1、Designation: F 2003 02Standard Practice forAccelerated Aging of Ultra-High Molecular WeightPolyethylene after Gamma Irradiation in Air1This standard is issued under the fixed designation F 2003; the number immediately following the designation indicates the year oforiginal adoption or, in the case o
2、f revision, the year of last revision. A number in parentheses indicates the year of last reapproval. Asuperscript epsilon (e) indicates an editorial change since the last revision or reapproval.1. Scope1.1 It is the intent of this practice to permit an investigatorto evaluate the oxidative stabilit
3、y of UHMWPE materials as afunction of processing and sterilization method. This practicedescribes a laboratory procedure for accelerated aging ofultra-high molecular weight polyethylene (UHMWPE) speci-mens and components for total joint prostheses. The UHM-WPE is aged at elevated temperature and at
4、elevated oxygenpressure, to accelerate oxidation of the material and therebyallow for the evaluation of its long-term chemical and me-chanical stability.1.2 Although the accelerated-aging method described bythis practice will permit an investigator to compare theoxidative stability of different UHMW
5、PE materials, it isrecognized that this method may not precisely simulate thedegradative mechanisms for an implant during real-time shelfaging and implantation.1.3 The accelerated aging method specified herein has beenvalidated based on oxidation levels exhibited by certain shelf-aged UHMWPE compone
6、nts packaged in air and sterilizedwith gamma radiation. The method has not been shown to berepresentative of shelf aging when the UHMWPE is packagedin an environment other than air. For example, this practice hasnot been directly correlated with the shelf life of componentsthat have been sealed in a
7、 low-oxygen package, such asnitrogen. This practice is not intended to simulate any changethat may occur in UHMWPE following implantation.1.4 This standard does not purport to address all of thesafety concerns, if any, associated with its use. It is theresponsibility of the user of this standard to
8、establish appro-priate safety and health practices and determine the applica-bility of regulatory limitations prior to use.2. Referenced Documents2.1 ASTM Standards:D 883 Terminology Relating to Plastics2F 648 Specification of Ultra-High Molecular-Weight Poly-ethylene Powder and Fabricated Form Surg
9、ical Implants3F 1714 Guide for Gravimetric Wear Assessment of Pros-thetic Hip-Designs in Simulator Devices3F 1715 Guide for Gravimetric Wear Assessment of Pros-thetic Knee-Designs in Simulator Devices32.2 ISO Standards:ISO 5834 Implants for surgeryUltra-high molecularweight polyethylene4ISO 14242 Im
10、plants for surgeryWear of total hip jointprostheses4ISO 14243 Implants for surgeryWear of total knee jointprostheses43. Terminology3.1 DefinitionsFor definitions of terms in this practicerelating to plastics, refer to Terminology D 883. For definitionsof terms in this practice relating to UHMWPE, re
11、fer toSpecification F 648 and ISO 5834.3.2 Definitions of Terms Specific to This Standard:3.2.1 oxidation, nthe incorporation of oxygen into an-other molecule (for example, UHMWPE) by means of achemical reaction, resulting in the formation of a chemicalcovalent bond.3.2.2 oxygen bomb, na pressure ve
12、ssel suitable for pre-conditioning of UHMWPE at an elevated temperature andpartial pressure of oxygen.4. Significance and Use4.1 This practice summarizes a method that may be used toaccelerate the oxidation of UHMWPE components usingelevated temperature and elevated oxygen pressure. Underreal-time c
13、onditions, such as shelf aging and implantation,oxidative changes to UHMWPE after sterilization using highenergy radiation may take months or years to produce changesthat may result in deleterious mechanical performance. The1This practice is under the jurisdiction of ASTM Committee F04 on Medical an
14、dSurgical Materials and Devices and is the direct responsibility of SubcommitteeF04.15 on Material Test Methods.Current edition approved Dec. 10, 2002. Published February 2003.2Annual Book of ASTM Standards, Vol 08.01.3Annual Book of ASTM Standards, Vol 13.01.4Available from American National Standa
15、rds Institute (ANSI), 25 W. 43rd St.,4th Floor, New York, NY 10036.1Copyright ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959, United States.method outlined in this practice permits the evaluation ofoxidative stability in a relatively short period of time (fo
16、rexample, weeks).4.2 This practice may also be used to oxidize UHMWPEtest specimens and joint replacement components prior tocharacterization of their physical, chemical, and mechanicalproperties. In particular, this practice may be used for acceler-ated aging of UHMWPE components prior to evaluatio
17、n in ahip or knee joint wear simulator as outlined in Guide F 1714(hip wear), Guide F 1715 (knee wear), ISO 14242 (hip wear),or ISO 14243 (knee wear), or combination thereof.5. Apparatus5.1 Combined ApparatusAn oxygen bomb (pressure ves-sel) apparatus that is capable of maintaining the desiredtemper
