Natural Synovial Joints Research Articles

Experimental study on boundary lubricity of superficial area of articular cartilage and synovial fluid

The boundary lubrication mechanism at the articulating surface of natural synovial joints has been the subject of much discussion in tribology. In this study, to elucidate the lubricating function of the superficial area of articular cartilage and synovial fluid (SF), cartilage specimens were processed with four different treatments: gentle and severe washing with detergent, incubation in NaCl solution, and trypsin digestion to selectively remove certain constituents from the cartilage surface. Subsequently, the frictional characteristics were examined in phosphate-buffered saline (PBS) and SF against glass. Angularly reciprocating sliding tests with a spherical glass probe and square articular cartilage specimens were performed at low contact loads in the mN range to extract the frictional behavior in the superficial area of the cartilage specimens. Meanwhile, the cartilage surface was observed to confirm the effects of treatments on the morphology of the cartilage surface using a fluorescence microscope and water-immersion methods. The coefficient of friction (COF) of the prepared cartilage specimens was varied from 0.05 to over 0.3 in PBS. However, a certain group of cartilage specimens exhibited a low COF of less than 0.1 with limited variation. For the low COF group of specimens, all four treatments increased the COF in PBS to different extents, and fluorescence microscopy revealed that the integrity of the cartilage surface was deteriorated by treatments. This means that the intact cartilage surface had lubricating constituents to maintain low friction, and the removal of such constituents resulted in the loss of the intrinsic boundary lubricity of the cartilage surface. The variation in the COF of the cartilage specimens was suppressed in SF because it had a clear boundary lubrication effect on the cartilage surface. The lubricating effect of SF could be confirmed even after degenerative treatment.

Milestones in Natural Lubrication of Synovial Joints

From years, the possibility to model, from the physical and mathematical point of view, the tribological phenomena acting in human natural synovial joints has been constituting a great challenge for researchers and scientists. This short review is aimed to summarize main findings on the lubrication modelling in human synovial joints through the years, starting from first insights, underlining the evolution of the proposed theories. The complexity of the natural human synovial joints, in which, biological, fluid dynamic, and tribological phenomena take place, makes this research area fascinating for scientists coming from several investigation fields. This manuscript underlines how, nowadays, on the basis of the findings achieved by scientists thorough the years, it is necessary a deep scientific cooperation between researchers from the different branches of the involved disciplines to achieve a greater knowledge about these tribo-systems taking in to account more points of view.

Biotribology of Synovial Cartilage: A New Method for Visualization of Lubricating Film and Simultaneous Measurement of the Friction Coefficient

A healthy natural synovial joint is very important for painless active movement of the natural musculoskeletal system. The right functioning of natural synovial joints ensures well lubricated contact surfaces with a very low friction coefficient and wear of cartilage tissue. The present paper deals with a new method for visualization of lubricating film with simultaneous measurements of the friction coefficient. This can contribute to better understanding of lubricating film formation in a natural synovial joint. A newly developed device, a reciprocating tribometer, is used to allow for simultaneous measurement of friction forces with contact visualization by fluorescence microscopy. The software allowing for snaps processing and subsequent evaluation of fluorescence records is developed. The evaluation software and the follow-up evaluation procedure are also described. The experiments with cartilage samples and model synovial fluid are carried out, and the new software is applied to provide their evaluation. The primary results explaining a connection between lubrication and friction are presented. The results show a more significant impact of albumin proteins on the lubrication process, whereas its clusters create a more stable lubrication layer. A decreasing trend of protein cluster count, which corresponds to a decrease in the thickness of the lubrication film, is found in all experiments. The results highlight a deeper connection between the cartilage friction and the lubrication film formation, which allows for better understanding of the cartilage lubrication mechanism.

The Effect of Synovial Fluid Composition, Speed and Load on Frictional Behaviour of Articular Cartilage.

