New joints for the Millennium: wear control in total replacement hip joints

A Perspective on Biotribology in Arthroplasty: From In Vitro toward the Accurate In Silico Wear Prediction

Nowadays hip arthroplasty is recognized as one of the most successful orthopedic surgical procedures, even if it involves challenges to overcome, such that lately, younger and more active patients are in need of total arthroplasty. Wear is still one of the main issues affecting joint prostheses endurance, and often causes loosening accompanied by implant failures. Actual in vitro wear tests executed by mechanical simulators have a long duration, are very expensive, and do not take into account all the possible daily activities of the patients; thus, the challenge to obtain a complete in silico tribological and dynamical model of (bio) tribo-systems could give the possibility to overcome the actual testing procedures and could contribute as a tool for a more accurate tribological design of human prostheses. This prospective paper is intended to underline actual research trends toward the challenge of having accurate numerical algorithms to be used both in preclinical testing and in the optimizations of the prostheses design. With this aim we depicted the possible in silico approach in artificial joints’ wear assessment over time, accounting for contact mechanics, numerical stress–strain analysis, musculoskeletal multibody, and synovial lubrication modelling (boundary/mixed, hydrodynamic, and elastohydrodynamic).

Mapping the Micro-Abrasion Mechanisms of CoCrMo: Some Thoughts on Varying Ceramic Counterface Diameter on Transition Boundaries In Vitro

The micro-abrasion wear mechanisms for CoCrMo against variable size alumina balls, representing typical artificial femoral head sizes, were investigated over a range of applied loads in foetal calf serum solution. SEM analysis of resulting wear scars displayed two-body and mixed-mode abrasion modes of wear. The wear factor, κ, was found to range between 0.86 and 22.87 (10−6 mm3/Nm). Micro-abrasion mechanism and wastage maps were constructed for the parameter range tested. A dominant two- to three-body abrasion regime was observed with an increasing load and ball diameter. The 28-mm ball diameter displayed the lowest wastage, with an increasing load. Proteins may act to reduce the severity of contact between abrasive particles and bearing surfaces. Wear volumes did not necessarily increase linearly with applied load and ball diameter; therefore, there is a need to develop more accurate models for wear prediction during micro-abrasion conditions. Wear mapping for hip replacements could provide a useful aid for pre-clinical hip wear evaluations and long-term performance.

Tribology and Dowson

It is with great sadness that we note the passing of Professor Duncan Dowson on 6th January 2020. Duncan was an esteemed member of the Editorial Board of this journal. He will be remembered as one of the founding fathers of tribology and as a true gentleman. He was the last living member of the Jost Committee, set up by the UK Government (1964–1966) to investigate the state of lubrication education and research, and to establish the requirements of industry in this regard [1]. This committee coined the term “tribology”.Duncan contributed to many areas of tribological research and established many of them, including elastohydrodynamic theory and biotribology.[...]

Aqueous Lubrication with the Molecularly Confined Films of Silicone-Based Amphiphilic Block Copolymer Aggregates

The confined film structures and tribological properties of the dilute aqueous solution of a silicone-based amphiphilic block copolymer, bis-isobutyl poly(ethylene glycol) (PEG)-14/amodimethicone (BIPA) copolymer, between mica surfaces were investigated. The BIPA copolymer existed as positively charged water-soluble aggregates in the solution. The adsorption behavior of the BIPA copolymer aggregates on a mica surface from solution was studied using an atomic force microscope (AFM); the result showed the immediate formation of a uniform adsorbed BIPA copolymer layer, followed by the gradual deposition of BIPA aggregates on the top of the adsorbed layer. Friction measurements were carried out using the surface forces apparatus (SFA) for the confined films of BIPA copolymer solution between mica surfaces, which revealed two different sliding film structures depending on the elapsed time after surface preparation. The sliding film consisting of two adsorbed BIPA copolymer layers was obtained for a relatively short elapsed time (not longer than 3 h), which had an extremely low friction coefficient μ (of the order of 10^-5). The sliding film on the following day (elapsed time of approximately 24 h) had the structure of a deposited/kinetically trapped BIPA aggregate layer confined between the opposing adsorbed layers, and the μ values were within the range from 10^-4 to 10^-3. Our results suggest that the different elapsed time ranges and resulting absence or presence of the intervening layer of trapped aggregates between the absorbed layers determine the tribological properties of the confined films. Molecular friction mechanisms are discussed for the two sliding structures, which give insight into using amphiphilic block copolymer aggregates for a new class of aqueous lubrication system to design extremely low friction interfaces.

Finite Element Analysis of Different Hip Implant Designs along with Femur under Static Loading Conditions

Background:

The hip joint is the largest joint after the knee, which gives stability to the whole human structure. The hip joint consists of a femoral head which articulates with the acetabulum. Due to age and wear between the joints, these joints need to be replaced with implants which can function just as a natural joint. Since the early 19^th century, the hip joint arthroplasty has evolved, and many advances have been taken in the field which improved the whole procedure. Currently, there is a wide variety of implants available varying in the length of stem, shapes, and sizes.

