Ultra high molecular weight polyethylene: From charnley to cross-linked

ULTRA HIGH MOLECULAR WEIGHT POLYETHYLENE: FROM CHARNLEY TO CROSS-LINKED STEPHEN LI, PhD

Ultra high molecular weight polyethylene (UHMWPE) has been in use since 1962 as the bearing in total hip replacements. However, during the last 39 years, there have been many attempts to improve the performance of this material. These attempts include adding fiber reinforcement, recrystallizing the material to higher crystallinity levels, modifying sterilization methods, modifying methods of manufacture, and, most recently, elevating the level of cross linking. A review of the basic science and clinical results of these changes in UHMWPE is provided. An overview of the newly elevated, introducted, cross-linked materials describing their similarity to and difference from UHMWPE with conventional levels of cross-linking is also given. KEY WORDS: cross-linking, wear, fracture, ultra high molecular weight polyethylene Copyright © 2001 by W.B. Saunders Company

Since 1962, the vast majority of total hip replacements have used ultra high molecular weight polyethylene (UHMWPE) as the bearing material for the acetabular cup. Since 1988, it is the only polymeric material that has been used as the acetabular bearing surface. Devices made of this material can function for over 15 years if well designed and well implanted? -6 However, beginning as early as 1977, there have been concerns over polyethylene wear. 7 It is now accepted that the generation of submicron-sized polyethylene particles is the key factor limiting the long-term function of a total hip replacement. The focus of this chapter is to review the history of the attempts to improve the performance of UHMWPE from carbon-reinforced UHMWPE to the recently introduced versions of elevated cross-linked products.

CONVENTIONAL UHMWPE UHMWPE has been in use since 1968 as an orthopaedic bearing material. The material is generically described as a linear polymer of ethylene with a molecular weight greater than 1 million. All of these UHMWPE resins are synthesized as a white, granular powder with diameters less than 500/~m and average diameters closer to 100/~m. In the early 1990s, there were over 10 different grades of UHMWPE available for use. These grades differed by their molecular weight, location of manufacture, and the addition of calcium stearate to minimize oxidation and improve processing. However, currently the list of available resins is down to 3. The resins are 1020 and 1050 (Ticona, League City, TX) and 1900 (Montel, Wilmington, DE).

From Medical Device Testing and Innovations, Sarasota, FL. Address reprint requests to Stephen Li, PhD, 20 Banff Dr, West Windsor, NJ 08550. Copyright© 2001 by W.B. Saunders Company 1048-6666/01/1104-0007535.00/0 doi: 10.1053/otor.2001.27837

~)88

None of these resins has calcium stearate added. The 1020 and 1050 resins differ by molecular weight: the 1020 resin has a nominal molecular weight of 2 million, whereas the 1050 has a nominal molecular weight of 4 to 5 million. The resins may either be produced in Frankfort, Germany or Houston, Texas by Hoechst/Celanese. Although you cannot tell the source of the resin from this designation, the location of manufacture is reflected in the lot numbers of these products. Lots that begin with 33 are produced in Texas, whereas lots that begin with 31 are produced in Germany. It is not known if there are performance differences between these sources. Montel 1900 is only available to a limited number of manufacturers who have agreed to limit the liability to Montel for use of the resin in medical devices. At this time, only Zimmer Orthopaedics and Biomet Orthopaedics (Warsaw, IN) continue to provide products made with 1900 resin. It should be noted that there are no data or experiences that clearly indicate that the grade of resin alone influences clinical performance. The UHMWPE resins are formed into acetabular components by one of 3 methods: (1) direct compression molding into an acetabular cup; (2) extrusion into bars followed by machining into a cup; and (3) compression molding into sheets followed by machining into a cup. Performance differences between these methods of manufacture will be discussed in a later section.

