Approximate closed-form solution of the synovial fluid film force in the human ankle joint with non-Newtonian lubricant

The aim of this paper is to propose an approximate closed form lubrication model of the human ankle joint by taking into account the porosity of the cartilage matrix and the non-Newtonian behaviour of the synovial fluid. The model is based on the theory of squeeze lubrication and introduce an original modified Reynolds equation obtained modelling the synovial fluid as a couple-stress fluid and the synovial fluid transport across the articular cartilage by using a modified Darcy's equation. This approach gives the advantage to obtain an analytical expression of the synovial pressure field and of the non-stationary fluid film force acting in the synovial joint during the squeeze motion in terms of couple-stress parameter, film thickness, and porosity parameter.     

The effect of porosity of articular cartilage on the lubrication of a normal human hip joint.

The effect of porosity of articular cartilage on the lubrication of a normal human hip joint has been studied. The poroelasticity equation of articular cartilage and the modified Reynolds equation for the synovial fluid lubricant have been successfully solved under squeeze-film motion and for the conditions experienced in a normal human hip joint. It has been shown that porosity of the articular cartilage depletes the lubricant film thickness, rather than increasing it, particularly when the lubricant film thickness becomes small. Furthermore, it has been shown that articular cartilage can be treated as a single-phase incompressible elastic material in the lubrication modelling under physiological walking conditions.     

Directed Ion Transport as Virtual Cause of Some Facilitated Friction–Lubrication Mechanism Prevailing in Articular Cartilage: A Hypothesis

Based on certain characteristics of the acid–base quasi-equilibria and on structural properties of the synovial inhomogeneous fluid in a model articular cartilage (mAC), we try to hypothesize on its facilitated friction–lubrication mechanism. We opt for a scenario that under departure from the acid–base, pH-dependent equilibrium, directed transport of protons (H⁺) is plausible, leading to a certain synergistic kinetic–thermodynamic pathway of the system as a whole. It can be viewed in such a way that protons, and virtually, other ions such as OH⁻; Ca²⁺, may pass through the (intra)micellar, possibly elongated spaces, playing their roles as if they were transported along temporarily formed ion (mainly, H⁺) transmembrane channels. Such a hypothetical scenario would thoroughly contribute to some electrostatics-aided, interstitial (synovial) biofluid pressurization, often reported by experimentalists as the appropriate mechanism of facilitating the lubrication in a real articular cartilage (rAC) in microrheological conditions, encountered in articulating joints of mammals.     

Analysis of normal stress effects in a squeeze film porous bearing

An analytical study of a porous bearing lubricated by a second-order fluid is considered. This investigation explains the working of general porous bearings and, in particular, describes the lubrication aspects of synovial joints. An approximate method for the solution of the governing fluid film equation and Darcy's equation for a porous region is used. Exact expressions for dimensionless pressure, load capacity and response time are obtained. The load capacity and response time for the diseased joint decrease compared with the healthy joint. The decrease in permeability of cartilage enhances the load capacity.     

Quantitative analysis of joint lubrication

A comprehensive theoretical analysis of the extent of elastohydrodynamic lubrication in human joints is presented. The analytical model is developed from existing experimental data on the geometry, loading, kinetics and elastic properties of the hip joint and the viscous properties of synovial fluid. Results of a computer-generated numerical solution of the lubrication equations are given which demonstrate that elastohydrodynamic lubrication does not persist within human joints. An alternative lubrication mechanism based on the information obtained from the analysis is discussed.     

A rational human joint friction test using a human cartilage-on-cartilage arrangement

Efficient lubrication is essential for synovial joint mobility in both health and disease. It is well known that extremely low friction is required for proper functioning of synovial joints. In several medical treatments, bio-lubricants are injected into human joints to maintain their proper functioning. In the course of developing and screening such bio-lubricants, it is important to measure their effect under conditions similar to the ones in vivo. To this end, a first attempt was made to test the friction of two slices of human articular cartilage sliding over each other under various working conditions in the presence of different lubricating fluids. The results can be used for future research in the field of joint lubrication.     

The synovial joint as an “intelligent” friction unit

A concept is advanced and a structural diagram is shown of the synovial joint as an “intelligent” friction unit possessing the properties of a cybernetic system. The joint demonstrates a record-low friction coefficient and compensation of cartilage wear as a triboengineering material maintaining human locomotor activity for many years. It is provided with a feedback system with the sympathetic center controlling the lubricating conditions of the movable joint of the articular bone ends, the lubricating fluid composition, and the elimination of debris from the articular capsule. The main reason why the joint fails is insufficient lubrication due to disruption of the exchange between the bloodstream and the joint synovial shell inducing a deficit of the lubricating synovial fluid. The disruption of the exchange is compensated by injecting synovial fluid substitutes. The paper shows the results of tribological studies and clinical trials providing evidence that the patient’s blood serum can be used as an absolutely compatible lubricating fluid to restore the functions of the articular synovial medium.     