Articular cartilage ensures smooth motion of natural synovial joints operating at very low friction. However, the number of patients suffering from joint diseases, usually associated with cartilage degradation, continuously increases. Therefore, an understanding of cartilage tribological behaviour is of great interest in order to minimize its degradation, preserving the reliable function of the joints. The aim of the present study is to provide a comprehensive comparison of frictional behaviour of articular cartilage, focusing on the effect of synovial fluid composition (i), speed (ii), and load (iii). The experiments were realized using a pin-on-plate tribometer with reciprocating motion. The articular cartilage pin was loaded against smooth glass plate while the tests consisted of loading and unloading phases in order to enable cartilage rehydration. Various model fluids containing albumin, γ-globulin, hyaluronic acid, and phospholipids were prepared in two different concentrations simulating physiologic and osteoarthritic synovial fluid. Two different speeds, 5 mm/s and 10 mm/s were applied, and the tests were carried out under 5 N and 10 N. It was found that protein-based solutions exhibit almost no difference in friction coefficient, independently of the concentration of the constituents. However, the behaviour is considerably changed when adding hyaluronic acid and phospholipids. Especially when interacting with γ-globulin, friction coefficient decreased substantially. In general, an important role of the interaction of fluid constituents was observed. On the other hand, a limited effect of speed was detected for most of the model fluids. Finally, it was shown that elevated load leads to lower friction, which corresponds well with previous observations. Further study should concentrate on specific explored phenomena focusing on the detailed statistical evaluation.

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.

Importance of adaptive multimode lubrication mechanism in natural synovial joints

The superior tribological performance in natural synovial joints with low friction and minimal wear appears to be actualized by not single lubrication mode but the synergistic combination of various modes from the fluid film lubrication to boundary lubrication corresponding to the severity of rubbing conditions. We have conducted the biphasic finite element analysis and some experimental studies for articular cartilage to elucidate this ingenious lubrication mechanism from the viewpoint of the adaptive multimode lubrication mechanism. In this paper, the effectiveness of different lubrication modes such as biphasic, boundary, gel-film and hydration lubrication particularly at low speed conditions is discussed.

High fatigue and wear resistance of phospholipid polymer grafted cross-linked polyethylene with anti-oxidant reagent

Event Abstract Back to Event High fatigue and wear resistance of phospholipid polymer grafted cross-linked polyethylene with anti-oxidant reagent Masayuki Kyomoto1, 2, 3, Toru Moro2, 4, Shihori Yamane1, 2, 3, Kenichi Saiga2, 3, Kenichi Watanabe2, 3, Sakae Tanaka4 and Kazuhiko Ishihara1 1 the University of Tokyo, Department of Materials Engineering, Japan 2 The University of Tokyo, Division of Science for Joint Reconstruction, Japan 3 KYOCERA Medical Corporation, Research Department, Japan 4 The University of Tokyo, Sensory & Motor System Medicine, Japan Introduction: Osteolysis caused by wear particles from polyethylene (PE) is a serious complication in the total hip arthroplasty (THA). To reduce wear, we have developed an articular cartilage-inspired technology (Aquala®) for surface modification with poly(2-methacryloyloxyethyl phosphorylcholine [MPC]) (PMPC) grafting. In natural synovial joints, fluid thin-film lubrication by the hydrated layer of cartilage is essential for the smooth motion of joints. Based on the biomimetic concept, a nanometer-scale layer of PMPC was formed on a cross-linked PE (CLPE) surface to better reproduce the ideal hydrophilicity and lubricity of the physiological joint surface. However, wear is only one of several important indicators of the clinical performance of acetabular liners. Oxidative degradation and fatigue fracture have been considered a potential limiting factor. The incorporation of the antioxidant vitamin E (VE) has been proposed recently to prevent the oxidation. Additionally, the fatigue fracture can be potentially avoided by using the VE. Our ultimate goal is to obtain not only high wear resistance but also high oxidative and fatigue resistances for life-long orthopedic bearings. We asked in this study: Will the hydration lubrication characteristics of the PMPC grafting layer and the oxidative stability of VE blended substrate affect fatigue fracture and wear resistance of CLPE liner under an accelerative aging condition? Materials and Methods: PE blended with 0.1 mass% VE was irradiated with gamma-rays (100 kGy) and annealed at 120°C for 12 h for cross-linking (HD-CLPE(VE)). HD-CLPE(VE) coated with benzophenone were immersed in a 0.5 mol/L aqueous MPC solution. The photoinduced graft polymerization on the HD-CLPE(VE) surface was carried out under ultraviolet irradiation of 5 mW/cm2 at 60°C for 90 min (PMPC-grafted HD-CLPE(VE)). All samples were then sterilized by a 25-kGy dose of gamma-rays under N2 gas. The obtained samples were accelerative aged according to the ASTM F2003. Impact-to-wear tests (ASTM F732) were conducted using a Pin-on-disk testing machine. The simulated wear test (ISO14242-3) was performed using a hip simulator. The wear particles isolated from the lubricant after wear test was observed by scanning electron microscopy. Results: In the impact-to-wear test, the PMPC-grafted surface characteristics did not appear to affect the impact fatigue resistance regardless of aging (Fig. 1B&C). Even after impact loads of 2.0 × 106 cycles, we did not observe either mechanical fracture or delamination in the sliding or backside surfaces of all groups. After 1.0 × 107 cycles of the hip simulator test, the PMPC-grafted HD-CLPE(VE) were found to show significantly low and stable wear compared with untreated HD-CLPE(VE) even after accelerative aging (Fig. 2). Substantially fewer wear particles isolated from lubricants were found for both PMPC-grafted HD-CLPE(VE) than for untreated HD-CLPE(VE). Discussion: The PMPC-grafted HD-CLPE(VE) provided high wear resistance. The PMPC-grafted layer leads to a significant reduction in the sliding friction between the surfaces because PMPC grafting causes formation of water thin films that can act as extremely efficient lubricants. The advantage of PMPC-grafting will be brought regardless of substrate characteristics. Conclusion: The PMPC-grafted HD-CLPE(VE) provides high fatigue and wear resistance even under an accelerative aging condition for life-long orthopedic bearings.