Material and Methods:

In this analytical study of femur, circular, oval, ellipse and trapezoidal-shaped stem designs are considered in the present study. The human femur is modeled using Mimics. CATIA V-6 is used to model the implant models. Static structural analysis is carried out using ANSYS R-19 to evaluate the best implant design.

Results:

All the four hip implants exhibited the von Mises stresses, lesser than its yielded strength. However, circular and trapezoidal-shaped stems have less von Mises stress compared to ellipse and oval.

Conclusion:

This study shows the behavior of different implant designs when their cross-sections are varied. Further, these implants can be considered for dynamic analysis considering different gait cycles. By optimizing the implant design, life expectancy of the implant can be improved, which will avoid the revision of the hip implant in active adult patients.

Early to mid-term outcome of Oxinium on Verilast highly cross-linked polyethylene bearing surface in uncemented total hip arthroplasty

INTRODUCTION

We report 5- to 10-year clinical and radiological outcomes of a series of uncemented Oxinium on Verilast (highly cross-linked polyethylene liners) total hip replacements (THAs) using the Anthology stem and R3/Reflection cup system.

METHODS

116 THAs were performed by 2 senior authors in 104 patients from 2005 to 2009. The mean age at time of operation was 63.8 (45-80) years. The average length of follow-up was 8.8 years. At final data collection 12 patients had died (13 THAs) and 3 patients (3 THAs) were lost to follow-up.

RESULTS

In the remaining 100 THAs, the preoperative Harris Hip Score of 39 improved to 91 and Oxford Hip Score improved from 16 to 44 (mean at 5 to 10 years). 100 THAs were available for radiographic analysis: all had stable bony ingrowth of both the stem and cup. Dorr's methods for wear in the 100 THAs showed an average wear <0.01 mm/year and no osteolysis; only 1 case showed 0.22 mm/year with some proximal femoral osteolysis but no cup lesions. There were no dislocations or revisions, survivorship in this series for both components for any cause as an endpoint was 100%. 1 intraoperative calcar fracture was sustained and treated with a cerclage wires, 2 peri-prosthetic fractures occurred and were treated with open reduction and fixation, both healed well with excellent outcomes.

DISCUSSION

The study has shown good clinical and radiological outcomes at 5-10 years and supports existing data on the potential benefits of this bearing performance in literature.

Wear of different materials for total hip replacement under adverse stop-dwell-start in vitro wear simulation conditions

Hip simulation is a common technique for pre-clinical evaluation of wear performance of total hip arthroplasty. Standard techniques replicate kinematics of walking patterns of a typical patient. Attention has focussed in developing simulations of other typical patient daily activities to improve accuracy of wear predictions. A method for simulating stop-dwell-start motion during patient walking and the effect on 36-mm metal-on-metal total hip arthroplasty was previously presented by the authors. This study sought to extend the previous work to look at the effect of these conditions on ceramic-on-ceramic, metal-on-polyethylene and ceramic-on-polyethylene bearings. Two stop-dwell-start protocols were used: one reproducing average patient movement patterns and one examining more severe conditions. For all materials tested, no significant increase in wear was observed under average stop-dwell-start conditions, suggesting the bearing types tested are robust to this type of activity. A significant increase in wear was observed for metal-on-metal, metal-on-polyethylene and ceramic-on-polyethylene bearings under severe stop-dwell-start conditions, this was attributed to depletion of lubricant in the bearing during the dwell period. A greater relative increase in wear was observed for metal-on-metal bearings compared with metal-on-polyethylene and ceramic-on-polyethylene bearings. This may be explained by the contributions of the different lubrication mechanisms in each bearing type. Wear of ceramic-on-ceramic was very low in all tests, suggesting normal measurement variation was masking any effect of the adverse conditions. It was not possible to determine any effect of the different activities. These results emphasise the importance of exploring adverse patient activity simulations. The increase in wear rate associated with an adverse activity such as seen in stop-dwell-start motion, has to be considered in the context of the frequency of the adverse activity cycle relative to other activities such as standard continuous walking, to determine the impact on the total wear in a given time period.

A computational parametric study on edge loading in ceramic-on-ceramic total hip joint replacements

Edge loading in ceramic-on-ceramic total hip joint replacement is an adverse condition that occurs as the result of a direct contact between the head and the cup rim. It has been associated with translational mismatch in the centres of rotation of the cup and head, and found to cause severe wear and early failure of the implants. Edge loading has been considered in particular in relation to dynamic separation of the cup and head centres during a gait cycle. Research has been carried out both experimentally and computationally to understand the mechanism including the influence of bearing component positioning on the occurrence and severity of edge loading. However, it is experimentally difficult to measure both the load magnitude and duration of edge loading as it occurs as a short impact within the tight space of hip joints. Computationally, a dynamic contact model, for example, developed using the MSC ADAMS software for a multi-body dynamics simulation can be particularly useful for calculating the loads and characterising the edge loading. The aim of the present study was to further develop the computational model, and improve the predictions of contact force and the understanding of mechanism in order to provide guidance on design and surgical factors to avoid or to reduce edge loading and wear. The results have shown that edge loading can be avoided for a low range of translational mismatch in the centres of rotation of the cup and head during gait at the level of approximately 1.0 mm for a cup at 45° inclination, keeping a correct cup inclination at 45° is important to reduce the edge loading severity, and edge loading can be avoided for a certain range of translational mismatch of the cup and head centres with an increased swing phase load.