18、ature with an accuracy of 62C by itself may be used,providing it incorporates the requirements of 5.2-5.4.5.2 Pressure VesselIf a combined apparatus is not used, itwill be necessary to enclose the specimens within a pressurevessel, also known as an “oxygen bomb,” capable of with-standing a static pr
19、essure of 690 kPa (100 psi). The pressurevessel shall be manufactured from stainless steel. The pressurevessel shall be equipped with either a regulator or a safetyrelease valve to maintain the internal pressure to the desiredvalue, when at equilibrium, to an accuracy of 67 kPa (61 psi).5.3 Because
20、oxygen-air mixtures will be maintained atelevated temperatures for weeks at a time, it is recommendedthat a laboratory that is performing aging at elevated pressuretake appropriate safety precautions. For this reason, the use ofa commercially available and properly validated “oxygenbomb” is recommen
21、ded.5The pressure vessel must be ofsuitable construction such that it does not leak, thereby leadingto the reduction of pressure during the two-week aging period.NOTE 1Oxygen flow and test interruption have been shown tosignificantly influence the outcome of accelerated aging studies. Conse-quently,
22、 the pressure vessel must maintain nearly constant pressure (thatis, within 67 kPa or 1 psi) throughout the duration of the testing period,or the results may not be reproducible or may be unreliable.5.4 Thermal ChamberIf a combined apparatus is notused, accelerated aging of the UHMWPE shall be condu
23、ctedusing a thermal chamber that can maintain the desired tem-perature with an accuracy of 62C. The spatial variation oftemperature within the thermal chamber shall be measuredusing thermocouples and verified to be less than 61C. Notethat the thermal chamber will need to be sufficiently large toacco
24、mmodate the pressure vessel, described in 5.2.5.5 Temperature ControllerThe combined apparatus orthermal chamber shall be equipped with a temperature control-ler, capable of controlling the heating rate with an accuracy of0.1C/min.6NOTE 2Temperature stability and test interruption has been shown tos
25、ignificantly influence the outcome of accelerated aging studies. Conse-quently, the pressure vessel must maintain nearly constant temperature(that is, within 61C) throughout the duration of the testing period, or theresults may not be reproducible or may be unreliable.6. Test Specimens6.1 The test s
26、pecimens shall be prepared in final formaccording to the requirements of any subsequent physical,chemical, or mechanical tests to be performed after acceleratedaging. For example, if the specimens will ultimately besubjected to hip joint simulation, they should be prepared infinal form according to
27、Guide F 1714 and ISO 14242.6.2 Finished specimens shall not be machined after accel-erated aging of (bulk) stock materials, because the acceleratedoxidation procedure outlined in this practice will result in aninhomogeneous distribution of chemical, physical, and hencemechanical properties through t
28、he thickness of an aged part.6.3 Test specimens shall be removed from their packagingprior to accelerated aging, because this practice is not intendedto reproduce the aging of UHMWPE that is stored in a lowoxygen environment.7. Validation of Apparatus7.1 Thermal Chamber ValidationUsing the calibrate
29、dtemperature sensor, validate the temperature of the acceleratedaging apparatus to within 61C of the aging temperature.7.1.1 Verify the calibration of the temperature sensor(s) thatwill be used to validate the thermal conditions in the acceler-ating aging apparatus. The temperature sensor shall be c
30、ali-brated as defined in the manufacturers instructions.7.2 Pressure Vessel ValidationVerify the integrity of thepressure vessel to within 67 kPa (61 psi) by conducting thefollowing 14-day (336 6 1 h) validation test:7.2.1 Increase the pressure of pure oxygen inside the vesselby 503 kPa (73 psi) at
31、70 6 1C.7.2.2 Throughout the duration of the validation test, thegage pressure shall not vary by 67 kPa (61 psi).7.2.3 Pressure vessels that are not capable of maintainingthe target gage pressure within the stated tolerance shall beconsidered invalid for the purposes of accelerated aging untilthe ex
32、cessive leaking has been rectified.7.3 The thermal chamber and pressure vessel shall bevalidated at least once per year, unless otherwise indicated bya specification or customer.8. Conditioning8.1 After high energy irradiation, specimens shall be main-tained at 23 6 2C (73.4 6 3.6F) for 28 days, sta
33、rting fromthe date of irradiation, prior to commencing accelerated aging,unless otherwise directed by the customer.8.2 After irradiation, specimens shall remain in their origi-nal packaging during the preconditioning period.8.3 Unirradiated specimens shall be maintained in a stan-dard laboratory env
