This study of retrieved 28 mm Metasul™ (cemented) revealed for the first time adverse wear effects created by impingement-subluxation in MOM. The 10 cases selected (with femoral stems) had annual follow-up 3–11 years. (1) Unequivocal evidence here shows that all heads routinely subluxed from the Metasul liners.

Femoral stems revealed well-demarcated notches (DN) on necks and trunnions (Fig. 1a: n = 6), shallow cosmetic blemishes (Fig. 1b CB: n = 4), and abrasion by cement (Fig. 1b: PMMA). As demonstrated by EOS radiographs, impingement locations varied with implant positioning, pelvic mobility and patient functionality – both anterior and posterior notching (Fig. 1). The first impingement notch occurred with head located (Fig. 2a), whereas the head had subluxed from the cup at 2^nd notch (Fig. 2b). The model demonstrated that patients gained 20° motion by such head-subluxation manoeuvres. It was surprising that there was no collateral damage evident on the liners. Even with severe notching of Ti6Al4V and CoCr stems, the cup rims generally appeared well-polished.

Femoral heads revealed macro-stripe damage on articular surfaces (Fig. 3), as did cups. Basal and polar macro-stripes on heads were always located at hip impingement positions. The equatorial stripes were formed at main-wear zone boundaries. Thus equatorial stripes were likely created by some form of rim-impact damage (micro-separation) or by local ingress of 3^rd-body wear particles under the cup rim.

Micro-grooving was evident within these macro-size stripes and frequently featured large raised lips (Fig. 3), interpreted as signs of adverse 3^rd-body wear mechanisms, and rarely described.(2) It would appear that large metal particulates were released during MOM impingement-subluxation manoeuvres and circulated the hip joint to producing severe 3^rd-body abrasion. Gradual decomposition of such large debris to nano-sized particulates under joint loading would then produce the often-referenced ‘self polishing’ effect of CoCr. EDS studies revealed metal smears on the CoCr surfaces containing the elements of titanium alloy (Ti, Al, V). This was further evidence of impingement-subluxation manoeuvres.(1, 3)

In-vivo cup wear patterns also appeared much larger than those produced in MOM simulators. Such differences likely reflected head-subluxation in vivo, whereby heads unconstrained by the subluxation maneuver were free to orbit up and even cross cup rims, i.e. “edge wear”. This appears to be the first study detailing the adverse wear mechanisms in MOM bearings. There are two limitations to our retrieval study, a) these wear results may not be representative for all MOM designs, and b) it is unknown whether such results have relevance to MOM cases continuing successfully.

Controversy exists over the role of fretting-corrosion in modular junctions of large-diameter metal-on-metal (MOM) heads given the many design plus alloy mix-and-match variations. Overall data was also scant regarding a) fitting stem trunnions to head tapers, b) role of taper angles, c) role of smooth vs threaded trunnion junctions, d) role of head neck-lengths and e) role of head diameters. While the “12: 14” taper has been used with small CoCr heads for 40 years, we could not find retrieval analyses on this European ‘gold-standard’. We therefore selected 10 femoral stems with 28 mm modular heads for analysis (3–8 years follow-up). Unique to this study were the threaded taper profiles on both stems and heads (Fig. 1).

Six stems were cemented Ti6Al4V (Alize, FH-Orthopedics, France) with 12/14 taper angle defined as 5° 42′. These represented Ti64: CoCr combinations from 2 vendors. The other four were CoCr stems including the CoCrMo (Protasul-2) and CoNiCrMo (Protasul-10) alloys (cemented and HA-coated; Sulzer, Switzerland). These CoCr: CoCr combinations from one vendor had “12/14” stem-taper defined as 5° 38′. Anatomical positioning of Metasul heads (Sulzer, Switzerland) was identified by main-wear zone maps. Femoral heads were then bi-valved in horizontal plane for direct imaging by interferometry (WLI) and SEM. Visual corrosion mapping (3) was recorded digitally in 4 anatomical views. Quantitative analysis used 1 to 5 taper zones with 6-replicate measurements per zone (Fig. 1).