Effect of an edge at cup rim on contact stress during micro-separation in ceramic-on-ceramic hip joints

Alumina ceramic total hip joint bearings have shown superior wear properties. The joint bearing may undergo adverse conditions such as micro-separation causing head contact on the cup rim. As a transition, an edge is formed between the cup bearing and the rim. The aim of this study was to predict the effect of the edge on contact stresses in order to better understand the mechanisms of wear. A finite element contact model was developed under the conditions of the head displacements 0.5-2 mm and vertical loads 0.5-3 kN. The edge contact produced the most severe stresses capable of causing elevated wear and damage to ceramic bearings. The study shows that the bearing design should be considered in association with clinical conditions to eliminate severe stress.

Poor performance of Enduron polyethylene liner in total hip arthroplasty: a minimum ten-year follow up and ultra-morphological analysis of wear particles

PURPOSE

The aim of the present study was to investigate the long-term outcome and the wear characteristics of two distinct types of ultra-high molecular weight polyethylene (UHMWPE) liners in total hip arthroplasty (THA).

METHODS

We conducted a retrospective clinical study on patients which were treated with total hip arthroplasty using either Enduron polyethylene (Enduron PE) or Trilogy polyethylene (Trilogy PE) liners based on a minimum of ten year follow up data. Morphological analyses of wear particles from tissue samples, which were harvested during revision surgeries, were also performed.

RESULTS

A total of 79 THAs in the Enduron group and 55 THAs in the Trilogy group were available for analysis. Kaplan-Meier survival with revision for wear-related complications as the endpoint of the Enduron PE liners was lower than that of Trilogy PE liners at ten years (93.5 % and 100 %, P = 0.03). The Enduron group had higher mean linear wear rate than that of the Trilogy group (0.20 ± 0.09 and 0.09 ± 0.03 mm/year, P < 0.01). The incidence of osteolysis for the Enduron group was higher than that of the Trilogy group (33.3 % and 12 %, P = 0.04). Under transmission electron microscopy, the Enduron group had more than 82 % of the particles less than 1.0 μm in size and more than 57 % of the particles less than 0.5 μm.

CONCLUSION

The long-term performance of Enduron liners was worse than that of Trilogy liners. Further clinical follow-up may be necessary in patients with Enduron PE liners in order to avoid catastrophic complications.

Incorporating corrosion measurement in hip wear simulators: An added complication or a necessity?

Corrosion is not routinely considered in the assessment of the degradation or the lifetime of total hip replacement bearing surfaces. Biomechanical simulations are becoming ever more complex and are taking into account motion cycles that represent activities beyond a simple walking gait at 1 Hz, marking a departure from the standard ISO BS 14242. However, the degradation is still very often referred to as wear, even though the material loss occurs due to a combination of tribological and corrosion processes and their interactions. This article evaluates how, by incorporating real-time corrosion measurements in total hip replacement simulations, pre-clinical evaluations and research studies can both yield much more information and accelerate the process towards improved implants.

Predictive wear modeling of the articulating metal-on-metal hip replacements

The lubrication regime in which artificial hip joints operate adds complexity to the prediction of wear, as the joint operates in both the full fluid film regime-specifically the elastohydrodynamic lubrication (EHL) regime-and the mixed or boundary lubrication regimes, where contact between the bearing surfaces results in wear. In this work, a wear model is developed which considers lubrication for the first time via a transient EHL model of metal-on-metal hip replacements. This is a framework to investigate how the change in film thickness influences the wear, which is important to further investigation of the complex wear procedure, including tribocorrosion, in the lubricated hip implants. The wear model applied here is based on the work of Sharif et al. who adapted the Archard wear law by making the wear rate a function of a relative film thickness nominalized by surface roughness for examining wear of industrial gears. In this work, the gait cycle employed in hip simulator tests is computationally investigated and wear is predicted for two sizes of metal-on-metal total hip replacements. The wear results qualitatively predict the typical wear curve obtained from experimental hip simulator tests, with an initial "running-in period" before a lower wear rate is reached. The shape of the wear scar has been simulated on both the acetabular cup and the femoral head bearing surfaces. © 2015 Wiley Periodicals, Inc. J Biomed Mater Res Part B: Appl Biomater, 105B: 497-506, 2017.

