JBJS | Wear Performance of Ultra-High Molecular Weight Polyethylene on Oxidized Zirconium Total Knee Femoral Components

The Journal of Bone & Joint Surgery, Volume 83, Issue 2 suppl 2

Scientific Article | November 01, 2001

Wear Performance of Ultra-High Molecular Weight Polyethylene on Oxidized Zirconium Total Knee Femoral Components

Myron Spector, PhD; Michael D. Ries, MD; Robert B. Bourne, MD; Willard S. Sauer, MS; Marc Long, PhD; Gordon Hunter, PhD

Myron Spector, PhD
Department of Orthopaedic Surgery, Brigham and Women’s Hospital,
75 Francis Street, Boston, MA 02115. E-mail address:mspector@rics.bwh.harvard.edu

Michael D. Ries, MD
Department of Orthopaedic Surgery, University of California, San Francisco,
500 Parnassus Avenue, Suite MU320W, San Francisco, CA 94143

Robert B. Bourne, MD
London Health Sciences Centre, University Campus, 339 WindermereRoad, London, ON N6A5A5, Canada

Willard S. Sauer, MS
Marc Long, PhD
Gordon Hunter, PhD
Orthopaedic Division, Smith and Nephew, Incorporated, 1450 Brooks Road, Memphis, TN 38116

In support of their research or preparation of this manuscript, one ormore of the authors received grants or outside funding from Smith andNephew. In addition, one or more of the authors received payments orother benefits or a commitment or agreement to provide such benefitsfrom a commercial entity (Smith and Nephew). Also, a commercialentity (Smith and Nephew) paid or directed, or agreed to pay or direct,benefits to a research fund, foundation, educational institution, or othercharitable or nonprofit organization with which the authors are affiliatedor associated.

The Journal of Bone & Joint Surgery. 2001; 83:S80-86

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Introduction

Wear debris can lead to osteolysis and aseptic loosening after total knee arthroplasty^1,2. Efforts to reduce wear of total knee replacements have focused primarily on improving implant design and the quality of ultra-high molecular weight polyethylene. Although these efforts have addressed issues related to fatigue wear of the ultra-high molecular weight polyethylene component, concerns remain about adhesive and abrasive wear caused by the hard counterface of the femoral component. Previous studies have shown that roughening of the condyles occurs clinically and that many observed scratches have a shape and orientation that can increase polyethylene wear (Figs. 1, 2, and 3)^3-8. Not only does volumetric wear of polyethylene increase with increasing counterface roughness, it also has been found that increasingly sharp peaks associated with counterface scratches increase the tendency for the production of submicrometer-sized debris that may be related to osteolysis⁹. These findings suggest that a hard counterface that resists roughening and provides low friction with ultra-high molecular weight polyethylene should reduce abrasive and adhesive wear and thereby prolong the survival of total knee replacements.

A metallic cobalt-chromium alloy (Co—28%Cr—6%Mo) is the standard material for femoral components. An alternative material, oxidized zirconium, was developed to improve, in comparison with cobalt-chromium alloy, resistance to roughening, frictional behavior, and biocompatibility¹⁰. In the present study, we compared the wear performances of these two hard counterfaces in a knee simulator.

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Fig. 1:: In a previous study, condyle surface roughness was found to be greater for retrieved cobalt-chromium femoral components than for components that had not been implanted^4,5.

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Fig. 2:: Under clinical conditions, cobalt-chromium femoral components develop scratches, some of which are oriented at an angle to the direction of motion, as seen on this interferometer image of a retrieved clinical specimen^4,5.

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Fig. 3:: Hard particles scratch cobalt-chromium surfaces, plowing up adjacent peaks that can increase abrasive wear of ultra-high molecular weight polyethylene, as seen on this interferometer image of a retrieved clinical specimen^4,5.

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Fig. 4:: Oxygen naturally diffuses into the zirconium alloy when it is heated in air, causing the original metal surface to transform to zirconium oxide (zirconia) ceramic.

