Pin-on-Disk Wear Testing of Biomaterials Used for Total Joint Replacements

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Pin-on-Disk Wear Testing of Biomaterials Used for Total Joint Replacements

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Radovan Zdero1, Leah E. Guenther2, Trevor C. Gascoyne2 Western University, London, ON, Canada1; Orthopaedic Innovation Centre, Winnipeg, MB, Canada2

1. BACKGROUND Osteoarthritis is a joint disease caused by cartilage deterioration in the shoulder, hip, and knee, thereby resulting in bone-on-bone grinding, functional impairment, and pain.1 Every year, about one million people worldwide are implanted with a total joint replacement (TJR) to restore function and reduce pain (Fig. 19.1A).2 TJRs are made from biomaterial combinations like metal-on-metal, ceramic-on-ceramic, and ceramic-on-polymer, but most commonly, metal-on-polymer.1,3 Metal components

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FIGURE 19.1 Total knee replacement (TKR). (A) Main components of a TKR implanted in artificial sawbones, (B) unused polymer component of a TKR, (C) explanted polymer component of a TKR with substantial wear. Images courtesy of Orthopaedic Innovation Centre, www.OrthoInno.com.

Experimental Methods in Orthopaedic Biomechanics. http://dx.doi.org/10.1016/B978-0-12-803802-4.00019-6 Copyright © 2017 Elsevier Inc. All rights reserved.

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may be fabricated from cobaltechrome alloy, stainless steel, or titanium alloy. Ceramic parts are usually manufactured from alumina. The polymer mostly employed is ultra-high molecular weight polyethylene (UHMWPE). Once implanted, TJRs experience forces and motions that create wear debris, which may cause osteolysis, device loosening, and revision surgery as early as 6e8 years post-arthroplasty (Fig. 19.1B and C).1 Implant manufacturers use commercially available joint simulators that mimic clinical conditions to optimize TJR wear performance, but these machines are expensive to set up and maintain. In response, researchers often use “classic” pin-on-disk (POD) wear testers, which are easy to use, relatively inexpensive, and can reasonably mimic clinical wear conditions.2,4e12 Therefore, this chapter explains how to perform POD wear testing of TJR biomaterials, as well as how to analyze, report, and interpret results.

2. RESEARCH QUESTIONS Typical research questions might include one or more of the following: • • • • • •

How does force, lubricant, motion, surface roughness, etc., affect a given biomaterial’s wear? Do biomaterials 1 vs. 2 vs. 3, etc., have a different wear performance for the same test conditions? Do biomaterials 1 vs. 2 vs. 3, etc., produce wear debris that has the same visual appearance? Do different POD research laboratories and wear testers produce the same wear results? Which POD wear testing parameters generate data most similar to clinically explanted TJRs? etc.

3. METHODOLOGY 3.1 GENERAL STRATEGY POD wear testing is done using established protocols for the most common TJR biomaterials, namely, UHMWPE articulating against cobaltechrome (CoCr).13e15 UHMWPE cylindrical pins and CoCr circular disks are fabricated, UHMWPE pins are presoaked in test lubricant, and both the pins and disks are mounted in a POD wear tester. Contact stress is applied to the POD, cyclic interfacial motion is employed for a certain number of cycles, and then pins are removed for weight change analysis (i.e., gravimetry). Both the pins and disks may then undergo surface roughness analysis (i.e., profilometry). Statistical comparisons of wear parameters (e.g., cumulative weight loss, average surface roughness, and wear rate) are made between test groups. Finally, correlations are made for wear outcomes vs. test groups (e.g., pin size, contact stress, number of cycles, etc.) to determine vital factors.

3. Methodology

GLOSSARY ✓ ✓ ✓ ✓ ✓ ✓

CoCr. Cobaltechrome alloy that is commonly used to construct components of a TJR. Lubricant. A fluid medium intended to mimic synovial fluid in human synovial joints. POD. Pin-on-disk configuration, which simulates TJR interfacial articulation. TJR. Total joint replacement, such as those used for the shoulder, hip, or knee. UHMWPE. Ultra-high molecular weight polyethylene used to fabricate TJRs. Wear. Degradation of surface quality and material due to interfacial articulation.

SAFETY FIRST ✓ ✓ ✓ ✓ ✓

Read the material safety data sheet for chemicals before use and exposure. Sodium azide is an extremely hazardous poison that should be used with caution. Handle test lubricants only while wearing goggles, gloves, a mask, and a lab coat. Clean test lubricant spills with soapy water, and then rinse with alcohol and air dry. Keep clothes, long hair, and fingers away from the POD tester’s moving parts during use.