Effects of home-based interval walking training on thigh muscle strength and aerobic capacity in female total hip arthroplasty patients: a randomized, controlled pilot study

Due to the reduced physical activity of patients who have undergone total hip arthroplasty (THA), there are no home-based exercise training regimens for preventing muscle atrophy and aerobic capacity impairment in these patients. We examined whether interval walking training (IWT) could prevented these issues. Twenty-eight female patients (∼60 years of age) who had undergone THA more than 2 months prior were randomly divided into IWT (n = 14) and control (CNT, n = 14) groups. The IWT subjects trained at a target of 60 min of fast walking at >70% peak aerobic capacity for walking (O_2peak) per wk for 12 wk, while those in the CNT maintained their previous sedentary life during the same period. We measured the energy expenditure of the daily physical activity, except during sleeping and bathing, every minute and every day during the intervention. We also measured the isometric knee extension (F_EXT) and flexion (F_FLX) forces, O_2peak, and anaerobic threshold during the graded cycling exercise (O_2AT) before and after the intervention. All subjects, except for one in IWT, completed the protocol. F_FLX increased by 23% on the operated side (P = 0.003) and 14% on the non-operated side of IWT (P = 0.006), while it only increased on the operated side of CNT (P = 0.03). The O_2peak and O_2AT in IWT increased by 8% (P = 0.08) and 13% (P = 0.002), respectively, and these changes were significantly higher in the IWT than in CNT group (both, P<0.05). In conclusion, IWT might be an effective home-based training regimen for preventing the muscle atrophy from reduced daily physical activity in THA patients.

Trial Registration

UMIN-CTR UMIN000013172

Retrieval analysis of alumina ceramic-on-ceramic bearing couples

BACKGROUND AND PURPOSE

Ceramic-on-ceramic (CoC) bearings have been in use in total hip replacement (THR) for more than 40 years, with excellent long-term survivorship. Although there have been several simulator studies describing the performance of these joints, there have only been a few retrieval analyses. The aim of this study was to investigate the wear patterns, the surface properties, and friction and lubrication regimes of explanted first-generation alumina bearings.

MATERIALS AND METHODS

We studied 9 explanted CoC bearings from Autophor THRs that were revised for aseptic loosening after a mean of 16 (range 7-19) years. The 3D surface roughness profiles of the femoral heads and acetabular cups (Srms, Sa, and Ssk) were measured to determine the microscopic wear. The bearings were imaged using an atomic-force microscope in contact mode, to produce a topographical map of the surfaces of the femoral heads. Friction tests were performed on the bearing couples to determine the lubrication regime under which they were operating during the walking cycle. The diametral clearances were also measured.

RESULTS

3 femoral heads showed stripe wear and the remaining 6 bearings showed minimal wear. The femoral heads with stripe wear had significantly higher surface roughness than the minimally worn bearings (0.645 vs. 0.289, p = 0.04). High diametral clearances, higher than expected friction, and mixed/boundary lubrication regimes prevailed in these retrieved bearings.

INTERPRETATION

Despite the less than ideal tribological factors, these first-generation CoC bearings still showed minimal wear in the long term compared to previous retrieval analyses.

The use of compliant layer prosthetic components in orthopedic joint repair and replacement: a review

The surgical repair or treatment of degenerative joint disease has traditionally involved the substitution of synthetic materials for one or both surfaces of the joint. Engineering thermoplastics, metals, and ceramics have either been widely accepted or experimentally evaluated for use as bearing surfaces in these prostheses. When engineering thermoplastics are used, the opposing surface is a metal or a ceramic, but metal-on-metal, metal-on-ceramic, and ceramic-on-ceramic have also been used or tested. Researchers have sought the opportunity to utilize materials with compressive mechanical properties more closely matching those of the natural articular cartilage. This review discusses the theory, testing, and application of elastomers for one bearing component of articular joint prostheses.

The current state of bearing surfaces in total hip replacement

We reviewed the literature on the currently available choices of bearing surface in total hip replacement (THR). We present a detailed description of the properties of articulating surfaces review the understanding of the advantages and disadvantages of existing bearing couples. Recent technological developments in the field of polyethylene and ceramics have altered the risk of fracture and the rate of wear, although the use of metal-on-metal bearings has largely fallen out of favour, owing to concerns about reactions to metal debris. As expected, all bearing surface combinations have advantages and disadvantages. A patient-based approach is recommended, balancing the risks of different options against an individual’s functional demands.

Cite this article: Bone Joint J 2014;96-B:147–56.

Effect of femoral head size on the wear of metal on metal bearings in total hip replacements under adverse edge-loading conditions

Metal-on-metal (MoM) bearings have shown low-wear rates under standard hip simulator conditions; however, retrieval studies have shown large variations in wear rates and mechanisms. High-wear in vivo has caused catastrophic complications and has been associated with steep cup-inclination angle (rotational malpositioning). However, increasing the cup-inclination angle in vitro has not replicated the increases in wear to the same extent as those observed in retrievals. Clinically relevant wear rates, patterns, and particles were observed in vitro for ceramic-on-ceramic bearings when microseparation (translational malpositioning) conditions were introduced into the gait cycle. In the present study, 28 and 36-mm MoM bearings were investigated under adverse conditions. Increasing the cup angle from 45° to 65° resulted in a significant increase in the wear rate of the 28 mm bearings. However, for the 36 mm bearings, head-rim contact did not occur under the steep cup-angle condition, and the wear rate did not increase. The introduction of microseparation to the gait cycle significantly increased the wear rate of the MoM bearings. Cup angle and head size did not influence the wear rate under microseparation conditions. This study indicated that high-in vivo wear rates were associated with edge loading due to rotational malpositioning such as high-cup-inclination angle and translational malpositioning that could occur due to several surgical factors. Translational malpositioning had a more dominant effect on the wear rate. Preclinical simulation testing should be undertaken with translational and rotational malpositioning conditions as well as standard walking cycle conditions defined by the ISO standard.

