Contact mechanics and elastohydrodynamic lubrication in a novel metal-on-metal hip implant with an aspherical bearing surface

Diameter and diametral clearance of the bearing surfaces of metal-on-metal hip implants and structural supports have been recognised as key factors to reduce the dry contact and hydrodynamic pressures and improve lubrication performance. On the other hand, application of aspherical bearing surfaces can also significantly affect the contact mechanics and lubrication performance by changing the radius of the curvature of a bearing surface and consequently improving the conformity between the head and the cup. In this study, a novel metal-on-metal hip implant employing a specific aspherical bearing surface, Alpharabola, as the acetabular surface was investigated for both contact mechanics and elastohydrodynamic lubrication under steady-state conditions. When compared with conventional spherical bearing surfaces, a more uniform pressure distribution and a thicker lubricant film thickness within the loaded conjunction were predicted for this novel Alpharabola hip implant. The effects of the geometric parameters of this novel acetabular surface on the pressure distribution and lubricant thickness were investigated. A significant increase in the predicted lubricant film thickness and a significant decrease in the dry contact and hydrodynamic pressures were found with appropriate combinations of these geometric parameters, compared with the spherical bearing surface.     

Experiments on Ultrasonic Lubrication Using a Piezoelectrically-assisted Tribometer and Optical Profilometer

Friction and wear are detrimental to engineered systems. Ultrasonic lubrication is achieved when the interface between two sliding surfaces is vibrated at a frequency above the acoustic range (20 kHz). As a solid-state technology, ultrasonic lubrication can be used where conventional lubricants are unfeasible or undesirable. Further, ultrasonic lubrication allows for electrical modulation of the effective friction coefficient between two sliding surfaces. This property enables adaptive systems that modify their frictional state and associated dynamic response as the operating conditions change. Surface wear can also be reduced through ultrasonic lubrication. We developed a protocol to investigate the dependence of friction force reduction and wear reduction on the linear sliding velocity between ultrasonically lubricated surfaces. A pin-on-disc tribometer was built which differs from commercial units in that a piezoelectric stack is used to vibrate the pin at 22 kHz normal to the rotating disc surface. Friction and wear metrics including effective friction force, volume loss, and surface roughness are measured without and with ultrasonic vibrations at a constant pressure of 1 to 4 MPa and three different sliding velocities: 20.3, 40.6, and 87 mm/sec. An optical profilometer is utilized to characterize the wear surfaces. The effective friction force is reduced by 62% at 20.3 mm/sec. Consistently with existing theories for ultrasonic lubrication, the percent reduction in friction force diminishes with increasing speed, down to 29% friction force reduction at 87 mm/sec. Wear reduction remains essentially constant (49%) at the three speeds considered.     

Hard-on-Hard Lubrication in the Artificial Hip under Dynamic Loading Conditions

The tribological performance of an artificial hip joint has a particularly strong influence on its success. The principle causes for failure are adverse short- and long-term reactions to wear debris and high frictional torque in the case of poor lubrication that may cause loosening of the implant. Therefore, using experimental and theoretical approaches models have been developed to evaluate lubrication under standardized conditions. A steady-state numerical model has been extended with dynamic experimental data for hard-on-hard bearings used in total hip replacements to verify the tribological relevance of the ISO 14242-1 gait cycle in comparison to experimental data from the Orthoload database and instrumented gait analysis for three additional loading conditions: normal walking, climbing stairs and descending stairs. Ceramic-on-ceramic bearing partners show superior lubrication potential compared to hard-on-hard bearings that work with at least one articulating metal component. Lubrication regimes during the investigated activities are shown to strongly depend on the kinematics and loading conditions. The outcome from the ISO gait is not fully confirmed by the normal walking data and more challenging conditions show evidence of inferior lubrication. These findings may help to explain the differences between the in vitro predictions using the ISO gait cycle and the clinical outcome of some hard-on-hard bearings, e.g., using metal-on-metal.     