人工軟骨候補材の疑似関節液中における潤滑状態とスクイズ膜の関係

Polyvinyl alcohol hydrogel (PVA-H) is proposed as a candidate material for cartilage of an artificial joint. PVA-H is expected to achieve the fluid film lubrication by the squeeze film. On the other hand, there are rarely studies about when the squeeze film formed and how effective the squeeze film was. The purpose of this study is to observe the formation of squeeze film between PVA-H and glass surface. In this study, contact area between the PVA-H and glass was observed by using total internal reflection method simultaneously with the measurement of friction force. A repeated loading-unloading mechanism so as to reproduce natural synovial joint was adopted in this study.

Biphasic and boundary lubrication mechanisms in artificial hydrogel cartilage: A review.

Various studies on the application of artificial hydrogel cartilage to cartilage substitutes and artificial joints have been conducted. It is expected in clinical application of artificial hydrogel cartilage that not only soft-elastohydrodynamic lubrication but biphasic, hydration, gel-film and boundary lubrication mechanisms will be effective to sustain extremely low friction and minimal wear in daily activities similar to healthy natural synovial joints with adaptive multimode lubrication. In this review article, the effectiveness of biphasic lubrication and boundary lubrication in hydrogels in thin film condition is focused in relation to the structures and properties of hydrogels. As examples, the tribological behaviors in three kinds of poly(vinyl alcohol) hydrogels with high water content are compared, and the importance of lubrication mechanism in biomimetic artificial hydrogel cartilage is discussed to extend the durability of cartilage substitute.

Shaft seal for separation of water and air with low frictional torque

A new type of shaft seal was specifically designed to separate water and air with a low frictional torque. The proposed water-sealing system, which was implemented as a shaft seal, has two hydrated seal lips fabricated from polyvinyl formal. An aqueous solution of polyethylene glycol, which is a non-Newtonian fluid, was injected between the seal lips to support the lubrication and sealing action at the dynamic seal faces. These configurations are based on a biomimetic approach, namely, the mechanism in a natural synovial joint. The water-sealing system exhibited a lower frictional torque than that of oil and mechanical seals, and the friction was not influenced by the applied water pressure. The prevention of water leakage from the water phase to the air phase was also confirmed for water pressure up to 0.8MPa. The proposed water-sealing system may contribute to the improvement of the recovery efficiency of natural energy systems utilizing water flows, tidal currents, and ocean currents, in which the rotating speed of a shaft is low under high water pressure.

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.

Superior lubricity in articular cartilage and artificial hydrogel cartilage

In healthy natural synovial joints, the extremely low friction and minimum wear are maintained by their superior load-carrying capacity and lubricating ability. This superior lubricating performance appears to be actualized not by single lubrication mode but by synergistic combination of multimode mechanisms such as fluid film, biphasic, hydration, gel film and/or boundary lubrication. On the contrary, in most artificial joints composed of ultra-high molecular weight polyethylene against metal or ceramic-mating material, boundary and/or mixed lubrication modes prevail and thus local direct contact brings down high friction and high-wear problems. To extend the durability of artificial joint, the reduction in friction and wear by improvement in lubrication mechanism is required as an effective design solution. In this paper, at the start, the mechanism of superior lubricity for articular cartilage is examined from the viewpoints of biphasic and boundary lubrication mechanism. Subsequently, the proposal of biomimetic artificial hydrogel cartilage is put forward to improve the lubricating modes in artificial joints. The tribological behaviours in two kinds of poly(vinyl alcohol) hydrogels are compared with that of natural cartilage. The importance in lubrication mechanism in artificial hydrogel cartilage is discussed.

