7The Polyethylene History 7 7

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Chapter 7 · The Polyethylene History – A. Bellare, M. Spector

7 The Polyethylene History

A. Bellare, M. Spector

Summary

Our understanding of the wear behavior of polyethylene (PE) components has deepened over the past few years as the adverse effects of gamma irradiation (in air) steril- ization have become understood.This understanding has led to methods to improve the wear performance of the polymer using cross-linking. However, it will still be several years before the clinical benefits of new methods of processing PE are clear and the benefit-risk ratio is established.

Introduction

Ultra-high-molecular-weight polyethylene (UHMWPE) is one of the principal materials employed in total knee arthroplasty. While the lubrication and friction of the metal-on-PE articulation provides the low-friction arthroplasty that Sir John Charnley sought,the wear of PE yields particulate debris that potentiates an osteolytic response, which remains a significant problem. The often rapid and extensive destruction of bone attributable to PE wear particles is so dramatic, and so challenges revision arthroplasty, that it has commanded the most attention in recent years.However,the actual incidence of this prob- lem remains somewhat in question. This point notwith- standing, the prevalence of PE wear particle-induced osteolysis is great enough to warrant changes in how the material is processed so as to improve its resistance to wear.Extrinsic factors that contribute to the wear of poly- ethylene are also being addressed: prosthetic designs that reduce stresses in the polymer; prosthetic designs and manufacturing processes that reduce the number of particles released from modular junctions, which can participate in three-body wear of polyethylene; and materials that may allow the production of more scratch- resistant metallic counterfaces.

It is well known that PE components of total joint replacement prostheses undergo processes that produce PE wear debris due to the articulation of the harder metallic component, usually a cobalt-chromium alloy, against the softer PE component. The generation of wear

debris not only damages the surface of the PE component but is also known to elicit a biological response that often results in bone resorption. This bone loss (referred to as osteolysis) can eventually lead to loosening of the pros- thetic device.The location and size of PE particle-induced osteolytic lesions often greatly complicate revision surgery. Work in recent years has focused on processing parameters that serve as the determinants of the resis- tance of PE to wear. The reduction of the amount of wear debris from,and surface damage to,PE would prolong the lifetime of such prostheses.

The objective of this chapter is to review the history of the use of polyethylene in total joint arthroplasty, as a basis for understanding the methods being employed to improve its performance. There are several prior reviews [26, 27] of this subject that can be accessed for useful reference.

Polyethylene Molecular Structure

UHMWPE has a very low frictional coefficient against metal and ceramics and is therefore used as a bearing surface for joint replacement prostheses. Moreover, the wear resistance of UHMWPE is greater than that of other polymers investigated for this application. Low strength and creep, however, present potential problems.

The term polyethylene refers to plastics formed from

the polymerization of ethylene gas. The possibilities for

structural variation of molecules formed by this simple

repeating unit for different molecular weight (e.g., crys-

tallinity, branching, and cross-linking) are so numerous

and dramatic, with such a wide range of attainable prop-

erties, that the term polyethylene refers to a wide array of

materials. The earliest type of polyethylene was made by

reacting ethylene at high (20 000–30 000 pounds per

square inch) pressure and temperatures of 200°–400°C

with oxygen as catalyst. Such material is referred to as

low-density polyethylene. A great amount of polyethylene

is produced now by newer,low-pressure techniques using

aluminum-titanium (Ziegler) catalysts. This is called lin-

ear polyethylene due to the linearity of its molecules, in

contrast to the branched molecules produced by high-

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7 degree of crystallinity attained with the regularly shaped molecules. Typically, there is no great difference in mole- cular weight between the low- and high-density varieties, e.g., 100 000–500 000. However, if the low-pressure process is used to make extremely long molecules, i.e., UHMWPE, the result is remarkably different. This mate- rial, with a molecular weight between 1 and 10 million, is less crystalline and less dense than high-density polyeth- ylene and has exceptional mechanical properties. It is ex- tremely tough and remarkably wear resistant; a 0.357 magnum bullet fired from 25 feet bounces back from a l-inch thick slab of UHMWPE. The material is used in very demanding applications (e.g., ore chutes in mining equipment) and is by far the most successful polymer used in total joint replacements. It far outperforms the various acrylics, fluorocarbons, polyacetals, polyamides, and polyesters which were tried for such purposes.

