The Journal of Bone & Joint Surgery

The Journal of Bone & Joint Surgery, Volume 86, Issue 6

Scientific Articles | June 01, 2004

Wear and Surface Cracking in Early Retrieved Highly Cross-linked Polyethylene Acetabular Liners

Letitia Bradford, MD¹; David A. Baker, PhD²; Jove Graham, PhD²; Arun Chawan, MS²; Michael D. Ries, MD¹; Lisa A. Pruitt, PhD²

¹ Department of Orthopaedics, 500 Parnassus Avenue, MU-320 West, University of California, San Francisco, San Francisco, CA 94143. E-mail address for L. Bradford: letitiabradford@yahoo.com
² Department of Mechanical Engineering-Bioengineering, 2121 Etcheverry Hall, University of California, Berkeley, Berkeley, CA 94720

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The authors did not receive grants or outside funding in support of their research or preparation of this manuscript. They did not receive payments or other benefits or a commitment or agreement to provide such benefits from a commercial entity. No commercial entity paid or directed, or agreed to pay or direct, any benefits to any research fund, foundation, educational institution, or other charitable or nonprofit organization with which the authors are affiliated or associated.

Investigation performed at the Department of Mechanical Engineering-Bioengineering, University of California, Berkeley, Berkeley, California

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J Bone Joint Surg Am, 2004 Jun 01;86(6):1271-1282

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Abstract

Background: A higher degree of cross-linking has been shown to improve the tribological properties of ultra-high molecular weight polyethylene in laboratory studies; however, its effect on in vivo behavior has not been well established. We investigated in vivo wear mechanisms in retrieved highly cross-linked polyethylene acetabular liners in order to determine if early in vivo wear behavior is accurately predicted by hip-simulator studies.

Methods: A total of twenty-four liners (twenty-one explanted and one unimplanted highly cross-linked liners and two explanted ethylene-oxide-sterilized non-cross-linked liners) were examined for this study. The average age of the patients was 59.9 years, and the average time in vivo was 10.1 months. Articular surface damage on the front and back sides of the liners was assessed with an optical scoring system. Surface quadrants were assigned a grade from 0 to 3 according to the observed wear mechanisms and the percentage of surface affected. The micromechanisms of liner damage were evaluated with use of scanning electron microscopy.

Results: The average front and back-side explant damage scores were 11 (range, 2 to 26.5) and 6.7 (range, 3.7 to 13.3), respectively. There was consistent evidence of early surface deformation and cracking. All explants exhibited some form of surface change, including surface cracking, abrasion, pitting, or scratching. The original machining marks on the liner surface were observed to be either unaltered, drastically distorted, or absent.

Conclusions: Highly cross-linked ultra-high molecular weight polyethylene acetabular liners that were retrieved at an average of ten months after implantation exhibited signs of surface damage that had not been predicted by in vitro hip-simulator studies. These devices had not failed clinically as a result of wear. The discrepancy between in vitro and in vivo wear surfaces may be due to variability in terms of in vivo lubrication and cyclic loading or may represent early surface damage mechanisms that are not well demonstrated by long-term simulator studies.

Figures in this Article

There is currently a great deal of interest regarding the use of highly cross-linked ultra-high molecular weight as an articulating surface for total hip replacements. Recent simulator studies have indicated that the Tpolyethylene wear of ultra-high molecular weight polyethylene is substantially reduced by increased cross-linking^1-8. Durasul (Sulzer Orthopedics, Austin, Texas) is a highly cross-linked ultra-high molecular weight polyethylene that is used for total hip replacements. High-magnification photomicrographs that have been made after simulator testing of these highly cross-linked liners have demonstrated preservation of surface machining marks. In laboratory studies, these liners have shown "no measurable wear" after five million and twenty million cycles (simulating five and twenty years of in vivo use)⁹. Although cross-linking has been shown to improve the tribological properties¹⁰ of ultra-high molecular weight polyethylene in laboratory tests, the in vivo behavior of this material has not been studied extensively.

Many manufacturers of hip arthroplasty devices have sought a high degree of cross-linking in order to minimize wear. The higher degree of cross-linking causes a restriction of chain mobility in the amorphous region of the polymer and results in limited plasticity in the polyethylene. This decreased tendency toward plastic damage is believed to be the primary mechanism for improving the wear resistance of ultra-high molecular weight polyethylene. It is thought that cross-linking mitigates texture evolution and cyclic softening of the polymer¹¹. However, this lack of plasticity could be detrimental to other mechanical properties such as toughness, ductility, and fatigue resistance. Thus, the fatigue resistance of the cross-linked polymer under high cyclic stresses remains an area of concern.