The WLI and SEM studies showed that non-contacting taper zones inside CoCr heads (Fig. 2) were threaded with pitch of 70 μm (PV: peak-valley depth = 5–7 μm). The non-contact zones on Sulzer stems had 130 μm pitch (PV = 4–8 μm) whereas Alize stems had 210 μm pitch (PV = 10–12 μm). Threads on both stem types were much coarser than CoCr heads; Ti64 stem threads were much coarser than CoCr stems. In contact zones, the Metasul threads had flattened (avg. roughness = 0.45 μm Ra). With CoCr stems there was little difference. Difference in pitch of stem-threads vs head-threads indicated there was no imprinting onto head tapers. Nor were there statistically significant differences evident in the contact zones along CoCr or Ti64 tapers. Small damaged areas (Fig. 3: arrows) may have been due to alternatively; initial machining, surgical impaction, in-vivo cold-welds, fretting, corrosion, or from surgical-removal. The as labeled “corrosion damage” was well within the “mild” grade for all implants.(3) Thus even with this considerable variety of design and material parameters, we were satisfied that these gold-standard taper junctions with threaded interfaces had performed very well with 28 mm MOM at 3–8 years follow-up.

Controversy has existed for decades over the role of fretting-corrosion in modular CoCr heads used with stems of CoCr vs Ti6Al4V. Since retrieval data on taper performance remains scant, we report here an18-year survivorship of a Ti6Al4V: CoCr combination (APR design; Intermedics Inc). Unique to this study were the threaded profiles present on both stem and head tapers (Fig. 1).

This female patient was revised for pain, osteolysis and recurrent hip dislocation at 17 years, 10 months. A prior MPE hip replacement performed for her severely dysplastic right hip had lasted 11 years. At this 2nd revision, the 28 mm CoCr head was found dislocated posteriorly and superiorly. Metallosis was evident in the tissues. The polyethylene liner showed extensive rim damage on both anterior and posterior aspects. The neck of her APR Revision stem (Intermedics Inc) had worn through the polyethylene rim and impinged on the metal cage. The cage was found loose, the liner had disassociated, and the peri-trochanteric areas were compromised by massive osteolysis. The femoral stem and head were removed together without disassembly. The femoral stem and acetabular construct were replaced by an ARCOS revision system using 36 mm head with a Freedom cup (cemented to Max-Ti cage; Biomet Inc.).

The complete femoral neck and head were bi-valved assembled in horizontal plane for direct imaging by interferometry and SEM (Fig. 1a). After sectioning the head separated from the stem. Quantitative imaging used 1 to 5 regions with 6-replicate measurements per region and differentiation into contact and non-contact zones (Fig. 1b). Visual corrosion mapping (3) was recorded digitally in 4 anatomical views (Figs 1b–f).

The thread profile on contact zone inside the head (Fig. 2a) had a pitch of approximately 40 μm and a peak-to-valley depth of 4 μm overall (Fig. 2b profile section of thread: PV = 2 μm). The thread profile on stem trunnion (Fig. 3a) had a pitch of approximately 125 μm and a peak-to-valley depth of 3.5 μm overall (Fig. 2b profile section of thread: PV = 1 μm). Thus the stem trunnion thread was much coarser than the head. Overall corrosion grading was judged very mild. Overall we were satisfied that this Ti6Al4V: CoCr combination taper junction with threaded interfaces had performed very well for 18 years. Nevertheless, our visual grading was subject to opinion and thus unrewarding. The continuing project will quantify the contacting and non-contacting regions of head and stem (Fig. 1b).

