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Scratching Vulnerability of Conventional vs Highly Cross-Linked Polyethylene Liners Because of Large Embedded Third-Body Particles
The Journal of Arthroplasty Vol. 27 No. 5 2012
Scratching Vulnerability of Conventional vs Highly Cross-Linked Polyethylene Liners Because of Large Embedded Third-Body Particles Anneliese D. Heiner, PhD,*y Alison L. Galvin, PhD,z John Fisher, PhD,z§ John J. Callaghan, MD,*t and Thomas D. Brown, PhD*y
Abstract: The hypothesis of this study was that acetabular liner vulnerability to scratching from femoral heads, roughened by third bodies embedded in the liner, is not significantly lower for highly cross-linked polyethylene (HXPE) than for conventional polyethylene (CPE). Six CPE and 6 HXPE acetabular liners were each reproducibly embedded with 5 cobalt-chromiummolybdenum (CoCrMo) beads then run for 10 000 cycles in a joint simulator. By visual rank ordering, there was low association between liner scratch severity and polyethylene type. The CPE and HXPE liner scratches were not significantly different in scratch peak-valley height or width or in liner roughness in the vicinity of the embedded beads. This model indicated that high cross-linking of polyethylene does not offer appreciable protection against severe scratching induced by large embedded third-body particles. Keywords: highly cross-linked polyethylene, hip arthroplasty, hip joint motion simulator, polyethylene, third-body wear. © 2012 Elsevier Inc. All rights reserved.
In total joint arthroplasty, third-body particles accessing the bearing space can dramatically increase wear, both directly by abrading the bearing surfaces and indirectly by the scratching of one surface, in turn accelerating wear of the other. Third bodies, such as porous ingrowth beads or pieces of bone cement, by definition, originate from other than the normal articulation surfaces. Thirdbody particles embedded in retrieved polyethylene acetabular liners have been documented by many studies [1-7] and have also been seen in polyethylene components from other total joint arthroplasty sites such as the knee, ankle, and elbow. Laboratory wear tests of polyethylene against metal have demonstrated increased polyethylene wear under third-body conditions [8,9].
From the *Department of Orthopaedics and Rehabilitation, University of Iowa, Iowa City, Iowa; yDepartment of Biomedical Engineering, University of Iowa, Iowa City, Iowa; zInstitute of Medical and Biological Engineering, University of Leeds, Leeds, UK; §Leeds Musculoskeletal Biomedical Research Unit, Leeds Teaching Hospitals Trust, Leeds, UK; and ∥Orthopaedics Department, VA Medical Center, Iowa City, Iowa. Submitted October 11, 2010; accepted October 6, 2011. The Conflict of Interest Statement associated with this article can be found at doi:10.1016/j.arth.2011.10.002. Reprint requests: Anneliese D. Heiner, PhD, University of Iowa, Department of Orthopaedics and Rehabilitation, Biomechanics Laboratory, 2181 Westlawn, Iowa City, Iowa 52242. © 2012 Elsevier Inc. All rights reserved. 0883-5403/2705-0014$36.00/0 doi:10.1016/j.arth.2011.10.002
For hip simulator testing of metal-on-polyethylene total hip arthroplasty implants, in the absence of thirdbody challenge, highly cross-linked polyethylene (HXPE) acetabular liners wear far less than do conventional polyethylene (CPE) liners [10]. However, HXPE may be relatively more susceptible to damage from large local asperities on the metal counterface, such as large scratches from third bodies, thus reducing the wear advantage of HXPE vs CPE under severe third-body conditions. Indeed, in some laboratory tests under conditions of third-body challenge, the wear rate of HXPE has been comparable with or even greater than that of CPE [8,11-14]. The scratch resistance of a material is a function of its hardness, and radiation dose and remelting have little effect on the hardness of polyethylene [10]. However, scratching includes lateral motion as well as axial motion, with only the latter being applied by depth-of-penetration hardness testing. Highly cross-linked polyethylene has compromised fracture toughness, fatigue resistance, and tensile properties, as compared with CPE [10,11], and these material properties could plausibly have an effect on the scratch resistance of polyethylene. Although liner scratching may not directly equate with outright wear, scratching, nevertheless, is a phenomenon from which no good can come. Scratching causes material to pile up in scratch lips. Analogous to cross-shearing effects for wear of nondamaged surfaces, the counterface scraping transversely across a scratch lip would tend to break off this
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piled-up material. Although the particle size of brokenoff scratch lip material would tend to be outside the most osteolytic range [15], its presence in the joint space would, nevertheless, be deleterious [16]. The hypothesis of this study was that acetabular liner vulnerability to scratching from femoral heads, roughened by large third bodies embedded at wear-critical locations [17] in the liner, is not significantly lower for HXPE than for CPE. In this study, to concentrate on liner scratching rather than on long-term wear (which could subsequently polish down or even erase these scratches [18]), the test period was kept short. To simulate third-body effects and to minimize differences between the specimens, a controlled, reproducible technique of third-body embedment into the liners was used. The scratching of the CPE vs HXPE liners was evaluated by visual rank ordering of pictures of scratched liners in terms of severity of damage, by profilometry of individual scratches, and by profilometry-based liner roughness measurements.
Materials and Methods Six CPE and 6 HXPE acetabular liners (Duraloc Enduron and Duraloc Marathon, respectively; DePuy Orthopaedics, Warsaw, Ind) were tested. The CPE material was not cross-linked, whereas the HXPE material was cross-linked with 5 Mrad and remelted. Both liner types were gas-plasma sterilized. The liners were 28 mm in diameter and were nonconstrained. Each liner was reproducibly embedded with 5 cobaltchromium-molybdenum (CoCrMo) beads (PREP F-75 Alloy Powder; Advanced Specialty Metals, Inc, Nashua, NH) measuring 300 to 320 μm in diameter (Fig. 1). The bead material was the alloy specified in ASTM F75-01 for use in surgical implants [19]. The bead size was one commonly used in cementless prostheses [20] and was within the range of particle sizes found embedded in retrieved polyethylene liners [2,5]. One bead was embedded at a site that an earlier finite element analysis had determined to be a maximally problematic site for accelerating liner wear because of roughening of the femoral head counterface [17]. This site was at a latitude of 45° and a longitude of 30° posteriorly from the line between the lateral-most point and the cup center, for a liner orientation of 45° tilt and 15° anteversion. The other 4 beads were embedded equidistantly around a 10-mm diameter circle centered about that location. The bead embedment procedure involved, first, creating indentations with larger beads at the specified sites on the liner then (cyanoacrylate) gluing the final beads into those indentations. This procedure was necessary to keep the final beads in place during pretest handling (including having the liner in a physiologic position in the joint motion simulator) and during the testing itself. The larger indentation beads, 400-μm diameter balls of alloy 316 stainless steel (New England
Fig. 1. Third-body beads embedded in acetabular liner: (A) bead positions for testing (bead diameter is increased for visibility and is not to scale) and (B) close-up.