POLYETHYLENE OXIDATION SHELF AGING In recent years, polyethylene material properties have been implicated as a factor in knee damage. Since the late 1970s, gamma irradiation has been the sterilization method of choice for orthopaedic implants. However, subsequent oxidation adversely affects both the material properties and the quality of the polyethylene. The majority of this postirradiation aging occurs between the time of sterilization and implantation (so-called "shelf aging"). There are three major consequences of oxidation: changes

Operative Techniquesin Orthopaedics,Vol 11, No 4 (October), 2001 : pp 288-295

in the quality of the material, changes in the mechanical physical properties of the material, and changes in wear resistance. Nonconsolidated polyethylene particles and subsurface bands of highly oxidized polyethylene have been found in shelf aged polyethylene devices, s-12

History of P o l y e t h y l e n e Oxidation in T o t a l J o i n t Replacements Since becoming commercially available in the late 1960s, the dominant method for the sterilization of UHMWPE components has been gamma irradiation from a Co 6° source. It has been known for some time that UHMWPE oxidizes after gamma sterilization and that physical properties may be adversely effected. 12-~5 However, these reports were made in an era when polyethylene wear was not considered a significant issue in total joint replacement performance. General interest in polyethylene was minor until 1988, when there was a direct indication that polyethylene debris could lead to bone resorption in mice. 16 In 1990, interest in oxidation, along with other issues, was rekindled as factors that could influence the generation of particulate debris were sought out. 17-2° However, it was not until several years later in 1993 and 1994 that it was reported that, if the degradation is severe enough, the quality of a polyethylene component can be adversely affected, as evidenced by the presence of nonconsolidated particles or subsurface white bands on cross-sectioning. 2~-z~In 1996, some manufacturers either modified the sterilization process to minimize degradation or abandoned gamma sterilization as a method of sterilization. These changes were made to minimize degradation. It should be noted, however, that there are still no FDA restrictions on the use of gamma sterilization for acetabular liners.

Quantifying Oxidation and Its Role in the Performance of Total Hip Replacements Although there was much laboratory research concerning oxidation of UHMWPE between 1990 and 1996, very few of the data were actually linked to clinical performance criteria such as the length of implantation or the wear rate of total hip replacements. Up until 1996, there were no studies that connected oxidation of polyethylene with the actual clinical wear of acetabular liners. This will be discussed below in detail. Postirradiation degradation of polyethylene is indicated by an increase in the density of the polyethylene. Unirradiated UHMWPE typically has density values < 0.94 g/mL. Immediately after irradiation, the density is typically > 0.94 g/mL. The change in density owing to post gamma in air irradiation aging is slow and linear. It gen-

erally takes over 4 years and density values > 0.95 g / m L for the formation of subsurface bands and nonconsolidated particles. Because it is known that oxidative degradation stops when the device is implanted, if a device is implanted less than 4 years after it is sterilized it will be below the 0.95 g / m L level and will perform in the expected manner. 24-26 However, we now know that, if a device should be implanted more than 4 years after it was gamma sterilized, then the adverse quality and physical property changes could arise. In contrast to degradation, cross-linking at the 2.5 to 4 mrad level can be beneficial in providing an increase in the wear resistance of polyethylene. This benefit of cross-linking occurs irmnediately after irradiation and does not require any postirradiation aging. The benefits of gamma irradiation at the 2.5 to 4 mrad level have been demonstrated in several studies. Wang et a127 compared the hip simulator wear of acetabular inserts that had been irradiated and aged for several years with inserts that had been recently irradiated. His results show that irradiated and aged inserts have similar or lower wear rates than unaged inserts. In 3 reports, the wear of gamma irradiated (both in air and in an inert atmosphere) inserts were shown to have 30% to 46% lower hip simulator wear than inserts that were sterilized by ethylene oxide gas. 2s-31 These results are explained by the fact that ethylene oxide gas neither causes degradation nor creates cross-linking of polyethylene. These results demonstrate that the wear benefit of cross-linking is greater than any potential detriment of degradation. It is currently believed that gamma irradiation of UHMWPE is the preferred method of sterilization. Problems of postirradiation aging are limited to decreases in fracture and fatigue resistance and not in decreased wear. These postirradiation issues can be minimized by irradiating components in a low oxygen environment (eg, vacuum, nitrogen, argon). The use of nonirradiation methods such as ethylene oxide gas provide products that will not oxidize due to irradiation but have higher wear rates because of the lack of cross-linking.

