D. Nečas

The effect of lubricant constituents on lubrication mechanisms in hip joint replacements

The aim of the present paper is to provide a novel experimental approach enabling to assess the thickness of lubricant film within hip prostheses in meaning of the contribution of particular proteins. Thin film colorimetric interferometry was combined with fluorescent microscopy finding that a combination of optical methods can help to better understand the interfacial lubrication processes in hip replacements. The contact of metal femoral head against a glass disc was investigated under various operating conditions. As a test lubricant, the saline solution containing the albumin and γ-globulin in a concentration 2:1 was employed. Two different mean speeds were applied, 5.7 and 22mm/s, respectively. The measurements were carried out under pure rolling, partial negative and partial positive sliding conditions showing that kinematic conditions substantially affects the formation of protein film. Under pure rolling conditions, an increasing tendency of lubricant film independently on rolling speed was detected, while the total thickness of lubricant film can be attributed mainly to albumin. When the ball was faster than the disc (negative sliding), a very thin lubricant film was observed for lower speed with no significant effect of particular proteins. The increase in sliding speed led to the increase of film thickness mainly caused due to the presence of γ-globulin. On the contrary, when the disc was faster than the ball (positive sliding), the film formation was very complex and time dependent while both of the studied proteins have shown any qualitative change during the test, however the effect of albumin seems to be much more important. Since a very good agreement of the results was obtained, it can be concluded that the approach consisting of two optical methods can provide the fundamental information about the lubricant film formation in meaning of particular proteins while the simultaneous presence of other constituents in model synovial fluid.

In situ observation of lubricant film formation in THR considering real conformity: The effect of diameter, clearance and material

The aim of the present study is to provide an analysis of protein film formation in hip joint replacements considering real conformity based on in situ observation of the contact zone. The main attention is focused on the effect of implant nominal diameter, diametric clearance and material. For this purpose, a pendulum hip joint simulator equipped with electromagnetic motors enabling to apply continuous swinging flexion-extension motion was employed. The experimental configuration consists of femoral component (CoCrMo, BIOLOX®forte, BIOLOX®delta) and acetabular cup from optical glass fabricated according to the dimensions of real cups. Two nominal diameters were studied, 28 and 36mm, respectively, while different diametric clearances were considered. Initially, a static test focused on the protein adsorption onto rubbing surfaces was performed with 36mm implants. It was found that the development of adsorbed layer is much more stable in the case of metal head, indicating that the adsorption forces are stronger compared to ceramic. A consequential swinging test revealed that the fundamental parameter influencing the protein film formation is diametric clearance. Independently of implant diameter, film was much thicker when a smaller clearance was considered. An increase of implant size from 28mm to 36mm did not cause a substantial difference in film formation; however, the total film thickness was higher for smaller implant. In terms of material, metal heads formed a thicker film, while this fact can be, among others, also attributed to clearance, which is more than two times higher in the case of ceramic implant.

Lubrication within hip replacements – Implication for ceramic-on-hard bearing couples

The objective of the present study is to clarify the lubrication processes within artificial joints considering the ceramic femoral heads focusing on the role of particular proteins. Two optical methods were employed; colorimetric interferometry and fluorescent microscopy. The experiments were conducted in ball-on-disc configuration, where the ball is made from ceramic (Sulox(TM), BIOLOX(®)delta) and the disc from optical glass. The measurements were realized under pure rolling, partial negative and partial positive sliding, to get a complex information about the protein film behaviour under various conditions. Moreover, two different speeds were investigated; 5.7 and 22 mm/s, respectively. The contact was lubricated by saline solutions containing albumin and γ-globulin in a ratio 2:1, while the total protein concentration was 10.5 mg/ml. Under pure rolling conditions, the film thickness gradually increases with time/rolling distance independently of material and rolling speed, while the dominant fluid constituent is albumin. In the case of negative sliding, the film formation is time/distance/speed dependent. At lower speed, both proteins contribute to film thickness; at higher speed, the effect of γ-globulin is not substantial. When the disc is faster, the character of film formation is similar to the metal component in the case of Sulox ceramic. Biolox ceramic shows a different behaviour, while for both materials, the contribution of γ-globulin increases with increasing speed. As most of the results can be well explained in terms of specific proteins, it can be concluded that the experimental approach is suitable for the investigation of protein film formation considering the ceramic materials.

Lubricant Rupture Ratio at Elastohydrodynamically Lubricated Contact Outlet

Elastohydrodynamically lubricated (EHL) contacts rarely exist as single contacts. Multiple contacts or single contacts subjected to the repeated over-rolling represent more often the case in practical applications. A typical example is the rolling element bearing. A lubricant rupture mechanism at each contact outlet determines the lubricant availability to the succeeding contact. This work presents a quantitative description of the lubricant film thickness rupture in EHL contact outlet with the use of the fluorescent microscopy. A rupture ratio of the film thickness between two diverging surfaces exiting the contact was measured for both pure rolling and rolling–sliding conditions. The influence of variation of several parameters such as lubricant properties, rolling speed or rolling element ellipticity to the lubricant rupture ratio was investigated. Understanding of the physical phenomena of the lubricant rupture extends further possibilities in both experimental and theoretical researches of the starved EHL.