Processing of Polyethylene

Implants of polyethylene are usually manufactured by the machining of components from bulk stock fabricated from as-synthesized polyethylene powder using mainly ram extrusion or compression molding. These processes involve application of heat and pressure to consolidate the powder into bulk components, followed by machining of the implant components, packaging, and sterilization.

Various grades of UHMWPE resins have been avail- able for orthopedic implant application, primarily from Ruhrchemie AG, which later changed its name to Hoechst, and is currently called Ticona. The early UHMWPE resin used by John Charnley was called RCH- 1000 for (R)uhr (CH)emie. This resin is similar to the current GUR 1020 UHMWPE resin. RCH-1000 was clas- sified as a form of HDPE (high-density polyethylene), which is why earlier papers refer to UHMWPE as HDPE [26]. Later, the orthopedic grade of polyethylene was called CHIRULEN. Since the 1990s, the UHMWPE resin used in implants has been called GUR or (G)ranular (U)HMWPE (R)uhrchemie. Common examples of poly- ethylene resins used today are GUR 1050 and GUR 1020.

The numbers following GUR refer to the following: the first digit refers to approximate or loose density (1); the second digit refers to presence (1) or absence (0) of cal- cium stearate, which has been used as a lubricant to as- sist processing; the third digit refers to molecular weight (2=2 million g/mole and 5=5 million g/mole), and the fourth digit (0) refers to the resin grade. Calcium stearate is no longer added to orthopedic-grade poly- ethylene, since reports showed increased levels of oxi- dation and fusion defects associated with calcium stearate [24, 40, 43, 46]. Another source of UHMWPE

properties compared to the Hoechst resins [45]. How- ever, Hi-Fax is no longer available, and GUR 1050 and GUR 1020 remain the only grades of polyethylene used in orthopedic implants.

The as-synthesized polyethylene resin particles are approximately 100 µm, but can be submicrometer in size as well. The broad size distribution of GUR 4150 (the dig- it “4” refers to the country code, USA, which was the nomenclature used for GUR resins.) powder particles have been measured by Pienkowski et al. [35, 36]. Each powder particle contains 10- 30-µm diameter aggregates comprising approximately 1-µm diameter nodules con- nected to each other by fibrils.Olley et al.[34] have shown that voids or defects remain along the resin boundaries even after the powder is "fully” consolidated into bulk components. The likely reason for the presence of defects is the high viscosity associated with the ultra-high mole- cular weight of the polyethylene that is required for high wear resistance. The incomplete consolidation of high- molecular-weight polyethylene resin compared with low- molecular-weight polyethylenes, however, is not a major concern. Gul et al. showed that there was no correlation between the degree of consolidation of UHMWPE pow- der particles and the rate of generation of particulate wear debris under the processing conditions that they used [23].

The nascent UHMWPE powder contains extended- chain crystals (thick lamellae) as well as thin lamellae [16]. The high melting temperature of 141°C observed us- ing a differential scanning calorimeter suggests that the powder contains mostly extended-chain crystals, as pre- sent in high-pressure crystallized UHMWPE. However, a study utilizing morphological, chemical, and molecular techniques indicated that a dual lamellar structure exist- ed.It is postulated that the fibrils of the polyethylene resin powder contain thick, extended-chain crystalline lamel- lae, while 20-nm thick lamellae (such as those present in bulk components manufactured using molding or ram extrusion) exist in the spherical domains [16].It is unclear why the powder morphology contains fibrils connecting spherical domains (sometimes referred to as the “cauli- flower” morphology). The presence of fibrils in the powder suggests that the as-synthesized powder has UHMWPE macromolecules trapped in a low-entangle- ment, aligned state compared with melt-crystallized UHMWPE. This low entanglement would assist in con- solidation of powder during molding or ram extrusion processes. A highly entangled state would make it harder to consolidate the powder, since it would require the chains from powder particles to disentangle and then re- entangle with the chains of the adjacent powder particles.