A Better Material?: Modifications to UHWMPE Poly II. In the late 1970s, an effort to reduce creep (cold flow) of UHMWPE and decrease wear led to the development of a composite of carbon fibers and UHMWPE. This composite, called Poly [I, was made by directly molding the fibers and polyethylene powder into tibial inserts, patellas, and acetabu]ar components. 32 The addition of carbon fibers increased compressive yield, flexural yield, tensile properties, and creep resistance, as seen in Table 1. Note also that the wear performance, as measured by pin

T A B L E 1. Summary Data for Carbon-Reinforced U H M W P E

% Carbon Fiber

Compressive Yield (MPa)

Flexural Yield (MPa)

Tensile Modulus (GPa)

Wear (:<10 -o g/cycle)

Deformation Under 1,000 psi (6.8 MPa)

0 10 15 20

151 191 200 225

20.9 29.6 31.0 38.7

1.0 1.4 2.0 2.7

38.0 3.8 5.2 4.8

1.14 0.06 0.3 0.3

ULTRA HIGH MOLECULARWEIGHT POLYETHYLENE

289

on disk tests, of the carbon-reinforced materials was also better than the nonreinforced polyethylene.3x-~4 However, these carbon-reinforced materials were found to have lower fatigue resistance compared with UHMWPE and suffered from manufacturing problems associated with incomplete molding, x~ Despite having better resistance to deformation, lower laboratory wear rates, and higher compressive and yield strength, its use was discontinued approximately 7 years after its introduction into the market place)" Hylamer UHMWPE.

Property % Crystallinity

Density Melting point Yield strength Tensile strength Elongation at break Modulus

Creep IZOD

415 GUR

Hylamer M

Hylamer

Units

50 0.934

57 0.946

68 0.955

% g/cc

135 23.3 33.8 339 1.39 2.3 950

147 26.5 37.9 369 2.01 1.2 1169

149

C

28.6 40.7 334 2.52 0.9 1196

MPa MPa % GPa % J/m

In the e a r l y 1990s, a n e w f o r m o f

UHMWPE was introduced that had significantly different physical properties from standard UHMWPE but did not contain any fillers or fibers. The property changes were achieved by controlling the morphology (crystalline structure) of the polymer using unusually high pressures, high temperatures, and very slow cooling rates. Before this development, many investigators had attempted to alter the polymer properties of UHMWPE by altering the material's morphology. This proved difficult because of the long lengths of tile UHMWPE polymer chains. It has now been shown by several investigators that the morphology of polyethylene can be increased without causing degradation using high temperatures (>250 ° C) and pressures (>2,800 atm)) 7-a~ Nonetheless, the goal of these studies was to examine the phenomenon of crystallization and not to establish structure-property relationships or to develop a new material to meet a specific application. The higher crystalline forms of UHMWPE made in this manner were t h o u g h t to exhibit higher strength and modulus, but the methods employed were not commercially feasible because of cost and scale limitations. More recently, a process for producing a range of higher crystalline forms of UHMWPE has been reported) ~,43This process allows the recrystallization of solid UHMWPE bars from 50% crystallinity to levels above 80% without degrading the starting material. Control of processing parameters allowed the generation of materials with a broad range of properties. The process increases elastic modulus values as much as 375'70 over that of conventional UHMWPE. At the same time, tensile yield strengths are increased up to 30%. The disproportionate increase in modulus compared with yield strength suggests that these materials may be at a disadvantage in total joint designs that have high contact and subsurface shear stresses. Processing 415 GUR (Ticona, League City, TX) at a lower pressure (235 MPa) provides a crystallinity increase of approximately 15%. This process has been used to create 2 commercial materials, Hylamer and Hylamer M (DuPont, Wilmington, DE). Both of these materials are recrystallized from 415 GUR and offer an opportunity to examine the effect of crystallinity on physical properties (Table 2)) 2 [t is important to note that the increase in density is caused by recrystallization of the UHMWPE without chain scission or loss of molecular weight. This is a different situation from tile earlier discussions of degradation in which density increased because of loss of molecular weight. As of 1996, there were nearly 100,000 Hylamer acetabular cups implanted but no large, prospective study has