The reliability of simulator wear-testing depends on the correlation of in vitro data with in vivo performance. In vivo wear behavior can be assessed radiographically and clinically as well as with the examination of clinically retrieved components. Retrieved components typically are examined following clinical failure of the bearing surface. Although clinical reports on the use of highly cross-linked ultra-high molecular weight polyethylene are quite limited, a series of highly cross-linked ultra-high molecular weight polyethylene liners have been retrieved. These liners were removed when early symptomatic loosening of the acetabular component developed because of a manufacturing defect of the metal shells, resulting in the recall of the devices¹². The purpose of the present study was to examine the behavior of highly cross-linked ultra-high molecular weight polyethylene after in vivo use and to determine if early in vivo wear mechanisms correlate with predictions based on in vitro wear-testing.

Materials and Methods

Materials and Methods | Results | Discussion | Acknowledgements | References

Retrieval of Components

Atotal of twenty-four liners were included in the present study. Twenty-one highly cross-linked ultra-high molecular weight polyethylene liners (Durasul; Sulzer Orthopedics) were explanted from twenty patients at the time of revision total hip arthroplasty. One patient had had a bilateral total hip arthroplasty, and therefore two explanted liners from this patient were included in the study. A single surgeon implanted and removed all of the highly cross-linked devices. All explanted specimens were removed in conjunction with the 2000 Sulzer Orthopedics Manufacturing Implant Device Recall¹³. The devices were removed as a result of symptomatic loosening of the acetabular component due to contamination of the acetabular shells with machining oil. Three liners that were not part of the Sulzer recall were utilized as controls for the retrieval study. One of the controls was a nonimplanted highly cross-linked acetabular liner (Durasul), and the other two were non-cross-linked ethylene-oxide-sterilized liners (Reflection; Smith and Nephew, Memphis, Tennessee) that had been implanted for eight and ten months. The former liner served as a non-implanted control for the highly cross-linked liners, and the latter two liners served as non-cross-linked controls with similar in vivo times. Both of the non-cross-linked control devices were removed for the treatment of recurrent dislocations. In all cases, a zirconia femoral head counterface was used. All explanted liners had an offset lip that was utilized as a reference point for the orientation of the liners. The study group included thirteen women and nine men. The average age of the patients was 63.4 years (range, thirty-eight to eighty-eight years). The retrieved implants had been in situ for an average of 10.1 months (range, two to twenty-four months).

Analysis of Retrieved Components

The liners were evaluated by gross examination and were photographed with use of a Sony Cyber-shot digital camera (Sony, Tokyo, Japan) with a 2.1-megapixel resolution and a 2.5-times zoom. Particular emphasis was placed on identifying common damage modes that are normally observed in association with retrieved hip and knee prostheses, including pitting, scratching, burnishing, abrasion, embedded particles, and permanent plastic deformation. Pitting was defined as a small depressed circular area on the surface of the device, measuring <1 mm in diameter, where material had been removed. Scratching was identified as unidirectional lines that had been cut into the polyethylene. Burnishing was characterized by shallow multidirectional swirled lines on the articular surface, resulting in a highly polished appearance. Abrasion was characterized by areas of roughened texture on the articular surface due to repeated rubbing against a counterface. Permanent plastic deformation was defined as areas of the component, particularly at the edges, where the polyethylene showed signs of flow and was deformed from its original shape. Particles of cement or metal that had become embedded in the polyethylene also were identified. Iatrogenic damage due to the removal process was identified by the observers and was excluded from the calculation of the overall surface-damage scores.

The front and back sides of each liner were evaluated with use of a qualitative optical scoring system by three separate observers, including two of the authors (J.G. and A.C.). None of the observers had any prior knowledge of the patient-related data. Figure 1 illustrates the standard position of reference. The offset lip of the liner was placed to the left, and the quadrants were labeled in a clockwise fashion beginning in the upper left-hand corner. The quadrants of the retrieved acetabular cups were graded according to the types of wear described above and the gross estimate of the percentage of the articulating area that was affected¹⁴. Grade 0 indicated that the type of wear was absent in a particular quadrant, grade 1 indicated that <10% of the surface area was affected, grade 2 indicated that 10% to 50% of the surface area was affected, and grade 3 indicated that >50% of the surface area was affected. The back side of each cup was divided in a similar fashion and was examined for evidence of surface damage with use of the same procedure.