There is a consensus that impingement, subluxation, and dislocation are major risks that can lead to failure in total hip arthroplasty (1). As well as producing edge-wear, such clinical events also may create additional loads of particulate debris (2). It has been suggested that the release of metal debris with collateral damage on metal-on-metal (MOM) bearings creates a particularly severe abrasive wear, hitherto not understood, and recently termed ‘micro-grooving’ (3,4). Perhaps related to this micro-grooving, large surface depressions have also been observed. These we labeled ‘Dongas’, from the South African term for a steep-sided gully created by erosion. The goal of this study was to examine Dongas found on retrieved MOM bearings and to correlate factors such as cause of revision, MOM diameter and Donga locations with respect to regions of normal and stripe wear. Our hypotheses were: (1) Dongas will be most visible in non-wear zones (NWZ) adjacent to the main-wear zone boundary (MWZ), (2) the 28 mm MOM, being inherently less stable compared to large-diameter MOM, will show a higher Donga frequency and (3) patients with subluxation or dislocation complaints will reveal a higher Donga frequency.

Five cases with 28 mm MOM, five of 34–38 mm, and five of 50–56 mm diameter were studied (N = 15). The MWZ was measured in each MOM head and the number of NWZ and MWZ Dongas recorded. Bearing damage was examined using a white-light interferometer (Zygo; 5x lens).

Dongas were mainly elliptical in shape, but sometimes highly irregular. They were commonly circumscribed by raised lips (Fig. 1). Donga “trails” were also found, appearing as a linear series of similar-sized Dongas (Fig. 2). Donga trails exhibited some variability with raised lips either lining only the opposite sides or circumscribing most of the perimeter. The Dongas were commonly found in NWZ, with less than 20% found in MWZ. For this set of 15 MOM bearings, large-diameter bearings showed the largest number of Dongas and the greatest frequency of Dongas resulted from either loose or migrating cups.

The high occurrence of dongas in the non-wear zone (supporting hypothesis-1) may be a result of particles swept into the bearing interface (2,5). The size of the Dongas and their frequent association to local micro-grooves indicated that these were the impact sites of circulating particles. Such large surface depressions (40–200 μm) have not been described previously and may be unique to MOM bearings (3,4). The observation that Dongas were most prevalent in cases with loose or migrating cups left hypothesis-2 unsatisfied. The much higher incidence of Dongas in the large-diameter MOM was surprising and negated hypothesis-3. Overall these new data relating Dongas and micro-grooves gives new credence to a hitherto unsuspected 3^rd-body abrasive wear mechanism due to repetitive subluxation or impingement.

The MOM controversy continues with many prevailing opinions as to the causes of failure in contemporary designs. There has been a great deal of focus on breakdown in fluid-film lubrication with respect to cup positioning and edge wear at its rim. However there has been very little discussion on the problems of 3^rd body abrasion. In only one study was there a description of unusually large abrasive marks on retrieved femoral heads (McKee Farrar MOM), revealing 100 μm wide scratches, attributed to circulating particles fractured during impingement episodes. With contemporary MOM devices, there is the potential for abrasion by particulates of CoCr, PMMA and Ti6Al4V. However it has been difficult to formulate a coherent simulator model for 3^rd-body abrasive wear, given the unpredictable nature of impingement damage releasing abrasive particles into the patient's hip joint. Thus this study sought to identify if metal or cement particulates were capable of creating 100 μm wide scratches as seen on retrieved MOM and develop a simulator model for 3^rd body abrasive testing on MOM bearings.

Six 38 mm CoCrMo bearings (DJO Inc., Texas) were run in a12-station hip simulator (SWM, Monrovia, CA) with cups mounted both anatomically and inverted (3 MOM each). The tests were run in standard simulator mode (Paul gait load cycle: 0.2–2 kN, frequency 1 Hz) with the addition of 5 mg of debris particles. Commercially available CoCr (ASTM F75) and titanium alloy (ASTM F136) particles and broken polymerized bone cement particles were used in the size range 50–200 μm. The simulator was run for only 10 cycles and the MOM parts removed for study. All bearings were ultrasonically cleaned and heads were examined using white light interferometry (WLI, Zygo Corp). Grooves were characterized using surface profiles to measure width, depth, and rim height. SEM imaging (EVO MA15, Zeiss) and EDS imaging (X flash detector 4010, Bruker AXS) was performed in areas of grooving and suspected transfer layers.