290

T A B L E 2. Effect of Crystallinity on Physical Properties

been published. There are some reports of Hylamer cups that have exhibited higher than expected wear, but it is unclear at this time what the overall performance of the material will be and why these components have performed in this manner)' DOES

HYLAMER

HAVE

HIGH WEAR?

Tile first clinical report regarding Hylamer was a brief communication by Chmell et al in 1996) 2 In this communication, the investigators describe tile performance of Hylamer in 143 of the 193 total hip replacements performed between 1990 and 1992. They reported that 5 of these liners were revised for eccentric wear and another was scheduled for revision. It was noted that the Hylamer cups were used with 7 different femoral component designs. Only 1 of tile 7 femoral component designs was made by DePuy (Warsaw, IN). No explanation for these cases of wear >0.38 m m / y were provided. However, it should be noted that this report was not part of a planned study but the collation of clinical data from 3 surgeons. In 1997, Livingston et a144 produced a full report on this topic. The investigators reported that 391 Hylamer liners had been used in primary total hip replacements between January 1991 and December 1993. Of these 391 cases, 191 were selected based on the criteria of 28 mm diameter size and a femoral component made by either DePuy or Osteonics (Rutherford, NJ). Both cemented and cementless systems were used. Additionally, the results of 50 cases with an Osteonics acetabular and femoral component were used for comparison. The retrieved implants were d M d e d into 4 groups and each group contained both components fixed with bone cement and cementless components. The results for each of these 4 groups are summarized in Table 3. Although the wear rate of the Group 1 cemented Hylamer liners had a wear rate of 0.13 m m / y , the authors conclude that Hylamer liners have average wear rates higher than those of conventional UHMWPE. They conclude that the average wear rate of the Hylamer liner is 0.27 m m / y while the wear rate of conventional UHMWPE is 0.12 m m / y . However, as Schmalzreid et al as pointed out in 1998, the authors clearly did not fully take into account the multifactorial nature of clinical wear. The patients who received Hylamer liners in this study were not randomly selected-- they received Hylamer liners a n d / o r alumina femoral heads because they were categorized as young and active. The patients that received liners made with conventional UHMWPE were generally not young, active patients. Based on the figures described STEPHEN LI

T A B L E 3. Wear Rates From 4 Patient Groups Group

Stem Manufacturer

Femoral Head

Liner

1

DePuy

CoCr

Hylamer

1A

DePuy

Alumina

Hylamer

2

Osteonics

CoCr

Hylamer

3

Osteonics

CoCr

Conventional

Cement

n

Age

Wear Rate (mm/y)