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Fig. 5:: Testing was conducted on a six-station, four-axis, physiological knee simulator.

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Fig. 6:: A 90% normal gait and 10% stair-climbing activity pattern was simulated in this study.

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Fig. 7:: Cobalt-chromium produced an aggregate wear rate of 4.68 ± 2.30 mm³/Mcycle after the wear-in period. (The dotted line is the aggregate wear regression. The solid line is the mean and the I-bar is the standard deviation at each measurement interval.)

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Fig. 8:: Oxidized zirconium produced an aggregate wear rate of 0.69 ± 0.52 mm³/Mcycle after the wear-in period. (The dotted line is the aggregate wear regression. The solid line is the mean and the I-bar is the standard deviation at each measurement interval.)

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Fig. 9:: Compared with cobalt-chromium, oxidized zirconium was associated with less ultra-high molecular weight polyethylene wear. (The dotted line is the aggregate wear regression, and the solid line is the mean at each measurement interval.)

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Fig. 10:: The ultra-high molecular weight polyethylene wear rate was 85% less for oxidized zirconium than for cobalt-chromium (regression and standard error).

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Fig. 11:: Oxidized zirconium tended to produce somewhat fewer submicrometer-sized polyethylene wear particles than did cobalt-chromium.

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Fig. 12:: The mean wear particle volume produced by oxidized zirconium tended to be less than that produced by cobalt-chromium (mean and standard deviation).

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Fig. 13:: All six tibial inserts exhibited burnishing consistent with adhesive and mild abrasive wear, as seen on this insert from the cobalt-chromium group.

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Fig. 14-A:: Scratches were observed on the cobalt-chromium femoral components (Fig. 14-A) but not on the oxidized zirconium femoral components (Fig. 14-B).

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Fig. 14-B:: Scratches were observed on the cobalt-chromium femoral components (Fig. 14-A) but not on the oxidized zirconium femoral components (Fig. 14-B).

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Fig. 15-A:: Figs. 15-A, 15-B, and 15-C The roughness of the cobalt-chromium condyles increased about five to ten times, whereas the oxidized zirconium condyles remained smooth.

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Fig. 15-B:: Figs. 15-A, 15-B, and 15-C The roughness of the cobalt-chromium condyles increased about five to ten times, whereas the oxidized zirconium condyles remained smooth.

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Fig. 15-C:: Figs. 15-A, 15-B, and 15-C The roughness of the cobalt-chromium condyles increased about five to ten times, whereas the oxidized zirconium condyles remained smooth.

Materials

The wear performances of three cast cobalt-chromium (ASTM [American Society for Testing and Materials Specification] F75) and three oxidized zirconium femoral components were compared. The polyethylene inserts were ethylene-oxide-sterilized, ram-extruded, GUR 1050 ultra-high molecular weight polyethylene. All tested components were medium-sized, cruciate-retaining, Genesis II‚ total knee replacement prostheses (Smith and Nephew, Memphis, Tennessee).

The oxidized zirconium components were produced from a wrought zirconium alloy (Zr—2.5%Nb) that is oxidized by thermal diffusion to create a zirconia surface about 5 m thick (Fig. 4). The oxidized component is then burnished to produce an articular surface at least as smooth as that of a cobalt-chromium component. The oxide is not an externally applied coating but rather a transformation of the original metal surface into zirconium-oxide ceramic. Previous testing has demonstrated that this oxide has excellent cohesion and adhesion for superior durability compared with coatings and the products of other surface-modification technologies^10-12. Oxidized zirconium thereby has the advantages of a ceramic counterface in terms of low friction, resistance to abrasive scratching, and immunity to corrosive roughening without the problem of brittleness that is associated with an all-ceramic component^13,14.