CoCr debris produced broad, curvilinear grooves with widths ranging from 20–170 μm, depths from 0.3–1.5 μm, raised rims, longitudinal striations and chatter marks. Titanium alloy debris produced arrays of very shallow scratches accompanying larger grooves. These larger grooves measured 20–110 μm wide and 0.4–1.9 μm deep.

EDS imaging showed the smears and islands contained the elements Ti, Al and V representative of the Ti6Al4V alloy. WLI imaging showed these metal deposits (250–900 um wide) were raised >10 um above the surface. Particularly conspicuous was evidence of considerable smearing on CoCr surfaces, with linear streaks ranging 150–300 μm wide. Bone cement debris proved incapable of grooving the CoCr surface, the only scratches observed were those comparable to normal carbide scratches.

It has come to light that one significant mechanism for MOM failure may be repeated subluxation or impingement episodes leading to edge wear and release of 3^rd body particles. This MOM debris-challenge model simulates a patient who experienced one subluxation or impingement event and then continues to walk normally until the next event occurs one week later. Our model assumes that 100–200 particles (debris size 100–200 μm) would be released into the joint space at each subluxation or impingement event. The question then becomes: what is the effect of the patient walking on that single dose of particulates over the next week (or 500,000 cycles in simulator test mode).

Nine 38 mm CoCrMo bearings (DJO Inc., Texas) were run inverted in a12-station hip simulator (SWM, Monrovia, CA). The test was run in standard simulator mode (Paul gait load cycle: 0.2–2 kN, frequency 1 Hz) with the addition of 5 mg of debris particles for the first 3 Mc, followed by 10 mg of debris particles from 3–5 Mc. Commercially available CoCr (ASTM F75) and titanium alloy (ASTM F136) particles and broken polymerized bone cement particles were used in the size range 50–200 μm. Serum was changed out every 500,000 cycles and a fresh dose of debris added. All bearings were ultrasonically cleaned and examined using white light interferometry (WLI, Zygo Corp) and SEM (EVO MA15, Zeiss). Wear rates were determined gravimetrically and serum discoloration was noted at each test interval.

Titanium alloy and CoCr debris produced darkened serum within the first hour of the test and remained so for the duration (500,000 cycles). Serum color with cement debris remained an opaque golden color throughout the test run. The debris challenge provoked the largest MOM wear response using Ti6Al4V particulates (6.7 mm³/Mc), slightly milder with CoCr particulates (4.5 mm³/Mc) and minimal with PMMA particulates (0.5 mm³/Mc). Compared to bone cement debris chambers (which had wear rates comparable to non abrasive MOM bearing tests), CoCr debris created a 9-fold higher MOM wear and titanium alloy debris created a 14-fold higher MOM wear. These observations indicated that only the metal debris elicited an ‘Adverse’ wear response with MOM bearings.