The effect of lubrication on the friction and wear of Biolox®delta

The performance of total hip-joint replacements depends strongly on the state of lubrication in vivo. In order to test candidate prosthetic materials, in vitro wear testing requires a lubricant that behaves in the same manner as synovial fluid. The current study investigated three lubricants and looked in detail at the lubrication conditions and the consequent effect on ball-on-flat reciprocating wear mechanisms of Biolox®delta against alumina. Biolox®delta, the latest commercial material for artificial hip-joint replacements, is an alumina-matrix composite with improved mechanical properties through the addition of zirconia and other mixed oxides. Three commonly used laboratory lubricants, ultra pure water, 25 vol.% new-born calf serum solution and 1 wt.% carboxymethyl cellulose sodium salt (CMC-Na) solution, were used for the investigation. The lubrication regimes were defined by constructing Stribeck curves. Full fluid-film lubrication was observed for the serum solution whereas full fluid-film and mixed lubrications were observed in both water and the CMC-Na solution. The wear rates in the CMC-Na and new-born calf serum were similar, but were an order of magnitude higher in water. The worn surfaces all exhibited pitting, which is consistent with the transition from mild wear to severe or "stripe" wear. The extent of pitting was greatest in the serum solution, but least in the water. On all worn surfaces, the zirconia appeared to have fully transformed from tetragonal to monoclinic symmetry. However, there was no evidence of microcracking associated with the transformed zirconia. Nevertheless, AFM indicated that zirconia was lost preferentially to the alumina grains during sliding. Thus, the current study has shown conclusively that the wear mechanisms for Biolox®delta clearly depend on the lubricant used, even where wear rates were similar.

Friction coefficients for mechanically damaged bovine articular cartilage

We used a pin-on-disc tribometer to measure the friction coefficient of both pristine and mechanically damaged cartilage samples in the presence of different lubricant solutions. The experimental set up maximizes the lubrication mechanism due to interstitial fluid pressurization. In phosphate buffer solution (PBS), the measured friction coefficient increases with the level of damage. The main result is that when poly(ethylene oxide) (PEO) or hyaluronic acid (HA) are dissolved in PBS, or when synovial fluid (SF) is used as lubricant, the friction coefficients measured for damaged cartilage samples are only slightly larger than those obtained for pristine cartilage samples, indicating that the surface damage is in part alleviated by the presence of the various lubricants. Among the lubricants considered, 100 mg/mL of 100,000 Da MW PEO in PBS appears to be as effective as SF. We attempted to discriminate the lubrication mechanism enhanced by the various compounds. The lubricants viscosity was measured at shear rates comparable to those employed in the friction experiments, and a quartz crystal microbalance with dissipation monitoring was used to study the adsorption of PEO, HA, and SF components on collagen type II adlayers pre-formed on hydroxyapatite. Under the shear rates considered the viscosity of SF is slightly larger than that of PBS, but lower than that of lubricant formulations containing HA or PEO. Neither PEO nor HA showed strong adsorption on collagen adlayers, while evidence of adsorption was found for SF. Combined, these results suggest that synovial fluid is likely to enhance boundary lubrication. It is possible that all three formulations enhance lubrication via the interstitial fluid pressurization mechanism, maximized by the experimental set up adopted in our friction tests.

Femoral head size does not affect ion values in metal-on-metal total hips

BACKGROUND

Metal-on-metal articulations can release substantial amounts of particles containing cobalt and chromium into the surrounding milieu, causing concern for cellular toxicity and adverse local soft tissue reactions. The diameter of the femoral head has been one of the variables that inversely affects wear of metal-on-metal total hip arthroplasty (THA). The oxidative stress of increased metal ions can be measured with serum markers. It is still controversial if larger femoral head diameters decrease wear rates in patients with metal-on-metal THA and if the increased metal ions alter the body's antioxidant status.

QUESTIONS/PURPOSES

We therefore (1) determined whole blood metal ions in patients with small (28 mm and 36 mm) and large (40 mm and 44 mm) diameter femoral heads; (2) measured oxidative stress markers (total antioxidants, nitrotyrosine, and peroxides); and (3) determined whether acetabular version or inclination influenced ion levels.

METHODS

One hundred four patients were retrospectively studied. We recorded Harris hip scores and UCLA activity scores. All patients were followed at 1 year.

