Medical Plastics and Biomaterials Magazine
MPB Article Index

Originally published March 1998

TECHNICAL PAPER SERIES

For more than 30 years, ultra-high-molecular-weight polyethylene (UHMWPE) has been used as a bearing material in total-joint-replacement prostheses.¹ Such orthopedic implants usually comprise a metal (typically a cobalt-chromium alloy) or ceramic component that articulates against a UHMWPE component during in vivo use. It has been well established that the longevity of such implants depends on the wear performance of the UHMWPE components.^1—4 The presence of particulate wear debris of UHMWPE that is generated due to the sliding of UHMWPE components against the metal or ceramic counterface has been linked to complications such as tissue inflammation, bone loss (or osteolysis), and implant loosening.^2—7 Osteolysis resulting from wear of UHMWPE is recognized as the leading problem in orthopedic surgery today. Although UHMWPE has superior wear characteristics compared with those of other polymers,¹ its resistance to wear must be improved for increased lifetime of joint-replacement prostheses.

New formulations of UHMWPE components have been developed in the past, with the goals of reducing creep and wear rates. For example, UHMWPE has been blended with carbon fibers to fabricate total-joint-replacement components known as Poly Two (Zimmer Inc., Warsaw, IN).⁸ However, while the devices manufactured using this blend had excellent resistance to creep, there was a decrease in fatigue resistance. In addition, no improvement in wear resistance was observed,⁸ and the material was ultimately discontinued for use in joint-replacement devices. More recently, high-pressure crystallization was employed to produce UHMWPE components with an increase in mechanical properties such as yield stress and Young's modulus.⁹ However, this material, known as Hylamer (DePuy-Dupont Orthopaedics, Newark, DE), has again not shown any improvement in laboratory wear tests,¹⁰ despite enhanced creep resistance and an increase in resistance to fatigue crack growth.¹¹ Early clinical results also indicate that Hylamer does not demonstrate increased resistance to wear in total-hip-replacement prostheses compared with conventional UHMWPE.^12,13

In recent years, a new approach has been adopted to improve the wear performance of UHMWPE. Instead of using novel processing methods such as high-pressure crystallization or physical blending, UHMWPE components have been modified via chemical methods. Cross-linking of UHMWPE macromolecules has been performed using cross-linking agents such as peroxides,¹⁴ and through gamma^15—17 or electron-beam irradiation.^18,19

The cross-linking of UHMWPE results in an interpenetrating network of high-molecular-weight polyethylene chains, with the potential benefit of increased strength in the interfacial region between resin particles of polyethylene components. Incomplete consolidation of resin particles has been observed in components of UHMWPE, and is believed to contribute to wear.¹ While there have been previous investigations of the effects of cross-linking on polyethylene morphology and mechanical properties,^20,21 these are the first studies that demonstrate the advantage of cross-linking UHMWPE for use in total-hip-replacement prostheses. Laboratory hip-simulator wear tests have shown that there is a decrease in UHMWPE wear rate corresponding to an increase in the degree of cross-linking.^14—19 However, it should be noted that these studies have not yet addressed the resistance of cross-linked UHMWPE to third-body wear, which is abrasion that results from the presence of hard particles that do not originate from either the UHMWPE or the metallic articulating component (e.g., bone chips, bone-cement particles, or debris from the metallic stem).

Although cross-linking of UHMWPE has been shown to improve performance in hip-simulator wear tests, mechanical tests conducted on cross-linked material have shown a reduction in several mechanical properties including Young's modulus, yield stress, ultimate tensile stress and strain-to-break.¹⁹ These results appear contradictory, since it is generally believed that the toughness of a polymer correlates with its wear performance. A better understanding of the relationship between the mechanical properties and wear performance of UHMWPE is required for the development of new wear-resistant polymeric components to be used in total-joint-replacement prostheses. In this study, the mechanical properties of three types of polyethylene with vastly different resistance to wear were compared. The goals were to identify the mechanical behavior and morphology of polyethylene that correspond to increased wear resistance. Because high-density polyethylene (HDPE) is known to have lower resistance to wear compared with UHMWPE,²² we used HDPE, UHMWPE, and cross-linked UHMWPE to identify those mechanical properties that correlate with improved wear performance.

EXPERIMENTAL

Pellets of HDPE (Petrothene LS 606-00) were obtained from the USI Division of Quantum Inc. (Cincinnati). The molecular weight of the HDPE was 55,000, with a polydispersity of M_w/M_n = 4.8 and a melt flow index of 9—11 g/10 min (ASTM D 1238). The pellets were compression-molded at 180°C and 10,000 psi into sheets of 1—2-mm thickness, using a Carver hydraulic press. The sheets were then slowly cooled to room temperature and annealed for 6 hours at 80°C. Molded sheets of GUR 415 resin (Hoechst Celanese, Houston, TX) were obtained from several commercial sources. Each sheet was heated in a vacuum oven to 180°C and slowly cooled to room temperature in order to remove any residual stresses from high pressures imposed during processing and to replace the previously unknown thermal history with a controlled thermal history.

