Medical Background - Penn Engineering



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

| | | | |

( C ( C ( C ( C ( C (

| | | | |

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.

[pic]

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.

Differential Scanning Calorimeter

A total of 19 samples were tested in the differential scanning calorimeter (DSC): 6 from the surface of the knee implant; 5 from the interior, or core, of the knee implant (defined as more than 3.5 mm below the surface); and 8 from the block sample. Each specimen was cut from the samples with a knife, ranging in weight from 2.5 to 7.5 mg. This sample was then placed in the aluminum pan, and the aluminum lid was pressed and sealed into the pan. The pan containing a specimen, as well as an empty aluminum pan (the reference pan), were placed into the chamber of the calorimeter. The mass of the sample was entered and the test was then run. The method of heating was initiated by holding the sample and reference pan at 55.0 (C for 0.5 minutes. Both the sample and reference pan were then independently and simultaneously heated so that each increased in temperature by 10 (C/min for 12 minutes, at which point a maximum temperature of 175(C was achieved. Nitrogen constantly and slowly flowed through the chamber to maintain atmospheric pressure and to prevent oxidation of the samples at high temperatures. Readings were constantly taken and plotted on a graph of heat flow (mW) vs. Temperature ((C) by the DSC software.

The resulting curve was a plot of power vs. time since the rate of temperature increases linearly with time. Because the goal of this experiment was to determine percent crystallinity, the maximum area under the graph (the heat of fusion, in J/g) was found. The data was also analyzed to determine the temperature of the transformation (onset melting temperature), as well as the peak temperature of transformation.

The published heat of fusion ((H) for a fully crystalline polyethylene material is 288 J/g 7. Since heat of transformation is directly related to the degree of crystallinity, the experimental value for the heat of transformation for each sample specimen was divided by 288 J/g to determine a value for the percent crystallinity.

Density Gradient Column

A sample from the surface of the knee implant, a sample from the interior of the implant, and a sample from the untreated block were placed in the density gradient column. Three color-coded glass standards of known density were already immersed in the column, providing a basis to determine the unknown density of the samples. This apparatus contained a total of 1180 mL of water and 700 mL of isopropanol. By having a greater concentration of water at the bottom of the column, the density at the bottom was close to that of water, approximately 1 g/cm3. Density of the mixture decreased linearly with height in the column. Each sample sank until it reached a point at which the density of the liquid in the column matched its own density. The samples were given time to settle; the final resting height of each sample was observed and recorded.

Hardness Test

Hardness measurements were taken with the Tukon Hardness test apparatus. Testing of hardness is done by effectively measuring a material’s resistance to indentation. Originally, the Rockwell Hardness Tester was to be used, but the smallest load choice was 15 kg ( a load far too great for the relatively soft polyethylene for this particular test.

A load of 500 grams was selected. A total of 8 measurements were taken. This test was performed on both the block polyethylene and the knee sample subjected to the 300 pound applied loads. Two measurements were taken on the unworn area of the specimen and two on the worn region of the specimen for each sample kind. An indentation in the shape of an inverted pyramid was made into the surfaces of the polyethylene. The values of the two diagonals were measured and averaged. This measured value was then converted to a Diamond Pyramid Hardness (DPH) value using a conversion device (similar to a slide rule) provided by the manufacturer.

Wear Test and Measurements

Any artificial prosthesis continuously experiences wear and friction inside the body due to everyday activities. To better ascertain any differences in wear properties due to gamma radiation of the knee implant, a wear/friction test was performed on both untreated block specimens and a specimen extracted from the sterilized knee.

The wear test machine was loaded into and powered by the Instron Model 1331. A polyethylene specimen was placed in the apparatus, as well as the stainless steel metal counterface, constituting a basis for an articulating surface. A normal force was provided by a spring connected to the counterface, perpendicular to the surface of the specimen. The sample remained stationary as the rest of the apparatus oscillated vertically, resulting in a frictional force and consequently, a wear indentation. (See figure 3)

Figure 3: Wear/Friction machine with loaded specimen for testing.