Frictional heating of total hip implants. Part 1: measurements in patients

Hip implants heat up due to friction during long lasting, high loading activities like walking. Thermal damage in the surrounding soft and hard tissues and deteriorated lubrication of synovial fluid could contribute to implant loosening. The goal of this study was to determine the implant temperatures in vivo under varying conditions. Temperatures and contact forces in the joints were measured in seven joints of five patients using instrumented prostheses with alumina ceramic heads and telemetry data transmission. The peak temperature in implants with polyethylene cups rose up to 43.1 degrees C after an hour of walking but varied considerably individually. Even higher temperatures at the joints are probable for patients with higher body weight or while jogging. The peak temperature was lower with a ceramic cup, showing the influence of friction in the joint. During cycling the peak temperatures were lower than during walking, proving the effect of force magnitudes on the produced heat. However, no positive correlation was found between force magnitude and maximum temperature during walking. Other individual parameters than just the joint force influence the implant temperatures. Based on the obtained data and the available literature about thermal damage of biological tissues a detrimental effect of friction induced heat on the stability of hip implants cannot be excluded. Because the potential risk for an individual patient cannot be foreseen, the use and improvement of low friction implant materials is important.     

Effect of swing phase load on metal-on-metal hip lubrication, friction and wear

There is renewed interest in metal-on-metal (MOM) total hip replacements (THRs), however, variable wear rates have been observed clinically. It is hypothesised that changes in soft tissue tensioning during surgery may alter loading of THRs during the swing phase of gait leading to changes in fluid film lubrication, friction and wear. This study aimed to assess the effect of swing phase load on the lubrication, friction and wear of MOM hip replacements. Theoretical lubrication modelling was carried out using elastohydrodynamic theory. All the governing equations were solved numerically for the lubricant film thickness between the articulating surfaces under the transient dynamic conditions with low and high swing phase loads. Friction testing was completed using a single axis pendulum simulator, simplified loading cycles were applied with low and high swing phase loads. MOM hip replacements were tested in a hip simulator, modified to provide different swing phase loading regimes; a low (100 N) and a high load (as per ISO 14242-1; 280 N). Results demonstrated that the performance of MOM bearings is highly dependent on swing phase load. Hence, changes in the tension of the tissues at surgery and variations in muscle forces may increase swing phase load, reduce lubrication, increase friction and accelerate wear. This may explain some of the variations that have been observed with clinical wear rates.     

Prediction of lubricating film thickness in UHMWPE hip joint replacements

An elastohydrodynamic lubrication model developed for a ball-in-socket configuration in a previous studies by the present authors (Jalali-Vahid et al., Thinning films and tribological interfaces, 26th Leeds-Lyon Symposium on Tribology, 2000, pp. 329-339) was applied to analyse the lubrication problem of a typical artificial hip joint replacement, consisting of an ultra-high molecular weight polyethylene (UHMWPE) acetabular cup against a metallic or ceramic femoral head. The cup was assumed to be stationary whilst the ball was assumed to rotate at a steady angular velocity and under a constant load. A wide range of main design parameters were considered. It has been found that the predicted lubricating film thickness increases with a decrease in the radial clearance, an increase in the femoral head radius, an increase in UHMWPE thickness and a decrease in UHMWPE modulus. However, the predicted lubricating film thicknesses are not found to be sufficiently large in relation to the surface roughness of the cup and head to indicate separation of the two articulating surfaces. It should also be noted that if the design features are unable to secure full fluid film lubrication, it may be preferable to select them for minimum wear rather than maximum film thickness. For example, an increase in head radius will enhance the film thickness, but it will also increase the sliding distance and hence wear in mixed or boundary lubrication conditions. Furthermore, it is pointed out that an increase in the predicted lubricant film thickness is usually associated with an increase in the contact area, and this may cause lubricant starvation and stress concentration at the edge of the cup, and adversely affect the tribological performance of the implant. The effect of running-in process on the lubrication in UHMWPE hip joint replacements is also discussed.     