3.2 MATERIALS AND TOOLS LIST • • • • • • • • • •

fixtures (mounting plate, acrylic parts, O-rings, etc.) fume hood microbalance POD wear tester and computer software profilometer (noncontact or contact type) scanning electron microscope (SEM) silicone sealant temperature control unit and/or hot plate test lubricant (e.g., bovine blood serumebased) test pins and disks

3.3 SPECIMEN PREPARATION Step 1. Determine the number of groups and specimens. Determine how many test groups, as well as how many specimens (i.e., pins and disks) per group are required. This is important in order to answer the research question in a statistically rigorous way. Consequently, a typical POD wear study can use anywhere from 3d25 specimens per test group.2,5,6,10,13 Step 2. Fabricate the pins. Prepare a polymer pin to mimic the articulating UHMWPE surface of a TJR (Fig. 19.2A).2,5,6,8,10,11,13,14 Obtain a long, cylindrical rod of medical grade UHMWPE, which may or may not be gamma irradiated with 2.5e5 Mrad. Use a band saw to cut the rod into preliminary segments that are about 20 mm long so the ends can be gripped during the fabrication process. Then, use a

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FIGURE 19.2 Specimen preparation for POD wear tests. (A) UHMWPE pin, (B) CoCr disk, (C) UHMWPE pins presoaking in bovine blood serumebased lubricant. Images courtesy of Orthopaedic Innovation Centre, www.OrthoInno.com.

lathe to reduce the outer surface to a 2e10 mm diameter, use a band saw to cut the rod into segments that are 12e13 mm long, and use a milling machine or lathe to make the ends flat with an average surface roughness of Ra
0.8 mm. Step 3. Fabricate the disks. Prepare a metal disk to mimic the articulating CoCr surface of a TJR (Fig. 19.2B).2,6,8,13 Obtain a medical grade CoCr plate with a 2e10 mm thickness. Cut out a circular disk using an industrial metal band saw or milling machine. Reduce the disk’s outer edge roughness using a grinding wheel or milling machine. Machine the disk’s outer edge to a 30e100 mm diameter using a lathe. Work the main surface by dry grinding, then wet grinding with #1200 and #2400 grit paper, and then cloth wheel polishing using 6-mm (and subsequently 1mm) diamond paste. The resulting surface should have an average roughness of Ra
0.8 mm. Step 4. Prepare the lubricant. In a fume hood, prepare a lubricant simulating human joint synovial fluid.5e11,14,15 Obtain commercially available, filtered, sterilized, non-iron-supplemented bovine blood serum. Then, dilute it with deionized water or phosphate buffered saline solution to a chosen bovine blood serum protein concentration of 20e72 g/L. To inhibit microbial growth, add an amount of sodium azide that is 0.2e0.3% of lubricant total mass. The lubricant can now be frozen at 15 to 20 C for up to 5 years or refrigerated at 2e8 C for up to 30 days prior to use. Step 5. Presoak the pins. Fully immerse each pin into a glass beaker containing about 150 mL of lubricant (Fig. 19.2C).2,5,6,11,14,16 Keep the lubricant at 37  3 C using a temperature control unit or hot plate for a minimum of 5 weeks to reproduce in vivo human body conditions. Use a watch glass to weigh down the pins. While most pins eventually will be wear tested, several pins can act as “soak controls” by permanently remaining in lubricant. This minimizes lubricant fluid uptake by pins during wear tests, as well as eliminating weight change measurement artifacts.

3. Methodology

Step 6. Clean the pins. Remove each pin from the lubricant bath in which it was presoaked and clean it, as follows: rinse it in warm water with a small amount of laboratory liquid detergent, wipe off any lubricant residue using a nonabrasive cloth, rinse it with deionized water, dry it with nitrogen gas, soak it in 95% methyl alcohol for 5 min, and dry it again with nitrogen gas.16 Step 7. Measure the pins. Use a precision microbalance with a 10-mg resolution to weigh each pin to the nearest 0.1 mg. Then, measure the surface roughness of each pin surface using a noncontact profilometer. Do this three times in order to get an average weight and an average surface roughness for each pin. Return the “soak control” pins to their lubricant baths, whereas the other pins are now ready to be wear tested. TIPS AND TRICKS ✓ The same researcher(s) should perform all POD wear tests for consistency in results. ✓ Metal disks can be successfully polished using a spinning cloth wheel and diamond paste. ✓ Apply silicone sealant along the inside edges of test chambers to prevent lubricant leaks.