To form a basis for testing, different normal forces were exerted on the unsterilized block samples to determine an optimal applied load. Consistency of surface contact area and measurable results were a prime consideration for load choice. Three different loads were tested on specimens from the untreated block sample of the polyethylene ( 100, 200 and 300 pounds. The applied load force was easily changed by changing the position of the spring with respect to its equilibrium position. Because the 100 and 200 pound loads resulted in unsatisfactory indentation in terms of depth and area on the surface of the polyethylene, a load of 300 pounds was chosen for the knee implant (See Figure A-1).

An oscilloscope was connected to the wear machine, and a load and displacement signal was generated. Thus, the frictional force in the vertical direction could be deduced from the signals. The coefficient of friction (() was then calculated for the samples according to

f = (N (Eq. 2)

where f is the frictional force measured by the oscilloscope in the vertical direction and N is the normal force supplied by the spring in the horizontal direction.

The depth of the indentation resulting from the wear test was measured using the optical microscope. The microscope was focused approximately 5 mm from the edge of the worn area of the sample. In 1 mm increments, the stage of the microscope was moved across the region of wear. The microscope was refocused at each point with the fine tuning adjustment. Each mark on the fine tuning adjustment knob corresponded to a 2 (m vertical move by the objective. In this manner, depth of the indentation due to wear was measured.

Results

Differential Scanning Calorimeter

The percent crystallinity for the samples was calculated, as outlined in the previous section. The value for the average onset temperature and crystallinity obtained by the DSC are shown in table 1 below. The onset temperature increased 1.37% from the interior to the surface of the knee sample, and was 3.85% higher for the surface sample than the block sample. The heat of fusion showed a 16.5% increase at the surface and a 16.9% increase at the interior. The average value for percent crystallinity showed an 8.51% increase at the surface and an 8.69% increase at the interior.

| |Onset Temp (oC) |Error |% Crystallinity(calc) |Error |

|Knee Sample (surface) |132.4 |1.79 |59.97 |1.99 |

|Knee Sample (interior) |130.6 |2.81 |60.15 |2.56 |

|Block Sample |127.4 |1.97 |51.46 |2.63 |

Table 1: The average values of onset temperature and the percent crystallinity (with respective error) obtained from DSC

Density Gradient Column

The density of the non-irradiated UHMWPE was 0.9257 g/cm3 ( 0.000375 g/cm3. For the knee sample, the density was 0.9448 g/cm3 ( 0.000375 g/cm3 at the surface and 0.9342 g/cm3( 0.000375 g/cm3 at the interior. Hence, 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.

Hardness Measurement

Hardness measurements between the worn and unworn areas were not found to be significantly different. Therefore, average measurements of hardness for the knee and block sample were found using all four respective measurements. Hardness was 5.43 ( 0.245 DPH and 4.63 ( 0.083 DPH, for the knee and block samples, respectively. There was an increasing hardness of 17.3% for the knee sample due to irradiation. Therefore, the knee sample was more resistant to indentations.

Friction Measurement

The sine waves, which represent the frictional force produced between the metal surface and the polyethylene surface for the knee and block samples are shown in figure 4. The frictional force for the block sample ranged from 13 lbs to 29 lbs. The normal force used for both samples was 300 lbs. The maximum coefficient of friction, calculated from equation 2, was found to be 0.0967 for the block. The range of the frictional force for the knee sample was found to be 30 lbs to 44 lbs. The maximum coefficient of friction for the knee sample was 0.1467. There was a 51.7% increase in maximum coefficient of friction from the block sample to the knee sample.

Figure 4: Voltage output for frictional force in vertical direction. Left: Knee sample (1 division corresponds to 20 lbs). Right: Block sample (1 division = 10 lbs).