A Model for Intra-Articular Heat Exchange in a Knee Joint

A mathematical model has been developed for the understanding of temperature distribution in knee joint. Temperature rises in knee joint as a result of frictional energy. This heated synovial fluid enters into the articular cartilage by the process of filtration and supplies heat to cartilage and bone. This cooled fluid again mixes well with the lubricant in the joint cavity. The problem is formulated as a two-region flow and diffusion model: flow and thermal diffusion within the intra-articular gap; and within the porous matrix covering the approaching bones at the joint. The solution of the coupled mixed boundary value problem is solved by using perturbation method. It has been observed that, in certain diseased and or old synovial joints, the movement of the fluid into or out of the cartilage resisted, and therefore, the temperature does rise. The temperature does rise in old and diseased joints as observed by varying the values of parameters from its normal values. These values refer to old age and/or diseases affecting degeneration of synovial fluid and or cartilage.     

Exploring possible connections between tribology and osteoarthritis

This paper is part of a continuing study aimed at exploring possible connections between tribology and mechanisms of synovial joint lubrication and degeneration. In a separate paper, the focus was on the tribological behaviour of natural and ‘normal’ synovial joints. The central thrust or purpose of the present paper is to stimulate discussion of ‘abnormal’ joint behaviour, in particular, degenerative joint disease or osteoarthritis, from the point of view of a tribologist, and in the light of our findings on cartilage wear. Some provocative questions are raised. For example, can a lack of ‘proper’ synovial joint lubrication or a specific biochemical ‘anti-wear’ agent lead to degeneration of the joint or more rapid removal of articular cartilage? Does osteoarthritic articular cartilage have poorer resistance to wear than ‘normal’ cartilage? It is not argued that arthritis is a tribological problem. However, it would seem that tribological connections with degenerative joint disease — and possibly other forms of arthritis — are indeed possible, but complex.     

Biotribology: Friction,wear, and lubrication of natural synovial joints

Studies have been carried out over the past several years to explore possible interconnections between tribology — the study of friction, wear, and lubrication — and arthrology, more specifically, mechanisms of synovial joint lubrication and degeneration. The focus of this paper is on the tribological behaviour of natural and so-called ‘normal’ synovial joints. A separate paper deals with possible connections between tribology and degenerative joint disease (e.g., osteoarthritis). The purpose of this paper is fourfold: (1) to present a summary of salient work on mechanisms of synovial joint lubrication; (2) to review the key findings of our in vitro wear studies made with bovine articular cartilage; (3) to discuss the significance of the cartilage wear studies in relation to existing joint lubrication theories; and (4) to describe a new device being used for studies of cartilage-on-cartilage deformation, friction, wear and damage under in vitro conditions.     

Ferrography: Its application to the study of human joint wear

Synovial fluid aspirates of 20 arthroplastic and 150 osteoarthritic joints were analyzed for evidence of wear particles. Ferrography, an industrial technique for the separation of particulate matter from samples of lubricating solutions, allows extraction of wear particles from synovial fluid. Bichromatic polarized microscopy and scanning electron microscopy permit identification and characterization of metallic, polyethylene and acrylic wear particles from arthroplastic joints as well as biological wear fragments of bone, cartilage, meniscus and synovium from osteoarthritic joints. With both techniques, the number and morphology of the wear particles within the synovial fluid specimens correlate with the rate and mechanism of wear as confirmed by examination of the joint implant or articular surfaces. Toxicity of the various types of wear particles was assessed by cytological examination of the fluid aspirate. Of significant interest is the identification of active phagocytosis of wear particles by synovial fluid white blood cells. This finding may implicate the particles as initiators of secondary inflammatory responses, as occurs in other arthritic diseases.Analysis of aspirated synovial fluid appears to be a useful method for studying the rates, mechanisms and biological responses to wear in both arthroplastic and degenerative joints. In surgical joint replacement, this technique holds much promise, not only as a test for wear and toxicity, but also as a means of assisting in the selection of materials and designs for superior articular implants. In osteoarthritic joints, this analysis provides a method for early diagnosis, serial assessment of therapy and prognostication concerning the future course of the disease. Possibly of most significance, in contrast to previous studies on intact articular surfaces, is the ability to study the principal site of degenerative changes, namely the wear particles. This ability may aid in the elucidation of the underlying cause of osteoarthritis.     