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Fig. 1: Scoring diagram of an acetabular liner. The offset lip was placed on the left, corresponding with Quadrants I and IV.

The maximum possible surface-damage score for one quadrant was 18, representing a grade of 3 for each of the six different types of wear. The damage scores for each quadrant were then combined to give an overall score; the maximum possible score was 72. The scores that had been assigned to the front and back sides of each liner by all three observers were averaged to create an average surface-damage score for each side. The zirconia ceramic heads corresponding with the most damaged liners were examined with scanning electron microscopy to assess the counterfaces of these liners.

Scanning Electron Microscopy

After a score had been assigned, the specimens were evaluated with use of a scanning electron microscope (JSM 6300; JEOL, Akishima, Japan). The specimens were cleaned ultrasonically in a methyl alcohol bath, followed by the application of 20 nm of gold or gold-palladium to aid in surface conduction during scanning electron microscopy. Each liner quadrant was thoroughly examined with the scanning electron microscope. The surface morphology was documented with both low and high-magnification representative photomicrographs. An average of 7.5 photomicrographs (range, one to twenty photomicrographs; median, 6.5 photomicrographs) were made for each liner. Attention was focused on areas of scratching, pitting, abrasion, and burnishing. These images were then correlated with the previously assessed damage scores.

Results

Materials and Methods | Results | Discussion | Acknowledgements | References

Analysis of Retrieved Components

All of the retrieved components demonstrated evidence of surface cracking, abrasion, scratching, or pitting on the articulating surfaces. The average front and back-side explant damage scores were 11 (range, 2 to 26.5) and 6.7 (range, 3.7 to 13.3), respectively. A two-tailed matched-pair t test was performed between pairs of the three independent observers. A significant difference in damage scores was seen when one pair of observers was compared (p < 0.002) but not when the other two pairs were compared (p > 0.05 and p > 0.28). The average scores for both the front and the back of each explanted liner are given in Table I. All of the liners exhibited the features in question. More specifically, nineteen (79%) showed evidence of pitting, seventeen (71%) showed evidence of abrasion, twenty-three (96%) showed evidence of scratching, two (8%) showed evidence of deformation, sixteen (67%) showed evidence of surface cracking, and one (4%) showed delamination. None of the devices exhibited burnishing or had embedded debris on the front side. Some liners had evidence of embedded debris on the back side.

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TABLE I Data on Explanted Acetabular Devices

				Average Wear Score
Case	Gender, Age (yr)	Involved Side	Duration In Vivo (mo)	Front	Back
1	F, 65	L	10	24	13.3
2	F, 82	L	10	10	8
3	M, 46	R	14	9	7.3
4	F, 74	R	5	12.5	6
5	F, 52	L	7	7	7.3
6*	F, 44	L	12	12.5	6.3
7*	F, 38	R	5	5.5	4
8	F, 38	L	7	12	6
9	M, 59	R	7	26.5	5
10	F, 43	L	16	7.5	4.6
11	M, 88	R	2	12	10.3
12	F, 62	L	24	2	5
13	F, 84	R	7	9	8.7
14	F, 72	L	9	21	5.7
15	M, 55	R	5	16.5	6.7
16	M, 43	R	18	17.5	6.7
17	F, 85	L	5	6.5	5.3
18	F, 58	R	8	6.5	10.7
19	M, 81	L	17	6	6.3
20	M, 51	R	3	13	8
21	F, 45	L	23	3	4
22†	M, 66	L	10	4.5	4.3
23†	M, 47	L	8	9	3.7

One patient with bilateral total hip arthroplasty. †Two control ethylene-oxide-sterilized cups.

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TABLE I Data on Explanted Acetabular Devices

				Average Wear Score
Case	Gender, Age (yr)	Involved Side	Duration In Vivo (mo)	Front	Back
1	F, 65	L	10	24	13.3
2	F, 82	L	10	10	8
3	M, 46	R	14	9	7.3
4	F, 74	R	5	12.5	6
5	F, 52	L	7	7	7.3
6*	F, 44	L	12	12.5	6.3
7*	F, 38	R	5	5.5	4
8	F, 38	L	7	12	6
9	M, 59	R	7	26.5	5
10	F, 43	L	16	7.5	4.6
11	M, 88	R	2	12	10.3
12	F, 62	L	24	2	5
13	F, 84	R	7	9	8.7
14	F, 72	L	9	21	5.7
15	M, 55	R	5	16.5	6.7
16	M, 43	R	18	17.5	6.7
17	F, 85	L	5	6.5	5.3
18	F, 58	R	8	6.5	10.7
19	M, 81	L	17	6	6.3
20	M, 51	R	3	13	8
21	F, 45	L	23	3	4
22†	M, 66	L	10	4.5	4.3
23†	M, 47	L	8	9	3.7

One patient with bilateral total hip arthroplasty. †Two control ethylene-oxide-sterilized cups.