Wear Measurement

The results for the wear test for the two samples were compared by graphing the depth of the worn area against the distance across the specimen at normal force of 300 lbs (Figure 5). In the graph, zero depth is surface level of the samples, while negative values indicate levels below surface level. The lighter shades of green and blue represent the worn area of each sample, respectively. The curve of depth versus distance for the block sample was deeper than that of the knee sample in the worn area; therefore the effective wear was greater for the block sample. At the edge of the worn area for the block sample, the level of material went up 12 microns (from 59 microns below the surface to 47 microns below) before it dropped. The maximum penetration for the block was 135 microns, and for the knee sample was 83 microns. This corresponds to a 39% decrease in maximum penetration for the knee sample, compared to the block sample.

Figure 5: Depth versus distance across the worn area of the specimen at 300 lbs normal force. The “wear zone” for both specimens (in lighter shades of their respective colors) begins at a distance of 5mm and ends at a distance of approximately 16mm. Maximum penetration (in red) occurs in the center of the wear zone.

Discussion

Gamma radiation unquestionably alters the structural and mechanical properties of ultra-high molecular weight polyethylene. The changes that occur have significance in the success rate of joint implants. Method of sterilization, therefore, becomes a primary concern in total joint replacements.

Effect on Structural Properties

Density increases in irradiated samples and increases further towards the surface of the irradiated sample. An increase 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. Different mechanisms of structural transformation are responsible. Although irradiated in nitrogen, the surface of the knee implant is exposed to residual O2 and H2O in the atmosphere15, in the packaging, and in vivo16. Consequently, bonds broken by irradiation (free radicals) are likely to bond to the reactive oxygen. The result is chain scission and the density increases, since the smaller chains can pack together more easily7. However, the amount of oxygen readily available is limited by its diffusion into the sample. Consequently, points in the sample that are far enough from the surface that oxygen from the surroundings does not diffuse, do not exhibit the same degree of oxidation. Ries et al. found that at 3.5 mm below the surface of the material, oxidation levels are independent of the surrounding atmosphere. However, above 3.5 mm, oxidation is found to be a function of oxygen concentration in the surroundings15. Goldman et al. confirmed that, up to 3.5 mm below the surface, chain scission ( caused by oxidation( is the mechanism that dominates the structural changes that arise from irradiation and ageing7.

The heat of fusion and onset melting temperature (Tm), calculated from the differential scanning calorimeter, show that the degree of cross-linking increases in the irradiated material. The heat of fusion of the irradiated samples increased by over 16% from the control sample. This corresponds to over an 8% increase in crystallinity. Since there was no apparent difference in heat of fusion between the interior of the knee implant and its surface, it can be inferred that the cross-linking remains relatively constant throughout the sample. Tm increased by a few degrees Celsius for all three samples and is indicative of an increase in the order of each sample; this is consistent with the observed changes in density as discussed above. Furthermore, the increase in melting temperature could be caused by an increase in the chain stiffness5. Since oxidative degradation is known to stiffen molecular chains, the increase in melting temperature is a good measure of the difference in the degree of oxidation between the interior and surface of the knee implant. The difference in melting temperature between the control and the interior sample indicates that the sample experiences a certain degree of oxidation, since no oxidation has occurred in the control sample. This is consistent with other studies15,19 and may be attributed to oxygen that is either dissolved or trapped in the material.

The stiffened chains caused by oxidation account for the increase in hardness at the surface of the irradiated sample. A 17.3% increase in hardness was observed. The fact that hardness was not significantly different in the worn and unworn areas indicates that the increase shows a change in the structure of the polyethylene; i.e. it is not affected by mechanical changes. Unfortunately, hardness tests were not performed at the interior of the irradiated sample due to machining limitations. Studies show that there is an appreciable difference in hardness between the interior and the surface of an irradiated material17. It would be interesting to see if hardness measurements in the interior would fall in between the control measurement and surface measurement.

