The effect of contact stress on cartilage friction, deformation and wear

Following hip hemiarthroplasty, a metal femoral head articulates against natural acetabular cartilage. Cartilage friction and wear may be influenced by variables including loading time, contact stress, contact area, sliding distance, and sliding speed. The aim of this study was to investigate the effect of these variables on cartilage friction, deformation and wear in a simulation using idealized geometry model. Bovine cartilage pins were reciprocated against metal plates to mimic a hemiarthroplasty articulation under static loading. The effective coefficient of friction (micro elf) under contact stresses (0.5 to 16 MPa), contact areas (12 and 64 mm2), stroke lengths (4 and 8 mm), sliding velocities (4 and 8 mm/s), and loading time (1 and 24 hours) were studied. The permanent deformation of cartilage (after 24 hours of recovery) with and without motion was recorded to assess cartilage linear wear. The micro eff was found to remain < 0.35 with contact stresses < or =4 MPa. Severe damage to the cartilage occurred at contact stresses > 8 MPa and significantly increased micro eff after 12 hours of reciprocation. In long-term, contact area had no significant effect on micro eff, and sliding distance and velocity only affected micro eff under low contact stresses. The cartilage linear wear increased with contact stress, sliding distance and velocity.     

The influence of continuous sliding and subsequent surface wear on the friction of articular cartilage.

Reciprocating motion friction tests were conducted upon cartilage-on-metal contacts while subjected to a constant load. Initial friction coefficients were compared with repeat friction coefficients following a sufficient load removal period. The repeat friction coefficients were marginally higher than the initial values and both were primarily dependent on the loading time. It was concluded that while a wear component had been identified, which modestly increased friction coefficients, the overriding parameter influencing friction was loading time. The authors postulate that fluid phase load carriage (being dependent on loading time) within the articular cartilage is largely responsible for low friction coefficients in the mixed and boundary lubrication regimes. This mechanism has been referred to as biphasic lubrication. Both synovial fluid and Ringer's solution were used as lubricants. Over the assessed 120 min loading time friction coefficients rose from 0.005 (for both lubricants) after 5 s to 0.50 and 0.57 for synovial fluid and Ringer's solution respectively. Synovial fluid was found to significantly reduce friction coefficients compared to Ringer's solution over broad ranges of the assessed loading times (p < 0.05). Stylus and non-contacting laser profilometry were successfully employed to provide reliable, quantitative and accurate measures of surface roughness. Laser profilometry before and after a continuous sliding friction test revealed a significant increase in surface roughness from Ra = 0.8 (+/- 0.2) micron to Ra = 2.1 (+/- 0.2) microns, (p < 0.0005); confirming that surface wear was occurring. Scanning electron microscopy (SEM) revealed the typical highly orientated collagen fibres of the superficial tangential zone. Environmental SEM (ESEM) of fully hydrated cartilage specimens provided largely featureless images of the surface which suggested that sample preparation for conventional SEM was detrimental to the authenticity of the cartilage surface appearance using SEM. Two distinct acellular, non-collagenous surface layers were identified using ESEM and transmission electron microscopy (TEM); respectively referred to as the boundary layer and surface lamina. The phospholipid/glycoprotein based boundary layer will provide boundary lubrication during intimate contact of opposing cartilage surfaces. The surface lamina, being a continuum of the proteoglycan interfibrillar matrix, is present to prevent fibrillation of the underlying collagen fibres. Both layers may contribute to the time dependent frictional response of articular cartilage. Although laser profilometry did reveal surface wear which was consistent with a small increase in friction, the primary variable controlling the friction coefficient was the period of loading.     

Correlation between friction of articular cartilage and reflectance intensity from superficial images

The lubrication mechanism of articular cartilage is characterized by an efficient performance. In this work, friction of articular cartilage was evaluated with in-site images of articular surface. The images were captured with the laser light reflected at the interface between a prism and articular cartilage. The attenuation of reflectance was associated with the increase of the contact of collagen network of articular cartilage. The light reflectance and friction coefficient for short sliding presented a significant positive correlation. Friction tests were also carried out for short (30 s) and long (300 s) preloading times. The results indicate that depletion of fluid film is responsible for the increase of friction and the recovery of the fluid film was observed for the long preloading after the early stage of sliding.     