Influence of Bio-Lubricants on the Tribological Properties of Ti6Al4V Alloy

Titanium alloy is one of the best materials for biomedical applications due to its superior biocompatibility, outstanding corrosion resistance, and low elastic modulus. However, the friction and wear behaviors of titanium alloys were sensitive to the environment including lubrication. In order to clarify the wear mechanism of titanium alloy under different lubrications including deionized water, physiological saline and bovine serum, the friction and wear tests were performed between Ti6Al4V plates and Si₃N₄ ball on a universal multi-functional tester. The friction and the wear rate of titanium alloy were measured under dry friction and three different lubrication conditions. The worn surfaces were examined by scanning electron microscopy. The results revealed that under the dry friction, the wear resistance of titanium alloy was the worst since the wear mechanism was mainly the combination of abrasive wear and oxidation wear. It was also found that Ti6Al4V alloy had low friction coefficient and wear rate under three lubrication conditions, and its wear mechanism was adhesive wear.     

Effect of bearing geometry and structure support on transient elastohydrodynamic lubrication of metal-on-metal hip implants

An effective lubrication can significantly reduce wear of metal-on-metal artificial hip joints. The improvement of the lubrication can be achieved through the optimisation of the bearing geometry in terms of a small clearance and/or the structural support such as a polyethylene backing underneath a metallic bearing in a sandwich acetabular cup form. The separate effects of these two factors on fluid film lubrication of 28 mm diameter metal-on-metal total hip joints under walking conditions were numerically investigated in this paper. The results show that a larger lubricant film due to the polyethylene backing can be significantly enhanced by the transient squeeze-film action, particularly during the stance phase, and a similar lubricant film can be developed for both the monolithic cup relying on the smaller clearance and the sandwich cup benefiting from the polyethylene backing. Both cup systems can function in a wide range of lubrication regimes, covering both mixed and fluid film, under the current design and manufacture conditions.     

Prediction of lubrication regimes in wrist implants with spherical bearing surfaces

The wrist joint is frequently affected by rheumatoid arthritis, resulting in wrist pain, deformity and ultimately loss of function. Artificial wrist implants have been introduced to treat the rheumatoid wrist, to attempt to alleviate pain and restore some function to the joint. The aim of this study was to predict the likely lubrication regimes that occur in wrist implants with spherical bearing surfaces. The implant was modelled as an equivalent ball-on-plane. Elastohydrodynamic lubrication theory was used to determine the minimum film thickness for the implant under different load, entraining velocity, lubricant viscosity, size of implant and material combinations. The results show that the highest film thickness is found in large implants, with high viscosity, high entraining velocity and low load. Hard-on-soft material combinations will operate with a boundary lubrication regime. Material combinations involving ceramic bearing surfaces have the potential to operate with a mixed lubrication regime.     

A hierarchy of computationally derived surgical and patient influences on metal on metal press-fit acetabular cup failure

The impact of anatomical variation and surgical error on excessive wear and loosening of the acetabular component of large diameter metal-on-metal hip arthroplasties was measured using a multi-factorial analysis through 112 different simulations. Each surgical scenario was subject to eight different daily loading activities using finite element analysis. Excessive wear appears to be predominantly dependent on cup orientation, with inclination error having a higher influence than version error, according to the study findings. Acetabular cup loosening, as inferred from initial implant stability, appears to depend predominantly on factors concerning the area of cup-bone contact, specifically the level of cup seating achieved and the individual patient's anatomy. The extent of press fit obtained at time of surgery did not appear to influence either mechanism of failure in this study.     

A novel sensor concept for optimization of loosening diagnostics in total hip replacement

The main reason for the revision of total hip replacements is aseptic loosening, caused by stress shielding and wear particle induced osteolysis. In order to detect an implant loosening early, the osseointegration of endoprosthetic implants must be measured exactly. Currently applied diagnostic methods, such as standard radiographs and clinical symptomatology, often result in an imprecise diagnosis. A novel radiation-free method to improve the diagnostic investigation of implant loosening is presented. The osseointegration of an implant can be identified using mechanical magnetic sensors (oscillators), which impinge on small membranes inside an implant component, e.g., the femoral hip stem. The maximum velocity after impingement of the oscillator depends on the osseointegration of the implant. Excitation of the oscillator is realized by a coil outside the human body. Another external coil is used to detect the velocity of the oscillator. To demonstrate the principle of the novel loosening sensor, an overdimensioned test device was designed to measure simulated loosening phases in the first experimental tests with different material layers. The overdimensioned test device of the loosening sensor showed significant differences in the various phases of fixation. Analysis of the membrane without any material layer in the case of advanced loosening resulted in a 23% higher maximum velocity compared to an attached artificial bone layer. Based on these preliminary results, the sensor system shows potential for the detection of implant loosening. Moreover, the proposed system could be used in experimental applications to determine the quality of bioactive coatings and new implant materials.     