The coefficient of friction (() is indicative of increased oxidation levels as well. Since oxidation is a form of chain scission, an increase in oxidation is accompanied by an increase in the fraction of lower molecular weight chains. A polymer of lower molecular weight tends to have a higher coefficient of friction5. The increase in the coefficient of friction was 51.7% in these tests. Clearly, irradiated samples transform into polymers of lower molecular weight, higher density, and increased levels of oxidative degradation.

Effect on Wear Properties

The structural changes, inferred from the results mentioned above, translate into changes in the mechanical properties. The wear properties, in particular, were the focus of this study. In this study, wear was determined as a dimensional change, to which two types of wear contribute, namely creep deformation and abrasive wear, or delamination. Creep deformation is defined as the time-dependent permanent deformation that occurs under stress5. In this type of wear, no mass is lost from the sample. Rather, the molecules adjust their positions under the stress applied. Conversely, abrasive wear is accompanied by a loss of mass from the sample. Molecules of polyethylene become delaminated (i.e. are peeled away) and the volumetric change is indicative of polyethylene debris, i.e. wear products. The degree to which the dimensional change is caused by these two mechanisms is difficult to determine quantitatively, without determining a change in mass. Since penetration was found to be on the order of microns, a change in mass is expected to be too small to be found on a conventional scale, especially considering additional particles that could contaminate the sample (e.g. particles left from the metal counterface).

Figure 5 shows that the irradiated sample undergoes a smaller dimensional change than the control sample. The maximum penetration decreased by 39%. Thus, total wear is reduced by gamma radiation. This is consistent with the increase in hardness, since hardness is a measure of a material’s resistance to indentation. However, inspection of the curves indicates the degree to which the two types of wear occur. The block (control) sample shows a sharp increase in volume of material outside the wear zone. Furthermore, at the beginning of the wear zone of the block sample, the depth decreases (i.e. the level is higher) by 12 microns before dropping drastically into the wear zone. Physically, this is a “bump” just at the edge of the wear zone that can be felt by rubbing one’s finger across the surface of the sample. The sharp increase outside the wear zone and the “bump” are indicative of creep deformation since polyethylene that is not present in the wear zone essentially relocates to another section of the sample. The knee sample does not show any indication that creep deformation is occurring since there is no “bump” and the curve flattens out outside the wear zone. Thus, the primary mechanism for the dimensional change in the irradiated sample must be abrasive wear.

This is consistent with many studies that relate a material’s structural changes with its wear properties. Oonishi et al. correlate an increase in hardness with a decrease in creep deformity, but an increase in brittleness6. If the mechanism of abrasive wear is viewed as microcracks occurring at the surface of the polymer, then the brittle nature of irradiated materials should lead to delamination. Furthermore, increased stiffness leads to decreased contact area at the bearing surface. The increased contact stress that results causes an exponential increase in wear15. Rose et al. submit that oxidation-stabilized defects limit the resistance to wear. Two mechanisms for these defects could be rupture of the intergranular fibrils or crack initiation by chain-end defects13. Finally, polyethylene wear is inversely correlated with molecular weight; i.e. the lower molecular weight, caused by chain scission, decreases the material’s resistance to abrasive wear4.

Sources of Error

Due to the small thickness of the knee implant, it was difficult to differentiate between interior and surface. A standard of 3.5 mm below the surface was set as the boundary between surface and interior but limited methods of extracting samples may not have ensured that this was maintained. Furthermore, the introduction of foreign objects into the pan during preparation might affect the results of the DSC.

The block sample was formed by compression molding of a resin. There are some disadvantages to this technique. The compressed material may display areas of varying density and mechanical properties from the surface to the center of the sample. This results if improper heat and pressures are applied20. Considering the small size of DSC specimens, this variance in density may be magnified and may cause inaccuracy in the results. In order to compare the block and knee samples, it was necessary to assume that gamma radiation was the only variable. However, forming of each sample may have had varying temperatures, pressures, or cooling rates. These would affect crystalline properties. Even if these are assumed to be equivalent, the machining process may have generated heat at the surface of the knee implant and again, would influence the properties.

