Effects of Gamma Radiation Sterilization on UHMWPE

Effects of Gamma Radiation Sterilization on UHMWPE

BE 210 Final Project

April 30, 1998

Group W7

Lytal Kaufman

Jenny Li

Alisa Plesco

Jonathan Rogers

Table of Contents

Section Page

1.  Abstract 3

2.  Background 4

·  Medical background 4

·  Properties of Polyethylene 5

·  Gamma Radiation Sterilization 6

·  Effects of Gamma Sterilization on Polyethylene 6

·  Gamma Sterilization in air vs. in an inert gas 7

·  Non-radiation Sterilization Methods 8

·  Experimental Objectives 8

3. Materials & Equipment 10

4.  Procedure 11

·  Differential Scanning Calorimeter 11

·  Density Gradient Column 12

·  Hardness Test 12

·  Wear Test and Measurements 12

5.  Results 15

·  Differential Scanning Calorimeter 15

·  Density Gradient Column 15

·  Hardness Measurement 15

·  Friction Measurement 16

·  Wear Measurement 16

6.  Discussion 18

·  Effect on Structural Properties 18

·  Effect on Wear Properties 19

·  Sources of Error 21

·  Consequences of Abrasive Wear 22

·  Choice of Sterilization Method 22

·  Conclusion 23

7.  References 25

8.  Appendix 28

Abstract

The purpose of this experiment was to evaluate the effects of gamma radiation sterilization on the material properties of ultra high molecular weight polyethylene (UHMWPE) and the extent to which these effects influence the choice of sterilization method. Four tests were performed on the polyethylene component of a Zimmer-brand knee implant and a pre-sterilized polyethylene block. From the differential scanning calorimeter, the onset temperature increased 1.37% from the interior to the surface of the knee sample and 3.85% from the surface sample to the block sample. The average value of heat of fusion increased 15.3% comparing the surface knee sample to the block sample. Using the density gradient column, an increase in density of 0.91% was observed between the control sample and the interior of the irradiated sample, and an increase of 2.04% was observed between the control and the surface of the irradiated sample. From the hardness testing, an increase in hardness of 17.3% was found for the irradiated knee sample. For the wear test, the maximum coefficient of friction between the metal and the polyethylene surfaces was found to be 51.7% higher for the irradiated sample than the control when the equal amount of normal force of 300 lbs was applied. The results of the wear test were plotted as depth versus distance across the specimen. The wear measurements showed a 39% decrease in maximum penetration for the irradiated sample compared to the block sample. The results indicated the irradiated sample was harder and more resistant to creep deformation. However, the irradiated sample exhibited a higher coefficient of friction and reduced abrasive wear resistance. All factors must be considered when choosing the best method of sterilization.

Background

Medical Background

The knee is a complex joint of three bones; the femur (thigh bone), the tibia (shin bone), and the patella (knee cap). When the knee is bent or straightened, the end of the femur rolls against the end of the tibia, and the patella glides in front of the femur. A cushion, which protects the joint from damage, is provided by the cartilage. A layer of cartilage, smooth soft tissue, covers the ends of the femur and tibia and lines the underside of the patella. Healthy cartilage along with the lubricating synovial fluid allow the knee to glide easily and absorb stress. Side to side mobility is provided by muscles and ligaments. The ligaments, a type of smooth tissue, hold the bones of the joint together and the muscles give the knee and leg the power needed for movement1.

The total knee replacement implant greatly resembles a real knee, and many different models use varying materials. Each knee implant contains a femoral component made of a metal, frequently cobalt chrome, a patellar component usually made of polyethylene, and a tibial component commonly made of either polyethylene or titanium covered by polyethylene. These components are often affixed with a bone cement, methyl methacrylate, but there has been a more recent alternative used which consists of a “rough” surface, which allows the bone to naturally grow and interlock with the artificial joint2.

The majority of joint replacement patients are those with osteoarthritis caused by the cracking and wearing away of cartilage due to normal use. However, joint replacements are not limited to cases of wear and tear, but also include cases of injuries that did not heal properly and chronic illness such as rheumatoid arthritis. Joint replacement candidates suffer from pain, stiffness, and loss of function of their joint3. The new implant relieves pain and improves the mobility of the joint.