Miniature Ball, Norfolk, Conn), were glued in place then compressed by a femoral head loaded to 263 N [20] for 1 hour in a materials testing machine. The indentation beads were then removed, leaving behind cavities in the liner surface. The final beads were then glued into those cavities. As measured by profilometry (described below), the scratching beads protruded on an average of 144 μm (SD, 15.7 μm) above the liner surface. Each liner was run for 10 000 level walking cycles at 1 Hz in a multiaxis joint motion simulator, articulating against a fresh 28-mm CoCr femoral head (Articul/Eze; DePuy). The metallic acetabular shell (Duraloc; DePuy), in which each liner was held, was oriented in 45° tilt and 15° anteversion. A femoral stem (Summit Basic; DePuy), to which each femoral head was attached, was oriented in 12° adduction. The joint simulator, described in more detail elsewhere [21], was actuated by a material testing machine (MTS 858 Bionix; MTS Systems Corp, Eden Prairie, Minn). This apparatus allowed control of rotations in all 3 degrees of freedom, in addition to control of axial load. Level walking waveforms from Kotzar et al [22] were used, converted for use in the multiaxis joint simulator, assuming a body weight of 620
744 The Journal of Arthroplasty Vol. 27 No. 5 May 2012 N. The test fluid was bovine calf serum (7.0 ± 0.5 gm% total protein; HyClone, Logan, Utah) with 20 mmol/L EDTA [23], diluted to 25% with distilled water. After testing, the heads and liners were photographed, using a dissecting microscope for close-ups. Liner scratching was evaluated by visual rank ordering of pictures of scratched liners for severity of damage, by stylus profilometry of individual scratches, and by stylus profilometry assessment of liner roughness. As a qualitative measure of severity of scratch damage, 3 experienced orthopedic surgeons specializing in arthroplasty rank ordered the pictures of the liner scratch fields according to visual appearance; the surgeons were blinded as to whether the liners were CPE or HXPE. Profiles of the liner scratches and assessment of liner roughness were performed using a contacting profilometer (Form Talysurf 5; Taylor-Hobson, Leicester, United Kingdom) with a cutoff of 0.85 mm and a resolution of 10 nm. For each liner, scratch profiles were measured at 4 consistent, equally radially spaced locations for the 3 most severe scratches apparent on each liner. The peak-valley height and width of the scratches were recorded at these locations. When the scratch at any given sampling location was too faint to measure, its height and width were recorded as zero. Liner roughness was measured in a consistent area around each bead; this area was a patch measuring 5 × 4 mm for the central bead location and one measuring 7 × 5.5 mm for the other bead locations. Liner roughness was also measured on a nonembedded tested control. The following liner roughness parameters were recorded: Ra, the average roughness of the surface; Rp, the maximum peak height of the surface profile; Rv, the maximum valley depth; Rt, the total height of the roughness profile; and Rsk, an index of whether the roughness was predominantly caused by peaks or valleys. Profiles of the head scratches were also measured to determine if the femoral heads that had articulated against the CPE vs the HXPE liners were similarly scratched. For each head, scratch profiles were measured at 12 radial locations around the specific scratch that corresponded to the central bead location. Profiles were also measured for all 5 bead location scratches for 2 heads. These heads were those that had been articulated against the liners that had the median rank-ordered scratching severity for each liner type. Peak lip height, valley depth, peak-valley height, and width of the scratches were recorded. Statistical analyses were done using Excel (Microsoft Corp, Redmond, Wash) and Minitab 16 Statistical Software (Minitab Inc, State College, Pa). Correlation between visual rank ordering of scratch severity and polyethylene type was measured by Spearman q. Agreement between the rank ordering of the 3 evaluators was determined by calculating the concordance rate between each pair [24]. The profilometry results for liner scratch dimensions and liner roughness
Fig. 2. Femoral head scratches resulting from third bodies embedded in the acetabular liner (white spots are light reflections).
were tested for differences between polyethylene types using a 1-tailed Student t test. The profilometry results for the head scratch dimensions for the central bead location were tested for differences between polyethylene types of the articulating liners using a 2-tailed Student t test. The profilometry results for head scratch dimensions were tested for differences between bead locations using the Kruskal-Wallis test followed by the Wilcoxon rank sum test (with Bonferroni correction) when needed for making multiple comparisons. Significance level was set at .05.
Results Both the heads and the liners displayed dramatic scratch damage. The heads each had 5 scratch families, clearly corresponding to the 5 embedded beads on their respective liners (Fig. 2). The damage to the acetabular liners also included macroscopically visible scratches (Fig. 3A and B). However, the amount of this liner scratching was unexpectedly variable. Removal of liner machining marks and wear (flattening) of the beads were also observed as were microscratches not visible to the naked eye. All beads were observed to be retained in their created indentations after testing, with no indication that they had detached and traveled. By surgeon visual rank ordering of liner scratch severity (Table 1), there was low association between scratch severity and polyethylene type (q = 0.177). There tended to be more HXPE liners than CPE liners in the more severe (upper half of the) rank orders. Concordance rates between the 3 surgeons were 80%, 82%, and 86% for surgeons 1 vs 2, 1 vs 3, and 2 vs 3, respectively, indicating reasonably consistent rankings. Liner scratch peak-valley height and scratch width were not significantly less for the HXPE liner scratches as
Conventional vs Highly Cross-Linked Polyethylene Liners Heiner et al
Fig. 3. Acetabular liner scratches resulting from third bodies embedded in the liner: (A) close-up around 1 embedded bead and (B) around all 5 embedded beads (this liner was ranked as the most [2 surgeons] or second most [1 surgeon] severely scratched).
compared with those on the CPE liners (Fig. 4). The average scratch peak-valley height for the HXPE liners tended to be higher than for CPE, by 12% (P = .16) for measurable scratches (unmeasurable scratches not inTable 1. Rank-Order Of Severity of Scratch Damage for CPE (C) and HXPE (H) Liners, as Measured by Visual Appearance of Liner Scratch Fields Surgeon Most severe
Least severe
1
2
3
H C H H H H C C C C H C
C C H H H H H C C C C H
C H C H H C C H H C H C
Three experienced orthopedic surgeons specializing in arthroplasty did the rank ordering and were blinded as to whether the liners were CPE or HXPE.
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Fig. 4. (A) Peak-valley height and (B) width of scratches on CPE and HXPE liners, with just the measurable scratch locations and with the nonmeasurable scratch locations (numbering 20 and 13 for the CPE and HXPE liners, respectively) set to zero and included in the calculations. Dispersion bars indicate SDs. These height and width measurements were not significantly less for the HXPE liner scratches, as compared with those on the CPE liners (α = .05).
cluded) and by 27% (P = .06) for all scratches (unmeasurable scratch dimensions recorded as zero), respectively. The average scratch width for the HXPE liners was slightly lower (0.7%, P = .46) than for CPE for measurable scratches and higher (13%, P = .16) than the CPE values for all scratches. Twenty scratch measurement locations (of 69) for the CPE liners and 13 scratch measurement locations on the HXPE liners (of 67) were unmeasurable because the scratch in those locations was too faint. None of the liner roughness measurements were significantly less for the HXPE liners, as compared with
Fig. 5. Roughness measurements for the scratched CPE and HXPE liners and for a tested control liner without bead embedment (no-bead control). Dispersion bars indicate SDs. None of the liner roughness measurements were significantly less for the HXPE liners, as compared with the CPE liners (α = .05).
