surfaces