The most common processes used to consolidate

polyethylene powder particles into bulk stock are com-

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pression molding into thick sheets and ram extrusion into rods. The final implant is usually machined from the bulk stock. These processes involve compaction of UHMWPE nascent powder at elevated temperatures, above melting temperature. They also utilize pressure to assist in consolidation. The final stage of processing in- volves annealing at elevated temperatures to remove residual stresses associated with processing and to in- crease the crystallinity of the components. Compression molded sheets of GUR 1020 and GUR 1050 UHMWPE resins 2.5–7.5 cm thick are commercially produced.

Ram extrusion is another common process employed to sinter nascent UHMWPE powder into 2.5- to 30-cm diameter rods that are several meters in length.Like com- pression molding, the extrusion processes are also fol- lowed by annealing at elevated temperatures. The bulk UHMWPE rods and sheets are generally uniform except for small spatial variations in anisotropy due to spatially non-uniform crystallization occurring due to the low thermal conductivity of polyethylene [2].

Direct compression molding of tibial and acetabular components has also been performed in some cases. The primary advantage of direct compression molding of im- plants is that the articular surfaces of the joint compo- nents are smooth, lacking machine marks or grooves.

However, by far the common choice for implant manu- facture is machining of compression molded sheets and ram extruded, rod stock of UHMWPE.

The Sterilization Issue

During the 1990s, sterilization of polyethylene compo- nents received much attention as studies began to show that sterilization can degrade the mechanical and wear properties of UHMWPE [4–11, 13, 18–20, 26, 33, 37–39, 41, 42].Until the mid 1990s,the common practice was to pack- age UHMWPE components of total joint replacements in air and thereafter sterilize the package using 25–37 kGy of gamma radiation. It is well known that radiation induces cross-linking, chain scission, and long-term oxidative degradation of polyethylene. In the polymer science field, the effects of ionizing radiation on post-irradiation aging of several types of polyethylene, including pressure-crys- tallized UHMWPE [6], have been studied in great detail, especially by Bhateja et al. [4–9]. Costa and co-workers demonstrated the detailed mechanism of oxidation and have shown that oxidation can also occur in ethylene oxide-sterilized UHMWPE, albeit to a much smaller ex- tent than in gamma radiation-sterilized UHMWPE [14, 17–20]. It is now well established that long-term post-irra- diation aging can have detrimental effects on both the morphology and the mechanical properties of UHMWPE [10, 11, 38, 39]. The effects of post-irradiation aging on TKRs have been well documented [42] in analyses of TKR

retrievals. The vast number of studies on gamma sterili- zation-induced oxidation of UHMWPE have resulted in several reviews that summarize various issues related to sterilization,its chemistry,and its effects on polyethyelene used in joint replacement prostheses [17, 26, 37].

It was originally believed that oxidation was associat- ed primarily with fatigue damage mechanisms such as de- lamination wear,which occurs in TKRs.However,it is now well established that the rate of particulate wear debris generation can also increase due to the molecular weight reduction and embrittlement in both tibial components and acetabular cups [3, 29]. Initially, gamma radiation in- creases resistance to wear debris generation due to the low level of cross-linking that accompanies gamma radiation.

However, with aging, oxidative effects begin to dominate and negate any initial benefits of gamma radiation, lead- ing to higher wear rates than unirradiated UHMWPE.

