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UHMWPE Biomaterials Handbook
Ultra-High Molecular Weight Polyethylene in Total Joint Replacement and Medical Devices
Academic Press
Copyright © 2009 Elsevier Inc.
All right reserved.
ISBN: 978-0-08-088444-8
Chapter One
A Primer on UHMWPE
Steven M. Kurtz, PhD
1.1 Introduction 1.2 What is a Polymer? 1.3 What is Polyethylene? 1.4 Crystallinity 1.5 Thermal Transitions 1.6 Overview of the Handbook References
1.1 INTRODUCTION
Ultra-high molecular weight polyethylene (UHMWPE) is a unique polymer with outstanding physical and mechanical properties. Most notable are its chemical inertness, lubricity, impact resistance, and abrasion resistance. These characteristics of UHMWPE have been exploited since the 1950s in a wide range of industrial applications (Figure 1.1), including pickers for textile machinery, lining for coal chutes and dump trucks, runners for bottling production lines, as well as bumpers and siding for ships and harbors. Over 90% of the UHMWPE produced in the world is used by industry.
For the past 45 years, UHMWPE has also been used in orthopedics as a bearing material in artificial joints. Each year, about 2 million joint replacement procedures are performed around the world, and the majority of these joint replacements incorporate UHMWPE. Despite the success of these restorative procedures, orthopedic and spine implants have only a finite lifetime. Wear and damage of the UHMWPE components has historically been one of the factors limiting implant longevity. In the past 10 years, highly crosslinked UHMWPE biomaterials have shown dramatic reductions in wear in clinical use around the world. The orthopedic community awaits confirmation that these reductions in wear will be associated with improved long-term survival, as expected.
UHMWPE comes from a family of polymers with a deceptively simple chemical composition, consisting of only hydrogen and carbon. However, the simplicity inherent in its chemical composition belies a more complex hierarchy of organizational structures at the molecular and supermolecular length scales. At a molecular level, the carbon backbone of polyethylene can twist, rotate, and fold into ordered crystalline regions. At a supermolecular level, the UHMWPE consists of powder (also known as resin or flake) that must be consolidated at elevated temperatures and pressures to form a bulk material. Further layers of complexity are introduced by chemical changes that arise in UHMWPE due to radiation sterilization and processing.
The purpose of this Handbook is to explore the complexities inherent in UHMWPE and an increasingly diverse field of UHMWPE biomaterials that include radiation crosslinking, composites, and antioxidants such as Vitamin E. This book is intended to provide the reader with a background in the terminology, history, and recent advances related to its use in orthopedics. A monograph such as this is helpful in several respects. First, it is important that members of the surgical community have access to up-to-date knowledge about the properties of UHMWPE so that this information can be more accurately communicated to their patients. Second, members of the orthopedic research community need access to timely synthesis of the existing literature so that future studies are more effectively planned to fill in existing gaps in our current understanding. Finally, this Handbook may also serve as a resource for university students at both the undergraduate and graduate levels.
This introductory chapter starts with the basics, assuming the reader is not familiar with polymers, let alone polyethylene. The chapter provides basic information about polymers in general, describes the structure and composition of polyethylene, and explains how UHMWPE differs from other polymers (including high density polyethylene [HDPE]) and from other materials (e.g., metals and ceramics). The concepts of crystallinity and thermal transitions are introduced at a basic level. Readers familiar with these basic polymer concepts may want to consider skipping ahead to the next chapter.
1.2 WHAT IS A POLYMER?
The ultra-high molecular weight polyethylene (UHMWPE) used in orthopedic applications is a type of polymer generally classified as a linear homopolymer. Our first task is to explain what is meant by all of these terms. Before proceeding to a definition of UHMWPE, one needs to first understand what constitutes a linear homopolymer.
A polymer is a molecule consisting of many (poly-) parts (-mer) linked together by chemical covalent bonds. The individual parts, or monomer segments, of a polymer can all be the same. In such a case, we have a homopolymer as illustrated in Figure 1.2. If the parts of a polymer are different, it is termed a copolymer. These differences in chemical structure are also illustrated in Figure 1.2, with generic symbols (A, B) for the monomers.
Polymers can be either linear or branched as illustrated in Figure 1.3. The tendency for a polymer to exhibit branching is governed by its synthesis conditions.
