Total Hip Arthroplasty (THA) is a well-established, cost-effective treatment for improving function and alleviating pain in patients who have disabling hip disease with excellent long-term results. Based on the excellent results, there is an ongoing trend for THA to be performed in younger and more active patients, having higher physical demands on their new total joints.

Polyethylene (PE) wear and its biological consequences are one of the main causes of implant failure in THA. Macrophages phagocytise PE wear particles and this will result in osteolysis and loss of periprosthetic bone. The risk of these complications can be estimated in relation to the amount of volumetric wear based on two assumptions: that the number of PE particles dispersed in the peri-prosthetic tissues is controlled by the amount of PE wear; and that the development of osteolysis and the resulting aseptic loosening is triggered by these PE particles. Based on these assumptions, a model was developed to estimate the osteolysis-free life of a THA, depending on the Linear Wear Rate (LWR) and femoral head size of the PE bearing.

A review of the literature was conducted to provide an estimate of the radiologic osteolysis threshold based on the volumetric wear of the PE bearing. This review demonstrates that this radiologic osteolysis threshold is approximated 670 mm3 for conventional PE. The osteolysis-free life of the THA was estimated by simply dividing this threshold volume by the annual Volumetric Wear Rate (VWR) of the bearing. The annual VWR is basically controlled by two parameters: (1) annual LWR and (2) head size, and was calculated by using published formulae.

For 28 mm heads, following osteolysis-free life was determined in function of the annual LWR. LWR: 10 µm/y => 116.6 years / LWR: 25 µm/y => 46.6 years / LWR: 50 µm/y => 23.3 years / LWR: 100 µm/y => 11.6 years. For 40 mm heads, following osteolysis-free life was determined in function of the annual LWR. LWR: 10 µm/y => 57.1 years / LWR: 25 µm/y => 22.9 years / LWR: 50 µm/y => 11.4 years / LWR: 100 µm/y => 5.7 years.

The osteolysis-free life determined by this model is in good agreement with the clinical results of PE bearings having a 28 mm head size and demonstrates that extreme low LWRs are mandatory to assure a descent osteolysis-free life for THA (PE bearings) using large heads, such as 40 mm. For such head sizes, small variations of the LWR may have large impacts on the osteolysis-free life of the THA.

INTRODUCTION

Total joint arthroplasty continues to gain acceptance as the standard of care for the treatment of severe degenerative joint disease, and is considered one of the most successful surgical interventions in the history of medicine. However, infection of these implants, called Periprosthetic Joint Infection (PJI), remains one of the biggest challenges facing orthopaedics today. PJI can lead to additional surgeries, revision, fusion and amputation.

Total joint arthroplasty continues to gain acceptance as the standard of care for the treatment of severe degenerative joint disease. However, the Periprosthetic Joint Infection (PJI) remains one of the biggest challenges facing orthopaedics today.

It is important to accurately diagnose PJI because its management differs from that of other causes of arthroplasty failure. The most common symptom of PJI is pain. In acute infection, the local signs and symptoms (e.g., severe pain, swelling, erythema, and warmth at the infected joint) of inflammation are generally present. On the other hand, chronic infection usually has a more subtle presentation, with pain alone, and is often accompanied by loosening of the prosthesis at the bone-implant interface. The diagnosis of PJI has proven quite challenging, as both acute and chronic infections can be difficult to differentiate from other forms of inflammation.

The reported literature on the diagnosis of PJI has focused on evaluated laboratory tests that were never developed specifically for the diagnosis of PJI. Because these tests were not made for the purpose of diagnosing PJI, it has been the responsibility of the orthopaedic community to evaluate and recommend their interpretation. This has resulted in significant confusion regarding the appropriate thresholds and optimal combination of these tests. These difficulties were the motivation for the development of a specific test for the detection of PJI. The promising diagnostic capabilities of synovial fluid biomarkers for PJI have already been reported in the literature. Studies have demonstrated that the alpha-defensin microbicidal peptide present in human neutrophils is an ideal biomarker for PJI due to the distinct separation it achieves between positive and negative results.

A specific test allowing to measure the concentration of the alpha-defensin in the synovial fluid has been developed.

The specificity and the sensitivity of this test for the detection of a PJI are respectively 96% and 97%. This test has been proven to have also a high reproducibility, its results not being influenced by antibiotics.