The purpose of this study was to determine the survivorship for a single MOM implant series and create an affordable clinical follow-up model. A retrospective cohort of 104 primary MOM THA procedures (94 patients) were all performed by one surgeon at three local hospitals and have reached 7–12 years follow-up. Fifteen patients are now deceased and 16 patients have been lost to follow-up. In the remaining 63 patients, 8 are bilateral providing a total of 71 THA for study. The clinical follow-up model included: hip score, X-rays, ultrasound, and metal ion testing. Due to the diversity of patient location, a variety of clinical labs were utilized for metal ion analysis. Statistical methods included a Kaplan-Meier survival curve. Hip scores were available for all 71 THA and of these 61 had a hip score (HHS) above 80 (86%). Fifty-three X-rays were available and of these 42 (79%) were in the safe zone (inclination ≤55 and anteversion ≤35). Thirty-six ultrasounds were performed and of these two yielded fluid collections (6%). Metal ion concentrations in either whole blood or serum have been documented for 32 of 63 patients (51%). Three outliers were identified within the metal ion concentrations; Co ranged 0.3–71.1 ppb (average 7.9, median 3.6), Cr ranged 0–26.9 ppb (average 4, median 2.1) and Ti ranged 2–100 ppb (average 51.8, median 52). Three patients have been revised by the original surgeon and two patients are currently scheduled to undergo revision surgery. Of the three patients revised, only one had a documented wear problem resulting from anterior subluxation, while the other two were revised for femur fracture and infection. These results represent survivorship of 94.7% at 9 years and may be a result of anterior lateral approach performed by one surgeon in which 79% of cups were well placed.

In an effort to understand the role of metal ion analysis and how it relates to revision surgery and implant wear, four revised MOM cases were reviewed. The first case was revised for acute infection and is representative of the low bearing wear predicted by MOM simulator studies. Two of the four cases had apparent anterior subluxation as a result of hip hyperextension occurring with long stride gaits. The last case is a true hypersensitivity response to CoCr ions.

All four MOM prostheses were implanted by one surgeon and revised by the same surgeon approximately 6–8 years postoperatively. The implants had been positioned satisfactorily with inclination angles 45°–55° and anteversion angles 28°–42°.

Patient A (76 y/o female) with bilateral MOM hip replacements, was revised at approximately 8 years due to infection and had moderately elevated ions at the time of revision surgery (Co = 5, Cr = 2.3, Ti = 4). Only the femoral head was retrieved in this case. Retrieval analysis identified a well defined main-wear zone and one polar stripe. The CMM indicated there was minimal wear overall (form factor = 11 μm).

Patient B (33 y/o male) with bilateral MOM hip replacements, was revised at approximately 8 years due to pain, popping/catching sensations, and elevated ions (Co = 33, Cr = 17, Ti = 90). Intraoperatively, the implant was observed subluxing superiorly from the acetabular cup with anterior rotation of the leg. Both the femoral head and acetabular cup were retrieved. Retrieval analysis identified a well defined main-wear zone and multi-directional polar stripe formations similar to those reported on dislocated implants (Figure 1)[McPherson 2012, 2013]. The CMM indicated that overall wear was significant (form factor > 100 μm).

Patient C (77 y/o female) was revised at approximately 6 years due to pain, suspected implant loosening, osteolytic cysts determined by CT, and highly elevated ions (co = 164, Cr = 45, Ti = 33). Intraoperatively, there was evidence of wear including darkly stained tissue and osteolytic cysts. Both the femoral head and acetabular cup were retrieved. Retrieval analysis identified a well defined main-wear zone and one polar stripe. CMM indicated considerable wear (head form factor > 200, cup form factor >300).

Patient D (45 y/o female) was revised at approximately 6 years due to pain, apparent reactive response joint effusion, and moderately elevated ions (Co = 5, Cr = 6, Ti = 71). Only the femoral head was retrieved. Retrieval analysis identified a well defined main-wear zone and one polar stripe. Minimal wear was indicated by CMM (form factor = 21).

These four cases demonstrate distinct failure models of MOM hips and their respective metal ion results. Due to the diversity of patient location, a variety of clinical labs were utilized for this patient population. Caution should be used in interpreting metal ion analysis, as there are still no standards.

Figure 1: Retrieval analysis of stripe wear identified on femoral head from patient B.

Figure 2: Femoral head from patient C showing broader polar stripe associated with anterior subluxation in comparison to narrow polar stripe found on femoral head from patient A.

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.