The effect of cartilage deformation on the laxity of the knee joint

In this paper, deformation of the articular cartilage layers is incorporated into an existing two-dimensional quasi-static model of the knee joint. The new model relates the applied force and the joint displacement, as measured in the Lachmann drawer test, and allows the effect of cartilage deformation on the knee joint laxity to be determined. The new model augments the previous knee model by calculating the tibio-femoral contact force subject to an approximate 'thin-layer' constitutive equation, and a method is described for finding the configuration of the knee under a specified load, in terms of a displacement from a zero-load reference configuration. The results show that inclusion of deformable cartilage layers can cause a reduction of between 10 and 35 per cent in the force required to produce a given tibial displacement, over the range of flexion angles considered. The presence of cartilage deformation was found to be an important modifier of the loading response but is secondary to the effect of ligamentous extension. The flexion angle dependence of passive joint laxity is much more strongly influenced by fibre recruitment in the ligaments than by cartilage deformation.     

Investigation of the friction and surface degradation of innovative chondroplasty materials against articular cartilage

Understanding the wear of the biomaterial-cartilage interface is vital for the development of innovative chondroplasty. The aim of this study was to investigate a number of biphasic materials as potential chondroplasty biomaterials. Simple geometry friction and wear studies were conducted using bovine articular cartilage pins loaded against a range of single-phase and biphasic materials. The frictions of each biomaterial was compared within simple and protein-containing lubricants. Longer-term continuous sliding tests within a protein containing lubricant were also conducted at various loading conditions to evaluate the friction and degradation for each surface. All single-phase materials showed a steady rise in friction, which was dependent on the loss of interstitial fluid load support from the opposing cartilage pin. All biphasic materials demonstrated a marked reduction in friction when compared with the single-phase materials. It is postulated that the biphasic nature of each material allowed an element of fluid load support to be maintained by fluid rehydration and expulsion. In the longer-term study, significant differences in the articular cartilage pin (surface damage) between the positive control (stainless steel) and the negative control (articular cartilage) was found. The potential biphasic chondroplasty materials produced a reduction in articular cartilage pin damage when compared with the single-phase materials. The changes in surface topography of the cartilage pin were associated with increased levels of friction achieved during the continuous wear test. The study illustrated the importance of the biphasic properties of potential chondroplasty materials, and future work will focus on the optimization of biphasic properties as well as long-term durability, such that materials will more closely mimic the biotribology of natural articular cartilage.     

Influence of hyaluronic acid on the time-dependent friction response of articular cartilage under different conditions

Therapeutic lubricant injections of hyaluronic acid are a relatively recent treatment for osteoarthritis. Their efficacy, however, in vivo has been subject to much debate. Frictional properties of cartilage-cartilage contacts under both static and dynamic loading conditions have been investigated, using healthy cartilage and cartilage with a physically disrupted surface, with and without the addition of a therapeutic lubricant, hyaluronic acid. Most of the cartilage friction models produced typical time-dependent loading curves, with a rise in static friction with loading time. For the dynamic loading conditions the rise in friction with loading time was dependent on the spatial (and time) variation in the load on the cartilage plate. For sliding distances of 4 mm or greater, when the cartilage plate was unloaded during sliding, the dynamic friction remained low whereas, with shorter sliding distances, the dynamic friction increased with increasing loading time. Static friction was higher than dynamic friction (under the same tribological conditions). The 'damaged' cartilage models produced higher friction than healthy cartilage under equivalent tribological conditions. It was shown that hyaluronic acid was an effective boundary lubricant for articular cartilage under static conditions with both healthy and damaged cartilage surfaces. Hyaluronic acid was less effective under dynamic conditions. However, these dynamic conditions had low friction values with the control lubricant because of the effectiveness of the intrinsic biphasic lubrication of the cartilage. It was only under the tribological conditions in which the cartilage friction was higher and rising with increasing loading time because of depletion of the intrinsic biphasic lubrication, that the role of hyaluronic acid as an effective therapeutic lubricant was demonstrated.     