Figure 2 is a low-magnification digital photomicrograph of a highly cross-linked liner (Case 1) that was retrieved after ten months in situ, demonstrating distortion of machining marks. Figure 3 represents a different liner (Case 3) that was retrieved after fourteen months in situ, demonstrating distortion or absence of machining marks in some areas. The average damage score for the front side of this liner was 9. Figures 4-A and 4-B illustrate the surfaces of a non-cross-linked liner (Case 23) that was retrieved after eight months in situ, demonstrating the presence of smooth abrasive scratches and evidence of ductile behavior.

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Fig. 2: Case 1. Low-magnification digital image of a highly cross-linked liner that was retrieved after ten months in situ, showing visible damage (×2.5).

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Fig. 3: Case 3. High-magnification image of a highly cross-linked liner that was retrieved after fourteen months in situ, illustrating distorted machining marks.

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Figs. 4-A and 4-B: Case 23. Fig. 4-A Low-magnification image of the worn area of an ethylene-oxide-sterilized acetabular liner that was retrieved after eight months in situ.

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Fig. 4-B: Higher-magnification image of the same specimen, demonstrating abrasive wear and ductile behavior.

Scanning electron photomicrographs demonstrated a consistent pattern of surface damage and cracking in all of the retrieved highly cross-linked liners. Figures 5-A, 5-B, 5-C, 6-A, 6-B, 7-A, 7-B, 8-A and 8-B depict a variety of damage modes that were observed in these devices. A consistent observation was the evolution of the machining marks. Figures 5-A, 5-B and 5-C illustrate the distortion of the initial machining marks on a liner (Case 2) that was retrieved after ten months in situ. The average damage score for the front side of this liner was 10. Figure 5-A shows the interface of normal and altered machining marks; note the transition between the two areas. Figures 5-B and 5-C illustrate surface cracking parallel to and perpendicular to the machining marks under low and high-power magnification. The machining marks typically were deformed. In some cases, small microcracks appeared perpendicular to the machining marks.

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Figs. 5-A, 5-B, and 5-C: Case 2. Fig. 5-A Low-magnification image of a highly cross-linked liner that was retrieved after ten months in situ, demonstrating the interface of normal and abnormal machining marks.

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Fig. 5-B: Low-magnification image showing the evolution of altered machining marks and surface cracking parallel to the marks.

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Fig. 5-C: Higher-magnification image illustrating cracking perpendicular to the machining marks.

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Figs. 6-A and 6-B: Case 9. Fig. 6-A Low-magnification image of a highly cross-linked liner that was retrieved after seven months in situ, demonstrating surface cracking along the articulating surface. The white arrows indicate cracks parallel to the machining marks, and the black arrows indicate cracks perpendicular to the machining marks.

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Fig. 6-B: High-magnification image illustrating pitting and surface tearing. The arrows point to de novo cracks that developed independent of the machining marks.

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Figs. 7-A and 7-B: Case 10. Fig. 7-A Low-magnification image of a highly cross-linked liner that was retrieved after sixteen months in situ, demonstrating an abraded region with obliteration of the machining marks.

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Fig. 7-B: Higher-magnification image of the abraded region, consistent with a high-density crystalline surface.

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Figs. 8-A and 8-B: Case 17. Fig. 8-A Low-magnification image of a highly cross-linked liner that was retrieved after five months in situ, demonstrating surface scratching without evidence of ductile behavior.

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Fig. 8-B: Higher-magnification image showing polyethylene scratching with extruded material.

Many explants exhibited a range of complex surface-damage mechanisms. Figures 6-A and 6-B show evidence of surface cracking, delamination, and pitting on a liner (Case 9) that was retrieved after seven months in situ. The average damage score for the front side of this liner was 26.5. The arrows in Figure 6-B indicate de novo cracks or surface tears that evolved completely separate from the machining marks. In comparison with those in Figure 5-C, these cracks did not initiate from the edges of the machining marks but rather developed away from the machining marks.