In vivo kinematics of two-component total ankle arthroplasty during non-weightbearing and weightbearing dorsiflexion/plantarflexion

Relatively high rates of loosening and implant failure have been reported after total ankle arthroplasty, especially in first and second generation implants. Abnormal kinematics and incongruency of the articular surface may cause increased loads applied to the implant with concomitant polyethylene wear, resulting in loosening and implant failure. The purpose of this study was to measure three-dimensional kinematics of two-component total ankle arthroplasty during non-weightbearing and weightbearing activities, and to investigate incongruency of the articular surfaces during these activities. Forty-seven patients with a mean age of 71 years were enrolled. Radiographs were taken at non-weightbearing maximal dorsiflexion and plantarflexion, and weightbearing maximal dorsiflexion, plantarflexion, and neutral position. 3D-2D model-image registration was performed using the radiographs and the three-dimensional implant models, and three-dimensional joint angles were determined. The implanted ankles showed 18.1±8.6° (mean±standard deviation) of plantarflexion, 0.1±0.7° of inversion, 1.2±2.0° of internal rotation, and 0.8±0.6mm of posterior translation of the talar component in the non-weightbearing activity, and 17.8±7.5° of plantarflexion, 0.4±0.5° of inversion, 1.8±2.0° of internal rotation, and 0.7±0.5mm of posterior translation in the weightbearing activity. There were no significant differences between the non-weightbearing and weightbearing kinematics except for the plantarflexion angle. Incongruency of the articular surface occurred in more than 75% of the ankles. Our observations will provide useful data against which kinematics of other implant designs, such as three-component total ankle arthroplasty, can be compared.     

An investigation of the effect of conformity of knee hemiarthroplasty designs on contact stress,friction and degeneration of articular cartilage: A tribological study

Hemiarthroplasty is a potentially attractive alternative to knee replacement for young, active patients, as it allows preservation of more bone stock for potential revisions. However, there has been limited success with hemiarthroplasty or spacers to date. The wear and degradation of the biomaterial–cartilage interface is of paramount importance in the design and success of hemiarthroplasties. A comprehensive understanding of the tribological performance of hemiarthroplasty implants in the natural joint is required. The objective of this study was to investigate the tribological response of bovine medial compartmental knees, both natural and hemiarthroplasty replaced, under physiological loads and motion. The conformity of these metallic hemiarthroplasties was varied (conforming plates with radius of 50 mm and radius of 100 mm and a flat plate design), in order to examine the effects of conformity and contact stress, on the friction, friction shear stress and cartilage degeneration. With decreasing conformity of hemiarthroplasty bearings, an increase in contact stress was found, which resulted in elevated friction, elevated friction shear stress and increased cartilage degeneration. A strong correlation was found between contact stress and wear and between friction shear stress and wear. This new and unique in vitro tribological simulation has shown the direct elevation of friction, surface fibrillation and biomechanical wear of cartilage, upon replacing the tibia with a hemiarthroplasty, particularly when using low conformity hemiarthroplasty designs.     