There are several factors that affect the overall success of the implant. The patient selection, including patient age, weight and activity status will affect the wear process. The implant selection, especially the materials and the sizes of the components used, affects the success of the implant. The polyethylene thickness has an effect on contact stresses. To minimize polyethylene wear, the design must maximize the thickness of the polyethylene insert within the limits of the anatomic construct4. The implant position and stabilization also influences the success of the implant. This is a surgical concern rather than a design issue. Surgeons must align the implant properly and fit the implant tightly so as to avoid debris entering through cracks as well as loosening. The last factor that is critical to polyethylene wear and possible ultimate failure of the implant is wound debridement and cleansing to eliminate third-body particulate debris. If the wound is not cleaned properly or if foreign particles enter the body during surgery, infections and other complications may occur. Improvements in the design of the implant as well as in the surgical techniques for implantation will increase the rate of success of implants.

Properties of Polyethylene

Polyethylene is a polymer whose repeating unit, or “mer”, is the hydrocarbon ethylene (C2H4) (Fig. 1). The molecule that is formed is a long chain consisting of a carbon backbone surrounded by hydrogen atoms5. Ultrahigh molecular weight polyethylene (UHMWPE) is generally classified as those polyethylene polymers whose molecular weight exceeds 1.75 million grams/mole6. The materials used for joint implants are generally on the order of 3 to 6 million7. Accompanying an increase in molecular weight is an improvement of many of the mechanical properties6, including a relatively low coefficient of friction and a superior resistance to wear5. It is primarily for these characteristics that an UHMWPE interface was chosen for joint implants6,8,9.

H H H H H

| | | | |

¾ C ¾ C ¾ C ¾ C ¾ C ¾

| | | | |

H H H H H

Figure 1: Two-dimensional section of a polyethylene chain. Polyethylene is a chain of ethylene “mers” (C2H4), connected by covalent bonds between carbon molecules. In this figure, the chain would extend to the left and right.

Gamma Radiation Sterilization

Gamma radiation is commonly used for the sterilization of medical devices. Cobalt 60 gamma sterilization has become the industry's method of choice because of its reliability in sterilizing thoroughly throughout the entire sample, the absence of chemical residues after sterilization, its flexibility, and its cost effectiveness10,11. Packages can be sterilized in bulk and at low temperatures, and products can be used immediately after sterilization since they do not acquire radioactivity12. 2.5 Mrad of gamma radiation is generally accepted for sterilization of medical equipment13.

Effects of Gamma Sterilization on Polyethylene

Numerous studies have considered the effects of sterilization on the polyethylene components of joint implants. Unfortunately, the results do not produce a fully conclusive verdict. All studies agree on the mechanism that produces the change in mechanical properties but do not necessarily agree on how these changes affect the success or failure rates of the joint implant.

Gamma radiation, one of the most commonly used methods for sterilization, has been known to rupture the molecular chain bonds of polymeric materials by a mechanism known as chain scission6,7,13-17. The broken chains leave free radicals in the material, which are capable of bonding to other molecules in the immediate vicinity. This can lead to cross-linking, a process where adjacent linear chains are joined to one another by covalent bonds5. A cross-linked polymer, in general, would be stronger5 but would have a decrease in ductility and an increased susceptibility to fatigue damage and wear15. Evidence of cross-linking can be found in density and/or crystallinity measurements since an increase in cross-linking effectively reduces the volume of open space between molecules.

Chain scission can also lead to oxidative degradation, in which the free ends react with oxygen present in the surroundings. The degree to which oxidation occurs depends on the amount of oxygen that can diffuse into the material. In environments with high oxygen content, high oxidation is expected while environments with low oxygen content will display lower degrees of oxidation. Diffusion of oxygen through the polymer specimen would follow Fick’s second law,

dC/dt = D(d2C/dx2) (Eq. 1)

where dC/dt represents the change in concentration with respect to time, D is the diffusion constant, and dC/dx represents the concentration gradient5. Thus, diffusion of oxygen into a polyethylene sample is time-dependent. At the time when the material becomes stabilized (i.e. will no longer readily react with molecules in its vicinity), oxygen would have diffused to a set distance from the surface. Consequently, evidence of oxidation would be observed only up to this point. Infrared microscopy (figure 1) was used by Wright Medical Technology, Inc. to show a consistent boundary line (approximately 1 to 2 mm below the surface) separating the surface layer and the core region with regard to oxidation levels17. During the time that polyethylene chains were reactive, oxygen diffused to a uniform depth within the sample.