746 The Journal of Arthroplasty Vol. 27 No. 5 May 2012 the CPE liners (Fig. 5). The HXPE values for Ra, Rp, Rv, and Rt were within 3.2% of the CPE values, and the absolute magnitude of the Rsk value for HXPE was 14% higher than the CPE value. The P values for Ra, Rp, Rv, Rt, and Rsk were .41, .46, .32, .40, and .33, respectively. The average roughness measures of Ra, Rp, Rv, and Rt were greater than the values for the nonembedded tested control liner. The negative Rsk values for the scratched liner locations indicated that the measured surfaces predominately had valleys rather than peaks. For the head scratches corresponding to the central bead location, the scratch dimensions for heads articulating against the HXPE liners were slightly (12%-20%) greater than for heads articulating against the CPE liners, with the differences in peak lip height and peak-valley height being statistically significant (Fig. 6A). The P values for scratch peak lip height, valley depth, peakvalley height, and width were .01, .07, .01, and .13, respectively. For the 2 heads for which all 5 bead location scratches were measured (Fig. 6B and C), there was a suggestion of inconsistent variability of the femoral head scratches between the 5 locations. For one head, the posterolateral bead location resulted in a head scratch that was noticeably shallower than the others (Fig. 6B), whereas for the other head, the central bead location resulted in a head scratch that was noticeably deeper (and with a higher peak) than the others (Fig. 6C). The head scratch height measures (peak lip height, valley depth, and peak-valley height) were on the order of a few micrometers, whereas the head scratch width was, with one exception (597 μm), on the order of a few hundred micrometers (the second maximum head scratch width measured was 275 μm).
Discussion In this experiment, for acetabular liners articulating with CoCr femoral heads, high cross-linking of the polyethylene did not offer significant protection against the severe scratching induced by the relatively large (300-320 μm) embedded third-body particles. By visual rank ordering, there was low association between liner scratch severity and polyethylene type, with a slight tendency for the HXPE liners to be judged as more severely scratched. By stylus profilometry, the scratch peak-valley height and scratch width on the HXPE liners were not significantly less than those for the CPE liners and, in fact, were usually slightly higher for the HXPE liners. Liner roughness measurements in the vicinity of the embedded beads were not significantly less for HXPE vs CPE, although both types had increased roughness as compared with a nonembedded control liner. The increase in surface roughness values for both types of polyethylene is indicative of local damage or wear in the vicinity of the beads. The negative skew (Rsk) may be associated with wear or with plastic deformation of the asperity peaks as well as with scratching, which can
Fig. 6. Peak lip height (Pp), valley depth (Pv), peak-valley height (Pt), and width (Pln) of scratches on femoral heads articulating against CPE and HXPE liners with embedded beads (A) for the head scratches corresponding to the central bead location for all liners and (B and C) for the head scratches corresponding to each of the 5 bead locations (see Fig. 1), as measured on the heads that were articulated against the liners having the median rank-ordered scratching severity for each liner type of (B) CPE and (C) HXPE. Dispersion bars indicate standard deviations; SD is significantly different (α = .05).
produce troughs [25]. This severe scratching of the polyethylene liners occurred within a relatively short test period, consisting of approximately 3 hours of physical testing at 1 Hz. The essential finding in this study, that HXPE liners do not show an advantage in scratch resistance vs CPE liners, was consistent with the findings of 2 retrieval studies of CPE and HXPE liners [18,26]. In a short- to medium-term retrieval study, there was no significant difference between the average
Conventional vs Highly Cross-Linked Polyethylene Liners Heiner et al
wear damage scratching scores between the CPE and HXPE liners [26]. In a short-term retrieval study, heavy scratching was more frequently noted on HXPE liners than on CPE liners [18]. The scratches on the femoral heads were of similar magnitude between the heads articulating against HXPE vs CPE liners, although there was a trend toward slightly more severe damage for the HXPE. All head scratch height measures (peak lip height, valley depth, and peak-valley height) were considerably smaller than the amount by which the embedded beads protruded from the (unloaded) liner. Upon load application by the femoral head, the embedded beads likely sank into the polyethylene liner, therefore decreasing the amount of bead protrusion. As the top of the embedded bead wore down, the exposed diameter of the bead would increase, thus increasing the swath width of the femoral head which the bead could potentially scratch. For one of the heads for which all 5 bead location scratches were measured, the central bead location resulted in a head scratch that was noticeably deeper (and with a higher peak) than the other 4 scratches on that head. As stated earlier, the central bead location was chosen as the site determined to be maximally problematic for accelerating liner wear because of roughening of the femoral head counterface [17]. Its resulting in the most severe head scratch among the 5 embedded beads was consistent with that study. For the liner results for which HXPE was less severely scratched than CPE (although not significantly so), post hoc power analyses was carried out to determine the detectable decreases available with the given sample sizes and SDs, with 80% power. The detectable decreases were 18%, 19%, and 20% for scratch width for measurable scratches, Rp, and Rt, respectively, whereas the experimentally measured decreases were only 0.7%, 0.8%, and 2.0%, respectively. Because of the intrinsically small differences in scratching resistance for these 2 variants of polyethylene, having a low type II error (ie, a low probability of incorrectly stating that the HXPE results were not lower than the CPE results) would require a prohibitively large number of measurements (324, 15 182, and 2689, respectively). However, for the reasonable number of measurements taken, there was no indication that the HXPE liners were substantially less scratched than the CPE liners. Laboratory wear studies for metal-on-polyethylene hip implants have used various means of simulating third-body challenge, including adding particles to the lubricant fluid; directly placing particles in the bearing space before testing; preroughening the counterface by a (usually) small number of discrete, controlled scratches; preroughening the counterface by more global means such as grit-paper abrasion; or by using retrieved components. Other than in studies using retrieved components, third-body challenge has typically been based on simulating the scratch geometries or the
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roughness values seen on retrieved components, often deliberately toward the higher range of damage severity. The direct use of retrieved components for laboratory wear testing has the drawbacks of limited availability and high variability in the amount and distribution of scratch damage on the components. One advantage of embedding third-body particles, as was done in the current study, is that the specific locations of the particles could be deliberately chosen and accurately reproduced. This is important because the location of the embedded debris is a strong determinant of the amount of the subsequent polyethylene wear ensuing [17]. Another advantage of the present approach is that the femoral head scratches were directly created in situ by these particles rather than being artificially created before the testing. This technique of third-body challenge could also be used in the study of other particle materials, particle sizes, and bearing surface materials. However, direct use of this bead embedment technique would not be compatible with determining component wear by gravimetric techniques because the recommended component cleaning technique could not be done after bead embedment and before testing (because the cleaning would likely remove the beads) and also might not be sufficient to remove all embedded material after testing (thus affecting weight measurement) [23]. One limitation of this particle embedment paradigm is that it involved only 1 batch of third bodies rather than addressing third-body entry as may often be the case in vivo [3,7,9,21]. The wear or flattening of embedded debris, as observed in this study and in total hip arthroplasty [27] and total knee arthroplasty [28] retrievals, may reduce the ability of a given embedded third-body particle to continue producing scratches on the opposing surface, if the particle becomes flush with the polyethylene surface. Another limitation is that because the third bodies were fixed in place, they could not cause the direct liner damage (or additional head damage) that mobile third bodies might [21,29]. However, considering the trade-off of the ability of the current protocol to provide well-prescribed, reproducible particle embedment sites, these limitations seem acceptable. Upon remelting of damaged polyethylene components, scratches, to some extent, can be eliminated, and machining marks can be recovered, with this shape memory effect indicating scratch damage associated with plastic deformation rather than wear [18,30-33]. Muratoglu et al [18,33] reported that HXPE liners and laboratory-damaged cylindrical specimens displayed these signs of shape recovery to a greater extent than did CPE ones, with the latter material retaining more of these signs of damage after remelting. Likewise, Mimnaugh et al [30] reported that, for liners that were scratched, tested in wear, and then remelted, machining marks reappeared for the HXPE liners but not for the
748 The Journal of Arthroplasty Vol. 27 No. 5 May 2012 CPE ones. On the other hand, Lazzarini et al [32] reported that HXPE liners, when tested under severe (although not specifically third body) laboratory conditions, had only slight recovery of machining marks when subsequently remelted, indicating that the liner damage was preponderantly caused by wear rather than by plastic deformation. Use of HXPE is now commonplace and may further increase because of widely publicized concerns with metal-on-metal bearings [34-36]. Because of the conflicting evidence as to whether HXPE scratching consists mainly of material removal vs mainly of plastic deformation, and given the widespread use of HXPE and the wide range of potential third-body challenges in vivo, it seems appropriate that increased attention be devoted to systematic study of third-body wear phenomena in this important class of materials.