Suggestions for improved wear performance of total knee replacements have included replacement of standard CoCr femoral components with ceramic. Yttria-stabilized zirconia (y-TZP) was introduced as high-strength and high toughness ceramic as an alternative to alumina ceramic. Since the introduction of zirconia in 1985, the clinical outcomes and successes for hip joint have been controversial. Y-TZP ceramics have been studied both experimentally and clinically. Magnesia-stabilized zirconia (Mg-PSZ) also appears promising for total knee replacements (TKR).

Mg-ZrO2 and CoCr femoral condyles were compared in the VanguardTM knee configuration (Biomet Inc, IN). Molded tibial inserts (GUR1050) were gamma-irradiation sterilization to 3.2-Mrad under argon. Knee simulation was conducted on a 6 station simulator (Shore Western Manufacturing, Monrovia, CA). Motion included 20 degrees of flexion/extension, 5 degrees of internal/external rotation and 5 mm of AP-translation. All knee components were subjected to 6 million cycles of normal walking (2.9 kN max, freq 1.4 Hz). Lubricant was 50% alfa-calf serum diluted to 20 mg/ml protein and using EDTA additive. Test duration was 6 million cycles (6-Mc), and wear was measured by weight-loss techniques.

For wear trending of CoCr/PE and MGZ/PE, linear wear trends were apparent from 1 to 6 Mc test duration. The control implants (CoCr/PE) showed excellent linear trending (regression coeff r> 0.99) with wears rate averaging 6.3 mm3/Mc. These data showed good control of experimental variance (< 10%). The ZrO2/PE combination showed good linear trending (r > 0.86) with wear rate averaging only 0.8 mm3/Mc. This set also showed good control of experimental variance (< 15%. The MGZ/PE wear was 8-fold reduced from that of CoCr/PE.

The laboratory knee wear simulation appeared very supportive of femoral condyles of Mg-stabilized zirconia. Such implants may provide excellent performance for active patients who may risk high wear rates over many years of use.

While there are many variation laboratory and clinical studies using metal-on-metal (MOM) bearings after introduction of the 28mm MOM THR in 1988, the mapping of wear phenomena in such retrieval cases has been mimimal. In laboratory study, 28mm MOM bearing’s wear-rate was low with “run-in” and “steady-state” than large diameter MOM without theory of fluid-filum lubrication. In clinical results were not superior to the same way of laboratory study. We present a detailed analysis of 33 retrieved MOM hip bearings with 1–11 years follow-up,

We compiled 33 retrieval cases (MetasulTM: Zimmer/CenterPulse Inc., Austin, TX) including clinical information, ion concentrations from ball diameters, cup designs and stripe wear damage. The bearing surfaces were mapped using reflected light microscope (RLM), white light interferometer (Zygo Newview 600, Zygo.) and SEM(XL-30 FEG). Wear maps were constructed according to types of surface wear identified.

Patients ranged from 36 to 76 years of age (Means: 56.9 years); 54% were males. Main causes for revision were progressive radiographic lines around the cups, osteolysis and pain. The 28mm ball diameter was used in 86% of cases (largest = 52mm ball). The CoCr liner incorporated a polyethylene adaptor in 75% of cases. Cup diameter > 50mm was present in 75% of cases. Eight femoral stems were recovered and all showed major impingement marks around the neck and five also had a metallosis (Mode-4A). Stripe wear was evident on 71% of CoCr balls with medial stripes twice as common as lateral. Stripe wear was identified in 25% of CoCr liners and extended 25–160° circumference around the liners. Clear liner rim damage was present in 10 (30%) and 3 demostrated severe damage of polyethelene adaptors.