Tribological Performance of Ti–Si-Based in Situ Composites

ABSTRACT In this study, a series of Ti–Si-based in situ composites was manufactured by means of a common argon arc melting technique and tribologically evaluated using a sliding ball-on-disc tester under simulated body fluid lubrication. The composite microstructure, mechanical properties, and surface roughness were characterized using light and scanning electron microscopy (SEM), vertical scanning interferometry (VSI), X-ray diffraction (XRD) analysis, and hardness measurements. The evolution of coefficients of friction (COFs) and the appearance of contacting surfaces showed that two the principal wear mechanisms were mixed elastohydrodynamic lubrication (EHL), typically followed by abrasive wear. The mixed EHL was due to the combined effect of serum solution lubrication and surface irregularities, which were produced during the routine surface preparation of samples. The mixed EHL provided the absence of wear and low and stable COFs, which did not depend on the phase composition, microstructure, or hardness of Ti–Si-based alloys. However, in most cases, the change in contact geometry led to the transition from mixed EHL to conventional boundary lubrication, accompanied by increased and unstable friction, adhesive material transfer of metal to the ceramic counterbodies, and abrasive wear. In this respect, the low wear resistance and high adhesion affinity of the titanium matrix of Ti–Si-based alloys should be improved.

On the observation of lubrication mechanisms within hip joint replacements. Part II: Hard-on-hard bearing pairs

The present study describes the lubrication mechanisms within artificial hip joints considering real conformity of rubbing surfaces. Part I is focused on hard-on-soft material combination, introducing the fundamentals of lubrication performance. These pairs have not been explored in terms of in situ observation before. The contact of metal femoral component articulating with transparent polymer acetabular cup was studied using a hip joint simulator. The film formation was evaluated by fluorescent microscopy method. Various model synovial fluids were employed while the key constituents, i.e. albumin, γ-globulin, and hyaluronic acid were fluorescently stained to determine its role in film formation process. Two types of the tests were performed. The first dynamic test aimed on the development of film thickness under constant load during motor driven swinging motion mimicking flexion-extension. Subsequently, a combined test was designed consisting of the three phases; static part with loading/unloading phase (1), pendulum swinging till spontaneous damping of the motion due to friction (2), and static observation under the constant load (3). The results clearly confirmed that the interaction of constituents of synovial fluid plays a dominant role and substantially influences the lubrication conditions. In particular, the main finding coming from the present study is that γ-globulin together with hyaluronic acid form relatively thin stable boundary layer enabling the enhanced adsorption of albumin, thus increasing the lubricant film. Part II of the present study is focused on hard-on-hard pairs while the main differences in film formation process are highlighted among others.

The Effect of Kinematic Conditions and Synovial Fluid Composition on the Frictional Behaviour of Materials for Artificial Joints

The paper introduces an experimental investigation of frictional behaviour of materials used for joint replacements. The measurements were performed using a ball-on-disc tribometer, while four material combinations were tested; metal-on-metal, ceramic-on-ceramic, metal-on-polyethylene, and ceramic-on-polyethylene, respectively. The contact was lubricated by pure saline and various protein solutions. The experiments were realized at two mean speeds equal to 5.7 mm/s and 22 mm/s and two slide-to-roll ratios, −150% and 150%. It was found that the implant material is the fundamental parameter affecting friction. In general, the metal pair exhibited approximately two times higher friction compared to the ceramic. In particular, the friction in the case of the metal varied between 0.3 and 0.6 while the ceramic pair exhibited friction within the range from 0.15 to 0.3 at the end of the test. The lowest friction was observed for polyethylene while it decreased to 0.05 under some conditions. It can be also concluded that adding proteins to the lubricant has a positive impact on friction in the case of hard-on-hard pairs. For hard-on-soft pairs, no substantial influence of proteins was observed. The effect of kinematic conditions was found to be negligible in most cases.

Frictional Properties of PVA Hydrogel

Total hip replacement is one of the most successful surgical treatments of modern medicine. Typically, at present, hard-on-hard bearing surfaces are widely used for components of artificial hip joints. Hard-on-hard means that both components have high modulus of elasticity in range of hundreds of GPa. However, these materials suffer from relatively high friction and wear rate. This is connected especially with occurring lubrication regime. To approach conditions presented in natural joints, it is necessary to think about artificial cartilage. One of the anticipated materials for artificial cartilage is polyvinyl alcohol (PVA) hydrogel. PVA hydrogel has water content about 85 % and its elastic modulus is approximately E ≈ 1.2 MPa, which is similar to natural cartilage. The main disadvantage of PVA hydrogel is its lower strength. In this study, commercial mini traction machine (MTM) was used to determine friction coefficient for various slide-to-roll ratios (SRR). Bovine serum was used as a lubricant and the tests were carried out under ambient temperature for three various speeds u1 = 25 mm/s; u2 = 50 mm/s; u3 = 100 mm/s and two different loads F1 = 5.2 N; F2 = 9.8 N, respectively. As expected, friction coefficient was very low, less than 0.05 under some conditions. In future, optical method based on the principle of fluorescent microscopy will be used for studying lubricant film thickness and protein adsorption on bearing surfaces.

Qualitative Analysis of Film Thickness in Rolling EHD Contact by Fluorescence Technique

Nowadays, it is well known that tribological processes play a significant role in a daily life. The elastohydrodynamic (EHD) regime of lubrication occurs in many applications including both technical and biological points of view. During the last fifty years there was a rapid evolution in experimental methods which help to better understand friction, lubrication and wear. In this study, mercury lamp induced fluorescence is employed to qualitative assess the film thickness in rolling EHD contact created by a steel ball and glass disk. Several experiments were performed to confirm the theory that the dependence between film thickness and entrainment speed should be linear in logarithmic. The results show satisfactory agreement with this prediction, so the method is correct and suitable for further research in the area of EHD contacts. This method also allows analyzing of compliant non-reflective contacts, which will be described in the next paper.