THE “GOLD STANDARD” Researchers should consult the American Society for Testing and Materials (ASTM) documents ASTM G99 (Standard test method for wear testing with a pin-on-disk apparatus), ASTM F732 (Standard test method for wear testing of polymeric materials used in total joint prostheses), and ASTM F2025 (Standard practice for gravimetric measurement of polymeric components for wear assessment). Previously published peer-reviewed journal articles may also be useful.

3.4 SPECIMEN TESTING Step 1. Choose a wear tester. The POD wear tester should produce relative motion between the pin and disk that generates a change in shear direction, which mimics in vivo TJR conditions and wear results (Fig. 19.3A).2,4,7,8,10,13,14 Specifically, the motion should be “multidirectional” or “circularly translating” between a stationary (or moving) pin and a moving (or stationary) disk. This is in contrast to some wear testers that only produce linear reciprocation or unidirectional rotation, which do not mimic TJR conditions. Also, the POD wear tester should be able to precisely control or monitor the force, speed, frequency, and lubricant temperature. Step 2. Secure the pins. Insert each pin perpendicular to the plane of motion (i.e.,  1 ) into its own overhead pin holder in the POD wear tester (Fig. 19.3B).13,14 A typical pin holder is composed of a hollow shaft made of metal or plastic into which the pin is press-fitted by hand or by a special hand tool and then brought down into direct contact with the disk.

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FIGURE 19.3 Typical POD wear tester. (A) Assembled POD wear tester, (B) UHMWPE pins secured in the holder, (C) CoCr disks secured in the test chamber. Images courtesy of Orthopaedic Innovation Centre, www.OrthoInno.com.

Step 3. Secure the disks. Insert each disk perpendicular to the plane of motion (i.e.,  1 ) into its own designated station in the test chamber of the POD wear tester (Fig. 19.3C).13,14 A typical test chamber is composed of an acrylic circular tube surrounding a stainless steel plate with or without through-holes and screws to accommodate different disk diameters. Place plastic plugs with O-rings into any empty through-holes to prevent lubricant from leaking out during wear testing. Apply a line of silicone sealant along the inside edges of the test chamber to reduce leakage. Step 4. Configure the tester. The POD wear tester should be configured manually or by computer control for each test group by choosing parameters that represent a particular TJR in vivo clinical condition, such as POD contact stress (¼applied force/cross-sectional area ¼ 1.1e3.5 MPa),2,4,7,11,14 sliding frequency (1 Hz),7,14 sliding speed (24e50 mm/s),7,14 sliding distance (25e35 mm/cycle),2,14 total number of cycles (1e7 Mc or million cycles),2,7,9,10 and lubricant temperature (37  3 C).7,14 Of course, different nonclinical levels may be used, depending on the research question. Step 5. Run the test. Begin wear testing with pins and disks in contact. Stop the test after every “interval” (i.e., every 250,000e500,000 cycles),2,5e7,10 and then perform these tasks: disassemble test chambers, dispose old lubricant, clean the pins, measure pin weight using a microbalance, measure pin surface roughness using a noncontact profilometer, photograph any interesting features on the disk’s wear path (e.g., cracks, discoloration, protrusions, etc.), add new lubricant to the test chamber, resecure the same pins, and continue wear testing under the same test conditions. Repeat this after every interval until the total number of wear cycles is completed.