Acknowledgments This study was funded by National Institutes of Health AR047653 and AR057780. The authors also thank DePuy Orthopaedics, Inc, for providing the implants, and Drs Richard Johnston and Nicolas Noiseux for rank ordering the liner scratches.
References 1. Shon WY, Baldini T, Peterson MG, et al. Impingement in total hip arthroplasty—a study of retrieved acetabular components. J Arthroplasty 2005;20:427. 2. Bloebaum RD, Zou L, Bachus KN, et al. Analysis of particles in acetabular components from patients with osteolysis. Clin Orthop Relat Res 1997;338:109. 3. Isaac GH, Wroblewski BM, Atkinson JR, et al. A tribological study of retrieved hip prostheses. Clin Orthop Relat Res 1992;276:115. 4. Sychterz CJ, Moon KH, Hashimoto Y, et al. Wear of polyethylene cups in total hip arthroplasty. A study of specimens retrieved post mortem. J Bone Joint Surg Am 1996;78:1193. 5. Lundberg HJ, Liu SS, Callaghan JJ, et al. Association of third body embedment with rim damage in retrieved acetabular liners. Clin Orthop Relat Res 2007;465:133. 6. Kim YH, Ritchie A, Hardaker C. Surface roughness of ceramic femoral heads after in vivo transfer of metal: correlation to polyethylene wear. J Bone Joint Surg Am 2005;87:577. 7. Sychterz CJ, Engh Jr CA, Swope SW, et al. Analysis of prosthetic femoral heads retrieved at autopsy. Clin Orthop Relat Res 1999;223. 8. Endo MM, Barbour PS, Barton DC, et al. Comparative wear and wear debris under three different counterface conditions of crosslinked and non-crosslinked ultra high molecular weight polyethylene. Biomed Mater Eng 2001; 11:23. 9. McKellop H, Shen FW, DiMaio W, et al. Wear of gammacrosslinked polyethylene acetabular cups against roughened femoral balls. Clin Orthop Relat Res 1999;369:73.
10. McKellop H, Shen FW, Lu B, et al. Development of an extremely wear-resistant ultra high molecular weight polyethylene for total hip replacements. J Orthop Res 1999; 17:157. 11. Shen FW, McKellop H. Surface-gradient cross-linked polyethylene acetabular cups: oxidation resistance and wear against smooth and rough femoral balls. Clin Orthop Relat Res 2005;430:80. 12. Sakoda H, Voice AM, McEwen HM, et al. A comparison of the wear and physical properties of silane cross-linked polyethylene and ultra-high molecular weight polyethylene. J Arthroplasty 2001;16:1018. 13. Galvin A, Kang L, Tipper J, et al. Wear of crosslinked polyethylene under different tribological conditions. J Mater Sci Mater Med 2006;17:235. 14. Fisher J, McEwen HM, Tipper JL, et al. Wear, debris, and biologic activity of cross-linked polyethylene in the knee: benefits and potential concerns. Clin Orthop Relat Res 2004; 428:114. 15. Glennon LP, Baer TE, Martin JA, et al. Sliding direction dependence of polyethylene wear for metal counterface traverse of severe scratches. J Biomech Eng 2008;130:051006. 16. Willert HG, Semlitsch M. Reactions of the articular capsule to wear products of artificial joint prostheses. J Biomed Mater Res 1977;11:157. 17. Lundberg HJ, Stewart KJ, Pedersen DR, et al. Problematic sites of third body embedment in polyethylene for total hip wear acceleration. J Biomech 2006;39:1208. 18. Muratoglu OK, Greenbaum ES, Bragdon CR, et al. Surface analysis of early retrieved acetabular polyethylene liners: a comparison of conventional and highly crosslinked polyethylenes. J Arthroplasty 2004;19:68. 19. ASTM F75-01 Standard specification for cobalt-28 chromium-6 molybdenum alloy castings and casting alloy for surgical implants. 2001. 20. McNie CM, Barton DC, Fisher J, et al. Modeling of damage to articulating surfaces by third body particles in total joint replacements. J Mater Sci Mater Med 2000;11:569. 21. Heiner AD, Lundberg HJ, Baer TE, et al. Effects of episodic subluxation events on third body ingress and embedment in the THA bearing surface. J Biomech 2008;41:2090. 22. Kotzar GM, Davy DT, Goldberg VM, et al. Telemeterized in vivo hip joint force data: a report on two patients after total hip surgery. J Orthop Res 1991;9:621. 23. ASTM F1714-96(2002) Standard guide for gravimetric wear assessment of prosthetic hip-designs in simulator devices. 2002. 24. Harrell FE. Regression modeling strategies: with applications to linear models, logistic regression, and survival analysis. New York: Springer-Verlag; 2001. 25. Cooper JR, Dowson D, Fisher J. Macroscopic and microscopic wear mechanisms in ultra-high molecular weight polyethylene. Wear 1993;162-164(Part 1):378. 26. Schroder DT, Kelly NH, Wright TM, et al. Retrieved highly crosslinked UHMWPE acetabular liners have similar wear damage as conventional UHMWPE. Clin Orthop Relat Res 2011;469:387. 27. Bauer TW, Ming J, D'Antonio JA, et al. Abrasive three-body wear of polyethylene caused by broken multifilament cables of a total hip prosthesis. A report of these cases. J Bone Joint Surg Am 1996;78:1244.
Conventional vs Highly Cross-Linked Polyethylene Liners Heiner et al 28. Que L, Topoleski LD, Parks NL. Surface roughness of retrieved CoCrMo alloy femoral components from PCA artificial total knee joints. J Biomed Mater Res 2000;53:111. 29. Raimondi MT, Vena P, Pietrabissa R. Quantitative evaluation of the prosthetic head damage induced by microscopic third-body particles in total hip replacement. J Biomed Mater Res 2001;58:436. 30. Mimnaugh KD, Yao JQ, Laurent MP, et al. The effect of entrapped bone particles on the surface morphology and wear of polyethylene. J Arthroplasty 2009;24:303. 31. Muratoglu OK, Ruberti J, Melotti S, et al. Optical analysis of surface changes on early retrievals of highly cross-linked and conventional polyethylene tibial inserts. J Arthroplasty 2003;18(7 Suppl 1):42.
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32. Lazzarini AM, Cottrell JM, Padgett DE, et al. Remelting of highly cross-linked polyethylene worn under laboratory conditions. Clin Orthop Relat Res 2007;465:128. 33. Muratoglu OK, Wannomae K, Christensen S, et al. Ex vivo wear of conventional and cross-linked polyethylene acetabular liners. Clin Orthop Relat Res 2005;438:158. 34. Medicines and Healthcare products Regulatory Agency. Medical Device Alert: All metal-on-metal (MoM) hip replacements (MDA/2010/033). 2010. 35. Crawford R, Ranawat CS, Rothman RH. Metal on metal: is it worth the risk? J Arthroplasty 2010;25:1. 36. http://www.fda.gov/MedicalDevices/Productsand MedicalProcedures/ImplantsandProsthetics/Metalon MetalHipImplants/ucm241769.htm. May 9, 2011.