Yes No Yes No Yes No Yes No

26 20 1 6 114 24 38 12

66.5 47.7 44 42.3 66.8 44 70 58

0.13 0.29 0.33 0.33 0.29 0.29 0.12 0.12

Abbreviation: CoCr, cobalt chromium. Data from Livingston et al. ~4

below, a 40-year-old patient would have an average wear rate of approximately 0.4 m m / y regardless of the polyethylene type or design of the liner and metal shell. Figure 1 in Schmalzreid et al's study is a plot of average wear versus average patient age for the 4 different groups represented in the report. 45 The correlation coefficient of the best fit line is 0.563, a poor fit. However, upon closer examination, 3 of the 4 groups (1, 1A, and 3) have acetabular components and femoral components that were supplied by the same manufacturer. The fourth group, Group 2, consisted of Hylamer acetabular liners articulating against an Osteonics femoral head. If the data in Fig 1 that correspond to a Hylamer liner wearing against an Osteonics (non-DePuy) femoral head is ignored, the remaining data lie on a straight line with a correlation coefficient of 0.9995. This strongly suggests that the wear of Hylamer liners against non-DePuy femoral heads differs from that of Hylamer liners and DePuy femoral heads. It also clearly suggests that the wear rate is highly dependent on patient age regardless of the combination of DePuy liner and femoral head. These data illustrate 2 key phenomena. Firstly, the Hylamer was placed in a much younger patient population and the wear rate is closely associated with patient age. Secondly, the association of wear and patient age is extremely linear when both acetabular components and femoral components are supplied by the same manufacturer. In the one group (Group 2), which had a DePuy Hylamer liner but an Osteonics femoral ball and stem, the wear rate was higher (off time line) than that of the other 3 groups that used all DePuy components. In 1998, Sychertz et al 4" presented the results of another clinical comparison of Hylamer and conventional UHMWPE. In this report, the radiographic wear rate of 80 Hylamer liners was compared with the radiographic wear of 140 Enduron liners. Enduron is a DePuy label for conventional UHMWPE. Both the Hylamer and Enduron liners were of the same design and articulated against either CoCr or Alumina heads supplied by DePuy. Although both 28- and 32-mm femoral heads were used, only time data for the 28-mm femoral head (same as the current case) will be discussed here. Time demographics associated with patients from the 2 groups are presented in Table 4. The wear rate at 3.6 years follow-up was lower for the Hylamer group than the conventional polyethylene group. Note that the Hylamer patients were 10 years younger than the Enduron patients and predominantly male. Both of these factors are usually ULTRA HIGH MOLECULAR WEIGHT POLYETHYLENE

associated with high wear rates. Despite these factors, the average wear of the Hylamer cups was less than that of the conventional UHMWPE control group. This is in direct contrast to the Livingston et al results discussed above and further demonstrates that the clinical wear rates of Hylamer are similar or better than conventional UHMWPE, even though the Hylamer components were placed in younger and probably more active patients. The differences in results between Livingston et al's and Sychertz et al's clinical studies is not known. Although it is clear that, at best, Hylamer did not provide any substantial benefit over that of conventional UHMWPE, it can also be said that, based on these reports, there is little evidence that the average wear rates of Hylamer acetabular liners are different from those of conventional UHMWPE when patient weight and activity are taken into consideration. Molded polyethylene.

In 1995, it w a s r e p o r t e d

that the

clinical wear rates of acetabular cups made by direct molding were 50% lower than those of cups made by other methods. The study compared the clinical wear rates of 162 all-polyethylene acetabular cups prepared by direct compression molding with 74 all-polyethylene cups made by machining extruded bar stock. All femoral components were cemented. Patient matching was performed by assigning 54 patients with an average age of 66 years, an average follow-up time of 6.7 years, and an average weight of 161 lbs to each group. The wear rate of the group with compression molded cups was 0.05 m m / y compared with 0.11 m m / y for the group with machined cups. Although time difference was statistically significant (P < 0.05), there were also several cofactors that may have played a role in wear rate. For instance, the machined cup group had Triad stems (Johnson & Johnson, Raynham, MA) and were all implanted by one of the investigators, whereas the molded cup group had TR28 stems (Zimmer, Warsaw, IN) and were also implanted by one of the in-

T A B L E 4. Patient Demographics and Wear

N Average age % Males 28 mm % Ceramic heads Average wear (rag/million cycles)