There are many limitations to such retrieval studies. These data are biased to cases that failed due to hip pain, radiographic signs of progressive osteolysis and some with high levels of metal ions. There was also the bias of having predominantly a CoCr sandwich design (polyethylene adaptor in 75% of cases). In early 1980s, the thin walled UHMWPE cup was introduced and used larger diameter balls for decreased risk of dislocation. However, unfortunally these big-ball cups produced significant PE wear debris, and diameter trends were returned to the Chanley’s small-ball paradigm again. In the same time (late of 1980’s), these second-generation MOM (28,32mm) was introduced for low wear characteristics alternate THR bearings, with sacrificing of joint stability and motion range. However, use of the small ball added well-known risks of impingement, subluxation and dislocation with rigid cups. In this study, using the ‘damage modes’ from McKellop, normal mode-1 wear occurred in only 14% of cases whereas modes 2–4 had an incidence approaching 30% each and signs of cup impingement were evident in 64% of cases. Thus summarizing MOM wear phenomena in “small” 28mm sandwich cup designs, there was retrieval evidence showing that damage modes 2–4 likely placed these patients at risk for adverse wear effects.

FDA approval of metal-on-metal (MOM: 28, 32mm) bearings has provided 10 years of clinical experiences in USA. However there has been no detailed mapping of wear phenomena in retrieval cases. We present an analysis of 28 cases, MOM retrievals with 1 to 10 years follow-up, radiographic reviews and metal ion studies. Ball diameters ranged from 28mm to 42mm. Two balls were the early design with skirts. Main indicators for revision were the progressive radiographic changes indicative of osteolysis, with associated hip pain. Approximately 54% of patients were males and ages ranged from 36 to 76 years of age. Only 7 femoral stems were recovered but all had impingement marks. Only three cases lacked any evidence of stripe wear and these were in very elderly patients. Approximately 85% of these cases showed some evidence of stripe wear and multiple stripes were clearly visible on 50% of the femoral balls. The medial ball stripes were twice as common as lateral. Stripe wear was identified in 25% of CoCr liners.

In the hip simulator studies generally show ‘run-in’ wear rates of 1–7mm3 per million cycles (Mc). We noted that above the 5mm3/Mc threshold, the serum generally appeared black. In contrast, the ‘steady-state’ wear rates of 0.1–1.6 mm3/Mc showed the true potential of MOM bearings. However there were often examples of higher wear (7–20 mm3/Mc), which gave confounding trends in published studies. Our studies of metal ions in the simulator lubricant provided a very accurate representation of MOM wear.

There are many limitations in comparing in-vitro to in-vivo wear performance. Our retrieval data are biased to cases that failed due to hip pain, had radiographic signs of progressive osteolysis and some showed high levels of metal ions. There was also the bias of having predominantly a CoCr sandwich design (polyethylene adaptor). Use of the small ball added the well-known risks of impingement, subluxation and dislocation with rigid cups. Using the ‘damage modes’ from McKellop, we found only normal Mode-1 wear to be rare in these cases, whereas Modes# 2–4 had an incidence approaching 30% each. Signs of impingement were evident in 85% of our cases. Thus summarizing these MOM wear phenomena in retrieved 28mm sandwich cups, the evidence implicated impingement and 3rd-body wear modes (#2–4) as the clinical risk for adverse wear effects at 10 years follow-up. The in-vitro wear studies have not yet simulated such adverse clinical effects.

Ceramic-on-ceramic bearings (ALX: pure alumina) have been used for human hip joints for almost 40 years. However an alumina matrix composite with zirconia (AMC) was introduced in year 2000 as a high-strength ceramic with almost double the fatigue resistance (AMC = 80.5%ALZ and 18vol% ZrO2). However we have not found any retrieval studies reported for this new ceramic bearing.