Combined effect of friction and macroscopic deformation on asperity flattening

The combined effect of friction and macroscopic plastic deformation on asperity flattening is studied. Crushing of a periodic array of wedge-like asperities is formulated as a rigid-viscoplastic periodic indentation problem with superimposed macroscopic deformation. A micromechanical framework is developed and the corresponding boundary value problem is solved using the finite element method. An anomalous regime of asperity flattening is predicted at low flattening rates, in which the effect of friction on asperity flattening is opposite to that observed in the absence of macroscopic deformation and also at high flattening rates. An incremental elastoplastic analysis confirms this finding.     

Tribological testing of articular cartilage of the medial compartment of the knee using a friction simulator

The aim of this study was to develop a tribological simulation of a unicondylar natural knee, to measure the friction and wear of articular cartilage (AC) against itself (AC-vs-AC) and against stainless steel (SS) simulating a hemiarthroplasty (AC-vs-SS). AC-vs-AC produced low levels of friction and no wear was found. AC-vs-SS showed higher levels of friction and significant AC wear. Using AC-vs-SS with elevated loading, the wear of AC was accelerated and through to bone. This study demonstrated the importance of contact stress in the design of a spacer hemiarthroplasty for the medial compartmental knee. Initial results showed the importance of contact mechanics, stress and biomaterial type in determining short-term tribological function and long-term clinical outcome of hemiarthroplasty.     

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.     

A study on the friction properties of poly(vinyl alcohol) hydrogel as articular cartilage against titanium alloy

Many biomaterials are being developed to repair or replace articular cartilage. One of these materials, poly(vinyl alcohol) (PVA) hydrogel prepared from aqueous solution of the polymer by freezing and thawing method may exhibit the mechanical properties required to withstand the harsh environment of diarthrodial joints. To better understand how PVA hydrogel friction is affected by different variable factors, a three-factor, three-level designed orthogonal experiment was developed. Factors include lubricant, sliding speed, and normal load. Friction coefficient of the PVA hydrogel was found to depend significantly on load and sliding speed. Lubricant had little effects on the friction coefficient. Friction coefficient of the PVA hydrogel decreased with the increase of sliding speed and the friction coefficient approximately increased linearly with the increasing load. Average friction coefficient decreased from 0.0447 to 0.0379 while the sliding speed increased from 0.06 to 0.22 m/s. Average friction coefficient increased from 0.0276 to 0.0546, almost increasing one time, while the load increased from 5 to 15 N.     

The potential of hydrogels as synthetic articular cartilage

The relatively poor mechanical properties of conventional synthetic hydrogels are illustrated and compared with those of articular cartilage. By using the composite structure of the natural material as a model a new family of hydrogels, based on interpenetrating polymer network (IPN) technology, has been developed. The underlying synthetic strategies are discussed and the properties of a novel representative network presented. IPN formation produces networks that are stiffer and stronger than the hydrogel copolymers of similar water content. In this behaviour these simple IPNs begin to mimic the properties of biological hydrogel composites. Thus, these materials have exciting potential for demanding in vivo applications.     

Load-displacement-time characteristics of articular cartilage

The load-displacement-time characteristics of cartilage are modelled in a simple analytical way, and the behavior under steady and oscillating loads is predicted numerically. Comparison with constant-load creep experiments shows fairly good agreement. The model predicts that under oscillating loads of typical physiological magnitude and frequency, the effect on displacement of fluid transport through the matrix is negligible in any one cycle.     

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.     

Microwave-enhanced fixation of rabbit articular cartilage

Cartilage, being highly aqueous, is difficult to preserve for electron microscopy without artefacts. Microwave-enhanced fixation is suggested as a standard method for block samples of this material, with dimensions of up to 12 × 7 × 3 mm. Cartilage samples from the tibial plateau of adult rabbits were fixed by conventional, cryo- or microwave-enhanced fixation. Constant or cyclical microwave irradiation of samples, immersed in fixatives, was carried out to varying final solution temperatures. Microwave-enhanced fixation and staining is shown to be both rapid and reproducible, giving fine structural preservation. Below 323 K microwave fixation always gave excellent preservation of the fine structure within seconds. At higher temperatures thermal artefacts were introduced. In this study the microwave-enhanced fixation is equal in quality to the best conventional immersion fixation and is nearly as fast as cryo-preservation. It provides a standardized, reproducible fixation for morphological studies on cartilage with good process control.     