A lateral flow version of this test (Synovasure PJI, distributed exclusively in Europe by Zimmer GmbH) has been recently developed. It allows reading the results in 10 minutes and it doesn't require any laboratories for its interpretation. Currently, this test device is in clinical evaluation in more than 200 European hospitals.

In case that the clinical evaluation of this test device is positive, this method will be a new paradigm for the diagnosis of periprosthetic joint infection.

Based on numerous national registries, cemented hip replacements have globally better long-term results than uncemented hip replacements. For example, following data have been published in national registries:

These registries demonstrated clearly that cemented fixation should be definitively preferred than uncemented fixation… Despite this evidence, uncemented fixation is more and more used in the majority of the countries performing total hip replacements.

A recent paper analysed the Swedish situation and may give some reasons for explaining this paradox. A Cox proportional hazards model was used to analyze the Relative Risk (RR) of revision for different type of implants and/or fixation for 170,413 total hip arthroplasties. The RR was adjusted for sex, age, and underlying diagnosis. If the RR is lower than 1, less revisions are seen with uncemented fixation and less revisions are seen with cemented fixation when the RR is higher than 1. The figure 1 summarizes the table 6 of this publication.

This figure naturally confirms that globally cemented fixation has a lower revision burden with an adjusted RR of 1.5 (revision of any component for aseptic loosening) than uncemented fixation. This difference in the revision is controlled by the cups, where the adjusted RR for uncemented cups is 1.8. Stems demonstrate an opposite behaviour with a lower revision burden for uncemented fixation with an adjusted RR of 0.4.

Analysing the revision rate of the 5 most common implants (cemented versus uncemented), the adjusted RR for aseptic loosening is lower than 1 for both cups and stems. The difference of the RR between all cups (RR: 1.8) and the 5 most common cups (RR: 0.5) indicates undeniably that some cup have a major influence on the revision rate of uncemented systems.

This analysis allows to draw following conclusions:•

In national registries, cemented fixation is globally superior.

Modern metal-on-metal bearings were reintroduced on the market by Prof. B.G. Weber in 1988. Since this date, more than 500,000 bearings have been implanted world-wide with excellent clinical results.

The goals of this presentation are to review critically the long-term published clinical results for metal-on-metal bearings (small and large diameter) and to investigate the current concerns (ions release, allergic reactions, pseudo-tumours…) about metal-on-metal bearings. Based on this review, the benefice-risk ratio of metal-on-metal bearings will be discussed.

Aim: The purpose of this study was to analyze the in-vivo wear rates of Birmingham Hip Resurfacing (BHR, Midland Medical Technologies Ltd., Birmingham, U.K.) explants and to contrast the results to the wear rates of conventional 28 mm metal-on-metal bearings (Metasul, Zimmer GmbH, Winterthur, Switzerland).

Methods: The wear rates, measured by a coordinate measuring machine (CMM5, SIP, Geneva, Switzerland), of 6 femoral components and two complete pairings of the BHR retrieved from 8 hips were contrasted to 43 28 mm heads from second generation metal-on-metal bearings (Metasul, Sulzer Orthopaedics Ltd., Winterthur, Switzerland).

Results: After 13 months (7 to 24) the BHR femoral heads showed a median volumetric wear rate of 2.9 mm³ (interquartile range: 0.8 to 7.1), slightly, but not significantly higher than the investigated 43 28mm Metasul heads (0.8 mm³/year, p = 0.067, 14 months [7 to 24] in-situ). One BHR case with a cup abduction angle of 70° showed a significant higher wear rate of 17.8 mm³. All BHR cases showed only small amounts of metallic particle histological and correspondingly, a mild histiocytic tissue response without foreign body granuloma formation.

Discussion: During the first two years after surgery the investigated BHR components showed wear rates substantially lower than conventional polyethylene bearings, comparable to Metasul bearings, implanted with very successful clinical results. But whether the wear rates will drop down after the so called running-in period comparable to conventional metal-on-metal bearings and provide young and active patients with a biologically acceptable particle volume over a long time period, still remains to be seen.

Introduction: Hip simulator and clinical retrieval studies have shown that metal-on-metal (MOM) hip implants commonly have biphasic wear. An initial high wear or running-in phase is generally followed by a low wear or steady-state phase. A number of hypotheses have been put forward to explain this biphasic phenomenon, including polishing of the metallic bearing surfaces and increasing conformity between the two articulating surfaces. The purpose of the present study was to compare the wear and lubrication of MOM hip implants between the running-in and steady-state periods.