RESULTS

The activity scores were similar in the two groups. There was no difference in metal ion levels or oxidative stress markers between patients with small- or large-diameter femoral heads. Acetabular inclination and anteversion had no effect on the metal ion levels.

CONCLUSIONS

The data suggest there is no difference in ion values in patients with large or small metal-on-metal THA and the increased metal ions do not alter the oxidant status of the patient.

Experimental friction coefficients for bovine cartilage measured with a pin-on-disk tribometer: testing configuration and lubricant effects

The friction coefficient between wet articular cartilage surfaces was measured using a pin-on-disk tribometer adopting different testing configurations: cartilage-on-pin vs. alumina-on-disk (CA); cartilage-on-pin vs. cartilage-on-disk (CC); and alumina-on-pin vs. cartilage-on-disk (AC). Several substances were dissolved in the phosphate buffered saline (PBS) solution to act as lubricants: 10,000 molecular weight (MW) polyethylene glycol (PEG), 100,000 MW PEG, and chondroitin sulfate (CS), all at 100 mg/mL concentration. Scanning electron microscopy photographs of the cartilage specimens revealed limited wear due to the experiment. Conducting the experiments in PBS solutions we provide evidence according to which a commercial pin-on-disk tribometer allows us to assess different lubrication mechanisms active in cartilage. Specifically, we find that the measured friction coefficient strongly depends on the testing configuration. Our results show that the friction coefficient measured under CC and AC testing configurations remains very low as the sliding distance increases, probably because during the pin displacement the pores present in the cartilage replenish with PBS solution. Under such conditions the fluid phase supports a large load fraction for long times. By systematically altering the composition of the PBS solution we demonstrate the importance of solution viscosity in determining the measured friction coefficient. Although the friction coefficient remains low under the AC testing configuration in PBS, 100 mg/mL solutions of both CS and 100,000 MW PEG in PBS further reduce the friction coefficient by ~40%. Relating the measured friction coefficient to the Hersey number, our results are consistent with a Stribeck curve, confirming that the friction coefficient of cartilage under the AC testing configuration depends on a combination of hydrodynamic, boundary, and weep bearing lubrication mechanisms.

Electrochemical Instrumentation of a Hip Simulator: A New Tool for Assessing the Role of Corrosion in Metal-On-Metal Hip Joints

Polyethylene wear debris induced osteolysis has triggered investigations to find alternative material combinations to the well-established metal-on-polyethylene hip implants. Owing to some early successful clinical cases, metal-on-metal (MoM) hip replacements have been attracting more and more interest. There is, however, considerable concern about the propensity of MoM hip replacements to release metal ions and fine, nanometre-scale metallic wear debris. The long-term effect from released metal ions and wear particles is still not clear.

To date, all the work on hip simulators focused on assessing mass losses damage has been referred to as ‘wear’. However, it is known in the field of tribocorrosion that mechanical removal of the passive layer on Co—Cr alloys can significantly enhance corrosion activity. In total joint replacements, it is possible that corrosion plays a significant role. However, no one has ever tried to extract, on a hip simulator, what proportion of the damage is due to mechanical processes and the corrosion processes. This paper describes the first instrumentation of an integrated hip joint simulator to provide in-situ electrochemical measurements in real time. The open circuit potential results are reported to assess the corrosion regime in the absence and presence of movement at the bearing surfaces. The importance of these measurements is that the real damage mechanisms can be assessed as a function of the operating cycle.

Wear and Creep of Highly Crosslinked Polyethylene against Cobalt Chrome and Ceramic Femoral Heads

The wear and creep characteristics of highly crosslinked ultrahigh-molecular-weight polyethylene (UHMWPE) articulating against large-diameter (36 mm) ceramic and cobalt chrome femoral heads have been investigated in a physiological anatomical hip joint simulator for 10 million cycles. The crosslinked UHMWPE/ceramic combination showed higher volume deformation due to creep plus wear during the first 2 million cycles, and a steady-state wear rate 40 per cent lower than that of the crosslinked UHMWPE/cobalt chrome combination. Wear particles were isolated and characterized from the hip simulator lubricants. The wear particles were similar in size and morphology for both head materials. The particle isolation methodology used could not detect a statistically significant difference between the particles produced by the cobalt chrome and alumina ceramic femoral heads.

Pitch-based carbon-fibre-reinforced poly (ether—ether—ketone) OPTIMA® assessed as a bearing material in a mobile bearing unicondylar knee joint

The introduction of unicondylar knee prostheses has allowed the preservation of the non-diseased compartment of the knee while replacing the diseased or damaged compartment. In an attempt to reduce the likelihood of aseptic loosening, new material combinations have been investigated within the laboratory. Tribological tests (friction, lubrication, and wear) were performed on metal-on-carbon-fibre-reinforced (CFR) poly (ether—ether—ketone) (PEEK) (pitch-based) mobile unicondylar knee prostheses up to 5×106 cycles. Both a loaded soak control and an unloaded soak control (both medial and lateral components) were used to compensate for weight change due to lubricant absorption. For this material combination the loaded soak control gave slightly lower wear for both the medial and the lateral components than did the unloaded soak control. The medial components gave higher steady state wear than the lateral components (1.70 mm3 per 106 cycles compared with 1.02 mm3 per 106 cycles with the loaded soak control). The results show that the CFR PEEK unicondylar knee joints performed well in these wear tests. They gave lower volumetric wear rates than conventional metal-on-ultra-high-molecular-weight polyethylene prostheses have given in the past when tested under similar conditions. The friction tests showed that, at physiological viscosities, these joints operated in the boundary—mixed-lubrication regime. The low wear produced by these joints seems to be a function of the material combination and not of the lubrication regime.

