A study on temperature regulation in synovialjoints

This paper deals with studies of the temperature regulation insynovial joints. During articulation, a certain amount ofintra-articular heat is generated due to joint activity andfrictional resistance. The movement of the interstitial fluidwithin the intra-articular gap and the exchange of fluid to andfrom the cartilage plays an important role in joint functionsincluding the regulation of heat. Analytically, the problem isformulated as a two-region flow and diffusion model: flow andthermal diffusion within the intra-articular gap and within theporous matrices covering the approaching bones at the joint. Thecoupled governing equations are solved as a two-region mixedboundary value problem with suitable boundary and matchingconditions and using perturbation technique. It has been observedthat the rise in temperature is more in the osteo-arthriticsynovial fluid as compared to normal and young synovial fluids.In normal subjects, there is hardly any rise intemperature.     

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.     

Rheological effects of synovial fluid on nutritional transport

Synovial fluid is an excellent source of nutrients for the cells of the cartilage, through which water and other solutes like glucose are permeable. It has long been established that the tissue imbibes and exudes fluid when deformed, metabolities are believed to move to and from the cells through the ground substance by mechanical effects and by diffusion. Local variation of the permeability within the articular cartilage plays an important role in nutritional transport. To account for the effects of structural configuration of tissue, rate, depth and amount of solute penetration, we have modelled the cartilage by mixture of two distinct constituents, i.e., an incompressible fluid phase and an incompressible porous solid phase. It is observed that when local permeability decreases, the concentration decreases. When fluid flows into the cartilage due to metabolism, the solid portion of the porous matrix increases. Due to the increased solidity of the cartilage matrix, less fluid enters into the cartilage and nutritional transport decreases. In the case of diseased joints the nutritional transport is very difficult, owing to increased rigidity or local variation of permeability within the cartilage. The concentration distribution at the same depth in articular cartilage for low-molecular-weight solutes is less than that for high-molecular-weight solutes. Thus, for low-molecular-weight solutes, the phenomenon of nutrition transport is diffusion dominated whereas for high-molecular-weight solutes, it is dominated by mechanical pumping action. The paper further concludes that in the process of imbibition and exudation, the cells of the middle area of the cartilage surface get more nutrition as compared to the cells at the periphery, so the earliest signs of cartilage degeneration appear in the unstressed areas. Therefore joint motion is assumed necessary for cartilage nutrition. It also concludes that as the viscoelastic parameter increases, the concentration decreases in the articular cartilage so that the cells of the cartilage get less nutrition and can deteriorate.     

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.     

Shear properties of articular cartilage of a bovine knee joint subjected to moderate and high loads: an experimental study.

Freshly excised bovine knee joints were subjected to oscillation under constant load on a specially designed knee joint articulating machine with the joints subjected to moderate and high loadings of 1471.5 and 2943 N respectively. Instantaneous and equilibrium shear moduli of the articular cartilage obtained from the experimental knee joints were measured on a mechanical indentor (DuPont 943 TMA) and compared with the corresponding values of the shear moduli of the cartilage obtained from the control knee joints. At moderate load, both the instantaneous and the equilibrium shear moduli exhibit significant increase in their values. However, at high load the constant shear moduli showed a decrease in its value whereas the value of the equilibrium shear modulus was observed to increase slightly.     

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.     

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.     

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.     

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.     

The effects of proteins on the friction and lubrication of artificial joints

The tribological testing of artificial hip and knee joints in the laboratory has been ongoing for several decades. This work has been carried out in an attempt to simulate the loading and motion conditions applied in vivo and, therefore, the potential for the success of the joint. However, several different lubricants have been used in these tests. The work documented in this paper compares results obtained using different lubricants and makes suggestions for future work. Hip joints and knee joints of different material combinations were tested in a friction simulator to determine their friction and lubrication properties. Both carboxymethyl cellulose (CMC) fluids and bovine serum (with CMC fluids added) were used as the lubricants. These were prepared to various viscosities to produce the Stribeck plots. Human synovial fluid, of just one viscosity, was used as the lubricant with some of the joints to give a true comparison with physiological lubricants. The results showed that, in most cases, the lubricant had a significant effect on the friction developed between the joint surfaces. This is thought to be due to the proteins that are present within the bovine serum adsorbing to the bearing surfaces, creating 'solid-like' films which rub together, protecting the surfaces from solid-to-solid contact. This would be beneficial in terms of wear but can either increase or decrease the friction between the contacting surfaces. It is important to simulate the conditions in vivo as closely as possible when testing these joints to try to obtain a better comparison between the joints and to simulate more accurately the way that these joints will operate in the body. In an attempt to simulate synovial fluid, bovine serum seems to be the most popular lubricant used at present. It would be beneficial, however, to develop a new synthetic lubricant that more closely matches synovial fluid. This would allow us to predict more accurately how these joints would operate long-term in vivo.     