Outcome of implant-supported overdenture treatment--a survey of 58 patients

Gerondontology 2012; doi: 10.1111/j.1741‐2358.2011.00524.x
Outcome of implant‐supported overdenture treatment – a survey of 58 patients Objective: The aim of this follow‐up study was to evaluate the long‐term outcome of implant‐supported or implant‐retained mandibular overdenture treatment. Material and methods: Altogether 112 patients treated with implant‐supported overdentures between 1985 and 2004 were invited to the follow‐up and 58 attended the clinical examination. The total amount of implants examined and still in use was 197. The average number of implants installed was three (range 2–4), and the mean length of the implants was 12 mm (range 8–21 mm). There were altogether 48 overdentures with a bar connection and 10 with a ball connection. Results: The most frequent prosthetic complications were technical: loosening of the retentive mechanism (39.7%) and breakage of the matrices (5.2%). The most common peri‐implant soft‐tissue findings were bleeding and slight hyperplasia. The implant‐supported overdentures of 19 patients (32.8%) had been renewed, while 39 patients (67.2%) still used their original overdentures, of which the oldest was 20 years old. Conclusion: The results of this long‐term follow‐up study show that the outcome of implant‐supported mandibular overdenture treatment was excellent. The patients were satisfied with the treatment, regardless of the attachment type used. Removable overdentures are more easy to clean and can be cleaned outside the patient’s mouth, whereas fixed‐implant full‐arch dentures in the edentulous mandible require much more time‐consuming hygiene. This kind of overdenture treatment is suitable also in the elderly, even though their ability to practice appropriate oral hygiene might be decreased.     

Tribological altruism: A sacrificial layer mechanism of synovial joint lubrication in articular cartilage

Boundary lubrication is characterized by sliding surfaces separated by a molecularly thin film that reduces friction and wear of the underlying substrate when fluid lubrication cannot be established. In this study, the wear and replenishment rates of articular cartilage were examined in the context of friction coefficient changes, protein loss, and direct imaging of the surface ultrastructure, to determine the efficiency of the boundary lubricant (BL) layer. Depletion of cartilage lubricity occurred with the concomitant loss of surface proteoglycans. Restoration of lubrication by incubation with synovial fluid was much faster than incubation with culture media and isolated superficial zone protein. The replenishment action of the BL layer in articular cartilage was rapid, with the rate of formation exceeding the rate of depletion of the BL layer to effectively protect the tissue from mechanical wear. The obtained results indicate that boundary lubrication in articular cartilage depends in part on a sacrificial layer mechanism. The present study provides insight into the natural mechanisms that minimize wear and resist tissue degeneration over the lifetime of an organism.     

Nanoscale design of snake skin for reptation locomotions via friction anisotropy.

Multi-mode scanning probe microscopy is employed to investigate the nanostructure of dermal samples from three types of snakes. Sophisticated friction modifying nanostructures are described. These include an ordered microfibrillar array that can function to achieve mission adaptable friction characteristics. Significant reduction of adhesive forces in the contact areas caused by the 'double-ridge' nanoscale microfibrillar geometry provides ideal conditions for sliding in forward direction with minimum adhesive forces and friction. Low surface adhesion in these local contact points may reduce local wear and skin contamination by environmental debris. The highly asymmetric, 'pawl-like' profile of the microfibrillar ends with radius of curvature 20-40 nm induces friction anisotropy in forward backward motions and serves as an effective stopper for backward motion preserving low friction for forward motion. The system of continuous micropores penetrating through the snake skin may serve as a delivery system for lubrication/anti-adhesive lipid mixture that provides for boundary lubrication of snake skins.     

Strain shielding in distal radius after wrist arthroplasty with a current generation implant: An in vitro analysis

A systematic review of peer reviewed articles has shown that the main cause for wrist arthroplasty revision is carpal and radial prosthetic loosening and instability. To improve arthroplasty outcomes, successive generations of implants have been developed over time. The problem with the current generation of implants is the lack of long-term outcomes data. The aim of the present work was to test the hypothesis that the current generation Maestro WRS implant has a stress, strain and stability behaviour which may be associated with a reduced risk of long-term radial component loosening. This study was performed using synthetic radii to experimentally predict the cortex strain behaviour and implant stability considering different load conditions for both intact and implanted conditions. Finite element models were developed to assess the structural behaviour of cancellous-bone and bone-cement, these models were validated against experimentally measured cortex strains. Measured cortex strains showed a significant reduction between intact and implanted states. Cancellous bone adjacent to the radial body component suffers a two to threefold strain reduction, comparing with the intact condition, while along the radial stem, in the axial direction, a strain increase was observed. It is concluded that the use of contemporary Maestro WRS implant changes the biomechanical behaviour of the radius and is associated with a potential risk of bone resorption by stress-shielding in the distal radius region for wrist loads in the range of daily activities.     