Several explants demonstrated unusual features in addition to the damage modes described previously. For example, Figures 7-A and 7-B illustrate the different and unusual morphological features of a liner (Case 10) that was retrieved after sixteen months in situ. The average damage score for the front side of this liner was 7.5. Figure 7-A is a low-magnification scanning electron microscopic image showing a central abraded region with obliteration of the machining marks. Figure 7-B is a higher-magnification view of the abraded area, demonstrating the crystalline appearance of the surface. Figures 8-A and 8-B illustrate the scratches on a liner (Case 17) that was retrieved after five months in situ. The average damage score for the front side of this liner was 6.5. This scratching mechanism was typical of that observed on several of the explants. However, unlike the non-cross-linked surfaces shown in Figures 4-A and 4-B, the surface adjacent to these scratches exhibits very little ductile behavior, resulting in a more sharply abraded appearance.

Discussion

Materials and Methods | Results | Discussion | Acknowledgements | References

The degree of cross-linking in ultra-high molecular weight polyethylene is achieved by controlling the dose of radiation to which the polymer is exposed under specific processing conditions. In general, highly cross-linked ultra-high molecular weight polyethylene is attained with use of a minimum of 50 to 100 kGy (5 to 10 Mrad) of gamma¹⁵ or electron-beam¹⁶ radiation. Irradiation typically is performed below the melting temperature of the polymer and is then coupled with a secondary annealing treatment to neutralize any remaining free radicals produced by irradiation. Cross-linking is known to change the molecular structure of the polyethylene; for example, some cross-linked ultra-high molecular weight polyethylene materials have a decreased crystallinity (approximately 20% to 30%) as compared with that of standard 1050 resins (approximately 50%)¹⁷. Cross-linking also can be achieved by means of peroxy-based chemistry with use of a minimum of 0.3% to 0.5% peroxide¹, but questions regarding the long-term stability of this material have limited its application as an orthopaedic implant.

Laboratory testing has indicated that these cross-linked materials have inferior resistance to crack propagation¹⁸. Fatigue is a clinically relevant problem for orthopaedic devices because of the cyclic nature of contact stresses at the articulating surface^19,20. Although fatigue wear mechanisms such as delamination and pitting are usually noted in association with total knee arthroplasty, they also may contribute to fatigue of modular acetabular components.

All of the explants in the present study displayed a combination of surface cracking, abrasions, pitting, and scratching. These findings are clearly different from those observed on the surfaces of highly cross-linked liners that have been tested in hip-simulator studies. Hip-simulator studies have demonstrated only occasional scratches on the surface of the acetabular liners, with maintenance of the original machining marks after long-term simulated use⁹. It would be expected that wear occurring in the absence of third-body debris would produce a visually smooth implant surface²¹. At the outset of the present study, one of the striking features of these retrieved devices was the deformation of the articular surface that was visible to the naked eye. Another striking feature was the average damage score for the retrieved specimens. It was higher than expected, considering that the liners had only been in clinical use for an average of 10.1 months. A number of the features of these retrieved implants did not appear to correlate with the predicted behavior of cross-linked and non-cross-linked ultra-high molecular weight polyethylene as reported in previous laboratory simulator studies⁹.

The distortion of the machining marks appears to be consistent with local cold flow or adhesive wear mechanisms. However, surface cracks or tears may be produced as a result of the decreased ductility and fatigue resistance associated with extensive cross-linking. In addition, these early in vivo wear mechanisms may produce small amounts of particulate debris. As previously mentioned, hip-simulator studies have demonstrated a substantial reduction in both the size and the number of wear particles generated by highly cross-linked polyethylene as compared with non-cross-linked polyethylene²². The observation of surface cracks or tears that develop during early in vivo use also may indicate that further subsurface cracking and fatigue wear mechanisms could develop with longer in vivo cyclic loading. As the machining marks transition into the bulk liner, a location for high stress concentrations and the initiation of cracking sites may be provided. This theory of the evolution of stress concentrations during articulation appears to correlate with the surface features seen in these retrieved implants. The cracking visualized in these devices initiated both from the edges of the original machining marks and separate from the machining marks. This surface cracking also suggests that there were high local contact stresses, possibly coupled with initial flaws at the articulating surface. Although the surfaces generally did not exhibit a large degree of ductility or cold flow, as has been seen in previous retrieval studies of non-cross-linked components^23,24, localized plasticity (as evidenced by areas of deformation of the machining marks) was observed in some instances.

