Metal-on-metal (MOM) and ceramic-on-metal (COM) studies in total hip arthroplasty (THA) documented adverse wear termed “edge loading”. Laboratory simulations necessitated cups steeply inclined to produce edge- loading, whereby cup rims could attenuate the normal wear patterns. Size of cup wear-pattern was therefore key in defining edge-loading. From prior simulator studies (‘Anatomic’ test: ISO-14242), we could demonstrate a linear relationship between size of cup wear-patterns and MOM diameters, cup wear-areas decreasing from 18% to 8%. However, retrieval studies (COM/ MOM) showed cup wear-patterns in vivo were much larger, typically covering 50–55% cup surfaces (Clarke 2013: Koper 2015). In prior MOM Anatomic simulator study (head oscillating, cup fixed), we noted areas worn on 60mm heads and cups averaging 1,668mm2 and 442mm2, respectively (Bowsher 2009). Thus, ratio ×3.77 described distributed area worn on heads relative to focal area worn in cups. In the orbital simulator, the only way to achieve larger cup wear areas was to reverse the component positions, i.e. cups oscillating, heads fixed. The overall goal for this project was to develop an understanding of how such edge-loading affected adverse-wear performance of THA in simulators.

60mm MOM (DJO, Austin TX) were chosen comparable to our prior study (Bowsher 2009) and cups were mounted inverted (oscillating) under fixed heads. Adaptors were machined to incline cup faces at 17o and 27o and, with the simulator's +/−23° motion, they experienced 40oand 50o cyclic peak oscillations, respectively. The orbital simulator was identical to that of prior study as was the test protocol (Bowsher 2009). Wear patterns on components were assessed visually and microscopically, taped and colored red to aid photography.

The 40° and 50° tests produced circular cup wear patterns that came progressively closer to the rims without actually producing edge-loading, creating average wear area of 1,663mm2. These proved identical to wear areas on heads (orbiting) in prior Anatomic test (1,668mm2). Using the hemispherical-area datum of 5,655mm2 for 60mm MOM, our test produced cup wear patterns with desired 29.4% coverage.

The value of ISTA conferences is that by definition these bring new arthroplasty ideas and technologies to the forefront. The international guideline for simulators (ISO-14242) has proven useful for standard ‘Anatomic’ cup tests that do not require edge-loading conditions. However, ours is the 1st simulator study to; (i) predict the size of THA wear patterns, (ii) show that ratio of head: cup wear-areas average ×3.8 in favor of mobile component, and (iii) demonstrated cups can be run Inverted to produce more clinically-relevant wear patterns that in edge- loading studies. The new learning experience was that studies of edge-loading in THA cups need to consider the ‘Inverted’ test in order to simulate clinically relevant tribo-mechanical parameters. Compared to Anatomic test, the Inverted-cup test has the advantage of (iv) producing larger cup wear areas, (v) clinically-relevant attenuation of wear patterns at cup rim, and (vi) intermittent edge-loading (instead of constant loading) judged likely to apply to a larger patient population at risk.

Use of “CPR” distance has proven clinical utility in stratifying risks of “steep cups” in MOM failures.[1, 4] The CPR indice has been defined as distance between point of intersection of the hip reaction force (Fig. 1: vector-R in contact patch) and closest point on the inner cup rim.[4] However, the CPR indice has limitations. It assumes that, (1) the hip load-vector (R) will be angled 10°-medial in all patients, (2) the contact patch will be same size in all patients, and (3) the contact patch will be invariant with increasing MOM diameter. In contrast it is known from retrieval studies that larger MOM bearings created much larger wear patches.[3] Furthermore, the size of cup wear-patches in MOM bearings can now be estimated with some certainty using simulator wear data.[2] Our objective was to develop an algorithm that would predict (i) contact-patch size for all cup designs and diameters, (ii) determine actual margin of safety (Fig. 1: MOS) for different laterally-inclined cups, and (iii) predict critical test angles for “steep” cup studies in hip simulators.

The ‘CPR-distance’ (Fig. 1) is subtended by the CPA angle, but the true margin of safety is the distance from edge of wear patch to cup rim, indicated here by MOS angle. In this algorithm the wear-patch size (CAP angle) is a key parameter, as derived from MOM wear data (Fig. 2). The CAP angles decrease with increasing MOM diameter, as defined by strong linear trend (R=0.998). The key 2nd parameter is cup inclination angle that juxtaposes the wear-pattern to the cup rim (CCI). For hemispherical cups the critical inclination is given by CCI = 90 – CAP/2, where articulation angle ABA = 180o. The cup bearing-surface is typically reduced < 180o(sub-hemispherical profile, instrumentation groove, rim bevel, etc). These effects are grouped under ‘rim-detail’, as defined by RD = (180-ABA)/2 (Fig. 1). Thus critical inclination any cup is given by CCI = 90o – (CAP/2) – RD = (ABA – CAP)/2. The margin-of-safety (Fig. 1) is then represented by the equation MOS = 100 – (CIA + CAP/2 + RD).