3. Methodology

3.5 RAW DATA COLLECTION Step 1. Record specimen characteristics. Before each wear test, ensure that important information is documented on pins and disks, such as physical dimensions, material name, and supplier. Lubricant name, composition, and bovine serum supplier should also be noted. Step 2. Record test groups. Each test group represents a change in some numerical or categorical factor that is controlled by the researcher (e.g., contact stress, sliding frequency, sliding speed, sliding distance, pin density, lubricant protein concentration, pin material, etc.) in order to assess its influence on wear performance (Table 19.1). Step 3. Record wear outcomes. Wear outcomes are quantities that are measured by the researcher (i.e., pin cumulative weight loss DW and pin average surface roughness Ra), which represent wear performance of the pin material (Table 19.1). Keep several things in mind. First, the weight loss of the UHMWPE wear pin is adjusted to account for fluid absorption during testing. At a certain test interval, the net gravimetric wear DW (i.e., net weight loss) of the UHMWPE wear pin is the apparent weight loss of the individual wear pin (Wloss) plus the average weight gain of the soak control pins (Sgain). Therefore, at a certain test interval, DW ¼ Wloss þ Sgain ¼ (Wi  Wf) þ (Sf  Si), where i is initial weight and f is final weight. Second, if time is of the essence, Ra can be measured before the study begins and after the very last interval of the study, rather than at each interval, since it can be a timeconsuming process. Third, the initial phase of a wear test is called “running-in wear,” which is a nonsteady-state phenomenon that occurs for many materials starting from 0 cycles to as high as 1 Mc. Data within this range may be collected since they provide some information about the material’s behavior, but they are ignored later when analyzing and presenting the study’s main results for steady-state wear. Step 4. Record wear surface features. Before the wear study begins and once all tests are done at the end of the last interval, obtain SEM images and/or Table 19.1 Raw POD wear test data.

Test Group

Wear Outcome

1

DW [mg] Ra [mm] DW [mg] Ra [mm] DW [mg] Ra [mm]

2 3

“Running-in Wear” Intervals

Steady-State Intervals

1

1

2

3

etc.

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etc.

etc. Ra, pin average surface roughness; DW, pin cumulative weight loss. Each numerical entry of DW and Ra measurements has an average  1 standard deviation or 95% confidence interval.

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photographs of the pin’s wear surface to detect any changes or damage (e.g., cracks, discoloration, protrusions, etc.).

3.6 RAW DATA ANALYSIS Step 1. Calculate correlation coefficients. Graphically plot all wear outcomes vs. test groups to visualize their interrelationship; then, calculate the correlation coefficient R for each line of best fit to determine the strength of the interrelationship. R > 0.8 is considered to indicate a strong correlation and that steady-state wear was achieved. Step 2. Perform statistical comparisons. Determine the criterion for statistical difference (e.g., P <0.01 or <0.05). Choose from among the various software programs for comparing two test groups (e.g., unpaired t-test) or two or more test groups influenced by multiple factors (e.g., analysis of variance, ANOVA). Parametric or nonparametric statistical tests are chosen, depending on whether or not results have a normal distribution. Step 3. Compute statistical power. Power analysis can be done after the study to ensure there were enough specimens per test group to detect all statistical differences that were actually present (i.e., was type II statistical error avoided?). Statistical power >80% is usually considered to indicate there were enough specimens per test group. Note that if good predictions of averages and standard deviations are available from prior studies, then the number of specimens and/or tests can be chosen before the study begins to ensure a power >80%. ENGINEER’S TOOLBOX Wear factor is a traditional way to report the pin material’s performance during POD wear testing. Wear factor is based on Archard’s Law, which states that the amount of wear debris from the pin is proportional to the applied force and sliding distance. This is expressed mathematically as _ K ¼ W/(rFD), where K is wear factor [mm3/(N$m)], W_ is wear rate of removed material [mg/Mc], r is material density [mg/mm3], F is applied force [N], and D is sliding distance [m/Mc]. (Note: The unit Mc means million cycles.)

4. Results

4. RESULTS Once all POD wear test data collection and analysis have been performed, it is then important to communicate and present the primary results in an understandable and concise manner to the reader of a journal article, conference paper, technical report, or book chapter. Step 1. Show the main steady-state results. Cumulative weight loss DW and average surface roughness Ra vs. number of cycles should be given (Fig. 19.4A and B). Statistical P values should be stated for pairwise comparisons between wear outcomes at each interval (e.g., DW for test group 1 vs. 2 vs. 3 at interval 1, etc.) and between wear outcomes for each test group (e.g., DW at interval 1 vs. 2 vs. 3 for test group 2, etc.). For each line of best fit, several items can be presented: an equation y ¼ mx þ b with slope m and intercept b, a linear correlation coefficient R, and its own P value to ensure that the slope of each line of best fit (i.e., slope ¼ m) is statistically different than a horizontal line (i.e., slope ¼ 0). (Note: “Running-in wear” data do not represent steady-state wear; thus, they are excluded from the lines of best fit. Also, data from soak control pins are not shown since they do not undergo wear testing and are only used to correct wear data for fluid absorption.) Step 2. Present steady-state wear rates. The slope m for each DW line of best fit is known as the “gravimetric wear rate” W_ in milligrams per million cycles

FIGURE 19.4 Steady-state wear outcomes. (A) Cumulative weight loss, (B) average surface roughness, (C) average wear rate. Each data point is an average  1 standard deviation or  95% confidence interval. P indicates the many statistical difference values obtained for each pairwise comparison between test groups and between intervals which can be indicated by symbols like asterisks (*), pounds (£), etc. Each line of best fit is represented by an equation y ¼ mx þ b, a linear correlation coefficient R, and its own statistical P value to ensure that its slope (i.e., slope ¼ m) is statistically different than a horizontal line (i.e., slope ¼ 0).