Materials and Methods: A standard 28mm Metasul^TM MOM bearing (Centerpulse Orthopedics, a Zimmer Company, Winterthur, Switzerland) was investigated. The wear testing was carried out using a 6-station AMTI hip simulator in the presence of 33% bovine serum and 67% Ringer solution (PH 7.2). The bearing surfaces of both the femoral and acetabular components were measured using a coordinate measurement machine at different stages of wear testing. The dimensional changes of the bearing surfaces due to wear were directly incorporated into the elastohydrodynamic lubrication analysis using an in-house developed code.

Results: The initial running-in period occurred during the first 1 million cycles, and little wear was observed subsequently up to 5 millions cycles. The maximum total wear depth was measured to be around 13 μm at 1 million cycles. The predicted average lubricant film thickness between the two articulating surfaces was increased from 0.024μm at the beginning, to 0.09μm at the end of the first 1 million cycles. For a given composite surface roughness of 0.03μm often quoted for the metallic bearing surfaces, such an increase in the lubricant film thickness represents a transition from a mixed to a fluid film lubrication regime.

Discussion: A large improvement in lubrication was predicted as a direct result of the running-in wear of the bearing surfaces. This was mainly due to the increased conformity between the two articulating surfaces and the decreased diametrical clearance. It was particularly noted that the improvement in lubrication after 1 million cycles was so significant that continuous fluid film lubrication was possible, leading to extremely low wear for up to 5 million cycles, and only material fatigue and start-up and stopping for wear measurements could cause a further increase in wear.

It is possible in theory to optimise the geometry of the metallic bearing surfaces, based on the worn components, to minimise the running-in wear. However, such an improvement in lubrication cannot be readily achieved because of difficulties in surgical techniques and position of the components.

Introduction: Laboratory simulator and clinical retrieval studies of metal-on-metal (MOM) total hip replacements have shown that the metallic alloy, the femoral head radius, the clearance between the acetabular cup and femoral head and the cup thickness can influence the contact mechanics, the lubrication and the wear of the articulation. MOM hip resurfacing procedures have received significant attention recently. The purpose of the present study was to compare the contact mechanics between a MOM hip resurfacing implant and a MOM total hip replacement under identical conditions.

Materials and Methods: A 50mm diameter DUROM^TM MOM hip resurfacing prosthesis and a 28mm diameter Metasul^TM MOM bearing system (Centerpulse Orthopedics, a Zimmer Company, Winterthur, Switzerland) were investigated. All implants were manufactured from wrought-forged high carbon cobalt chromium alloy (Pro-tasul 21WF^TM). The diameters of the DUROM^TM femoral head and acetabular cup were 50mm and 50.145mm respectively, and the corresponding wall thickness of the acetabular component was around 4mm. The diameters of the Metasul^TM femoral head and acetabular cup were 28mm and 28.12mm. Three-dimensional finite element models were created to simulate the contact between the bearing surfaces of both the femoral head and the acetabular cup fixed to a three dimensional anatomically positioned pelvic and femoral bone consisting of both cortical (with 1mm thickness) and cancellous regions. The load applied to both models was 3200N.

Results: The maximum contact pressure at the bearing surfaces was found to be around 22MPa for the DUROM^TM and the contact area between the femoral and acetabular components was predicted to be 237mm². For the Metasul^TM bearing under identical conditions, the maximum contact pressure and the contact area predicted were approximately 47MPa and 74mm² respectively.

Discussion: A large reduction in the contact pressure, which should improve overall tribological performances, was noted for the DUROM^TM hip resurfacing prosthesis, as compared with the Metasul^TM bearing. The main reasons for this reduction were the large diameter of the articulation and the small acetabular cup thickness of the DUROM^TM system. In contrast, the Metasul^TM bearing has a smaller head diameter, and relies on a polyethylene backing underneath the metallic cup inlay to reduce the contact pressure at the articulating surfaces.