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.     

Effect of Magnetic Field in Lubrication of Synovial Joints

A two region flow model has been developed in this paper in the presence of external magnetic field for the better understanding of synovial joint lubrication mechanism. The model consists of two parallel porous cartilageous surfaces separated by a thin film of non newtonian lubricant representing the synovial fluid which is assumed to behave like a paramagnetic fluid system. In this paper, we have represented the cartilage by a mixture of two interacting continua and synovial fluid by viscoelastic fluid. A transverse magnetic field is applied to the system. We have used the modified form of Darcy’s law given by Zahn and Rosenweig; to describe the penetration dynamics of magnetic fluids through porous media. Because of exact solution not being possible for the governing non-linear partial differential equations, the perturbation method has been used to obtain approximate solutions. The results have been obtained by computational techniques and compared by results available in the literature. In this paper, the possibility of increased efficiency of joint lubrication, particularly in diseased states by the application of applied magnetic fields has been explored. The applied magnetic field increases the load carrying capacity. Which helps in sustaining greater loads. Similarly, the viscoelastic parameter describes the increase in the concentration of the suspended hyaluronic acid molecules which, in turn, increases the overall viscosity of the lubricant, which also helps in sustaining greater loads.     

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.     

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 discovery of the synovial lymphatic stomata and lymphatic reabsorption in knee effusion

To illustrate the mechanism of lymphatic reabsorption in knee joint effusion. The current investigation employed transmission electron microscopy (TEM) and scanning electron microscopy (SEM) techniques to reveal the ultrastructure of the knee synovial membrane in New Zealand rabbits and human. Ultrastructural changes of the synovial lymphatic stomata were observed by using trypan blue absorption and sodium hydroxide (NaOH) digestion methods, and the animal models of synovitis. New Zealand rabbits and human synovial membranes were composed of two types of synovial cells: type A and type B. No lymphatic stomata were found among type A synovial cells, whereas lymphatic stomata with the diameters ranging 0.74–3.26 µm were found in type B synovial cells, and some stomata were closed. After the NaOH digestion, a number of sieve pores, similar to lymphatic stomata in size and shape, were observed in the dense fibrous connective tissue underneath the type B synovial cells. After injecting trypan blue into the rabbit knee joint cavity, absorption of trypan blue through the lymphatic stomata was observed, suggesting the absorption function of the synovial lymphatic stomata. In the rabbit knee joint synovitis models, the synovial lymphatic stomata diameter enlarged. Some macrophages migrated from the lymphatic stomata, indicating that the synovial lymphatic stomata were involved in the joint effusion absorption and inflammatory response. Our study is the first to report the existence of synovial lymphatic stomata in the New Zealand rabbits and human knee joints. Lymphatic stomata may have an important role in the reabsorption of joint effusion. Microsc. Res. Tech. 78:479–484, 2015. © 2015 Wiley Periodicals, Inc.     

Wear and lubrication in an artificial knee joint replacement

A theoretical study is made of the lubrication mechanism occuring in knee joint replacement under restricted motion. The idealised model has been shown to produce results, consistent with those in normal situations. Effects of increase in concentration of suspended particles in the analysis are similar to that of increase in concentration of hyaluronic acid molecules in synovial fluid. Important deductions are made for load capacity and volume wear rate and it has been shown that the slip velocity plays an important role in maintaining the self-adjusting nature of human joints.     

Isolation and analysis of articular joints wear debris generated in vitro

The total replacement of damaged or diseased synovial joints represents one of the greatest advances in orthopaedic surgery of the 20th century. Whereas replacements are available for the shoulder, ankle, elbow, and knee, hip accounts particularly for the most surgical interventions. In France, 100,000 hip joints per year are replaced and all the implants consist of a sliding pair represented by a hard counter face, either metal or ceramic, and commonly a softer polymer. Ultra high molecular weight polyethylene (UHMWPE) was first used in joint replacement in the early 1960s. Since that time, it had been the dominant polymer for bearing surfaces in orthopaedic surgery. However, generation of UHMPWE wear debris from bearing surfaces in patients is the major problem for long term implants. Both volume and morphology of the wear particles determine the response of the body to debris, and subsequent effects on secure fixing.This paper presents a review of the type of particles most frequently found in biopsies of tissues from explanted prostheses. Indeed, the size and the amount of these debris are very important factors for a better understanding of wear and corrosion wear processes in artificial joints. Real wear particles are also described in this paper.