A flat-on-flat surface was used as the standard for the metal counterface. However, due to the inexact nature of the polishing, the surfaces may not have been completely flat. Furthermore, a foreign particle, such as dust or metal debris, could come between the articulating surfaces, acting as a third abrasive. The roughness should not have changed drastically, but even a minor change would have significant consequences since the wear factor is proportional to the counterface roughness raised to a power greater than one21. Stainless steel, which was used for the counterface, has largely been abandoned in clinical implant use due to its tendency for scratching and wear4. With a larger choice of metals, a more widely used metal for implants, could have been chosen.

Consequences of Abrasive Wear

This increase in abrasive wear in the irradiated polyethylene component can have severe effects on the success or failure of a joint implant. Clinical data has shown that polyethylene debris particles can cause osteolysis, or bone resorption, in the joint and around the implant. Osteolysis occurs when macrophages surround foreign particles and in the process, release inflammatory mediators that stimulate bone resorption ( the dissolution of hydroxyapatite( by osteoclasts22,23. Aseptic loosening of the prosthesis, in many cases, is commonly accompanied by severe bone loss in the periprosthetic tissue[McGee]. Polyethylene debris has been isolated within these tissues and ranged in size from shards measuring over 100 (m in length to submicron particles23,24. Studies show that evidence of osteolysis can be observed using radiographic techniques between 3 and 6 years after implantation25. As of 1984, studies showed that, at 8 years after implantation, 12% of implants needed removal due to aseptic loosening. By 10 years after implantation, over 95% of the patients exhibit calcar resorption26, a sign that osteolysis is ensuing. Subsequent to the emergence of a “calcar crater” is debonding between the cement and the femoral component. Debris is then pushed into the component-cement interface, and a linear pattern of osteolysis along the femoral stem ensues25.

Choice of Sterilization Method

The changes in mechanical properties that can be attributed to gamma radiation may influence the manufacturer’s choice for sterilization procedure. An advantage is the formation of a stronger, harder material that is more resistant to creep deformation, causing loosening at the articulating surface. The disadvantages include higher friction, decreased abrasive wear resistance, and a reduced fatigue resistance. Higher friction can result in a loosening of the bone-implant interface since the increased friction produces a moment around the bone-implant interface. Decreased abrasive wear resistance causes osteolysis (as discussed above) and the reduced fatigue resistance, attributed to brittleness caused by oxidative degradation15,27, makes the material more susceptible to fracture over a given number of cycles. The degree to which these results influence the choice of sterilization procedure is dependent not only on the changes in mechanical properties but also on surgical technique and patient cases. McGee et al. found that the incidence of osteolysis depends on the access of wear particles to the implant-bone interface. They showed that as the degree of bone apposition to the implant increased, the incidence of osteolysis was reduced. Thus, poor fixation and implant instability results in connective tissue at the implant-bone interface, allowing access to polyethylene debris24. This was confirmed by Kobayashi et al., whose studies found that tibial polyethylene components that were fixed without cement accumulated larger numbers of polyethylene particles, compared to those fixed with cement23. It may be for this reason that Zimmer Knee Systems are not cleared for use in the United States without bone cement28. Furthermore, they found a higher number of polyethylene particles in total knee replacements than in total hip replacements. This may be attributed to higher contact stresses in the knee, compared to the hip. The higher contact stress in the knee also explains the higher incidence of fatigue failure in knee implants15. Finally, the activity of the patient may be a factor. It was formerly assumed that patients took 1 million steps per year but it has been shown that younger patients are expected to take 4 million steps per year4. In other words, gamma radiation sterilization may not be a major concern for a polyethylene component used in a well-cemented hip implant of an elderly patient, but may be a consideration for use in an uncemented knee implant of a younger, more active patient. Essentially, the choice of sterilization method must be reviewed in the context of how the implants are used and what is required of them.