Orthopedic implant manufacturers have recognized the effect of oxidation on degradation of polyethyelene. Cur- rently, some implant manufacturers sterilize UHMWPE using non-radiation methods, such as ethylene oxide or gas plasma sterilization. Other orthopedic manufacturers have resorted to packaging of components in low oxygen environments, such as vacuum-foil packaging, or packag- ing in nitrogen or argon gas. These methods should de- crease the rate of oxidation during storage. However, it is not yet known whether in vivo oxidation rates would even- tually affect the clinical performance of conventional UHMWPE, packaged in low oxygen environments and then sterilized using gamma radiation.

Modified Forms of Polyethylene

The problems associated with wear of PE components in joint replacement prostheses has prompted work direct- ed toward the development of new forms of PE to improve wear resistance.

One approach used to reduce surface damage and sub-surface crack growth in knee components is through development of new prosthetic designs that increase the contact area between components, thereby reducing stress in PE. Such methods, based on measurements and calculations of contact stress on components, have led to the development of thicker and more conformal PE com- ponents that are expected to reduce catastrophic failure and delamination wear.

Other approaches to reduce wear rates in PE aim at

altering the form of polyethylene through alteration of

the number or size of the crystallites or the molecular

bonding of the molecular chains in the noncrystalline

domains of the polymer. More recent methods that have

been used to realize these goals include: (1) processing

techniques apply high pressures to the polymer, and (2)

the use of cross-linking chemistry.

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7 In the 1980s and early 1990s attempts to improve the per- formance of polyethylene turned to carbon fiber rein- forcement and to heat-pressing. In an effort to reduce creep a fiber reinforced polymer composite was produced by blending carbon fibers with UHMWPE (Poly Two, Zimmer, Warsaw, IN). The composite was directly mold- ed into tibial inserts and patellar components [15]. The material was also used for the fabrication of acetabular cups in total hip replacements [15].As reported in a review of UHMWPE [27], although the Poly Two devices had a significantly higher creep resistance (p<0.03), “they required extraordinary quality-control measures, had lower fatigue resistance compared with ultra-high mole- cular weight polyethylene, and demonstrated no clinical improvement in the rate of wear”.The use of Poly Two was discontinued approximately 7 years after its introduction into the marketplace.

Heat pressing was another method employed in an attempt to improve the tribology of polyethylene. How- ever, several investigations of retrieved heat-pressed tibial inserts [12, 21, 30] demonstrated delamination. In one study [12], 52% of 33 retrieved components showed severe delamination within 4 years of implantation. Light microscopy revealed a surface layer separated from the insert with a clear line of demarcation 250–580 µm below the articulating surface.

High-Pressure Forms of Polyethylene

In the early 1990s, high-pressure crystallization was em- ployed to produce PE components (Hylamer, DuPont, Wilmington, DE) with an increase in mechanical proper- ties such as yield stress and modulus of elasticity [27].

Two forms of high-pressure PE,which vary in certain me- chanical properties (viz., modulus of elasticity), have been introduced into the clinic for total hip and knee arthroplasty.The primary difference in the bulk structure of conventional and high-pressure forms of PE is that in conventional PE the degree of crystallinity is 50%–55%

while it is 68%–75% in the high-pressure form. The high- pressure process facilitates thickening of crystallites in PE from 0.025 µm in conventional processes to approxi- mately 0.2 µm in the high-pressure form.While the high- pressure PE is more resistant to deformation by creep and fatigue crack growth, it has never been shown to have a substantially higher wear resistance in laboratory wear tests.

Recent clinical results indicate that the linear wear rate and consequential incidence of osteolysis and revi- sion rate for the high-pressure form of PE are greater than for conventional PE. Why the clinical wear rate for this

properties, is not entirely clear. It may be that the high- pressure form is less resistant to oxidation than was pre- viously appreciated. If so, implementation of sterilization methods that do not favor oxidation may result in im- proved performance of this form of PE. However, the fact that wear of PE devices is as multifactorial as it is indicates that additional laboratory investigations of high-pressure forms of PE will be required before their wear perfor- mance can be more fully explained.

Cross-Linked Polyethylene

Cross-linking is currently being used in an attempt to improve the wear performance of PE [26, 28, 31, 32, 44].