Evaluation of a superior lubrication mechanism with biphasic hydrogels for artificial cartilage

To extend the durability of artificial joints, biomimetic artificial hydrogel cartilage is proposed as a way of improving the lubrication mechanism in artificial joints. The application of hydrogels with properties similar to those of articular cartilage can be expected to duplicate the superior load-carrying capacity and lubricating ability of natural synovial joints. Frictional behaviors with three kinds of poly(vinyl alcohol) (PVA) hydrogels with high water content were examined in reciprocating tests. Interstitial fluid pressure, von Mises stress and fluid flow were compared in biphasic finite element analysis, and frictional behavior was evaluated in terms of biphasic lubrication and surface lubricity. Hybrid gel prepared by a combination of cast-drying and freeze-thawing methods showed superior low friction.     

The Fractal Structure of Equine Articular Cartilage

The naturally occurring structure of articular cartilage has proven to be an effective means for the facilitation of motion and load support in equine and other animal joints. Cartilage has been found to be a complex and dynamic medium, which has led to an incomplete understanding of the nature and operating mechanisms acting within a joint. Although cartilage has biphasic and triphasic properties, it is believed that the performance of equine articular joints is influenced by the surface roughness of the joint cartilage (Ateshian et al., '98; Chan et al., 2011; Yao and Unsworth, '93). Various joint types with different motions and regimes of lubrication have altered demands on the articular surface that may affect cartilage surface properties. In research performed on freshly harvested samples, equine articular cartilage has been shown to possess a multiscale structure and a fractal dimension. It is thought that by determining the fractal dimension (D) of articular cartilage, a better understanding of the friction, wear, and lubrication mechanisms for biomechanic surfaces can eventually be reached. This study looks at the fractal dimensions of three different articular cartilage surfaces in the equine carpus: the radiocarpal, midcarpal, and carpometacarpal surfaces. The three surfaces provide an ideal comparison of fractal dimensions for a different range of motion, geometry, and loading. In each sample, identical treatment was performed during measurement by a stylus profilometer. SCANNING 34: 418–426, 2012. © 2012 Wiley Periodicals, Inc.     

A Biphasic Model for Hip-Joint Replacement

The ball-and-socket geometry of the hip joint makes kinematic analysis of the joint motion relatively straightforward in comparison to other joints. The load-carrying surfaces of both ball and socket are covered with tough viscoelastic material known as cartilage. A number of lubrication theories have been proposed in the literature to account for the low coefficient of friction and low wear observed in healthy joints. The actual mechanism by which joints are capable of sustaining large repetitive loads with virtually no wear and with very little friction has not been fully understood. Therefore, analytical studies are presented for the understanding of the lubrication mechanism occurring in hip-joint replacements under restricted motion during standing or in the supporting phase during walking. The viscoelastic fluid has been considered to represent the synovial fluid in the fluid-film region. The problem described here has been analyzed in two regions (the porous matrix and the fluid-film region) separately along with suitable matching and boundary conditions at the interface. It has been concluded that the effect of the viscoelastic parameter for a particular gap is to increase the load capacity, indicating positive effects of the increase in concentration of suspended particles in the lubricant region. It has been observed that the coefficient of friction decreases with increasing values of the viscoelestic parameter. This is due to the fact that as the viscoelastic parameter increases, the concentration of hyaluronic acid molecules increases. It may also be noted from the results that the coefficient of friction decreases with increasing values of slip parameter. This shows that the slip velocity occurring at the porous boundary helps in maintaining normal functioning of human joints.     

Recent developments in the tribology of artificial joints

A history of the tribological development of artificial joints compares how these are lubricated with the mechanisms involved in human joints. It is concluded that while healthy human joints are lubricated by fluid film action, all current artificial joints at best are lubricated by mixed lubrication and hence wear is taking place throughout the life of the prosthesis. A new concept in artificial joints is described. Soft elastic layers simulate articular cartilage and if selected carefully can develop full fluid film lubrication with consequential low friction and minimal wear.     

Tribology of human and artificial joints.