Contact of dual mobility implants: effects of cup wear and inclination

Cup wear and inclination on the pelvic bone are significant factors, which change the contact of the articulating surfaces, thus, impacting the long-term performance of hip implants. This paper presents a finite element (FE) analysis of the contact of the dual mobility implants under the influence of cup wear and inclination. A 3D FE model of the implant was developed with the application of equivalent physiological loading and boundary conditions. Effects of cup inclination angle ranging from 45° to 60° and the wear depth ranging from 0 to 2.46 mm equivalent to up to 30 years of the implant's life on the contact pressure and von Mises stress were investigated. Simulation results show that the contact pressure and von Mises stress decrease significantly with a modest wear depth and remains quite in-sensitive to the cup inclination angle and wear depth up to 1.64 mm. With wear depth further up to 2.46 mm, the cup thickness (i.e. cup thinning on worn region) may be more predominant than increasing of contact area between the cup and the head. The wear on the inner surface of the cup is found to rule out the overall contact pressure and stress in the implant. Furthermore, individual and combined effects of both important parameters are analysed and discussed with respect to available clinical/laboratory studies.     

Dependence of nanoscale friction and adhesion properties of articular cartilage on contact load

Boundary lubrication of articular cartilage by conformal, molecularly thin films reduces friction and adhesion between asperities at the cartilage-cartilage contact interface when the contact conditions are not conducive to fluid film lubrication. In this study, the nanoscale friction and adhesion properties of articular cartilage from typical load-bearing and non-load-bearing joint regions were studied in the boundary lubrication regime under a range of physiological contact pressures using an atomic force microscope (AFM). Adhesion of load-bearing cartilage was found to be much lower than that of non-load-bearing cartilage. In addition, load-bearing cartilage demonstrated steady and low friction coefficient through the entire load range examined, whereas non-load-bearing cartilage showed higher friction coefficient that decreased nonlinearly with increasing normal load. AFM imaging and roughness calculations indicated that the above trends in the nanotribological properties of cartilage are not due to topographical (roughness) differences. However, immunohistochemistry revealed consistently higher surface concentration of boundary lubricant at load-bearing joint regions. The results of this study suggest that under contact conditions leading to joint starvation from fluid lubrication, the higher content of boundary lubricant at load-bearing cartilage sites preserves synovial joint function by minimizing adhesion and wear at asperity microcontacts, which are precursors for tissue degeneration.     

Frictional heating of total hip implants. Part 2: finite element study

Due to higher friction artificial hip joints warm up more than natural joints during walking and other continuous activities. This could lead to thermal damage in the surrounding tissues and be a reason for long-term implant loosening, an effect which has not yet been investigated. In vivo measurements with instrumented implants showed temperatures inside the prosthetic head up to 43.1 degrees C (Part 1 of this work). Based on the experimental data a finite element model was developed to calculate the temperatures in the tissues surrounding the hip implant to determine whether these tissues can heat up to critical levels. Various parameters were investigated which could account for the variations in the measured temperatures in the patients, including the perfusion rate in tissues, the volume of synovial fluid, and different implant materials. We found that the synovial fluid is most endangered by thermal damage and consequent deterioration of lubricating properties. Implants with a cobalt-chromium head and a polyethylene cup are unfavourable as they can elevate the temperature in the synovia to more than 46 degrees C. With regard to thermal properties stems made from cobalt-chromium alloys are superior to titanium stems, by better conducting heat to the femur and minimizing the synovial fluid temperature. Factors determining the temperatures during walking are insufficiently known or cannot be determined in the individual patient. Therefore, the risk of a thermally induced implant loosening cannot currently be estimated. Under unfavourable conditions such a risk exists, however. General improvements of implant materials and clinical studies on the possibility of implant loosening due to high temperatures are therefore required.