Scratching Vulnerability of Conventional vs Highly Cross-Linked Polyethylene Liners Because of Large Embedded Third-Body Particles Anneliese D. Heiner, PhD,*y Alison L. Galvin, PhD,z John Fisher, PhD,z§ John J. Callaghan, MD,*t and Thomas D. Brown, PhD*y
Abstract: The hypothesis of this study was that acetabular liner vulnerability to scratching from femoral heads, roughened by third bodies embedded in the liner, is not significantly lower for highly cross-linked polyethylene (HXPE) than for conventional polyethylene (CPE). Six CPE and 6 HXPE acetabular liners were each reproducibly embedded with 5 cobalt-chromiummolybdenum (CoCrMo) beads then run for 10 000 cycles in a joint simulator. By visual rank ordering, there was low association between liner scratch severity and polyethylene type. The CPE and HXPE liner scratches were not significantly different in scratch peak-valley height or width or in liner roughness in the vicinity of the embedded beads. This model indicated that high cross-linking of polyethylene does not offer appreciable protection against severe scratching induced by large embedded third-body particles. Keywords: highly cross-linked polyethylene, hip arthroplasty, hip joint motion simulator, polyethylene, third-body wear. © 2012 Elsevier Inc. All rights reserved.
In total joint arthroplasty, third-body particles accessing the bearing space can dramatically increase wear, both directly by abrading the bearing surfaces and indirectly by the scratching of one surface, in turn accelerating wear of the other. Third bodies, such as porous ingrowth beads or pieces of bone cement, by definition, originate from other than the normal articulation surfaces. Thirdbody particles embedded in retrieved polyethylene acetabular liners have been documented by many studies [1-7] and have also been seen in polyethylene components from other total joint arthroplasty sites such as the knee, ankle, and elbow. Laboratory wear tests of polyethylene against metal have demonstrated increased polyethylene wear under third-body conditions [8,9].
From the *Department of Orthopaedics and Rehabilitation, University of Iowa, Iowa City, Iowa; yDepartment of Biomedical Engineering, University of Iowa, Iowa City, Iowa; zInstitute of Medical and Biological Engineering, University of Leeds, Leeds, UK; §Leeds Musculoskeletal Biomedical Research Unit, Leeds Teaching Hospitals Trust, Leeds, UK; and ∥Orthopaedics Department, VA Medical Center, Iowa City, Iowa. Submitted October 11, 2010; accepted October 6, 2011. The Conflict of Interest Statement associated with this article can be found at doi:10.1016/j.arth.2011.10.002. Reprint requests: Anneliese D. Heiner, PhD, University of Iowa, Department of Orthopaedics and Rehabilitation, Biomechanics Laboratory, 2181 Westlawn, Iowa City, Iowa 52242. © 2012 Elsevier Inc. All rights reserved. 0883-5403/2705-0014$36.00/0 doi:10.1016/j.arth.2011.10.002
For hip simulator testing of metal-on-polyethylene total hip arthroplasty implants, in the absence of thirdbody challenge, highly cross-linked polyethylene (HXPE) acetabular liners wear far less than do conventional polyethylene (CPE) liners [10]. However, HXPE may be relatively more susceptible to damage from large local asperities on the metal counterface, such as large scratches from third bodies, thus reducing the wear advantage of HXPE vs CPE under severe third-body conditions. Indeed, in some laboratory tests under conditions of third-body challenge, the wear rate of HXPE has been comparable with or even greater than that of CPE [8,11-14]. The scratch resistance of a material is a function of its hardness, and radiation dose and remelting have little effect on the hardness of polyethylene [10]. However, scratching includes lateral motion as well as axial motion, with only the latter being applied by depth-of-penetration hardness testing. Highly cross-linked polyethylene has compromised fracture toughness, fatigue resistance, and tensile properties, as compared with CPE [10,11], and these material properties could plausibly have an effect on the scratch resistance of polyethylene. Although liner scratching may not directly equate with outright wear, scratching, nevertheless, is a phenomenon from which no good can come. Scratching causes material to pile up in scratch lips. Analogous to cross-shearing effects for wear of nondamaged surfaces, the counterface scraping transversely across a scratch lip would tend to break off this
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piled-up material. Although the particle size of brokenoff scratch lip material would tend to be outside the most osteolytic range [15], its presence in the joint space would, nevertheless, be deleterious [16]. The hypothesis of this study was that acetabular liner vulnerability to scratching from femoral heads, roughened by large third bodies embedded at wear-critical locations [17] in the liner, is not significantly lower for HXPE than for CPE. In this study, to concentrate on liner scratching rather than on long-term wear (which could subsequently polish down or even erase these scratches [18]), the test period was kept short. To simulate third-body effects and to minimize differences between the specimens, a controlled, reproducible technique of third-body embedment into the liners was used. The scratching of the CPE vs HXPE liners was evaluated by visual rank ordering of pictures of scratched liners in terms of severity of damage, by profilometry of individual scratches, and by profilometry-based liner roughness measurements.
Materials and Methods Six CPE and 6 HXPE acetabular liners (Duraloc Enduron and Duraloc Marathon, respectively; DePuy Orthopaedics, Warsaw, Ind) were tested. The CPE material was not cross-linked, whereas the HXPE material was cross-linked with 5 Mrad and remelted. Both liner types were gas-plasma sterilized. The liners were 28 mm in diameter and were nonconstrained. Each liner was reproducibly embedded with 5 cobaltchromium-molybdenum (CoCrMo) beads (PREP F-75 Alloy Powder; Advanced Specialty Metals, Inc, Nashua, NH) measuring 300 to 320 μm in diameter (Fig. 1). The bead material was the alloy specified in ASTM F75-01 for use in surgical implants [19]. The bead size was one commonly used in cementless prostheses [20] and was within the range of particle sizes found embedded in retrieved polyethylene liners [2,5]. One bead was embedded at a site that an earlier finite element analysis had determined to be a maximally problematic site for accelerating liner wear because of roughening of the femoral head counterface [17]. This site was at a latitude of 45° and a longitude of 30° posteriorly from the line between the lateral-most point and the cup center, for a liner orientation of 45° tilt and 15° anteversion. The other 4 beads were embedded equidistantly around a 10-mm diameter circle centered about that location. The bead embedment procedure involved, first, creating indentations with larger beads at the specified sites on the liner then (cyanoacrylate) gluing the final beads into those indentations. This procedure was necessary to keep the final beads in place during pretest handling (including having the liner in a physiologic position in the joint motion simulator) and during the testing itself. The larger indentation beads, 400-μm diameter balls of alloy 316 stainless steel (New England
Fig. 1. Third-body beads embedded in acetabular liner: (A) bead positions for testing (bead diameter is increased for visibility and is not to scale) and (B) close-up.