Studies of human joint lubrication mechanisms have led to the conclusion that under normal healthy conditions they are fluid film lubricated. The main features responsible for allowing this mechanism to operate are the dynamic nature of the loading and the compliance of the bearing surfaces (articular cartilage). In contrast, artificial joints, being made of much more rigid materials, have been demonstrated to be lubricated by a mixed regime, where some load is carried by the fluid film and some by solid to solid contact. Since some surface contact takes place then wear remains a problem and friction is much higher than in human joints. The use of compliant surface bearings for artificial joints has been explored and shown to be of great advantage, reproducing the effects of natural joints. However, elastomeric materials are known to degrade in aqueous solutions so this aspect has been examined to ensure a reasonable life in the human body. Joints of the lower limb--hip, knee, and ankle--have similar load and motion patterns and behave in a similar way in terms of lubrication. Joints of the hand are not in any way similar in their behaviour and so a typical upper limb joint, the finger, has been studied to see if improvements can be made to the design of replacement artificial joints. Novel suggestions like plastic on plastic joints have been shown to be an alternative which is worthy of further consideration.     

The Effect of Surface-Active Phospholipids on the Lubrication of Osteoarthritic Sheep Knee Joints: Friction

The synovial fluid aspirate from human joints that have experienced serious traumatic injury has been shown to have lower concentrations of phospholipids when compared with healthy joints. Previous studies provide evidence that synovial fluid constituents, specifically dipalmitoyl phosphatidylcholine (L-DPPC), are highly surface active, capable of rapidly depositing a layer of phospholipids onto glass. Such research has demonstrated that the adsorbed surface layers of synovial surfactant are excellent lubricants in vitro, significantly reducing the coefficient of friction under physiological loading in human knee joints. This study aimed to investigate the effect of concentration of L-DPPC lubricant solutions on the coefficient of friction of worn articular cartilage on steel. A pin-on-disc apparatus was used to measure the coefficient of friction of sheep-knee articular cartilage on steel under unidirectional sliding at physiological conditions of load and speed. Concentrations of L-DPPC solution between 100 times less and 100 times more than is normally present in synovial fluid were tested. All specimens were tested following a period of unlubricated induced wear. Trials were carried out at ambient temperature and between 33–37°C (representative of in vivo joint temperature). Friction measurement results demonstrated a reduction in the coefficient of friction of worn articular cartilage against steel with increasing concentrations of L-DPPC in lubricant solution.     

Development of artificial articular cartilage

Attempts have been made to develop an artificial articular cartilage on the basis of a new viewpoint of joint biomechanics in which the lubrication and load-bearing mechanisms of natural and artificial joints are compared. Polyvinyl alcohol hydrogel (PVA-H), 'a rubber-like gel', was investigated as an artificial articular cartilage and the mechanical properties of this gel were improved through a new synthetic process. In this article the biocompatibility and various mechanical properties of the new improved PVA-H is reported from the perspective of its usefulness as an artificial articular cartilage. As regards lubrication, the changes in thickness and fluid pressure of the gap formed between a glass plate and the specimen under loading were measured and it was found that PVA-H had a thicker fluid film under higher pressures than polyethylene (PE) did. The momentary stress transmitted through the specimen revealed that PVA-H had a lower peak stress and a longer duration of sustained stress than PE, suggesting a better damping effect. The wear factor of PVA-H was approximately five times that of PE. Histological studies of the articular cartilage and synovial membranes around PVA-H implanted for 8-52 weeks showed neither inflammation nor degenerative changes. The artificial articular cartilage made from PVA-H could be attached to the underlying bone using a composite osteochondral device made from titanium fibre mesh. In the second phase of this work, the damage to the tibial articular surface after replacement of the femoral surface in dogs was studied. Pairs of implants made of alumina, titanium or PVA-H on titanium fibre mesh were inserted into the femoral condyles. The two hard materials caused marked pathological changes in the articular cartilage and menisci, but the hydrogel composite replacement caused minimal damage. The composite osteochondral device became rapidly attached to host bone by ingrowth into the supporting mesh. The clinical implications of the possible use of this material in articular resurfacing and joint replacement are discussed.     

Role of ultrafiltration of synovial fluid in lubrication of human joints

Introducing the concept of mixture theory of two interacting continua for the poroelastic cartilage and the micromorphic approach for the synovial fluid, a two-region flow model has been developed in order to study the lubrication characteristics of synovial joints. The fluid transport in the deformable porous cartilaginous matrix is computed from a simple analysis of the coupled equations of motion and the resulting flow into the intra-articular gap. As the gap closes, ultrafiltration of the suspending medium increases the load carrying capacity and closure time. It ultimately leads to the formation of a lubricant gel on the surfaces when the gap reduces to the order of surface asperities.