Since 1977 we did implant all alumina (Al2O3) bearings total hip prostheses. A lot of data were documented concerning tissue reaction, in vivo wear behaviour, fractures, and clinical outcome. Ceramic materials retrieved at revision were analysed. In some cases, wear was as low as a few microns for a 15-year period in use. This is two thousand times less than a regular metal on polyethylene sliding couple. and 100 times less than a metal on metal prosthesis. Fracture mechanism is related to crack propagation into the material. During the first period, the fracture rate was in the range of 2%; it then dropped to less than 0.1 %. Few fractures could not be explained by technical or design mistakes. Clinical outcome: More than 4000 total hips in selected young and /or active patients were implanted. In a recently reported series of consecutive patients operated by P.Boutin during the year 1980, 118 hips in 106 patients were included. Mean age was 62 years. At the twenty-year follow-up evaluation, forty-five patients (fifty-one hips) were still alive and had not been revised, twenty-five patients (twenty-five hips) had undergone revision of either or both components, twenty-seven patients (thirty hips) had died from unrelated causes, and nine patients (twelve hips) were lost to follow-up. The mean Merle d’Aubigné hip score was 16.2 ± 1.8 at the latest follow-up. Survival of the cup at twenty years with revision for any reason as the end-point was 85.6 percent for cementless cups versus 61.2 % for cemented cups, respectively. Survival of the stem at twenty years with revision for any reason as the end-point was 84.9 % for cementless stems versus 87.3% for cemented stems. Wear of the prosthetic components was undetectable on plain radiographs. No fracture of the alumina socket or head was recorded. Another study concerned a more recent design of the socket which consisted in a metal back titanium alloy shell covered with a pure titanium mesh with an alumina liner. The nine year survival rate was 98.4% with revision for aseptic loosening as the end point. Conclusion This alumina on alumina bearing provides interesting results without any physical limitation specially in young and active patients.

Metal-on-metal (MOM) bearings for artificial hip joints have attracted significant attention recently as a way of reducing wear and consequently wear particle induced periprosthetic osteolysis, which is the major cause of failure. One of the most widely used MOM total hip implants is the Metasul system (Zimmer GmbH), in which a thick polyethylene backing is used underneath the metallic inlay. The purpose of this study was to investigate the effect of the polyethylene backing on the transient lubrication under dynamic loading and velocity conditions representative of walking. A 28mm diameter Metasul bearing was analysed, and the predicted lubricant film thickness was compared with that for an all metallic cup. The predicted transient lubricant film thickness in the Metasul system was found to be significantly greater than the corresponding prediction from the all metal cup. Therefore it was concluded that the polyethylene backing may play an important role in the lubrication and overall tribological performance of the Metasul bearing system with a diameter of 28mm.

Introduction: A renewal of interest in large metal-on-metal bearings has been seen due to the introduction of resurfacing prostheses. According to lubrication theory, large metal-on-metal bearings may obtain a film fluid lubrication. The mode of lubrication may be described by the lambda coefficient λ, which is the ratio between the thickness of the lubricant hc and the root mean square roughness of the bearing Rq. If this coefficient λ is higher than 3, a fluid film lubrication is expected. To have this situation, the following parameters must be optimized: diametral clearance and roughness. This presentation investigates the role of these two parameters, based on two commercially available products.

Methods: To determine the λ coefficient, the thickness hc of the lubricant must be determined, as well as the roughness of the bearing Rq. The Hamrock – Dawson equation (1) allows the determination of the thickness hc as a function of the bearing parameters. The roughness Rq is measured by a stylus profilometer.

Results: With a typical load of 3000 N, an angular velocity of 1 rad/s, and a viscosity of 0.005 Pas, the Hamrock – Dawson equation gives the following film thickness hc for a 50 mm metal-on-metal bearing with different diametral clearances:

Diametral clearance [μm] 100 150 200 250 300<

The following roughnesses Rq were measured for two types of resurfacing prosthesis:

As cast CoCr alloy (BHR by MMT): 23 ± 6 nm

This large difference in the λ coefficients indicates that the lubrication mode of these two different prostheses is probably different. Based on this analysis, the wrought-forged component has ideal lubrication (λ > 3) whereas the as cast does not reach ideal lubrication (λ < 3).

Conclusions: This investigation shows that minute differences in the geometry and in the roughness of a metal-on-metal prosthesis may significantly influence their lubrication behaviour as well as the wear resistance.