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FIGURE 19.5 Polymer pin surfaces. (A) SEM image of pin surface showing concentric machining marks (prestudy), (B) SEM of pin surface showing wear (poststudy). Images courtesy of Orthopaedic Innovation Centre, www.OrthoInno.com.

[mg/Mc], which should also be statistically compared between test groups (Fig. 19.4C). The material with the lower wear rate is considered to have superior resistance to wear. Step 3. Illustrate wear patterns. Provide SEM images and/or photos of prestudy and poststudy pin wear surfaces to identify wear mechanisms of interest, such as abrasion, burnishing, deformation, delamination, embedded debris, pitting, and scratching (Fig. 19.5). ALTERNATIVES AND ADAPTATIONS ✓ Biomaterial options. POD wear tests can be done for a variety of biomaterial combinations, such as metal-on-metal, metal-on-polymer, ceramic-on-ceramic, ceramic-on-polymer, bone-on-metal, cartilage-on-metal, etc., representing different TJRs. ✓ Joint simulators. These commercially available machines perform wear tests on actual TJRs that would be implanted in the shoulder, spine, hip, and knee. The six degree-of-freedom forces and motions are those experienced in the human body; however, these machines are very expensive.

5. DISCUSSION After completing all POD wear tests, data collection, data analysis, and data presentation, then final results can be considered and interpreted in the broader context of some important clinical, biomechanical, and/or technological considerations, as follows. Osteoarthritis of weight-bearing synovial joints (e.g., shoulder, hip, knee) causes cartilage wear, bone-on-bone grinding, functional impairment, and pain.1 Annually, one million people worldwide receive a TJR for this condition.2 Most TJRs work via metal-on-polymer articulation at the joint interface, but in vivo forces and motions

7. Quiz Questions

create polymer wear debris through wear mechanisms like abrasion, burnishing, deformation, delamination, embedded debris, pitting, and scratching.3,17 Subsequently, polymer wear debris launches a cascade of important events: auto-immune response / osteolysis (i.e., bone resorption) / implant loosening / implant failure / revision surgery.1 Standardized POD wear test protocols have been developed in order to predict and reproduce in vivo clinical wear of TJRs.13,14 However, the studies that have been particularly successful at doing this have incorporated several key factors, such as a contact stress of 1.1e3.5 MPa.2,4,7,11,14 Multidirectional or circularly translating motion that changes the cross-shear direction,2,4,7,8,10,13,14 and bovine blood serumebased lubricant, which permits some polymer surface scratching via third body wear.4e11,14 Even so, the standardized test protocols do not always fully incorporate the latest research findings and, thus, may need to be occasionally updated to reflect clinical conditions. POD wear testing of UHMWPE can produce a range of numerical results, depending on the type of UHMWPE used and the experimental conditions. For example, average wear rate W_ can range from 1.69d13.55 mg/Mc (standard UHMWPE),2,5,8,9 0.41e2.06 mg/Mc (generic cross-linked UHMWPE),2 0.15e6.43 mg/Mc (commercial cross-linked UHMWPE),5,6 and 4.28e36.68 mg/ Mc (explanted UHMWPE from TJR patients).7 Similarly, average surface roughness Ra can reach values of 0.88 mm (standard UHMWPE),2 0.68 mm (generic crosslinked UHMWPE),2 and 0.007e0.012 mm (commercial cross-linked UHMWPE).6

6. SUMMARY • • • • • •

Osteoarthritis is a joint disease that can be addressed by surgical implantation of a TJR. TJRs can experience different types of wear mechanisms leading to component damage. POD wear testers have been developed that reasonably reproduce in vivo TJR conditions. POD wear tests assess the effects of contact stress, material, sliding type, and lubrication. POD wear tests can provide pin cumulative weight loss, surface roughness, and wear rate. POD wear tests on UHMWPE pins can yield a wide range of numerical results.