Influence of synovia constituents on tribological behaviors of articular cartilage

Abstract The extremely low friction and minimal wear in natural synovial joints appear to be established by effective lubrication mechanisms based on appropriate combination of articular cartilage and synovial fluid. The complex structure of cartilage composed of collagen and proteoglycan with high water content contributes to high load-carrying capacity as biphasic materials and the various constituents of synovial fluid play important roles in various lubrication mechanisms. However, the detailed differences in functions of the intact and damaged cartilage tissues, and the interaction or synergistic action of synovia constituents with articular cartilage have not yet been clarified. In this study, to examine the roles of synovia constituents and the importance of cartilage surface conditions, the changes in friction were observed in the reciprocating tests of intact and damaged articular cartilage specimens against glass plate lubricated with lubricants containing phospholipid, protein and/or hyaluronic acid as main constituents in synovial fluid. The effectiveness of lubricant constituents and the influence of cartilage surface conditions on friction are discussed. In addition, the protectiveness by synovia constituents for intact articular cartilage surfaces is evaluated.

J028025 河川流発電のための生体模倣シール(Bio-Star)のトライボロジー特性(第2報)([J028-02]人々のくらしとバイオ(2))

Renewable energy research has been promoted to slow down climate change and maintain economic growth. Streamflow is valued as a source of energy, and environmentally friendly and low-friction sealing system that employs biomimetic technologies is proposed for its utilization. A dynamo-electric generator and ancillary systems with waterproof construction are installed under water. Lip seals with a rotating shaft function to prevent the ingress of water from the outside and excellent frictional properties with various rotation speeds are required to improve the power generation efficiency with constantly changing water flow. The sealing system was successfully developed using a hydrated material (polyvinyl formal; PVF) that mimics articular cartilage in a natural synovial joint. Polyethylene glycol (PEG) dissolved in distilled water, which is a non-Newtonian fluid, was used as a lubricant. These materials have low toxicity and low environmental impact. The biomimetic sealing system exhibits excellent frictional properties with extremely low ingress of water.

Importance of adaptive multimode lubrication mechanism in natural and artificial joints

The healthy natural synovial joints maintain excellent load-carrying capacity and lubricating properties with extremely low friction and minimum wear even under heavily loaded conditions. The superior lubricating performance appears to be actualized by not single lubrication mode but the synergistic combination of various modes depending on the severity of operating conditions. This mechanism is called adaptive multimode lubrication and the application of this good working lubrication mechanism to artificial joints with soft layer is expected to contribute to remarkable improvement in longevity of joint prostheses. However, some of detailed mechanisms in natural synovial joints have not yet been clarified. In this article, the effectiveness of biphasic lubrication in natural synovial joints was examined by biphasic finite element analysis under both the on–off loading (migrating contact) and the continuous loading (continuous contact) to cartilage. Then, the method to suppress the gradual rising in friction for articular cartilage under the continuous loading is discussed. Finally, the effectiveness of fibre reinforcement in hydrogel artificial cartilage was examined in walking simulator test.

C225 バイオミメティックベアリングの潤滑モードと軸受温度の関係(OS-13: 生体熱工学)

An environmentally friendly and low-friction bearing for applications such as tidal power or streamflow generation is proposed using biomimetic technologies. A biomimetic bearing was successfully developed using an ultra-high molecular weight polyethylene (UHMWPE) or a hydrated material, polyvinyl formal (PVF), which mimics articular cartilage in a natural synovial joint. A non-Newtonian fluid, polyethylene glycol (PEG) dissolved in distilled water, was used as a lubricant, and the biomimetic bearing exhibited excellent frictional properties to reduce the mechanical loss.

Effectiveness of adsorbed film and gel layer in hydration lubrication as adaptive multimode lubrication mechanism for articular cartilage