Hylamer

Enduron

80 54.2 56 56 45 0.15

140 64.4 45 61 16 0.20

NOTE. Data from Sychertz et al. 16

291

TABLE

5, Clinical Wear of Cross-Linked HDPE

Stem

Femoral Ball

Polyethylene

n

Wear Rate (mm/y)

T28 SOM SOM SOM

T28 SOM Ceramic Ceramic

Nonirradiated UHMWPE 100 Mrad HDPE Nonirradiated UHMWPE 100 Mrad HDPE

15 19 71 9

0.25 0.076 0.098 0.072

NOTE. Data from Oonishi et al,49

vestigators. Nonetheless, this study clearly suggested that the polyethylene manufacturing method may influence clinical performance. More recently, this finding was verified in a hip simulation experiment in which direct compression molded polyethylene cups. 47 The molded cups were made from 1900 resin and gamma sterilized in an inert atmosphere. The machined cups were made from Hospital for Special Surgery (HSS) reference UHMWPE (4150) and gamma sterilized in air. The hip simulator wear rates of the molded and machined cups were 14 and 31 mg/million cycles respectively (P < 0.01). This represents a 55% reduction in wear rate. Rasquinha et a148 reported that the average linear head penetration rate for 235 directly molded, all polyethylene, cemented cups at a mean follow-up time of 6 years was 0.075 m m / y . This is 56% lower than the rate of 0.17 m m / y reported previously for the machined, uncemented, metal backed cups of the same design. These 2 clinical reports and the hip simulation data suggest that clinical wear rates may be reduced to less than 0.1 m m / y by the use of direct compression molded cups. As will be discussed later, osteolysis is unlikely for at least 10 years of use. Elevated cross-linked U H M W P E .

Since 1971, i t has b e e n

known that the wear properties of polyethylene are greatly improved with increasing levels of gamma irradiation. Between 1971 and 1978, Oonishi et aP 9 implanted high-density polyethylene acetabular liners that had been irradiated at 100 Mrads. The prosthesis was named SOM after the initials of Shikita, Oonishi, and Mizuho Company. It was made with COP alloy, which contains stainless steel and 20% cobalt. Several clinical studies were conducted to compare the wear of unirradiated versus highly irradiated, high-density polyethylene (HDPE) against stainless steel and ceramic femoral heads. It should be noted that in cases in which the acetabular or femoral

TABLE

Source Total dose Irradiation rate (Mrad/h) Irradiation temperature Post-irradiation melt UHMWPE

components were loosened or migrated, metal-backed components and those not having well-defined radiographs were excluded from evaluation. The results of the included cases are provided in Table 5. The wear rate of the highly cross-linked (100 Mrads) HDPE was less than that of unirradiated UHMWPE. However, the wear rate of the highly cross-linked HDPE was 0.07 m m / y , which is not much lower than the 0.1 m m / y reported for UHMWPE irradiated at 2.5 to 4 Mrads. Furthermore, 0.07 m m / y is substantially higher than the 0 wear rate obtained from irradiating UHMWPE with doses greater than 20 Mrads. The 0.07 m m / y rate means either that irradiating HDPE is not the same as irradiating UHMWPE or that hip simulations underestimate the clinical wear for highly cross-linked UHWMPE. It is difficult to use these results for direct comparison to contemporary versions of elevated cross-linked products because of differences in starting material, femoral ball materials, and designs. In 1996, Wroblewski s° reported the clinical performance of XLP, an UHWMPE that has been cross-linked using a silane-coupling agent. They implanted 19 XLP cups in 17 patients. Wroblewski 5° found that there was a "bedding-in period" in which the femoral head penetration into the liner was between 0.2 and 0.4 mm, which corresponded to an average head penetration rate of 0.29 m m / y . After 2 years, the average wear rate decreased to 0.022 m m / y . This contrasted with the steady state 0.07 m m / y rate of the metal against UHMWPE gamma sterilized between 2.5 to 4 Mrads. The third example of an elevated cross-linked cup was an acetabular cup that was exposed to 10 Mrad irradiation in the presence of acetylene gas. The acetylene gas was used to provide higher levels of cross linking on the surface of the liner. Although over 400 of these devices were implanted in the late 1970s and early 1980s, there has only been complete clinical and radiographic follow-up in 61 cases. Forty-one of these 61 cases showed no detectable wear, whereas the others showed wear averaging 0.10 mm/y. In the past few years, several manufacturers have introduced UHMWPE acetabular liners that have been irradiated above 4 Mrads with both electron beam and gamma irradiation. In most cases, the irradiation is followed by a heating step that quenches the free radicals and inhibits oxidative degradation. This heating step is generally conducted above the melting point of the UHMWPE. As an example, the Marathon elevated cross-linked UHMWPE is produced by irradiating an extruded bar of UHMWPE at