Applicability of the new algorithm can be visualized with a 48mm MOM (cup ABA=160o) run in a standard simulator test (Fig. 3). The algorithm predicts that with cup at 40o inclination there is good margin of safety (11.8o), representing a 5mm distance. This would become much reduced at CIA = 50o, while true edge-wear appears at the 60o test inclination (Fig. 3. EW = −8.2o). For clinical comparison with ‘CPR-distances’, the algorithm shows that positioning the wear patch 10o-medial (Figs. 1, 3) has margin of safety averaging 11.5 mm (MOS) less than was predicted by the CPR indice. While CPR has shown clinical utility, it is believed that compensating for actual size of cup wear-patterns provides a more realistic risk assessment for different MOM diameters in different cup positions. Thus the new algorithm permits accurate depiction of cup wear-patterns for use in both clinical and simulator studies.

Retrieval studies of metal-on-metal (MOM) resurfaced hips revealed cup “edge wear” as a common failure mechanism [Morlock-2008]. Retrieval analysis of total hip arthroplasty (THA) also demonstrated extensive rim wear (Fig. 1: 190–220o arcs), typically across the superior cup [Clarke-2013]. Such wear patterns have not been demonstrated in hip simulator studies. The simulator “steep cup” models typically had motion arcs (flexion, etc.) input via the femoral head [Leslie-2008, Angadji-2009]. With fixed-inclination cups this produces constant loading of cup rim against the head (Fig. 2a). This is unlikely to be the physiological norm, unless patients walk constantly on the rims of mal-positioned cups. More likely the patients produce edge-wear intermittently due to functional and postural variations. Therefore a novel simulator model is proposed in which the cup undergoes edge-wear intermittently at one extreme of flexion (Fig. 2a). Our study objective using this new simulator model (Fig. 2a, b) was to (i) demonstrate MOM wear-rates and wear patches as a function of these dynamic-inclinations (40 o, 50 o, 70o), and (ii) compare the simulator data to MOM retrievals (Fig. 1).

Two simulator studies were run, both using 60mm MOM. Four bearings were run to 1-million cycles (1Mc) with cups peaking at 40 and 50° dynamic-inclinations, thus providing control data with no edge-wear. In 2nd study, 4 MOM were run with cups given a dynamic-inclination of 70° to produce edge-wear effects. In study-2 currently at 2.5Mc duration, the femoral heads showed the two classical wear phases with run-in at 1.7mm³/Mc and steady-state at 0.084mm³/Mc (Fig. 3a). Wear-rate for cups at 2.34mm³/Mc was 40% higher than heads and continued to rise linearly with time (Fig. 3a). At 2.5Mc, cup wear averaged ×5.7 greater than heads and resulting wear-patterns extended 85°−225° around cup rim (Fig. 3b: average 151°). In study-1, wear patches in cups with 40° dynamic-inclination approached within 12.4mm of the cup rim as denoted by circumferential grooves. This margin-of-safety (MOS) represented a 24°angle. The cup wear-patch averaged area of 1,760mm2. With cups run at 70o dynamic-inclination, the wear patches were transferred an additional 30o towards the rim thereby representing a 6° transfer across the rim.

This is the 1st wear study to use the new dynamic-inclination test mode to better simulate cup function in vivo. It was particularly satisfying to see the similarity in wear-patterns between retrieval (Fig. 1) and simulator cups (Fig. 3b). It is also the 1st study to monitor sites and magnitudes of cup wear areas and to purposely produce “edge wear”. The cups with 40° and 50° dynamic-inclinations had large margins of safety. With 70° dynamic-inclination the margin of safety was lost - effectively there was a 6° transfer of the wear patch across the cup rim. Even this apparently small effect at one location in each gait cycle sufficiently perturbed MOM performance that wear increased by an order of magnitude. Notably this was all cup wear and not by femoral head participation. The study continues but at 2.5Mc duration the cups revealed 5-fold greater wear than heads.

Introduction

In this study, we aimed to investigate the effect of the topography of the female taper surface on taper wear.

Hip simulator studies on MOM bearings have historically involved ‘custom’ cetabular cups. I.e. having neither beaded layers nor biological coatings.

The aim of this study was to investigate wear using such MOM bearings and evaluate the potential wear and evaluate the potential for error in the gravimetric assessment.

Six x 38 mm HC Co-Cr bearings were supplied (Global and IO International Orthopaedics). The cups were received in ‘off-the-shelf’ condition with a cast Co-Cr beaded/HA-coated backing. To remove the HA-coating, the cups were pre-soaked in lemon juice for 4 days (articular surfaces shielded). Custom plastic fixtures were machined to fit the beaded contours of the cups. Test duration was 5Mc inorbital hip simulator (Shore-Western). MOM wear was estimated from serum ion contamination. Serum samples were digested and assessed using ICP/MS (Weck Labs Inc, CA).