Methods

Wear performance was evaluated with use of a six-station, four-axis, physiological knee simulator (AMTI [Advanced Medical Technologies, Incorporated], Watertown, Massachusetts) (Fig. 5). The 90% normal gait and 10% stair-climbing activity pattern was based in part on fluoroscopy measurements of patients with a cruciate-retaining total knee replacement (Fig. 6)^5,6. Cyclic frequency was 1 Hz, and test duration was more than five million cycles. The test lubricant, 50% bovine serum with sodium azide and EDTA, was recirculated at 37C and replaced every six to seven days (approximately 0.5 Mcycle).

Polyethylene wear was measured gravimetrically by weighing each tibial insert every time the test lubricant was replaced. Two soak controls were used to correct for fluid absorption. The weight measurements were converted to volumetric wear loss with use of a density for ultra-high molecular weight polyethylene of 0.93 g/cm³. Aggregate wear rates were determined for each test group by pooling the measurements for all three inserts after the wear-in period and calculating the slope with linear regression analysis.

The number and size of submicrometer-sized wear particles were determined on scanning electron microscope images. The test lubricant from each test station was collected from the 4.0 to 4.6-Mcycle interval, prepared with acid digestion and vacuum filtration (with 0.05-m-pore-size filters), and then analyzed separately¹⁵. The particle analysis protocol was being developed in a parallel effort when this study was being conducted, so samples from only one test interval were analyzed.

Four roughness measurements were performed, at preselected locations from 0 to 45 of flexion, on each condyle of each femoral component after testing for comparison with pretest values. Measurements were performed with use of a Surfcom 575A Profilometer (Tokyo Seimitsu, Tokyo, Japan) with a 2-m-radius stylus tip and a cut-off length of 0.25 mm. The roughness was characterized with use of three parameters: Ra, the average surface displacement (of peaks and valleys) from the mean surface line; Rpm, the average peak height above the mean surface line; and Rpk, the average peak height above the main bearing surface.

Results and Discussion

Wear Rate

Measurements of the ultra-high molecular weight polyethylene inserts tested against the oxidized zirconium femoral components indicated weight gain until about 1.9 Mcycle. This suggests that this portion of the test was a "wear-in period" during which the rate of fluid absorption by the ultra-high molecular weight polyethylene exceeded the wear rate of the tibial insert. Thereafter, the weight loss in both femoral groups was roughly linear with time. Because of this extended wear-in period, testing was extended to about 6.1 Mcycle. To focus on the linear wear behavior, aggregate wear rates were calculated only from measurements made from 1.9 to 6.1 Mcycle.

The aggregate wear rate for the cobalt-chromium group was 4.68 ± 2.30 mm³/Mcycle (regression and standard error) (Fig. 7). The aggregate wear rate for the oxidized zirconium group was 0.69 ± 0.52 mm³/Mcycle (Fig. 8), which was 85% less than that for the cobalt-chromium group (p = 0.097) (Figs. 9 and 10).

It was noted that the measurement scatter in the cobalt-chromium group was unusually large when compared with that in the oxidized zirconium group and that in other test series conducted on this simulator. Subsequent investigations failed to reveal the reason for this variation, but they did support the wear rates determined in this study for both counterface materials. Because of this and the 90% confidence level in the comparison despite the small number of samples, it is reasonable to expect oxidized zirconium to provide advantages over cobalt-chromium in terms of wear of ultra-high molecular weight polyethylene under clinical conditions.

Wear Particles

Analysis of the polyethylene wear particles in the test lubricant indicated that, in comparison with cobalt-chromium, oxidized zirconium produced 23% fewer submicrometer-sized particles (p = 0.25) (Fig. 11), with a 48% reduction in average particle volume (p = 0.35) (Fig. 12). These comparisons lacked significance within a 90% confidence limit.

Although not significant, the trends in the wear particle analysis suggest that oxidized zirconium produces no more and perhaps fewer submicrometer-sized particles than cobalt-chromium does. This finding is in contrast to the findings with some forms of cross-linked ultra-high molecular weight polyethylene, which may reduce gravimetric wear rates dramatically while increasing the number of submicrometer-sized wear particles¹⁶.