Miniature Ball, Norfolk, Conn), were glued in place then compressed by a femoral head loaded to 263 N [20] for 1 hour in a materials testing machine. The indentation beads were then removed, leaving behind cavities in the liner surface. The final beads were then glued into those cavities. As measured by profilometry (described below), the scratching beads protruded on an average of 144 μm (SD, 15.7 μm) above the liner surface. Each liner was run for 10 000 level walking cycles at 1 Hz in a multiaxis joint motion simulator, articulating against a fresh 28-mm CoCr femoral head (Articul/Eze; DePuy). The metallic acetabular shell (Duraloc; DePuy), in which each liner was held, was oriented in 45° tilt and 15° anteversion. A femoral stem (Summit Basic; DePuy), to which each femoral head was attached, was oriented in 12° adduction. The joint simulator, described in more detail elsewhere [21], was actuated by a material testing machine (MTS 858 Bionix; MTS Systems Corp, Eden Prairie, Minn). This apparatus allowed control of rotations in all 3 degrees of freedom, in addition to control of axial load. Level walking waveforms from Kotzar et al [22] were used, converted for use in the multiaxis joint simulator, assuming a body weight of 620
744 The Journal of Arthroplasty Vol. 27 No. 5 May 2012 N. The test fluid was bovine calf serum (7.0 ± 0.5 gm% total protein; HyClone, Logan, Utah) with 20 mmol/L EDTA [23], diluted to 25% with distilled water. After testing, the heads and liners were photographed, using a dissecting microscope for close-ups. Liner scratching was evaluated by visual rank ordering of pictures of scratched liners for severity of damage, by stylus profilometry of individual scratches, and by stylus profilometry assessment of liner roughness. As a qualitative measure of severity of scratch damage, 3 experienced orthopedic surgeons specializing in arthroplasty rank ordered the pictures of the liner scratch fields according to visual appearance; the surgeons were blinded as to whether the liners were CPE or HXPE. Profiles of the liner scratches and assessment of liner roughness were performed using a contacting profilometer (Form Talysurf 5; Taylor-Hobson, Leicester, United Kingdom) with a cutoff of 0.85 mm and a resolution of 10 nm. For each liner, scratch profiles were measured at 4 consistent, equally radially spaced locations for the 3 most severe scratches apparent on each liner. The peak-valley height and width of the scratches were recorded at these locations. When the scratch at any given sampling location was too faint to measure, its height and width were recorded as zero. Liner roughness was measured in a consistent area around each bead; this area was a patch measuring 5 × 4 mm for the central bead location and one measuring 7 × 5.5 mm for the other bead locations. Liner roughness was also measured on a nonembedded tested control. The following liner roughness parameters were recorded: Ra, the average roughness of the surface; Rp, the maximum peak height of the surface profile; Rv, the maximum valley depth; Rt, the total height of the roughness profile; and Rsk, an index of whether the roughness was predominantly caused by peaks or valleys. Profiles of the head scratches were also measured to determine if the femoral heads that had articulated against the CPE vs the HXPE liners were similarly scratched. For each head, scratch profiles were measured at 12 radial locations around the specific scratch that corresponded to the central bead location. Profiles were also measured for all 5 bead location scratches for 2 heads. These heads were those that had been articulated against the liners that had the median rank-ordered scratching severity for each liner type. Peak lip height, valley depth, peak-valley height, and width of the scratches were recorded. Statistical analyses were done using Excel (Microsoft Corp, Redmond, Wash) and Minitab 16 Statistical Software (Minitab Inc, State College, Pa). Correlation between visual rank ordering of scratch severity and polyethylene type was measured by Spearman q. Agreement between the rank ordering of the 3 evaluators was determined by calculating the concordance rate between each pair [24]. The profilometry results for liner scratch dimensions and liner roughness
Fig. 2. Femoral head scratches resulting from third bodies embedded in the acetabular liner (white spots are light reflections).
were tested for differences between polyethylene types using a 1-tailed Student t test. The profilometry results for the head scratch dimensions for the central bead location were tested for differences between polyethylene types of the articulating liners using a 2-tailed Student t test. The profilometry results for head scratch dimensions were tested for differences between bead locations using the Kruskal-Wallis test followed by the Wilcoxon rank sum test (with Bonferroni correction) when needed for making multiple comparisons. Significance level was set at .05.
Results Both the heads and the liners displayed dramatic scratch damage. The heads each had 5 scratch families, clearly corresponding to the 5 embedded beads on their respective liners (Fig. 2). The damage to the acetabular liners also included macroscopically visible scratches (Fig. 3A and B). However, the amount of this liner scratching was unexpectedly variable. Removal of liner machining marks and wear (flattening) of the beads were also observed as were microscratches not visible to the naked eye. All beads were observed to be retained in their created indentations after testing, with no indication that they had detached and traveled. By surgeon visual rank ordering of liner scratch severity (Table 1), there was low association between scratch severity and polyethylene type (q = 0.177). There tended to be more HXPE liners than CPE liners in the more severe (upper half of the) rank orders. Concordance rates between the 3 surgeons were 80%, 82%, and 86% for surgeons 1 vs 2, 1 vs 3, and 2 vs 3, respectively, indicating reasonably consistent rankings. Liner scratch peak-valley height and scratch width were not significantly less for the HXPE liner scratches as
Conventional vs Highly Cross-Linked Polyethylene Liners Heiner et al
Fig. 3. Acetabular liner scratches resulting from third bodies embedded in the liner: (A) close-up around 1 embedded bead and (B) around all 5 embedded beads (this liner was ranked as the most [2 surgeons] or second most [1 surgeon] severely scratched).
compared with those on the CPE liners (Fig. 4). The average scratch peak-valley height for the HXPE liners tended to be higher than for CPE, by 12% (P = .16) for measurable scratches (unmeasurable scratches not inTable 1. Rank-Order Of Severity of Scratch Damage for CPE (C) and HXPE (H) Liners, as Measured by Visual Appearance of Liner Scratch Fields Surgeon Most severe
Least severe
1
2
3
H C H H H H C C C C H C
C C H H H H H C C C C H
C H C H H C C H H C H C
Three experienced orthopedic surgeons specializing in arthroplasty did the rank ordering and were blinded as to whether the liners were CPE or HXPE.
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Fig. 4. (A) Peak-valley height and (B) width of scratches on CPE and HXPE liners, with just the measurable scratch locations and with the nonmeasurable scratch locations (numbering 20 and 13 for the CPE and HXPE liners, respectively) set to zero and included in the calculations. Dispersion bars indicate SDs. These height and width measurements were not significantly less for the HXPE liner scratches, as compared with those on the CPE liners (α = .05).
cluded) and by 27% (P = .06) for all scratches (unmeasurable scratch dimensions recorded as zero), respectively. The average scratch width for the HXPE liners was slightly lower (0.7%, P = .46) than for CPE for measurable scratches and higher (13%, P = .16) than the CPE values for all scratches. Twenty scratch measurement locations (of 69) for the CPE liners and 13 scratch measurement locations on the HXPE liners (of 67) were unmeasurable because the scratch in those locations was too faint. None of the liner roughness measurements were significantly less for the HXPE liners, as compared with
Fig. 5. Roughness measurements for the scratched CPE and HXPE liners and for a tested control liner without bead embedment (no-bead control). Dispersion bars indicate SDs. None of the liner roughness measurements were significantly less for the HXPE liners, as compared with the CPE liners (α = .05).