Cross-linking of UHMWPE was performed using the electron-beam irradiation facility at the High-Voltage Research Laboratory of the Massachusetts Institute of Technology. UHMWPE sheets of 2-mm thickness were subjected to doses of 2.5, 5.0, 10.0, and 20.0 Mrd (25, 50, 100, and 200 kGy) at room temperature. The sheets were then placed in a vacuum oven, evacuated, and heated to 180°C for 30 minutes in a nitrogen gas—filled environment to initiate the cross-links and quench the free radicals generated in the polymer by the electron beam. Finally, the sheets were slowly cooled to room temperature.

The HDPE, UHMWPE, and cross-linked UHMWPE sheets were machined into dog bone—shaped tensile specimens (ASTM D 638M-III) using an Omax water-jet cutting machine. Tensile and strain-recovery tests were performed using an Instron-4201 tensile tester, with a crosshead speed of 10 mm/min used for all experiments. Four to six specimens were used for each tensile test to calculate experimental error. In the case of the strain-recovery tests, initial nominal strains of 75, 150, and 225% were imposed on each specimen prior to strain recovery.

Differential scanning calorimetry (DSC) was performed on each specimen using a Perkin-Elmer DSC-7 to obtain the percentage crystallinity for each type of polyethylene. Each sample was heated from room temperature to 170°C, using a heating rate of 10°C/min. A heat-of-fusion value of 293 J/g was used for 100% crystalline polyethylene to calculate the percentage crystallinity in each specimen.

Table I. Degree of crystallinity for samples of HDPE, UHMWPE, and cross-linked UHMWPE.

RESULTS AND DISCUSSION

A combination of DSC, tensile tests, and strain-recovery tests showed that—in addition to differences in wear characteristics—there were discernible differences in the morphology and mechanical properties of HDPE, UHMWPE, and cross-linked UHMWPE. DSC measurements of HDPE, UHMWPE, and cross-linked UHMWPE showed that the degree of crystallinity in UHMWPE was significantly lower than that of HDPE (see Table I). This is attributed to the large number of entanglements present in UHMWPE. Because polyethylene has a high rate of crystallization, the long, entangled chains of UHMWPE do not have sufficient time to disentangle and fold into the growing crystallites, thereby resulting in a larger number of tie molecules and entanglements in the amorphous region between crystallites. There is a concomitant reduction in the number of loose chain folds in the crystallites of UHMWPE compared with those of HDPE (as depicted in Figure 1). It is well known that an increase in the number of tie molecules leads to an increase in toughness in polyethylene.²³ Therefore, the larger number of tie molecules and chain entanglements associated with high-molecular-weight polyethylenes are believed to be the reason for their improved resistance to wear.

Figure 1. A schematic showing the arrangement of macromolecules of polyethylene and the crystallites of HDPE (left), UHMWPE (center), and cross-linked UHMWPE (right).

Cross-linking of UHMWPE led to a further lowering of the degree of crystallinity (see Table I). The reduction in crystallinity from 0-Mrd UHMWPE to 2.5-Mrd UHMWPE was large, followed by a minor reduction in crystallinity with higher doses of radiation (or a higher degree of cross-linking). This suggests that constraints imposed on the crystallizing chains due to the formation of a cross-linked network structure had a larger role in reducing the degree of crystallinity in UHMWPE than did the presence of cross-links themselves.

Figure 2. An engineering stress-versus-nominal-strain plot for UHMWPE subjected to E-beam doses of 0, 2.5, 5.0, 10.0, and 20 Mrd. Each curve has been offset by 10% for clarity.

Tensile tests performed on UHMWPE and cross-linked UHMWPE showed that there was a monotonic reduction in the yield stress, ultimate tensile stress, and strain-to-break with an increase in the degree of cross-linking (see Figure 2). This finding is in agreement with a previous study on the effects of radiation cross-linking on mechanical properties of UHMWPE.¹⁹ The systematic reduction in toughness (area under the stress-strain curve) with increasing degrees of cross-linking suggests that cross-linked UHMWPE should be less resistant to wear. However, hip-simulator tests have shown that there is a monotonic decrease in wear rates with increasing doses of radiation. One plausible explanation for these seemingly contradictory results is that the toughness of UHMWPE—defined by the area under the stress-strain curve—correlates with third-body wear, a mechanism of wear that was not present in the hip-simulator wear tests performed on cross-linked UHMWPE.