6, Contemporary Highly Cross-Linked U H M W P E

Control

Cross Fire*

Durasul ~

Longevity~

Marathon §

Gamma 2.5-4 0.2 Ambient No Extruded 4150

Gamma 10 0.2 Ambient No Extruded 1050

E-beam 9.5 57 120°C Yes Extruded 1050

E-beam 10 57 50 ° Yes Compression molded 1050

Gamma 5 0.2 Ambient Yes Extruded 1050

"Osteonics, Rutherford, NJ. tSulzer, Austin, TX. tZimmer, Warsaw, IN. §Johnson & Johnson, Warsaw, IN.

292

STEPHEN LI

T A B L E 7. Average Yield Stress Values Dose

Not Heat Treated

TABLE

Melt Heat Treated

Gamma Irradiated 2.5 10 20

23.5 23.4 23.7 Electron Beam Irradiated

20.7 20.6 19.6

2.5 10

23.2 23.8

20.0 20.2

5 Mrads. This bar is then heated above its melting point to quench the free radicals. The irradiated and heat-treated bar is then machined into an acetabular liner, packaged, and sterilized with a gas plasma that does not cause any further cross-linking. The different production details of these elevated cross-linked products are summarized in Table 6. The laboratory hip simulator wear rates of all products irradiated above 5 Mrads is 0. Physical and chemical properties. T h e p h y s i c a l p r o p e r -

ties, modulus, yield, elongation-to-break, creep, and fracture of cross-linked U H M W P E can be significantly changed by irradiation or postirradiaton treatments. A report by Premnath et al presents the changes in properties with one particular cross link treatment, sl Note in Table 7 that, although the yield strength appears to increase slightly with increasing doses of irradation, the postirradiation heat treatment performed to eliminate oxidation reduces the yield strength of the material significantly (P < 0.03).

FRACTURE TOUGHNESS A property that is not often evaluated is the fracture resistance of UHMWPE. A measure of the fracture toughness can be obtained using the J integral method. In this test, the energy required to grow a crack certain distance is determined. Figure 1 shows a plot of selected J integral curves for g a m m a in air sterilized, melt-stabilized samples. All irradiated samples demonstrated a decrease in fracture toughness over unirradiated controls. This indi80

6O 2.5 Mrad

~ 4o

8. Effect of Irradiation Dose on Fracture Toughness

Irradiation Dose

J (kJ/m2)