746 The Journal of Arthroplasty Vol. 27 No. 5 May 2012 the CPE liners (Fig. 5). The HXPE values for Ra, Rp, Rv, and Rt were within 3.2% of the CPE values, and the absolute magnitude of the Rsk value for HXPE was 14% higher than the CPE value. The P values for Ra, Rp, Rv, Rt, and Rsk were .41, .46, .32, .40, and .33, respectively. The average roughness measures of Ra, Rp, Rv, and Rt were greater than the values for the nonembedded tested control liner. The negative Rsk values for the scratched liner locations indicated that the measured surfaces predominately had valleys rather than peaks. For the head scratches corresponding to the central bead location, the scratch dimensions for heads articulating against the HXPE liners were slightly (12%-20%) greater than for heads articulating against the CPE liners, with the differences in peak lip height and peak-valley height being statistically significant (Fig. 6A). The P values for scratch peak lip height, valley depth, peakvalley height, and width were .01, .07, .01, and .13, respectively. For the 2 heads for which all 5 bead location scratches were measured (Fig. 6B and C), there was a suggestion of inconsistent variability of the femoral head scratches between the 5 locations. For one head, the posterolateral bead location resulted in a head scratch that was noticeably shallower than the others (Fig. 6B), whereas for the other head, the central bead location resulted in a head scratch that was noticeably deeper (and with a higher peak) than the others (Fig. 6C). The head scratch height measures (peak lip height, valley depth, and peak-valley height) were on the order of a few micrometers, whereas the head scratch width was, with one exception (597 μm), on the order of a few hundred micrometers (the second maximum head scratch width measured was 275 μm).
Discussion In this experiment, for acetabular liners articulating with CoCr femoral heads, high cross-linking of the polyethylene did not offer significant protection against the severe scratching induced by the relatively large (300-320 μm) embedded third-body particles. By visual rank ordering, there was low association between liner scratch severity and polyethylene type, with a slight tendency for the HXPE liners to be judged as more severely scratched. By stylus profilometry, the scratch peak-valley height and scratch width on the HXPE liners were not significantly less than those for the CPE liners and, in fact, were usually slightly higher for the HXPE liners. Liner roughness measurements in the vicinity of the embedded beads were not significantly less for HXPE vs CPE, although both types had increased roughness as compared with a nonembedded control liner. The increase in surface roughness values for both types of polyethylene is indicative of local damage or wear in the vicinity of the beads. The negative skew (Rsk) may be associated with wear or with plastic deformation of the asperity peaks as well as with scratching, which can
Fig. 6. Peak lip height (Pp), valley depth (Pv), peak-valley height (Pt), and width (Pln) of scratches on femoral heads articulating against CPE and HXPE liners with embedded beads (A) for the head scratches corresponding to the central bead location for all liners and (B and C) for the head scratches corresponding to each of the 5 bead locations (see Fig. 1), as measured on the heads that were articulated against the liners having the median rank-ordered scratching severity for each liner type of (B) CPE and (C) HXPE. Dispersion bars indicate standard deviations; SD is significantly different (α = .05).
produce troughs [25]. This severe scratching of the polyethylene liners occurred within a relatively short test period, consisting of approximately 3 hours of physical testing at 1 Hz. The essential finding in this study, that HXPE liners do not show an advantage in scratch resistance vs CPE liners, was consistent with the findings of 2 retrieval studies of CPE and HXPE liners [18,26]. In a short- to medium-term retrieval study, there was no significant difference between the average
Conventional vs Highly Cross-Linked Polyethylene Liners Heiner et al
wear damage scratching scores between the CPE and HXPE liners [26]. In a short-term retrieval study, heavy scratching was more frequently noted on HXPE liners than on CPE liners [18]. The scratches on the femoral heads were of similar magnitude between the heads articulating against HXPE vs CPE liners, although there was a trend toward slightly more severe damage for the HXPE. All head scratch height measures (peak lip height, valley depth, and peak-valley height) were considerably smaller than the amount by which the embedded beads protruded from the (unloaded) liner. Upon load application by the femoral head, the embedded beads likely sank into the polyethylene liner, therefore decreasing the amount of bead protrusion. As the top of the embedded bead wore down, the exposed diameter of the bead would increase, thus increasing the swath width of the femoral head which the bead could potentially scratch. For one of the heads for which all 5 bead location scratches were measured, the central bead location resulted in a head scratch that was noticeably deeper (and with a higher peak) than the other 4 scratches on that head. As stated earlier, the central bead location was chosen as the site determined to be maximally problematic for accelerating liner wear because of roughening of the femoral head counterface [17]. Its resulting in the most severe head scratch among the 5 embedded beads was consistent with that study. For the liner results for which HXPE was less severely scratched than CPE (although not significantly so), post hoc power analyses was carried out to determine the detectable decreases available with the given sample sizes and SDs, with 80% power. The detectable decreases were 18%, 19%, and 20% for scratch width for measurable scratches, Rp, and Rt, respectively, whereas the experimentally measured decreases were only 0.7%, 0.8%, and 2.0%, respectively. Because of the intrinsically small differences in scratching resistance for these 2 variants of polyethylene, having a low type II error (ie, a low probability of incorrectly stating that the HXPE results were not lower than the CPE results) would require a prohibitively large number of measurements (324, 15 182, and 2689, respectively). However, for the reasonable number of measurements taken, there was no indication that the HXPE liners were substantially less scratched than the CPE liners. Laboratory wear studies for metal-on-polyethylene hip implants have used various means of simulating third-body challenge, including adding particles to the lubricant fluid; directly placing particles in the bearing space before testing; preroughening the counterface by a (usually) small number of discrete, controlled scratches; preroughening the counterface by more global means such as grit-paper abrasion; or by using retrieved components. Other than in studies using retrieved components, third-body challenge has typically been based on simulating the scratch geometries or the
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roughness values seen on retrieved components, often deliberately toward the higher range of damage severity. The direct use of retrieved components for laboratory wear testing has the drawbacks of limited availability and high variability in the amount and distribution of scratch damage on the components. One advantage of embedding third-body particles, as was done in the current study, is that the specific locations of the particles could be deliberately chosen and accurately reproduced. This is important because the location of the embedded debris is a strong determinant of the amount of the subsequent polyethylene wear ensuing [17]. Another advantage of the present approach is that the femoral head scratches were directly created in situ by these particles rather than being artificially created before the testing. This technique of third-body challenge could also be used in the study of other particle materials, particle sizes, and bearing surface materials. However, direct use of this bead embedment technique would not be compatible with determining component wear by gravimetric techniques because the recommended component cleaning technique could not be done after bead embedment and before testing (because the cleaning would likely remove the beads) and also might not be sufficient to remove all embedded material after testing (thus affecting weight measurement) [23]. One limitation of this particle embedment paradigm is that it involved only 1 batch of third bodies rather than addressing third-body entry as may often be the case in vivo [3,7,9,21]. The wear or flattening of embedded debris, as observed in this study and in total hip arthroplasty [27] and total knee arthroplasty [28] retrievals, may reduce the ability of a given embedded third-body particle to continue producing scratches on the opposing surface, if the particle becomes flush with the polyethylene surface. Another limitation is that because the third bodies were fixed in place, they could not cause the direct liner damage (or additional head damage) that mobile third bodies might [21,29]. However, considering the trade-off of the ability of the current protocol to provide well-prescribed, reproducible particle embedment sites, these limitations seem acceptable. Upon remelting of damaged polyethylene components, scratches, to some extent, can be eliminated, and machining marks can be recovered, with this shape memory effect indicating scratch damage associated with plastic deformation rather than wear [18,30-33]. Muratoglu et al [18,33] reported that HXPE liners and laboratory-damaged cylindrical specimens displayed these signs of shape recovery to a greater extent than did CPE ones, with the latter material retaining more of these signs of damage after remelting. Likewise, Mimnaugh et al [30] reported that, for liners that were scratched, tested in wear, and then remelted, machining marks reappeared for the HXPE liners but not for the
748 The Journal of Arthroplasty Vol. 27 No. 5 May 2012 CPE ones. On the other hand, Lazzarini et al [32] reported that HXPE liners, when tested under severe (although not specifically third body) laboratory conditions, had only slight recovery of machining marks when subsequently remelted, indicating that the liner damage was preponderantly caused by wear rather than by plastic deformation. Use of HXPE is now commonplace and may further increase because of widely publicized concerns with metal-on-metal bearings [34-36]. Because of the conflicting evidence as to whether HXPE scratching consists mainly of material removal vs mainly of plastic deformation, and given the widespread use of HXPE and the wide range of potential third-body challenges in vivo, it seems appropriate that increased attention be devoted to systematic study of third-body wear phenomena in this important class of materials.