Keep in mind that the conceptual models of polymer structure illustrated in Figures 1.2 and 1.3 have been highly simplified. For example, it is possible for a copolymer to have a wide range of substructural elements giving rise to an impressive range of possibilities. In industrial practice, polyethylenes, including UHMWPE, are frequently copolymerized with other monomers (e.g., polypropylene) to achieve improved processing characteristics or to alter the physical and mechanical properties of the polymer. For example, according to ISO 11542, which is the industrial standard for UHMWPE, the polymer can contain a large concentration of copolymer (up to 50%) and still be referred to as "UHMWPE." However, most of the UHMWPEs used to fabricate orthopedic implants are homopolymers, and so we will restrict our further discussion to polymers with only a single type of monomer.
The principal feature of a polymer that distinguishes it from other materials, such as metals and ceramics, is its molecular size. In a metallic alloy or ceramic, the elemental building blocks are individual metal atoms (e.g., Co, Cr, Mo) or relatively small molecules (e.g., metal carbides or oxides). In a polymer, however, the molecular size can comprise over 100,000 monomer units, with molecular weights of up to millions of g/mol.
The molecular chain architecture of a polymer also imparts many unique attributes, including temperature and rate dependence. Some of these unique properties are further illustrated in the specific case of UHMWPE in subsequent sections of this chapter. For further background on general polymer concepts, the reader is referred to texts by Rodriguez and Young.
1.3 WHAT IS POLYETHYLENE?
Polyethylene is a polymer formed from ethylene (C2,H4),which is a gas having a molecular weight of 28g/mol. The generic chemical formula for polyethylene is —(C2,H4)n—, where n is the degree of polymerization. A schematic of the chemical structures for ethylene and polyethylene are shown in Figure 1.4.
For an ultra-high molecular weight polyethylene, the molecular chain can consist of as many as 200,000 ethylene repeat units. Put another way, the molecular chain of UHMWPE contains up to 400,000 carbon atoms.
There are several kinds of polyethylene (LDPE, LLDPE, HDPE, UHMWPE), which are synthesized with different molecular weights and chain architectures. LDPE and LLDPE refer to low density polyethylene and linear low density polyethylene, respectively. These polyethylenes generally have branched and linear chain architectures, respectively, each with a molecular weight of typically less than 50,000g/mol.
High density polyethylene (HDPE) is a linear polymer with a molecular weight of up to 200,000g/mol. UHMWPE, in comparison, has a viscosity average molecular weight of 6 million g/mol. In fact, the molecular weight is so ultra-high that it cannot be measured directly by conventional means and must instead be inferred by its intrinsic viscosity (IV).
Table 1.1 summarizes the physical and mechanical properties of HDPE and UHMWPE. As shown in the table, UHMWPE has a higher ultimate strength and impact strength than HDPE.
Perhaps more relevant from a clinical perspective, UHMWPE is significantly more abrasion- and wear-resistant than HDPE. The following wear data for UHMWPE and HDPE was collected using a contemporary, multidirectional hip simulator. Based on hip simulator data, shown in Figure 1.5, the volumetric wear rate for HDPE is 4.3 times greater than that of UHMWPE.
In the early 1960s, UHMWPE was classified as a form of high-density polyethylene (HDPE) among members of the polymer industry. Thus, Charnley's earlier references to UHMWPE as HDPE are technically accurate for his time but have contributed to some confusion over the years as to exactly what kinds of polyethylenes have been used clinically. From a close reading of Charnley's works, it is clear that HDPE is used synonymously with RCH-1000, the trade name for UHMWPE produced by Hoechst in Germany. With the exception of a small series of 22 patients who were implanted with silane-crosslinked HDPE at Wrighington, there is no evidence in the literature that lower molecular weight polyethylenes have been used clinically.
1.4 CRYSTALLINITY
One can visualize the molecular chain of UHMWPE as a tangled string of spaghetti over a kilometer long. Because the chain is not static, but imbued with internal (thermal) energy, the molecular chain can become mobile at elevated temperatures. When cooled below the melt temperature, the molecular chain of polyethylene has the tendency to rotate about the C-C bonds and create chain folds. This chain folding, in turn, enables the molecule to form local ordered, sheetlike regions known as crystalline lamellae. These lamellae are embedded within amorphous (disordered) regions and may communicate with surrounding lamellae by tie molecules. All of these morphological features of UHMWPE are shown schematically in Figure 1.6.
The degree and orientation of crystalline regions within a polyethylene depends upon a variety of factors, including its molecular weight, processing conditions, and environmental conditions (such as loading), and will be discussed in later chapters of this work.