Counterface Roughness

The tibial inserts exhibited burnishing consistent with adhesive and mild abrasive wear (Fig. 13). Cracking or other evidence of fatigue wear was not observed.

Even though abrasive conditions were not intentionally created in the simulator, the cobalt-chromium condyles exhibited numerous scratches, some deep, early in the wear test. These scratches were oriented in the anterior-posterior direction and aligned with the motion pattern against the tibial insert (Fig. 14-A). This finding is consistent with observations from other simulator tests^5,17,18. In contrast, the oxidized zirconium condyles remained visually pristine throughout the test (Fig. 14-B).

Consistent with the visual observations, post-test roughness measurements indicated that the oxidized zirconium condyles remained smoother than the cobalt-chromium condyles (Figs. 15-A, 15-B, and 15-C). Before testing, the mean roughness values for the two hard counterfaces were within 0.02 m of each other. For oxidized zirconium, the Rpm (p = 0.27) and Rpk (p = 0.80) values did not change significantly during testing, although the Ra increase of <0.01 m was significant (p << 0.01). Cobalt-chromium exhibited significant roughening (with increases of about five to ten times in the Ra, Rpm, and Rpk values; p << 0.01).

Clinical Implications

Compared with cobalt-chromium, oxidized zirconium femoral components reduced the ultra-high molecular weight polyethylene wear rate by nearly an order of magnitude. This suggests that the use of oxidized zirconium may increase the lifetime of ultra-high molecular weight polyethylene tibial insert and patellar components. Additional work is needed to determine whether the associated reduction in the generation of submicrometer-sized particles is significant and whether such a reduction could reduce the potential for debris-induced osteolysis and component loosening.

The lack of scratches on the oxidized zirconium components and the presence of scratches on the cobalt-chromium components provide a visual confirmation of the superior abrasion resistance of oxidized zirconium. These scratches probably contributed very little to the difference in wear rates because they were aligned with the sliding motion^6,19. Thus, the wear behavior in these simulator tests was probably dominated by adhesive wear (rather than abrasive or fatigue wear), and the difference in wear rates was probably reflective of the difference in sliding friction.

With its more "wettable" (hydrophilic) ceramic oxide surface, oxidized zirconium has a lower coefficient of friction against ultra-high molecular weight polyethylene than does cobalt-chromium¹⁰. This means that synovial fluids tend to be attracted to the surface and to spread out to provide better sliding lubrication. The ceramic oxide maintains this low friction characteristic because it resists surface roughening through its immunity to oxidative wear (adhesion and corrosion of the passive metal surface) and its excellent abrasion resistance¹⁴.

A parallel investigation indicated that the quantity and properties of synovial fluid vary greatly among patients with a total knee replacement and that these differences may affect lubrication in the joint²⁰. A more wettable surface may be better able to maintain good lubricity when lubricating fluids are scarce. As lubrication is lost, friction can increase and adhesive wear can accelerate. Therefore, oxidized zirconium may have a greater advantage over cobalt-chromium when lubricating fluids are present in considerably smaller amounts than were employed in this simulator test.

Scratches on femoral components retrieved from patients are sometimes oriented at an angle to the sliding motion^3-5, which is in contrast to the observed alignment of the scratches on the cobalt-chromium in this study. Our test also lacked intentional additions of hard particles, such as bone cement and bone debris, that are sometimes found in the joint. These differences between simulator and clinical conditions suggest that abrasive wear can supplement the adhesive wear of ultra-high molecular weight polyethylene observed in this study. Thus it is reasonable to expect that, in a more abrasive environment, the wear benefits of oxidized zirconium over cobalt-chromium exhibited in this study could be magnified.

Conclusions

The greater lubricity and resistance to roughening of oxidized zirconium likely explain the reduction in ultra-high molecular weight polyethylene wear. In a more abrasive environment, this advantage is expected to be magnified. On the basis of these results, it appears that the use of an oxidized zirconium femoral component may contribute to the reduction of wear-related complications in total knee replacement.

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