Acknowledgments This study was funded by National Institutes of Health AR047653 and AR057780. The authors also thank DePuy Orthopaedics, Inc, for providing the implants, and Drs Richard Johnston and Nicolas Noiseux for rank ordering the liner scratches.
References 1. Shon WY, Baldini T, Peterson MG, et al. Impingement in total hip arthroplasty—a study of retrieved acetabular components. J Arthroplasty 2005;20:427. 2. Bloebaum RD, Zou L, Bachus KN, et al. Analysis of particles in acetabular components from patients with osteolysis. Clin Orthop Relat Res 1997;338:109. 3. Isaac GH, Wroblewski BM, Atkinson JR, et al. A tribological study of retrieved hip prostheses. Clin Orthop Relat Res 1992;276:115. 4. Sychterz CJ, Moon KH, Hashimoto Y, et al. Wear of polyethylene cups in total hip arthroplasty. A study of specimens retrieved post mortem. J Bone Joint Surg Am 1996;78:1193. 5. Lundberg HJ, Liu SS, Callaghan JJ, et al. Association of third body embedment with rim damage in retrieved acetabular liners. Clin Orthop Relat Res 2007;465:133. 6. Kim YH, Ritchie A, Hardaker C. Surface roughness of ceramic femoral heads after in vivo transfer of metal: correlation to polyethylene wear. J Bone Joint Surg Am 2005;87:577. 7. Sychterz CJ, Engh Jr CA, Swope SW, et al. Analysis of prosthetic femoral heads retrieved at autopsy. Clin Orthop Relat Res 1999;223. 8. Endo MM, Barbour PS, Barton DC, et al. Comparative wear and wear debris under three different counterface conditions of crosslinked and non-crosslinked ultra high molecular weight polyethylene. Biomed Mater Eng 2001; 11:23. 9. McKellop H, Shen FW, DiMaio W, et al. Wear of gammacrosslinked polyethylene acetabular cups against roughened femoral balls. Clin Orthop Relat Res 1999;369:73.
10. McKellop H, Shen FW, Lu B, et al. Development of an extremely wear-resistant ultra high molecular weight polyethylene for total hip replacements. J Orthop Res 1999; 17:157. 11. Shen FW, McKellop H. Surface-gradient cross-linked polyethylene acetabular cups: oxidation resistance and wear against smooth and rough femoral balls. Clin Orthop Relat Res 2005;430:80. 12. Sakoda H, Voice AM, McEwen HM, et al. A comparison of the wear and physical properties of silane cross-linked polyethylene and ultra-high molecular weight polyethylene. J Arthroplasty 2001;16:1018. 13. Galvin A, Kang L, Tipper J, et al. Wear of crosslinked polyethylene under different tribological conditions. J Mater Sci Mater Med 2006;17:235. 14. Fisher J, McEwen HM, Tipper JL, et al. Wear, debris, and biologic activity of cross-linked polyethylene in the knee: benefits and potential concerns. Clin Orthop Relat Res 2004; 428:114. 15. Glennon LP, Baer TE, Martin JA, et al. Sliding direction dependence of polyethylene wear for metal counterface traverse of severe scratches. J Biomech Eng 2008;130:051006. 16. Willert HG, Semlitsch M. Reactions of the articular capsule to wear products of artificial joint prostheses. J Biomed Mater Res 1977;11:157. 17. Lundberg HJ, Stewart KJ, Pedersen DR, et al. Problematic sites of third body embedment in polyethylene for total hip wear acceleration. J Biomech 2006;39:1208. 18. Muratoglu OK, Greenbaum ES, Bragdon CR, et al. Surface analysis of early retrieved acetabular polyethylene liners: a comparison of conventional and highly crosslinked polyethylenes. J Arthroplasty 2004;19:68. 19. ASTM F75-01 Standard specification for cobalt-28 chromium-6 molybdenum alloy castings and casting alloy for surgical implants. 2001. 20. McNie CM, Barton DC, Fisher J, et al. Modeling of damage to articulating surfaces by third body particles in total joint replacements. J Mater Sci Mater Med 2000;11:569. 21. Heiner AD, Lundberg HJ, Baer TE, et al. Effects of episodic subluxation events on third body ingress and embedment in the THA bearing surface. J Biomech 2008;41:2090. 22. Kotzar GM, Davy DT, Goldberg VM, et al. Telemeterized in vivo hip joint force data: a report on two patients after total hip surgery. J Orthop Res 1991;9:621. 23. ASTM F1714-96(2002) Standard guide for gravimetric wear assessment of prosthetic hip-designs in simulator devices. 2002. 24. Harrell FE. Regression modeling strategies: with applications to linear models, logistic regression, and survival analysis. New York: Springer-Verlag; 2001. 25. Cooper JR, Dowson D, Fisher J. Macroscopic and microscopic wear mechanisms in ultra-high molecular weight polyethylene. Wear 1993;162-164(Part 1):378. 26. Schroder DT, Kelly NH, Wright TM, et al. Retrieved highly crosslinked UHMWPE acetabular liners have similar wear damage as conventional UHMWPE. Clin Orthop Relat Res 2011;469:387. 27. Bauer TW, Ming J, D'Antonio JA, et al. Abrasive three-body wear of polyethylene caused by broken multifilament cables of a total hip prosthesis. A report of these cases. J Bone Joint Surg Am 1996;78:1244.
Conventional vs Highly Cross-Linked Polyethylene Liners Heiner et al 28. Que L, Topoleski LD, Parks NL. Surface roughness of retrieved CoCrMo alloy femoral components from PCA artificial total knee joints. J Biomed Mater Res 2000;53:111. 29. Raimondi MT, Vena P, Pietrabissa R. Quantitative evaluation of the prosthetic head damage induced by microscopic third-body particles in total hip replacement. J Biomed Mater Res 2001;58:436. 30. Mimnaugh KD, Yao JQ, Laurent MP, et al. The effect of entrapped bone particles on the surface morphology and wear of polyethylene. J Arthroplasty 2009;24:303. 31. Muratoglu OK, Ruberti J, Melotti S, et al. Optical analysis of surface changes on early retrievals of highly cross-linked and conventional polyethylene tibial inserts. J Arthroplasty 2003;18(7 Suppl 1):42.
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32. Lazzarini AM, Cottrell JM, Padgett DE, et al. Remelting of highly cross-linked polyethylene worn under laboratory conditions. Clin Orthop Relat Res 2007;465:128. 33. Muratoglu OK, Wannomae K, Christensen S, et al. Ex vivo wear of conventional and cross-linked polyethylene acetabular liners. Clin Orthop Relat Res 2005;438:158. 34. Medicines and Healthcare products Regulatory Agency. Medical Device Alert: All metal-on-metal (MoM) hip replacements (MDA/2010/033). 2010. 35. Crawford R, Ranawat CS, Rothman RH. Metal on metal: is it worth the risk? J Arthroplasty 2010;25:1. 36. http://www.fda.gov/MedicalDevices/Productsand MedicalProcedures/ImplantsandProsthetics/Metalon MetalHipImplants/ucm241769.htm. May 9, 2011.