Figure 3. An engineering stress-versus-nominal-strain plot for HDPE, UHMWPE (m415), and cross-linked UHMWPE (x415).

To determine the relationship between mechanical behavior, morphology, and wear resistance of polyethylene, we compared the tensile behavior of HDPE, UHMWPE, and 5-Mrd-irradiated, cross-linked UHMWPE, as shown in Figure 3. It can be observed that HDPE, which has the lowest resistance to wear, stretched to higher strains with little or no strain hardening compared with UHMWPE and cross-linked UHMWPE. A similar deformation behavior has been observed in solution-crystallized UHMWPE, although the strain-to-break is substantially higher in solution-crystallized UHMWPE compared with HDPE.²⁴ The primary reason for the similarity between the deformation behavior of solution-crystallized UHMWPE and melt-crystallized, low-molecular-weight HDPE is that, in both cases, there are a lower number of chain entanglements and a higher number of chain folds compared with melt-crystallized, cross-linked UHMWPE.

A study in which UHMWPE was crystallized from solutions of various concentrations showed that the material's strain-to-break was reduced and its strain hardening increased with an increase in polymer concentrations.²⁵ Since higher polymer concentrations contain a larger number of chain entanglements, it was deduced that entanglements were responsible for strain hardening and the reduction in strain-to-break. Based on these previous studies and our observations on HDPE and UHMWPE, it can be concluded that entanglements play an important role in wear resistance and must be increased to improve the wear performance of UHMWPE. The results also suggest that if a polyethylene specimen demonstrates a large degree of strain hardening in a tensile test, it is likely to exhibit high wear resistance as well.

Figure 4. An engineering stress-versus-nominal-strain plot showing strain recovery after a 75% nominal strain imposed in HDPE, UHMWPE, and cross-linked UHMWPE.

It has been observed that the multidirectional motion of metal against UHMWPE leads to higher wear rates in hip-simulator wear tests.²⁶ One theory proposes that the mechanism of wear involves orientation of UHMWPE followed by fracture of the oriented UHMWPE when the wear path changes. The observations led us to hypothesize that the ability of polyethylene to recover imposed strain should be beneficial for wear performance. Strain-recovery tests showed that cross-linked UHMWPE recovered a larger amount of strain compared with UHMWPE (see Figure 4). Also, both of the UHMWPEs recovered a much larger amount of strain than did the HDPE. The strain recovery behavior persisted even at higher imposed strain, as shown in Figure 5, although the ability of HDPE to recover strain fell sharply with increasing applied strains. The strain recovery behavior of the more wear-resistant UHMWPE can also be attributed to the presence of entanglements, which behave as physical cross-links in such high-rate deformation tests. The strain energy stored in this network structure upon application of large, plastic strain leads to recovery upon release of the load. Oriented HDPE, which contains very few chain entanglements, is unable to recover strain to the same extent as does UHMWPE.

Figure 5. A plot of percentage strain recovery versus imposed strain for HDPE, UHMWPE (m415), and 5-Mrd cross-linked UHMWPE (x415).

CONCLUSION

Wear performance of polyethylene can be predicted by its mechanical behavior and morphology. This study indicated that increased amounts of strain hardening and strain recovery correlate with an increase in wear resistance of polyethylene in hip-simulator wear tests. Since a higher number of chain entanglements lead to strain hardening and strain recovery, processing methods that maximize the number of chain entanglements should be used to fabricate polyethylene components that are intended for load-bearing applications. However, it must be noted that whereas a higher number of entanglements can result in improvements in wear performance, the concomitant reduction in crystallinity leads to a reduction in resistance to creep, which may be undesirable for certain applications.

ACKNOWLEDGMENTS

The authors wish to thank Professor Robert E. Cohen of the Massachusetts Institute of Technology and Professor Myron Spector of Harvard Medical School/Brigham & Women's Hospital for their valuable insights on the morphology and wear performance of UHMWPE. Kenneth Wright of the High-Voltage Research Laboratory of the Massachusetts Institute of Technology is gratefully acknowledged for his assistance with the electron-beam irradiation. An earlier version of this paper was presented at the 23rd Annual Meeting of the Society for Biomaterials, held in New Orleans, April 30—May 4, 1997.

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Seema H. Bajaria is a mechanical engineer in the commercial space division of Lockheed Martin Missiles and Space (Sunnyvale, CA). She performed this study at the Massachussetts Institute of Technology, where she obtained a BS in mechanical engineering. Anuj Bellare, PhD, received his doctorate through the program in polymer science and technology at MIT and spent two years as a postdoctoral research associate at Princeton University. He is currently an instructor of orthopedic surgery (biomaterials) at Harvard Medical School and Brigham & Women's Hospital in Boston.

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