The effect of component size and orientation on the concentrations of metal ions after resurfacing arthroplasty of the hip

Increased concentrations of metal ions after metal-on-metal resurfacing arthroplasty of the hip remain a concern. Although there has been no proven link to long-term health problems or early prosthetic failure, variables associated with high metal ion concentrations should be identified and, if possible, corrected. Our study provides data on metal ion levels from a series of 76 consecutive patients (76 hips) after resurfacing arthroplasty with the Articular Surface Replacement. Chromium and cobalt ion concentrations in the whole blood of patients with smaller (<or= 51 mm) femoral components were significantly higher than in those with the larger (>or= 53 mm) components (p < 0.01). Ion concentrations in the former group were significantly related to the inclination (p = 0.01) and anteversion (p = 0.01) of the acetabular component. The same relationships were not significant in the patients with larger femoral components (p = 0.61 and p = 0.49, respectively). Accurate positioning of the acetabular component intra-operatively is essential in order to reduce the concentration of metal ions in the blood after hip resurfacing arthroplasty with the Articular Surface Replacement implant.

The influence of wear paths produced by hip replacement patients during normal walking on wear rates

Variation in wear paths is known to greatly affect wear rates in vitro, with multidirectional paths producing much greater wear than unidirectional paths. This study investigated the relationship between multidirectional motion at the hip joint, as measured by aspect ratio, sliding distance, and wear rate for 164 hip replacements. Kinematic input from three-dimensional gait analysis was used to determine the wear paths. Activity cycles were determined for a subgroup of 100 patients using a pedometer study, and the relationship between annual sliding distance and wear rate was analyzed. Poor correlations were found between both aspect ratio and sliding distance and wear rate for the larger group and between annual sliding distance and wear rate for the subgroup. However, patients who experienced a wear rate <0.08 mm/year showed a strong positive correlation between the combination of sliding distance, activity levels, and aspect ratio and wear rate (adjusted r(2) = 55.4%). This group may represent those patients who experience conditions that most closely match those that prevail in simulator and laboratory tests. Although the shape of wear paths, their sliding distance, and the number of articulation cycles at the hip joint affect wear rates in simulator studies, this relationship was not seen in this clinical study. Other factors such as lubrication, loading conditions and roughness of the femoral head may influence the wear rate.

Metal ion levels in the blood of patients after hip resurfacing: a comparison between twenty-eight and thirty-six-millimeter-head metal-on-metal prostheses

BACKGROUND

Metal ion toxicity, metal hypersensitivity, and metal carcinogenicity are causes for concern for patients with metal-on-metal hip replacements. Thus, understanding the biological fate of metal ions, and consequently their long-term systemic effects, is of great interest to patients and surgeons alike.

METHODS

Inductively coupled plasma mass spectrometry was used to measure the levels of cobalt, chromium, and molybdenum ions in the blood of control patients (preoperative control pre-resurfacing patients), patients with a metal-on-polyethylene total hip prosthesis, patients with a metal-on-metal total hip prosthesis with either a 28 or 36-mm femoral head, and patients with a hip resurfacing prosthesis. Since cobalt and chromium ions have the potential to induce oxidative stress through irreversible biochemical damage to macromolecules, the levels of ions were correlated to the concentration of three oxidative stress markers in the plasma of these patients.

RESULTS

The median cobalt level was significantly lower (p < 0.001) in the 36-mm metal-on-metal total hip arthroplasty group (1.8 parts per billion [1.8 microg/L]) compared with the 28-mm metal-on-metal total hip arthroplasty group (2.5 parts per billion [2.5 microg/L]) and the hip resurfacing group (2.3 parts per billion [2.3 microg/L]) at six months postoperatively. The median chromium level was also significantly lower (p < 0.01) in the 36-mm metal-on-metal total hip arthroplasty group (0.25 parts per billion [0.25 microg/L]) compared with the 28-mm metal-on-metal total hip arthroplasty group (0.35 parts per billion [0.35 microg/L]) and the hip resurfacing group (0.50 parts per billion [0.50 microg/L]) at six months postoperatively. However, neither the median cobalt levels nor the median chromium levels were significantly different among the three metal-on-metal groups at one year. The median levels of molybdenum were not significantly different among the three groups at either six months or one year. In addition, there was no significant difference in the plasma concentration of oxidative stress markers in patients with metal-on-metal bearings compared with that in control patients.