Wear maps were generated on three retrieved AMC femoral heads (28 and 36mm diameters) using x-ray diffraction, roughness and SEM imaging techniques. The wear study ran a physiologically appropriate, micro-separation test on 36mm ceramic balls and liners (AMC/ALZ). Wear rates were determined for the four combinations of balls and cups (ALX:AMC) with mapping of main-wear and stripe-wear zones, surface-roughness and analysis of debris morphology. In addition, the zirconia transformation to monoclinic phase was studied in AMC bearings

The retrieval study showed for the first time the wear phenomena occurring on three retrieved AMC femoral heads (at 1, 3, 6 years). Two had been paired with alumina liners and one with a polyethylene liner. Case-1 featured a 36mm ball in an UHMWPE socket, case-2 was an intact 28mm AMC ball and case-3 had a fractured ball from an IDE study. Laser interferometry and SEM were used to image ceramic wear and x-ray diffraction for analysis of transformation in the zirconia phase. Main-wear zones, stripe-wear zones, metal contamination and sites of implant impingement were also characterized. Surface roughness and in-vivo aging were quantified for both non-worn and worn areas. The SEM studies showed well-preserved articular surfaces, some with faint parallel scratches still evident. The latter likely represented the manufacturer’s original polishing marks. Multiple stripe-wear sites were identified with roughness 25–65nm (Sa) whereas polished main-wear zones averaged very low at 2–3nm. Metal impingements sites stained black with transfer of titanium increased roughness up to 140nm. Mildly worn areas of case-2 AMC ball averaged 10% transformation in the zirconia phase (tetragonal to monoclinic). In the stripe-wear zones, the monoclinic phase increased to 30%. The taper-bore and fracture surfaces in case-3 averaged 30% to 40% monoclinic, respectively. The stripe-wear zones and black metal contamination on these retrieved 28mm balls were correlated to multiple impingement sites on the rim of the alumina liners and titanium shells.

The laboratory model produced stripe wear on the ceramic balls and liners. The AlX/AlX controls produced the highest run-in and steady-state wear rates at 6.3 and 2mm3/Mc respectively). In contrast, the AMC/AMC combination produced the lowest wear rates at 0.5 and 0.1 mm3/Mc, respectively). With hybrid ball:cup combinations (AlX:AMC; AMC:AlX) the wear rates were similar and showed a 3-fold reduction compared to controls. In hybrid pairings, the AMC ceramic wore preferentially more than its AlX counterpart, regardless if present as a ball or cup implant. Thus the AMC ball contributed 66% to AMC/AlX total wear whereas the ALZ ball contributed only 33% of the total AlZ/AMC wear.

This study appears to be the first documentation of wear in retrieved AMC bearing surfaces. In general, the AMC surfaces worn in-vivo corresponded well to our in-vitro wear model. The stripe-wear zones in AMC femoral heads had rougher surfaces and higher monoclinic transformation than the main-wear zone. Overall the AMC ceramic appeared more resistant to stripe-wear effects created by the micro-separation and impingement phenomena.

From 1985 metal-on-metal (MOM) designs of resurfacing (RSA) and total hip arthroplasties (THR) have been available over a large diameter range (28–60mm). In-vitro studies indicated satisfactory low wear performance for all designs and diameters tested (wear = 0.1 to 7 mm3). While reports from many centers have been encouraging, some have reported adverse effects. We reviewed clinical and metal ion studies in large diameter retrievals and compared these to 28mm MOM cases. Patients with the latter THR ranged 36–76 years of age and were followed 9–11 years. Main finding in our revisions was osteolysis and pain. The 28mm ball was represented 86% of cases; 71% balls had stripe wear. For liners, 25% had circumferential stripe wear and impingement was evident in 64% cases. Seven cemented stems were recovered with impingement marks; 26 stems were undamaged and therefore not revised. Using the concept of ‘damage modes’ from McKellop, normal wear mode #1 was evident in only 14% of 28mm retrievals whereas incidence of ‘abnormal’ modes #2-4 approached 30% each. Thus the 28mm MOM appeared susceptible to impingement risks with CoCr liners. Summarizing MOM retrievals, damage modes 2–4 were most likely implicated in revisions. The performance of such ‘small diameter’ THRs will be contrasted to our large diameter THR and RSA experience. The questions to be reviewed include, how much of the reported MOM adversity was predictable and how much risk was due to

wear of small diameter MOM,