Effects of ageing on the biomechanical properties of rat articular cartilage

Previous studies have demonstrated that male Sprague Dawley (SD) rats experience age-related bone loss with the same characteristics as that in ageing men. As articular cartilage, like bone, is a critical component of the health and function of the musculoskeletal system, the authors hypothesized that articular cartilage in the untreated male SD rats could be a suitable model for studying the age-related deterioration of articular cartilage in men. To test this hypothesis, male SD rats were killed at between 6 and 27 months. The right femur of each rat was removed. The effects of ageing on the structural integrity of the distal femoral articular cartilage were studied by biomechanical testing with a creep indentation apparatus. The aggregate modulus, Poisson's ratio, permeability, thickness, and percentage recovery of articular cartilage were determined using finite element/non-linear optimization modelling. No significant differences were observed in these biomechanical properties of the distal femoral articular cartilage as a function of age. Therefore, untreated male SD rats appear to be unsuitable for studying the age-related changes of articular cartilage as they occur in men. However, and more intriguingly, it is also possible that ageing does not affect the biomechanical properties of articular cartilage in the absence of cartilage pathology.     

The fate of implanted autologous chondrocytes in regenerated articular cartilage

Autologous chondrocyte implantation (ACI) is used to treat some articular cartilage defects. However, the fate of the cultured chondrocytes after in-vivo transplantation and their role in cartilage regeneration remains unclear. To monitor the survival and fate of such cells in vivo, the chondrocytes were labelled with a lipophilic dye and the resultant regenerated tissue in dogs examined. It was found that, 4 weeks after implantation, the osteochondral defects were filled with regenerative tissue that resembled hyaline cartilage. Fluorescence microscopy of frozen sections of the regenerated tissue revealed that the majority of cells were derived from the DiI-labelled implanted chondrocytes. From these results, it was concluded that a large population of implanted autologous chondrocytes can survive at least 4 weeks after implantation and play a direct role in cartilage regeneration. However, it remains unknown whether other cells, such as periosteal cells or bone marrow stromal stem cells, are involved in the regeneration of cartilage after ACI.     

The effect of friction on the usability of touchpad

To investigate the degradation of the touchpad usability by surface wear, the touchpad with new, worn, plastics and paper surfaces are examined in terms of performance and frictional response. It is found that the friction coefficient of the surface affect the usability. It is concluded that the major factors controlling the friction was surface roughness. It is also found that friction coefficient is affected by the difficulty of the task. The reasons for this is the human tendency to be cautious, he/she try to reduce the load to be precise. Then the load dependence of friction coefficient results in high friction. The other reason is mental sweating, which will be increased when the task is difficult.     

The preparation of human articular cartilage for scanning electron microscopy

This paper compares sixteen preparative techniques thought to be of advantage in the study by scanning electron microscopy (SEM) of human articular cartilage surfaces. The adequacy of surface preservation obtained with the techniques, was judged subjectively, first, by the reproducibility of secondary electron images of normal cartilage, and second, by comparing the results with those obtained by reflected light microscopy of the fresh unfixed cartilage surface over a magnification range of × 20 – × 240. Adequate surface preservation was confirmed when cartilage surfaces had been dehydrated through ethanol to propylene oxide and vacuum dried; dehydrated through amyl acetate and quenched in Freon before freeze-drying; dehydrated and passed through amyl acetate at low temperature before freeze drying. Valuable information can be obtained from different specimens by varying the technique of preparation. At different ages, different surface features are best preserved. In a systematic study it has been found essential to adopt a uniform preparative method and to control the results by reflected light microscopy. Even with the most perfect preparation, the surface appearances cannot be identical with those that function under load in vivo.