7. QUIZ QUESTIONS 1. Why does osteoarthritis often lead to surgical implantation of a TJR? 2. What are three wear mechanisms that are commonly experienced by an implanted TJR?

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3. What are the different types of motion that can be created by various POD wear testers? 4. What are the main factors that researchers can control during a typical POD wear test? 5. Calculate the wear factor of a UHMWPE pin during a POD wear test with these pin characteristics: gravimetric wear rate (10 mg/Mc), applied force (157 N), density (0.95 mg/mm3), and sliding distance (30 mm/cycle) (answer: 2.2  106 mm3/(N$m)).

REFERENCES 1. Garino JP, Beredjiklian PK. Adult reconstruction and arthroplasty: core knowledge in orthopaedics. Philadelphia (PA, USA): Mosby Elsevier; 2007. 2. Harsha AP, Joyce TJ. Comparative wear tests of ultra-high molecular weight polyethylene and cross-linked polyethylene. Proceedings of the Institution of Mechanical Engineers (Part H): Journal of Engineering in Medicine 2013;227(5):600e8. 3. Hosseinzadeh HRS, Eajazi A, Shahi AS. “The bearing surfaces in total hip arthroplasty: options, material characteristics, and selection” (Chapter 10). In: Fokter S, editor. Recent advances in arthroplasty. Rijeka (Croatia): InTech; 2012. Available free online at: http:// cdn.intechopen.com/pdfs-wm/26863.pdf. 4. Baykal D, Siskey RS, Haider H, Saikko V, Ahlroos T, Kurtz SM. Advances in tribological testing of artificial joint biomaterials using multidirectional pin-on-disk testers. Journal of the Mechanical Behavior of Biomedical Materials 2014;31:117e34. 5. Brandt JM, Vecherya A, Guenther LE, Koval SF, Petrak MJ, Bohm ER, et al. Wear testing of crosslinked polyethylene: wear rate variability and microbial contamination. Journal of the Mechanical Behavior of Biomedical Materials 2014; 34:208e16. 6. Guenther LE, Turgeon TR, Bohm ER, Brandt JM. The biochemical characteristics of wear testing lubricants affect polyethylene wear in orthopaedic pin-on-disc testing. Proceedings of the Institution of Mechanical Engineers (Part H): Journal of Engineering in Medicine 2015;229(1):77e90. 7. Kurtz SM, MacDonald DW, Kocagoz S, Tohfafarosh M, Baykal D. Can pin-on-disk testing be used to assess the wear performance of retrieved UHMWPE components for total joint arthroplasty? Biomed Research International 2014:1e6, 581812. 8. Saikko V. A multidirectional motion pin-on-disk wear test method for prosthetic joint materials. Journal of Biomedical Materials Research 1998;41(1):58e64. 9. Saikko V. A hip wear simulator with 100 test stations. Proceedings of the Institution of Mechanical Engineers (Part H): Journal of Engineering in Medicine 2005;219(5): 309e18. 10. Saikko V. Effect of contact pressure on wear and friction of ultra-high molecular weight polyethylene in multidirectional sliding. Proceedings of the Institution of Mechanical Engineers (Part H): Journal of Engineering in Medicine 2006;220(7):723e31. 11. Saikko V, Ahlroos T. Type of motion and lubricant in wear simulation of polyethylene acetabular cup. Proceedings of the Institution of Mechanical Engineers (Part H): Journal of Engineering in Medicine 1999;213(4):301e10.

References

12. Wright KMJ, Dobbs HS, Scales JT. Wear studies on prosthetic materials using the pinon-disc machine. Biomaterials 1982;3:41e8. 13. ASTM G99. Standard test method for wear testing with a pin-on-disk apparatus. West Conshohocken (PA, USA): American Society for Testing and Materials (ASTM); www.astm.org. 14. ASTM F732. Standard test method for wear testing of polymeric materials used in total joint prostheses. West Conshohocken (PA, USA): American Society for Testing and Materials (ASTM); www.astm.org. 15. Serum Source International Inc., Charlotte (NC, USA), www.serumsourceintl.com. 16. ASTM F2025. Standard practice for gravimetric measurement of polymeric components for wear assessment. West Conshohocken (PA, USA): American Society for Testing and Materials (ASTM); www.astm.org. 17. Brown TD, Bartel DL. What design factors influence wear behavior at the bearing surfaces in total joint replacements? Journal of the American Academy of Orthopaedic Surgeons 2008;16(Suppl. 1):S101e6.

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