Conclusion

The effects of gamma radiation sterilization, and subsequent oxidation, on the material properties of ultra-high molecular weight polyethylene have significant ramifications for their use in implantation. Considering all of the aforementioned consequences of the gamma sterilization process, specifically increased abrasive wear and the production of polyethylene debris products, improved techniques of gamma sterilization or alternative forms of sterilization for polyethylene must be investigated.

References

1. Lenox Hill Hospital. How do we replace a knee. Center for Total Joint Replacement. New York City, NY.

2. Lenox Hill Hospital. What is Total Joint Replacement. Center for Total Joint Replacement. New York City, NY.

3. Lenox Hill Hospital. Who Needs a Total Joint Replacement. Center for Total Joint Replacement. New York City, NY.

4. Cuckler, John M. Factors Influencing the Limitations of Polyethylene. Orthopedics – Special Online Edition. . Presented at the Continuing Medical Education program, Current Concepts in Joint Replacement. Mt. Sinai Medical Center and Case Western Reserve University, December 12-14, 1996.

5. Callister, William D. Jr. Materials Science and Engineering: An Introduction. Fourth edition. John Wiley and Sons, Inc. New York, NY. 1997.

6. Oonishi, H.; Ishimaru, H. and Kato, A. Effect of cross-linkage by gamma radiation in heavy doses to low wear polyethylene in total hip prostheses. Journal of Materials Science: Materials in Medicine. 7 (1996): 753-763.

7. Goldman, M.; Gronsky, R.; Ranganathan, R. and Pruitt, L. The effects of gamma radiation sterilization and ageing on the structure and morphology of medical grade ultra high molecular weight polyethylene. Polymer 1996, Vol. 37, No. 14: 2909-2913.

8. Charnley, John. Low Friction Arthroplasty of the Hip. Springer-Verlag: Berlin, 1979.

9. Gilbertson, LN. Wear of polyethylene in Total Hip Replacement. Research Laboratories of Zimmer, Inc., Warsaw, Indiana, 1995.

10. Isomedix. Choosing Your Sterilization Process, .

11. Gammaster home. Gammaster( Irradiation.

12. ZEUS, Inc. Gamma Sterilization ().

13. Rose, RM; Goldfarb, EV; Ellis, E; and Crugnola, AN. Radiation Sterilization and the Wear Rate of Polyethylene. Journal of Orthopaedic Research. 1984; 2:393-400.

14. Biomet, Inc. Oxidation and Sterilization Issues with UHMWPE. ArCom( Polyethylene – A Solid Story.

15. Ries, M.D., Weaver, K., Rose, R.M., Gunther, J., Sauer, W., and Beals, N.: Fatigue strength of polyethylene after sterilization by gamma irradiation or ethylene oxide. Clinical Orthopaedics and Related Research. 333:87-95, 1996.

16. Sun, D.C.; Wang, A.; Stark C.; Dumbleton J.H. Development of Stabilized UHMWPE Implants with Improved Oxidation Resistance via Crosslinking. Scientific Exhibition Presented at the 63rd Annual Meeting of the American Academy of Orthopaedic Surgeons, February 22-26, 1996, Atlanta, Georgia, USA.

17. Wright Medical Technology, Inc. Effects of Sterilization Methods on Ultra-High Molecular-Weight Polyethylene. Technical Monograph. Arlington, TN. 1995. ().

18. McKellop, HA and Clark, IC. Evolution and evaluation of materials-screening machines and joint simulators in predicting in vivo wear phenomena. In Functional Behavior of Orthopedic Biomaterials. Volume II: Applications. Ed. Paul Ducheyne and Garth W. Hastings. CRC Press, Inc: Boca Raton, FL, 1984. pp. 51-86.