0 2.5 5 10

45 29 23 16

cates that the a m o u n t of energy required to produce a crack in cross-linked UHMWPE materials can be lower than the a m o u n t of energy to produce the same crack length in a non-cross-linked material. Table 8, which compares the energy required to grow a crack a distance of 0.5 m m as a function of irradiation dose, is based on Figure 1. Loss of fracture toughness also translates into loss of fatigue strength. In a model that simulates rim loading of acetabular liners, the loss of fatigue strength owing to elevated cross-linking and a post-irradiation heat treatment is much greater than file loss of fatigue strength caused by oxidation, s2 In this rim crack model, an acetabular liner is completely supported by the rim. A razor notch is placed u n d e r the rim at the location of maximum stress. If the UHMWPE is gamma sterilized in air at less than 2.5 Mrads, there is no growth of the razor notch even after 1 million cycles at 2.5 kN load. However, if the UHMWPE is acceleratedly aged to the level of a liner that was postirradiation shelf aged for 5 years, the crack growth under the same conditions increases to an average 11 m m / m i l lion cycles. This result corresponds to clinical experience within the ACS liner (Depuy, Warsaw, IN). The ACS liner was recalled after several reports of rim fracture. However, upon closer examination, it was found that the rim fractures only occurred when the UHMWPE was significantly oxidized. In comparison, if the same model is used on liners prepared by irradiation and melt quenching, the resultant crack growth rates are substantially higher (Table 9). This indicates that, under certain conditions, the combination of elevated doses of irradiation and melt quenching may drastically reduce both the fracture and fatigue resistance of UHMWPE. The clinical concern is that lower wear rates for reduced fracture and fatigue resistance will result in more fracture-related failures in acetabular liners. So h o w much w e a r i m p r o v e m e n t do you need? T h e d i f ferent versions of elevated cross-linked UHMWPE mate-

rials introduced in the last few years have reduced laboratory hip simulator wear rates from over 30 rag/million

T A B L E 9. Effect of Cross-Linking Methods and Fatigue Fracture

2O

0.2

0.4

0.6

0.8

Average crack growth (mm)

Fig 1. Effect of irradiation dosage on J integral fracture toughness. ULTRA HIGH MOLECULAR WEIGHT POLYETHYLENE

Treatment

Crack Growth Rate (ram/million cycles)

None Acceleratedly aged to 5-year shelf life 2.5 Mrads 5 Mrads + melt quenching 10 Mrads + melt quenching

0 11 0 150 25,000

293

T A B L E 10. Wear Versus Incidence of Osteolysis Number

Wear Rate

Number Osteolysis

% Osteolysis 100 80 43 0

8

>0.3

8

10 21 9

0.2-0.3 0.1-0.2 <0.1

8 9 9

cycles to nearly zero. However, this reduction, as discussed above, has come at the expense of lower mechanical properties and lower fracture and fatigue resistance. It is reasonable to question how much wear reduction is actually needed to avoid osteolysis. A recent paper by Dowd et al addresses this issue, as Dowd et al reported on the analysis of radiographic wear and osteolysis in 48 primary total hip replacements with at least a 10-year follow-up. The acetabular liners were 32 mm Arthropor design (Joint Medical Products, Raynham, MA) with a CoCr stem. The results of Dowd et al's study are summarized in Table 10. These results indicate that, when the head penetration rate is less than 0.1 r a m / y , there is no osteolysis at 10 years. It is not known if osteolysis will develop at longer implant times. The 0.1 m m / y clinical wear rate corresponds to approximately 15 mg/million cycles on a hip simulator with 28 mm diameter components. This indicates that, if the hip simulator wear rate is less than 15 mg/million cycles, no osteolysis will occur within a 10 year period. This wear rate can be achieved with the use of elevated levels of cross-linking, and, thus, any complications owing to the loss of UHMWPE properties caused by higher irradiation dosages can be avoided or minimized.

CONCLUSION In the last 10 years, many changes have occurred in the manufacturing, sterilization, and posttreatments of the UHMWPE used in acetabular liners. Most of these changes provided a laboratory benefit but the actual benefit to the patients has been, to date. minimal. The only exception appears to be the use of directly molded liners, which appear to provide a clinical wear benefit that was predicted by laboratory data. The recent introduction of elevated cross-linked UHMWPE has traded the benefits of significantly improved laboratory wear for significantly reduced laboratory measured fracture toughness. Only through careful monitoring, study, and follow-up will the overall benefit to the patient be determined.

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