34、ironment of 23 6 2C (73.4 6 3.6F) for40 6 1 h prior to commencing accelerated aging.9. Procedure9.1 Specimen OrientationTest specimens shall be arrayedwithin the test chamber or oxygen bomb such that all relevantsurfaces have equivalent access to oxygen during the test. Forexample, with hip and knee
35、 components, the articulating5Pressure vessels available from Advantec MFS, Inc., 6691 Owens Drive,Pleasanton CA, 94588-3335 have been found satisfactory for this purpose.6Air convection ovens available from Cole Parmer Instrument Company,http:/ have been found satisfactory for this purpose.F2003022
36、surface which may subsequently be subjected to wear simula-tion shall not be obstructed or covered by other parts ormaterials that might interfere with uniform access of thesurface to oxygen.9.2 PressurizationThe pressure vessel shall be filled atroom temperature and purged with oxygen at least thre
37、e timesprior to starting the aging experiment. For example, at astandard laboratory environment, a change in pressure of 62.56 1 psi will be needed, such that the pressure increases to 503kPa (73 psi) as the at the target aging temperature of 70C isreached.9.3 Standard Relative HumidityWater shall n
38、ot be addedto the pressure vessel during accelerated aging. The user shouldbe aware that adding water to the test chamber may affect theoxidation mechanism during the accelerated aging process.9.4 Initial Temperature and Heating RateThe pressurevessel is initially at standard laboratory temperature
39、(23 62C) and will be gradually raised to the aging temperature of70C. The initial heating rate will be 1.0 6 0.1C/min.9.5 Accelerated AgingSpecimens are to be aged at aconstant temperature of 70 C and at an equilibrium gagepressure of 503 kPa (73 psi, 5 atmospheres) of pure oxygen for336 6 1 h (14 d
40、ays) prior to subsequent testing. There are tobe no interruptions of the aging period (that is, no opening ofthe pressure vessel).9.6 Recording During the TestTemperature and pressurerecordings should be logged daily during the test period to noteany potential changes of the experimental conditions.
41、9.7 Guide for Subsequent TestingSpecimens shall besubjected to further testing within two weeks after acceleratedaging.10. Reporting of Specimen Preparation and TestConditions10.1 The written report shall include details regarding thepreparation of the test samples, the chronology of the acceler-ate
42、d aging, and the storage conditions for the test samples.10.2 Test Sample PreparationThe investigator shall listthe size, shape, and method of manufacture of the test samples.The report shall also contain the type of resin used, themanufacturer/supplier of the UHMWPE, and any subsequentprocesses tha
43、t were performed on the test articles after manu-facture, such as sterilization or high energy irradiation.10.3 ChronologyThe report shall list the time at which thetest specimens were manufactured, subsequently sterilized, andlater aged. The report will also report the time that anysubsequent analy
44、sis or testing was performed on the ageditems.10.4 Test Sample Storage ConditionsIt is important todocument the storage conditions of the test samples before andafter accelerated aging. The report shall indicate the environ-mental conditions (that is, storage in air versus nitrogen) andtemperature u
45、nder which the specimens were stored.11. Keywords11.1 aging; oxidation; preconditioning; stability; UHM-WPE; UHMW PE; ultra-high molecular weight polyethyleneAPPENDIX(Nonmandatory Information)X1. RationaleX1.1 Post-irradiation aging results in degradative changesto the physical, chemical, and mechan
46、ical properties of UHM-WPE. (2, 3) Even under ambient conditions, oxidation ofirradiated UHMWPE evolves at a slow pace, with a degrada-tion rate measured in years. (2) As a result, accelerated agingtest methods have been developed in the past four years toaccelerate the oxidation process in UHMWPE a
47、nd therebyassess oxidative stability during a comparatively short timeperiod.X1.2 Oxidation of UHMWPE proceeds in a complexcascade of chemical reactions, which may be accelerated byincreasing the temperature or by increasing the concentrationof available oxygen, or both. (4) Consequently, in several
48、studies, post-irradiation aging has been simulated using acombination of thermal oxidation and elevated oxygen pres-sure. (1, 5, 6) Despite the variation in test conditions reportedby these studies, accelerated oxidation protocols have increas-ingly been employed not only to characterize the effects
49、 ofgamma sterilization in air, but also to evaluate the oxidationresistance of UHMWPE sterilized by alternative methods.X1.3 Accelerated oxidation test methods for UHMWPE arenot without their limitations. Even though the method outlinedin this practice is now widely used for accelerated agingUHMWPE specimens prior to mechanical testing, the questionremains as to whether or not the thermal technique preciselyrecreates the morphology and mechanical properties of shelf-aged UHMWPE. (7, 8) Although research is still needed toelucidat