The majority of the HA-coating was removed from the cups after four days of soaking inlemon juice after 21 days of soaking all cup weights appeared atable (within 1 mg). Reflected-light microscopy (RLM) showed no descernible signs of HA and the total weight loss due to HA remval averaged∼400mg.

During hip simulator there was no visual evidence of lost or broken beads, 3rd body abrasion etc (Sa<30nm). Both gravimetric and metal ion analysis showed consistent wear trends for all MOM cups. The MOM with the highest wear (predicted by ion analysis) demonstrated 1.2 mm (3)/Mc)OWR) at 5Mc. In comparsion, gravimetric analysis predicted an OWR of 1.3 mm (3)Mc for the same MOM, a difference of only 8%. Soaking beaded-HA cups in lemon juice and BCS proved effective in removing the coating.

The beaded cups remained stable in weight during the wear study and caused little discrepancy in gravimetric analysis (8%).

The method described did not lead to breaking of beads, elevated 3rd-body abrasion, cup damage or distorted wear scars.

Introduction and Aims: Metal-on-metal (MOM) hips offer an attractive solution for hip arthroplasty. However, concerns remain over the optimum metallurgical condition of the cobalt chrome alloy and also the optimum bearing diameter to produce to least possible wear.

Method: Hip bearings of 28, 40 and 56mm high carbon cobalt chrome diameter were tested, all were HIPed and Solution annealed, apart from four 40mm bearings were left ‘as cast’. Radial clearances were controlled at 110 microns, initial surface finish was Sa 0.05 microns, and the sphericity deviation was 4–8 microns. A multi-directional biaxial rocking hip joint simulator was used. All bearing couples were initially subjected to three million cycles of standard physiological walking (2450 N max, 1 Hz) followed by four million cycles of severe gait tests, i.e. slow walking (0.62 Hz) and fast jogging (4500 N max, 1.75 Hz).

Results: In the diameter study, the 56mm bearings produced the lowest wear rates for all patient activities simulated followed by the 40 and 28mm bearings. However, the running-in wear was greatest for the 56mm bearings followed by the 28 and 40mm bearings. All surface wear patterns showed very similar characteristics, suggesting similar wear processes. This would indicate that larger diameters are indeed better in terms of wear in the long term, but may produce slightly more wear initially, which has been reflected in increased ion release in short term studies.

Comparing the 40mm as cast and heat-treated bearings, running-in wear was observed for both material groups in the first million cycles, generating wear rates of 2.3 mm3/million cycles and 2.4 mm3/million cycles for the HIPed/solution heat treated and as cast components respectively, indicating no statistical difference (p > 0.9). Under steady-state wear conditions, the combined normal walking wear rate was also similar for both groups, showing 0.48 mm3/106 cycles (p > 0.2). Under simulated fast jogging cycles, the results again showed no statistical difference in wear performance between the two groups (p > 0.3), generating approximately a 10-fold increase in volumetric wear compared to normal walking, showing 4.4 mm3/106 cycles.

Conclusion: This wear testing program, which used both standard and adverse testing conditions, has shown that heat treatments (HIP and solution anneal) do not affect the wear rates of cast cobalt chrome alloy. Further, this testing has confirmed that larger diameter bearings do indeed produce less wear in the long term.

Large diameter metal on metal hip bearings have been shown to display exceptionally low wear in vivo. However, as these components are often cast, they may be heat treated to improve homogeneity, although it has been suggested that this may adversely affect wear. Therefore a hip simulator study was commissioned to investigate this further.

Multi-station hip simulator testing was carried out on 40 mm diameter high carbon cast cobalt chrome alloy components, all having similar radial clearances (~100mm), surface finishes (0.01mm Ra) and sphericity deviations (< 10mm). Three bearing couples were hot isostatic pressed (HIPed) and solution heat treated, generating a fine carbide structure, and three bearing couples were left as cast, creating a coarse carbide structure. All sockets were mounted in an MTS hip simulator, inclined at 35° to the horizontal, and subjected to standard walking conditions (2450 N max) using 25% bovine serum as a test lubricant. Wear was calculated gravimetrically using temperature controls.

Running-in wear was observed for both groups generating a similar combined head and cup mean wear rate of 2.3 mm3/106 cycles. The mean steady-state wear rate (SE) for the as cast and HIPed components was 0.38 (0.13) and 0.57 (0.11) mm3/106 cycles respectively showing no statistical difference (p > 0.2). Wear was generally higher for the cups than the heads.

These wear rates are two orders of magnitude lower than published wear rates of metal-UHMWP E under similar conditions and one order of magnitude lower than lightly crosslinked UHMWPE articulating against CoCrMo under the same conditions.

In this test, the effect of HIPing and solution heat treatment on metal-metal wear would therefore appear to be insignificant. This is in contrast to the published influences of both bearing diameter and bearing tolerances, i.e. sphericity and radial clearance on the wear of metal-metal hip joints.