Features similar to those observed in the present study were previously described by Rostoker²⁵, who believed that the described features represented folding of the polyethylene; however, it is important to note that the responsible micromechanisms in the present study appeared to be different from those in the earlier work. The surfaces in the present study demonstrated an increased jaggedness and appeared to be the precursors for a brittle fracture mode representing the evolutionary onset of surface damage.

The multidirectional abrasive scratching that was observed in the retrieved implants was consistent with the multidirectional wear patterns that are seen in hip-joint simulator studies and that are expected in the hip. Bragdon et al. showed increased wear of ultra-high molecular weight polyethylene when multidirectional motion was tested in hip-simulator studies²⁶. Fibrillar rearrangement along the direction of sliding was reported in a study of ultra-high molecular weight polyethylene liners that were subjected to repeated abrasion²⁷. These fibrils were thought to have an increased brittleness at the surface layer, leading to an increased susceptibility to surface fracture. Such scratches may deform the surface of cross-linked materials less than that of non-cross-linked materials. As seen in Figures 8-A and 8-B, little of the partially extruded material appears along the scratch edges, which could potentially liberate smaller particles.

Figures 4-A and 4-B show a non-cross-linked ethylene-oxide-sterilized liner with a smearing effect of the original machining marks representing the cold flow phenomenon. Cold flow was described by Chang et al. as a surface phenomenon that causes deformation of the material under the influence of shear forces in the absence of increased temperatures²⁸. Because the machining marks can flow or be rubbed down to a smooth surface over time, they do not crack or break off²³. While plastic deformation has been shown to be responsible for texture developments associated with wear in non-cross-linked ultra-high molecular weight polyethylene, deformation also contributes to resistance to crack growth. Furthermore, this plasticity may play a critical role in cold flow and the lack of surface cracking in ethylene-oxide-sterilized (non-cross-linked) explants.

Pitting has long been associated with fatigue damage and commonly has been observed in retrieved implants sterilized with gamma radiation in air, especially tibial plateaus^20,29. Pitting was observed in many of the retrieved implants in the present study, suggesting that fatigue wear mechanisms may have been active in these devices. Additionally, some specimens revealed evolution of the machining marks that was suggestive of fatigue crack growth striations (Figs. 5-B and 5-C).

Previous hip-simulator studies have demonstrated no loss of machining marks in vitro. However, the retrieved liners in the present in vivo study demonstrated a marked distortion of the machining marks. Similar modes of surface damage are not seen in simulator studies, which indicates that the simulators may not accurately predict the in vivo wear mechanisms of electron-beam highly cross-linked ultra-high molecular weight polyethylene. It is difficult to attribute these findings to a geographical or patient-specific problem because similar findings have been reported by other investigators³⁰. The discrepancy between in vitro and in vivo findings may be due to the variability of in vivo lubrication and cyclic loading or may represent early wear mechanisms that are not well represented by long-term in vitro simulator studies.

Different methods are used to produce highly cross-linked ultra-high molecular weight polyethylenes, and the retrieved implants in the present study represent only a subgroup of materials with similar, but not identical, characteristics. The results of the present study reflect early in vivo findings, which may not necessarily correlate with long-term wear behavior or with the in vivo behavior of highly cross-linked polyethylene liners produced by other methods. The present study suggests that the laboratory conditions that are currently utilized do not fully represent in vivo conditions. It is also important to note that the liners in the present study had not failed clinically as a result of wear. However, further modifications in simulator protocols may help to more accurately predict early in vivo wear behavior.

Acknowledgements

Materials and Methods | Results | Discussion | Acknowledgements | References

Note: The authors thank Meera Sankaran and Maneka Sinha for assistance with retrieval damage scoring, Ron Wilson for assistance with the scanning electron microscopy, and Dr. Harold Reynolds for providing the retrievals.

References

Materials and Methods | Results | Discussion | Acknowledgements | References

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Topics

ethylenes ; hip region ; oxides ; polyethylene ; magnification ; abrasion ; simulators ; ultra-high molecular weight polyethylene

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Letitia Bradford, David A. Baker, Jove Graham, Arun Chawan, Michael D. Ries, Lisa A. Pruitt; Wear and Surface Cracking in Early Retrieved Highly Cross-linked Polyethylene Acetabular Liners. The Journal of Bone & Joint Surgery. 2004 Jun;86(6):1271-1282.

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