19. Johnson, T and Devanathan, D. Nitrogen Packaging and Gamma Radiation Sterilization of UHMWPE. Research Laboratories of Zimmer, Inc., Warsaw, Indiana, 1996.

20. Biomet, Inc. Resin and Consolidation Issues with UHMWPE. ArCom( Processed Polyethylene.

21. Lancaster, JG; Dowson, D; Isaac, GH and Fisher, J. The wear of ultra-high molecular weight polyethylene sliding on metallic and ceramic counterfaces representative of current femoral surfaces in joint replacement. Proceeding of the Institution of Mechanical Engineers. 1997, Vol. 211 Part H. 17-24.

22. Fox, Stuart Ira. Human Physiology, Fourth Edition. Wm. C. Brown Publishers: Dubuque, Iowa, 1993.

23. Kobayashi, A; Bonfield, W; Kadoya, Y; Yamac, T; Freeman, MAR; Scott G and Revell, PA. The size and shape of particulate polyethylene wear debris in total joint replacements. Proceeding of the Institution of Mechanical Engineers. 1997, Vol. 211 Part H. 11-15.

24. McGee, MA; Howie, DW; Neale, SD; Haynes, DR and Pearcy, MJ. The role of polyethylene wear in joint replacement failure. Proceeding of the Institution of Mechanical Engineers. 1997, Vol. 211 Part H. 65-72.

25. Mallory, Thomas H. The Radiographic Identification of Osteolysis: A View from the Box. Orthopedics – Special Online Edition. . Presented at the Continuing Medical Education program, Current Concepts in Joint Replacement. Mt. Sinai Medical Center and Case Western Reserve University, December 12-14, 1996.

26. Ducheyne, P. The fixation of permanent implants: A functional assessment. In Functional Behavior of Orthopedic Biomaterials. Volume II: Applications. Ed. Paul Ducheyne and Garth W. Hastings. CRC Press, Inc: Boca Raton, FL, 1984. pp. 3-20.

27. Sauer, Willard L.; Weaver, Kevin D.; Beals, Neil B. Fatigue performance of ultra-high-molecular weight polyethylene: effect of gamma radiation sterilization. Biomaterials, 1996, Vol. 17 NO. 20.

28. Zimmer, Inc. MGII Total Knee System. Warsaw, Indiana.

Additional References

29. Baird, Donald G. and Collias, Dimitris I. Polymer Processing: Principles and Design. Butterworth-Heinemann: Boston, 1995.

30. Bioengineering Laboratory II Manual. Experiment 5: Material Science of the Artificial Hip. University of Pennsylvania. Philadelphia, PA. Spring 1997. .

31. Bigsby, RJA; Hardaker, CS and Fisher, J. Wear of ultra-high molecular weight polyethylene acetabular cups in a physiological hip joint simulator in the anatomical position using bovine serum as a lubricant. Proceeding of the Institution of Mechanical Engineers. 1997, Vol. 211 Part H. 265-269.

32. Semlitsch, M and Willert, HG. Clinical wear behaviour of ultra-high molecular weight polyethylene cups paired with metal and ceramic ball heads in comparison to metal-on-metal pairings of hip joint replacements. Proceeding of the Institution of Mechanical Engineers. 1997, Vol. 211 Part H. 73-87.

33. Silver, Frederick and Doillon, Charles. Biocompatibility: Interactions of Biological and Implantable Materials. Vol. 1: Polymers. VCH Publishers, Inc., New York, 1989.

Appendix

Figure A-1: Wear measurements on three block samples at 100, 200, and 300 lbs of normal force. The sample subjected to 300 lbs normal force had best surface contact and least margin of error.

-----------------------

[pic]

[pic]

[pic]

[pic]

[pic]

................
................

In order to avoid copyright disputes, this page is only a partial summary.

Google Online Preview   Download