With various daily activities, the effectiveness of adsorbed film formed on a gel-like layer at the uppermost superficial articular cartilage in natural synovial joints becomes important to control the friction and wear of articular cartilage in mixed or boundary lubrication regime as an adaptive multimode lubrication mechanism. Furthermore, in the case where the adsorbed film has been removed, the proteoglycan gel layer is expected to preserve low friction and protect against the wear of bulk cartilage tissue with an effective hydration lubrication mechanism. Besides, it is indicated that the biphasic lubrication plays an important role in lowering of friction in articular cartilage containing a large amount of water. At the present stage, however, the detailed relationship between adsorbed films and hydrated gel layers has not yet been elucidated. In this article, the frictional behaviours of articular cartilage on a glass plate were observed in the reciprocating tests with the restarting process after interruption and unloading. The lubricating effectiveness of adsorbed films in hyaluronate (HA) solutions was examined using intact and partially damaged cartilage specimens. The role of albumin and γ-globulin in relation to the surface conditions of gel layer is discussed. The restarting friction immediately after reloading became lower as a result of recovery of the effective interstitial fluid pressurization and hydration and adsorbed films have a significant effect on the frictional behaviour at local contacts. To clarify the molecular phenomena taking place under rubbing condition, in situ observations of the forming adsorbed film were conducted. The measurements were performed using the fluorescent staining method for protein and HA molecules at low contact pressures.

Development of Early Concepts of Lubrication in Natural Synovial Joints

Despite a significant increase in the number of lubrication studies of articualr cartilage and synovial joints in recent times, the early development of lubrication concepts is particularly important and has played an important role in the understanding of the complex lubrication mechanism in these natural bearings. The majority of the important lubrication concepts were developed before 1970s. The aim of this paper is to present a record of early major lubrication concepts developed for natural synovial joints up to the 1970s; to present a few selected recent studies to interpret the results and contradictions and to highlight the importance of a systematical approach and reinforce the concept of the multi-mode lubrication. Some recently proposed, unique lubrication mechanisms for the natural synovial joints are also discussed.

Cartilage-mimicking, Super Lubricious Bearing Surface Extends Longevity of Artificial Joint Replacements

Artificial joint replacement is an effective treatment for patients with severe arthritis whose number has been increasing due to the expansion of the elderly population. In artificial joint replacements with considerably less efficient lubrication, polyethylene (PE) wear leading to osteolysis and aseptic loosening limits the longevity of total hip arthroplasty (THA). As a result, the quality of all artificial joint replacements is becoming increasingly important. In natural synovial joints under physiological conditions, fluid thin-film lubrication by the hydrated intermediate layer (hydration lubrication) of cartilage is known to be essential for the smooth motion of the joints. A surface gel layer of poly (2-methacryloyloxyethyl phosphorylcholine) (MPC) with cartilage-mimicking structures on a cross-linked PE (CLPE) surface, which may realize ideal hydrophilicity and lubricity resembling the physiological joint surface, has been developed for reducing wear. The suggested approaches for bio-mimicking are surely novel in the field of orthopaedic biomaterials science. The cartilage-mimicking, super lubricious surface of the MPC-grafted CLPE could extend longevity of artificial joint replacement.

Cartilage-mimicking, high-density brush structure improves wear resistance of crosslinked polyethylene: a pilot study.

In natural synovial joints under physiologic conditions, fluid thin-film lubrication by a hydrated layer of the cartilage is essential for the smooth motion of the joints. The considerably less efficient lubrication of artificial joints of polyethylene is prone to wear, leading to osteolysis and aseptic loosening and limiting the longevity of THA. A nanometer-scale layer of poly(2-methacryloyloxyethyl phosphorylcholine) (PMPC) with cartilage-mimicking brushlike structures on a crosslinked polyethylene (CLPE) surface may provide hydrophilicity and lubricity resembling the physiologic joint surface. We asked whether the photoirradiation time during graft polymerization would affect the density and stability of the PMPC layer and the PMPC-grafted surface would enhance the durability of artificial joints. We investigated the effect of photoirradiation time and the resultant characteristics of the PMPC layer on the durability of the CLPE. For each of the PMPC-grafted CLPE surfaces with various photoirradiation times (six groups: 0 [untreated CLPE], 11, 23, 45, 90, and 180 minutes), 18 sample pieces (total of 108 samples) were evaluated in surface analyses, and four cups (total of 24 samples) were evaluated in a hip simulator test. The density of the PMPC layer increased with an increase in the photoirradiation time. The hip simulator test confirmed the PMPC-grafted CLPE with a high density of the PMPC layer exhibited minimal wear as compared with the untreated CLPE. High-density PMPC grafting appears essential for maintaining the high wear resistance of the PMPC-grafted CLPE. To obtain a high-density PMPC layer, the photoirradiation time must be greater than 45 minutes. The cartilage-mimicking, density brushlike structure of the PMPC-grafted CLPE could extend high durability to acetabular cups in THA. Our in vitro findings suggest the wear performance of CLPE acetabular cups in THA can be improved by this approach.