A first introduction of a successful metal-on-metal (m-o-m) articulation was made by G.K. McKee in 1956 using a cast CoCrMo alloy for the head and cup. Long-term clinical investigations of m-o-m and polyethylene-on-metal articulation showed similar 20 years follow-up survival rates. This and very good wear results obtained with some of the first generation m-o-m articulation led to a re-introduction of the m-o-m articulation in 1988. A healthy hip joint has a very low friction coefficient and almost no wear due to the optimal lubrication, which, under normal conditions, completely separates the two articulation surfaces. On the other hand all artificial hip prostheses are unable to produce or maintain a permanent lubrication film. Therefore, the surfaces of the prostheses are always subject to wear. Compared to polyethylene liners, wearing at an average linear rate of 0,1 to 0,2mm per year, m-o-m articulations showed generally very little wear. In vitro simulations of the second generation m-o-m articulation on a Stanmore hip simulator showed a steady wear rate of 5.6±7.3μm per million cycles, with a higher wear rate during the running-in phase of about one million cycles. The analysis of over 200 second-generation m-o-m retrieved hip implants showed an average linear wear rate of approximately 5μm per year after the running-in period, with a follow up time of up to ten years. There’s a great concern about the incidence of cancer after a total m-o-m hip replacement. It is very difficult to find a causal relationship between THR and cancer occurrence, as in some studies many cancers were detected within two years after THR, which indicate rather an associative relationship. However, the summarized results do not indicate an increased cancer risk after m-o-m total hip replacements. Over 130,000 m-o-m articulations have been implanted since 1988 and the clinical results have been excellent matching or surpassing current gold standards for hip replacement.

Aims: The purpose of the present study was to investigate the contact mechanics at the articulating surfaces in metal-on-metal hip implants. Methods: A 28mm diameter Metasul (from Sulzer Orthopedics Ltd.) was analysed in the present study. Both the femoral head and the acetabular cup were manufactured from matching cobalt chromium alloy. The cobalt chromium alloy acetabular inlay was thermo-mechanically bonded to an ultra high molecular weight polyethylene (UHMWPE) backing, which was in turn inserted into a titanium shell with a snap-þt for cementless þxation. The radial clearance between the femoral head and the acetabular cup was 60μm. Finite element method (ABAQUS 6.2) was used to model the contact at the articulating surfaces between the femoral head and the acetabular cup, under a load of 3.2kN. Results: The average contact pressure at the bearing surfaces was found to be about 45MPa. This was considerably lower than 63MPa if the UHMWPE backing was replaced by cobalt chromium alloy. It was also interesting to compare the present result with the use of a larger femoral head or a reduced clearance. In order to match the average contact pressure of 45MPa, it was found to be necessary to increase the femoral head radius to 18mm for a given radial clearance of 60μm or to decrease the radial clearance to 35μm for a þxed femoral head radius of 14mm. Conclusions: The use of an UHMWPE backing underneath a cobalt chromium alloy cup signiþcantly reduces the contact stresses experienced at the articulating surfaces in metal-in-metal hip implants.

Aseptic loosening due to particle disease is one of the major problems for THA. Since simulator studies are expensive, there is a need for screening tests to evaluate different pairings. Recent publications have shown that multiaxial motions are important in reproducing appropriate wear behaviour. The objective of the study was to design a simple bidirectional pin-on-disc screening test with multidirectional motions allowing on-line dimensional wear measurements and to compare the measured in-vitro wear rate with the observed in-vivo wear rate.

The multidirectional motions were obtained by a rotating pin on a rotating disc. With one disc rotation, the pin rotates 4 times, which leads to 4 changes in the wear direction. The wear characteristics were: load 3.45 MPa, lubricant 33% calf serum – 67% Ringer’s solution, temperature 37°C. The wear rate was measured on-line by the dimensional change in the height of the pin and by the conventional gravimetric method.

Following results were obtained: Wear Coefficient [mm3/Nm]: 1.446 10-6 ± 0.447 10-6 [CoCrMo – UHMWPE] – 0.515 10-6 ± 0.136 10-6 [Al2O3- UHMWPE] /Linear Wear Rate [mm/106 cycles]: 0.132 ± 0.041 [CoCrMo – UHMWPE] – 0.047 ± 0.012 [Al2O3- UHMWPE] /Gravimetric Wear Rate [mg/106 cycles]: 2.104 ± 0.709 [CoCrMo – UHMWPE] – 0.769 ± 0.205 [Al2O3- UHMWPE].

This bi-directional screening test with shear motions has the following characteristics: Gives the three wear parameters /Shows s significant difference between the CoCrMo – UHMWPE and the Al203 – UHMWPE pairing /Demonstrates a relatively good agreement between the in-vivo and the in-vitro measurements.

This type of screening tests will provide some earlier information concerning the tribological behaviour of pairing for total hip joints.