The crystalline lamellae are microscopic and invisible to the naked eye. The lamellae diffract visible light, giving UHMWPE a white, opaque appearance at room temperature. At temperatures above the melt temperature of the lamellae, around 137°C, UHMWPE becomes translucent. The lamellae are on the order of 10–50nm in thickness and 10–50µm in length. The average spacing between lamellae is on the order of 50nm.
The crystalline morphology of UHMWPE can be visualized using transmission electron microscopy (TEM), which can magnify the polymer by up to 16,000 times. An ultramicrotomed slice of the polymer is typically stained with uranyl acetate to improve contrast in the TEM. The staining procedure makes the amorphous regions turn gray in the micrograph. The lamellae, which are impervious to the contrast agent, appear as white lines with a dark outline. From the TEM micrograph in Figure 1.7, one can appreciate the composite nature of UHMWPE as an interconnected network of amorphous and crystalline regions.
1.5 THERMAL TRANSITIONS
As already indicated, one of the distinguishing characteristics of polymeric materials is the temperature dependence of their properties. Returning to our conceptual model of an UHMWPE molecule as a mass of incredibly long spaghetti, one must also imagine it to be jiggling and writhing with thermal energy. Generally speaking, many polymers undergo three major thermal transitions: the glass transition temperature (Tg), the melt temperature (Tm), and the flow temperature (Tf).
The glass transition (Tg) is the temperature below which the polymer chains behave like a brittle glass. Below Tg, the polymer chains have insufficient thermal energy to slide past one another, and the only way for the material to respond to mechanical stress is by stretching (or rupture) of the bonds constituting the molecular chain. In UHMWPE, the glass transition occurs around—120°C.
As we raise the temperature above Tg, the amorphous regions within the polymer gain increased mobility. When the temperature of UHMWPE rises above 60–90°C, the smaller crystallites in the polymer begin to melt. The melting behavior of semicrystalline polymers, including UHMWPE, is typically measured using differential scanning calorimetry (DSC). DSC measures the amount of heat needed to increase the temperature of a polymer sample. Some representative DSC data for UHMWPE is shown in Figure 1.8.
The DSC trace for UHMWPE shows two key features. The first feature of the DSC curve is the peak melting temperature (Tm), which occurs around 137°C and corresponds to the point at which the majority of the crystalline regions have melted. The melt temperature reflects the thickness of the crystals as well as their perfection. Thicker and more perfect polyethylene crystals will tend to melt at a higher temperature than smaller crystals.
In addition, the area underneath the melting peak is proportional to the crystallinity of the UHMWPE. DSC provides a measure of the total heat energy per unit mass (also referred to as the change in enthalpy, ΔH) required to melt the crystalline regions within the sample. By comparing the change in enthalpy of a UHMWPE sample to that of a perfect 100% crystal, one can calculate the degree of crystallinity of the UHMWPE. Most bulk UHMWPEs are about 50% crystalline.
As the temperature of a semicrystalline polymer is raised above the melt temperature, it may undergo a flow transition and become liquid. Polyethylenes with a molecular weight of less than 500,000g/mol can be observed to undergo such a flow transition (Tf). However, when the molecular weight of polyethylene increases above 500,000g/mol, the entanglement of the immense polymer chains prevents it from flowing. UHMWPE does not exhibit a flow transition for this reason.
1.6 OVERVIEW OF THE HANDBOOK
This Handbook is organized into three main sections. The first section, which consists of three chapters, reviews the basic scientific, engineering, and clinical foundations for UHMWPE. For example, in Chapter 2, we explain how UHMWPE must be formed into bulk components from the resin powder using extrusion or compression molding techniques. In Chapter 3, we review the techniques associated with sterilization and packaging of UHMWPE implants. Chapters 5–12 cover the basic clinical applications of UHMWPE in the lower extremities, upper extremities, and the spine.
The second part of the Handbook is focused on the development of UHMWPE biomaterials technologies for orthopedics. Chapters 13 and 14 summarize the state of the art as it pertains to remelted and annealed highly crosslinked UHMWPE, and Chapters 15 and 16 describe the advances in stabilization of highly crosslinked and conventional UHMWPE using doped and blended vitamin E. Chapter 17 covers advances in UHMWPE composites, including the history of Poly II, and Chapter 18 describes advances in creating microcomposites of UHMWPE and hyaluronan, a biomolecule that promotes lubrication in cartilage. Chapter 19 summarizes advancements in high pressure crystallization and crosslinking of UHMWPE, including the history of Hylamer. The last chapter in this section, Chapter 20, is a compendium of the processing, packaging, and sterilization information for first- and second-generation highly crosslinked UHMWPE materials that are currently used in hip and knee arthroplasty.
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