Cross-linking of PE converts the otherwise linear, high- molecular-weight PE macromolecule into an interpene- trating network structure of polymer chains. This type of molecular structure is often quantitatively characterized by the molecular weight between cross-links (usually referred to as M

_c

), i.e., the higher the M

_c

, the lower the density or degree of cross-linking junctions. Cross-link- ing of PE can be performed using cross-linking agents such as peroxides and silanes, and by the use of gamma or electron beam radiation. Laboratory hip simulator wear tests have shown that there is a decrease in wear rate with an increase in degree of cross-linking of PE. These studies, and the clinical results of a few trials that em- ployed cross-linked PE acetabular cups several years ago, provide compelling evidence that cross-linking can re- duce wear rates in acetabular components.

While cross-linking has been found to improve the performance of PE components in total hip arthroplasty, the trade-off in other mechanical properties has raised questions about the potential problems with such an approach and focused attention on the indications that might best benefit from its use [1, 25].With an increase in cross-linking density comes an undesirable change in mechanical properties such as reduced elongation- to-failure, which occurs largely in the case of radiation cross-linking but to a lesser extent in the case of peroxide cross-linking.Associated with this reduction in the elon- gation-to-break is a reduction in the energy required to propagate a crack and the resistance of cross-linked parts to cyclic loading. It is possible that while cross-linking may improve wear resistance it may place components at greater risk of fracture.

There may be a reduction in the degree of crystallinity

with increasing cross-link density, which may affect

properties such as modulus of elasticity and creep. The

modulus can be expected to decrease with decreasing

crystallinity, but the effects on creep are less certain. In

polymers with little or no crystallinity (namely rubbers),

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increasing cross-link density decreases creep. The effects of cross-linking on the creep of semi-crystalline poly- mers such as UHMWPE is less clear; this can be an im- portant issue because the early radiographic studies of highly cross-linked polyethylene in hip prostheses may principally be measuring creep. Cross-linking a crys- talline polymer may introduce defects and constraints that can decrease the overall crystallinity since the crys- tals have to grow around the defects. It has been shown (for example, comparing Hylamer to conventional UHMWPE) that lower crystallinity results in lower resis- tance to creep deformation. Processing of UHMWPE after cross-linking can be altered to restore the crys- tallinity (e.g., by annealing after the melting). Alterna- tively, the melt can be pressurized so that crystallization occurs under pressure, and increases crystallinity com- pared with melting without any applied pressure. In that case, crystallinity is not lost, and the cross-linking may additionally assist in resisting creep deformation.

Also of interest is the observation using laboratory uniaxial,reciprocating wear tests that cross-linked PE had a larger increase in wear rates compared with non-cross- linked PE when a rougher counterface was used. It is not yet known whether this increased susceptibility to a rough counterface is due to a reduced degree of crys- tallinity in cross-linked PE or to a reduced elongation-to- failure.

Because of the questions related to the change in cer- tain properties with cross-linking, only an intermediate degree of cross-linking (an equivalent radiation dose of 50–100 kGy rather than 200 kGy) is being employed by many, so that there is a balance between increase in wear resistance and reduction in mechanical properties. Some have shown that, although there is a strong dependence of radiation dose (or cross-link density) on wear resis- tance in hip simulator tests, similar doses led to less im- provements in wear rates using a knee simulator.One rea- son for lower sensitivity of cross-linking to wear in the knee simulator is that linear wear tracking does not lead to high volumetric wear rates even for conventional PE, and it is difficult to conclude whether cross-linking is appropriate for knee components using smooth counter- face,linear tracking; some believe that a low level of cross- linking may be beneficial for the knee prosthesis.

Acknowledgements.

M. Spector, Director, Tissue Engi- neering,VA Boston Healthcare System, was supported by the Department of Veterans Affairs, Veterans Health Ad- ministration, Rehabilitation Research and Development Service; M. Spector is a Research Career Scientist.

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