Oxidation is known to stiffen molecular chains7 and oxidation levels could, therefore, be measured with hardness tests. Furthermore, wear rates would be affected by oxidation. The process of oxidative degradation essentially shortens the length of the polymer chains by binding to oxygen rather than to ethylene “mers”. The shortened chains have a lower molecular weight than the original chains. This increase in the fraction of low molecular weight material has been proven to increase the wear rate18.

Figure 2: Cross-section of UHMWPE components showing an oxidized surface layer and a core region with minimal oxidation. 17

Gamma Sterilization in Air vs. in an Inert Gas

Gamma radiation in an inert environment, such as argon or nitrogen, (as opposed to air, where oxygen is prevalent) can decrease the extent to which oxidation occurs. Studies show that surface oxidation levels of nitrogen-aged components are approximately 66% less than those of oxygen-aged components and that the “white band”, seen in figure 2, is not visible in components irradiated and aged in nitrogen19. However, a significant degree of oxidation has been shown to occur even under inert conditions13,15-17. A likely explanation is that oxygen is either dissolved or trapped within the material13,15. Another explanation is that residual O2 and H2O are present in the surrounding gas15. A third, and possibly most feasible, explanation is that short-term oxidation is completely eliminated by sterilizing in the inert gas, but during shelf storage, oxygen may leak into the package and cause oxidation. Continued oxidation may also occur in vivo, but this is thought to occur to a lesser extent16. In other words, short-term oxidation is avoided by irradiation in an inert gas but long-term oxidation is more difficult to prevent.

Non-radiation Sterilization Methods

Ethylene oxide gas (EtO), a chemical sterilant, may be used to avoid severe changes in the mechanical properties of polyethylene. EtO does not break polymer bonds, as gamma radiation does, and therefore does not promote cross-linking and oxidative degradation15,17. However, EtO is a toxic substance and may leave toxic residuals, such as ethylene chlorohydrin and ethylene glycol14. It is also only a surface sterilant and is not as reliable as gamma radiation would be.

Experimental Objectives

Many studies show that UHMWPE evolves by means of gamma radiation into a material of lower molecular weight, higher density, and increased levels of oxidation. This translates into changes in the mechanical properties, which can be found in the laboratory. The extent to which these changes contribute to failure rates of joint implants has been a source of conflict in recent years. Many of these conflicts revolve around the wear properties. Biomet Inc. prefers the method of gamma sterilization in the presence of an inert gas since oxidation is “minimal” and the additional cross-linking improves the abrasive wear resistance14. Conversely, Wright Medical Technology, Inc. prefers EtO sterilization since fatigue wear, causing pitting and delamination, is more likely to occur in gamma-radiated materials due to its increased oxidation17. The goal of this experiment is to verify the structural changes that occur, to determine the mechanical effects of gamma-radiation sterilization on UHMWPE, specifically its wear properties, and to evaluate the extent to which these effects influence the choice of sterilization method.

Materials & Equipment

·  Polyethylene component of a Zimmer-brand knee implant (sterilized)

·  Polyethylene block from Zimmer (unsterilized)

·  Counterfaces: stainless steel flat ended circular cylinders (cross sectional area approximately 64mm2)

·  Perkin-Elmer AD-4 Autobalance

·  TA Instruments Differential Scanning Calorimeter (DSC)

·  Density gradient column

·  Tukon Hardness Machine

·  Friction/Wear testing machine – designed by Alex Radin of the mechanical testing lab at the University of Pennsylvania

·  Instron Model 1331

·  Tectronix 5223 Digitizing Oscilloscope

·  Olympus Optical Microscope, Model BH-2

The polyethylene “block”, supplied by Zimmer, was compression molded from the polymer resin. The polyethylene component of the knee implant is the tibial articulating surface of Zimmer’s MGII Total Knee System. It was machined from a compression-molded sheet, similar to the “block”, and then irradiated and packaged in nitrogen gas.

The stainless steel counterfaces were polished to a 5 micrometer finish.

Procedure

In order to determine the effects of the gamma radiation sterilization process on the material properties of the ultra-high molecular weight polyethylene, four different tests were performed.