CONCLUSIONS

The blood metal ion levels in the hip resurfacing group were similar to those in the 28 and 36-mm-head metal-on-metal total hip arthroplasty groups. This study suggests that the increased metal ion levels had no effect on oxidative stress markers in the blood of these patients.

Activity levels and polyethylene wear of patients 10 years post hip replacement

BACKGROUND

Wear of the acetabular cup and implant loosening in total hip arthroplasty are thought to be affected by individual patient activity levels.

METHODS

Activity levels of 100 patients with unilateral primary hip replacements were measured using a digital pedometer 10-years post-operatively.

FINDINGS

Activity rates of hip replacement patients 10 years post surgery were found to fall with increasing age strata, with the age group 70-74 years showing significantly reduced activity rates compared to the 55-64 years age group and the age group greater than 80 years showing significantly reduced activity rates compared to both the 55-64 years age group and the 65-69 years age group. However, a clear correlation between age and activity was not found. Acetabular cup wear showed no relationship with patient activity levels.

INTERPRETATION

Progressive reduction in activity levels with more elderly hip replacement patients may inform long term planning of hip arthroplasty and implant choice.

[Hip resurfacing: current state of knowledge]

SUMMARY

The tribologic quality of metal-on-metal bearings has enabled a second generation of hip resurfacing techniques. Compared with a conventional hip prosthesis, this type of arthroplasty has many advantages: sparing femoral (and acetabular) bone stock, preservation of hip joint biomechanics (femoral offset, leg length), better recovery for high-level sports activities, easier revision, less risk of dislocation, less risk of extension to the shaft in the event of osteolysis. Hip resurfacing can thus be considered as true "minimally invasive bone surgery". There are however specific complications of resurfacing, including femoral neck fracture and collapse of the femoral head. All of the conventional approaches can be used for hip resurfacing procedures, but a precise operative technique is mandatory. The key to success is a proper position of the femoral piece. Certain biomechanical rules for implantation are required in order to limit the cam effect, spare femoral bone, and maintain harmonious loading. Hip resurfacing can be indicated for young and/or active subjects for whom restoration of hip biomechanics offers a real advantage over conventional hip arthroplasty. The mid-term results have been encouraging, both clinically and radiographically.

Understanding the differences between the wear of metal-on-metal and ceramic-on-metal total hip replacements

Hip simulator studies have been carried out extensively to understand and test artificial hip implants in vitro as an efficient alternative to obtaining long-term results in vivo. Recent studies have shown that a ceramic-on-metal material combination lowers the wear by up to 100 times in comparison with a typical metal-on-metal design. The reason for this reduction remains unclear and for this reason this study has undertaken simple tribometer tests to understand the fundamental material loss mechanisms in two material combinations: metal-on-metal and ceramic-on-ceramic.

A simple-configuration reciprocating pin-on-plate wear study was performed under open-circuit potential (OCP) and with applied cathodic protection (CP) in a serum solution using two tribological couples: firstly, cobalt—chromium (Co—Cr) pins against Co—Cr plates; secondly, Co—Cr pins against alumina (Al2O3) plates. The pin and plate surfaces prior to and after testing were examined by profilometry and scanning electron microscopy.

The results showed a marked reduction in wear when CP was applied, indicating that total material degradation under the OCP condition was attributed to corrosion processes. The substitution of the Co—Cr pin with an Al2O3 plate also resulted in a dramatic reduction in wear, probably due to the reduction in the corrosion—wear interactions between the tribological pair.

Lubrication regimes in lumbar total disc arthroplasty

A number of total disc arthroplasty devices have been developed. Some concern has been expressed that wear may be a potential failure mode for these devices, as has been seen with hip arthroplasty. The aim of this paper was to investigate the lubrication regimes that occur in lumbar total disc arthroplasty devices. The disc arthroplasty was modelled as a ball-and-socket joint. Elastohydrodynamic lubrication theory was used to calculate the minimum film thickness of the fluid between the bearing surfaces. The lubrication regime was then determined for different material combinations, size of implant, and trunk velocity. Disc arthroplasties with a metal-polymer or metal-metal material combination operate with a boundary lubrication regime. A ceramic-ceramic material combination has the potential to operate with fluid-film lubrication. Disc arthroplasties with a metal-polymer or metal-metal material combination are likely to generate wear debris. In future, it is worth considering a ceramic-ceramic material combination as this is likely to reduce wear.

Lubrication regime of the contact between fat and bone in bovine tissue

Fat pads are masses of encapsulated adipose tissue located throughout the human body. Whilst a number of studies describe these soft tissues anatomically little is known about their biomechanics, and surgeons may excise them arthroscopically if they hinder visual inspection of the joint or bursa. By measuring the coefficient of friction between, and performing Sommerfeld analysis of, the surfaces approximating the in vivo conjuncture, this contact has been shown to have a coefficient of friction of the order of 0.01. The system appears to be lubricated hydrodynamically, thus possibly promoting low levels of wear. It is suggested that one of the functions of fat pads associated with subtendinous bursae and synovial joints is to generate a hydrodynamic lubricating layer between the opposing surfaces.