1、Designation: F 2003 02 (Reapproved 2008)Standard 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
2、 or, in the case of revision, the year of last revision. A number in parentheses indicates the year of last reapproval. Asuperscript epsilon () 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 o
3、xidative stability 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 te
4、mperature and at 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
5、of different UHMWPE 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-ag
6、ed UHMWPE components 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
7、 been sealed in a low-oxygen package, such asnitrogen. This practice is not intended to simulate any changethat may occur in UHMWPE following implantation.1.4 The values stated in SI units are to be regarded asstandard. The values given in parentheses are mathematicalconversions to inch-pound units
8、that are for information onlyand are not considered standard.1.5 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 establish appro-priate safety and health practices and determine the applica-
9、bility of regulatory limitations prior to use.2. Referenced Documents2.1 ASTM Standards:2D 883 Terminology Relating to PlasticsF 648 Specification for Ultra-High-Molecular-Weight Poly-ethylene Powder and Fabricated Form for Surgical Im-plantsF 1714 Guide for Gravimetric Wear Assessment of Pros-theti
10、c Hip-Designs in Simulator DevicesF 1715 Guide for Wear Assessment of Prosthetic KneeDesigns in Simulator Devices32.2 ISO Standards:4ISO 5834 Implants for surgeryUltra-high molecularweight polyethyleneISO 14242 Implants for surgeryWear of total hip jointprosthesesISO 14243 Implants for surgeryWear o
11、f total knee jointprostheses3. 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, refer toSpecification F 648 and ISO 5834.3.2 Definitions of Terms Specific to This Standard
12、: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 vessel suitable for pre-conditioning of UHMWPE at an elevated temperature andpartial pressu
13、re of oxygen.4. Significance and Use4.1 This practice summarizes a method that may be used toaccelerate the oxidation of UHMWPE components using1This practice is under the jurisdiction ofASTM Committee F04 on Medical andSurgical Materials and Devices and is the direct responsibility of SubcommitteeF
14、04.15 on Material Test Methods.Current edition approved May 1, 2008. Published June 2008. Originallyapproved in 2002. Last previous edition approved in 2002 as F 2003 02.2For referenced ASTM standards, visit the ASTM website, www.astm.org, orcontact ASTM Customer Service at serviceastm.org. For Annu
15、al Book of ASTMStandards volume information, refer to the standards Document Summary page onthe ASTM website.3Withdrawn.4Available from American National Standards Institute (ANSI), 25 W. 43rd St.,4th Floor, New York, NY 10036, http:/www.ansi.org.1Copyright ASTM International, 100 Barr Harbor Drive,
16、 PO Box C700, West Conshohocken, PA 19428-2959, United States.elevated temperature and elevated oxygen pressure. Underreal-time conditions, such as shelf aging and implantation,oxidative changes to UHMWPE after sterilization using highenergy radiation may take months or years to produce changesthat
17、may result in deleterious mechanical performance. Themethod outlined in this practice permits the evaluation ofoxidative stability in a relatively short period of time (forexample, weeks).4.2 This practice may also be used to oxidize UHMWPEtest specimens and joint replacement components prior tochar
18、acterization of their physical, chemical, and mechanicalproperties. In particular, this practice may be used for acceler-ated aging of UHMWPE components prior to evaluation 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 I
19、SO 14243 (knee wear), or combination thereof.5. Apparatus5.1 Combined ApparatusAn oxygen bomb (pressure ves-sel) apparatus that is capable of maintaining the desiredtemperature 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
20、 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 pressure of 690 kPa (100 psi). The pressurevessel shall be manufactured from stainless steel. The pressurevessel shall be equipped
21、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 oxygen-air mixtures will be maintained atelevated temperatures for weeks at a time, it is recommendedthat a laboratory that is pe
22、rforming aging at elevated pressuretake appropriate safety precautions. For this reason, the use ofa commercially available and properly validated “oxygenbomb” is recommended. The pressure vessel must be ofsuitable construction such that it does not leak, thereby leadingto the reduction of pressure
23、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, the pressure vessel must maintain nearly constant pressure (thatis, within 67 kPa or 1 psi) throughout the duration of the testi
24、ng 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 conductedusing a thermal chamber that can maintain the desired tem-perature with an accuracy of 62C. The spatial variation oftemperatu
25、re 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 toaccommodate the pressure vessel, described in 5.2.5.5 Temperature ControllerThe combined apparatus orthermal chamber shall be equippe
26、d with a temperature control-ler, capable of controlling the heating rate with an accuracy of0.1C/min.NOTE 2Temperature stability and test interruption has been shown tosignificantly influence the outcome of accelerated aging studies. Conse-quently, the pressure vessel must maintain nearly constant
27、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 specimens shall be prepared in final formaccording to the requirements of any subsequent physical,chemical, or mechanical tests to
28、be performed after acceleratedaging. For example, if the specimens will ultimately besubjected to hip joint simulation, they should be prepared infinal form according to Guide F 1714 and ISO 14242.6.2 Finished specimens shall not be machined after accel-erated aging of (bulk) stock materials, becaus
29、e the acceleratedoxidation procedure outlined in this practice will result in aninhomogeneous distribution of chemical, physical, and hencemechanical properties through the thickness of an aged part.6.3 Test specimens shall be removed from their packagingprior to accelerated aging, because this prac
30、tice is not intendedto reproduce the aging of UHMWPE that is stored in a lowoxygen environment.7. Validation of Apparatus7.1 Thermal Chamber ValidationUsing the calibratedtemperature sensor, validate the temperature of the acceleratedaging apparatus to within 61C of the aging temperature.7.1.1 Verif
31、y 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 cali-brated as defined in the manufacturers instructions.7.2 Pressure Vessel ValidationVerify the integrity of thepressure vessel t
32、o 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 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 ve
33、ssels that are not capable of maintainingthe target gage pressure within the stated tolerance shall beconsidered invalid for the purposes of accelerated aging untilthe excessive leaking has been rectified.7.3 The thermal chamber and pressure vessel shall bevalidated at least once per year, unless ot
34、herwise 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, starting fromthe date of irradiation, prior to commencing accelerated aging,unless otherwise directed by the customer.8.2 After irrad
35、iation, specimens shall remain in their origi-nal packaging during the preconditioning period.8.3 Unirradiated specimens shall be maintained in a stan-dard laboratory environment of 23 6 2C (73.4 6 3.6F) for40 6 1 h prior to commencing accelerated aging.9. Procedure9.1 Specimen OrientationTest speci
36、mens 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 components, the articulatingF 2003 02 (2008)2surface which may subsequently be subjected to wear simula-tion shall not be obstruc
37、ted 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 three timesprior to starting the aging experiment. For example, at astandard laboratory
38、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 not be addedto the pressure vessel during accelerated aging. The user shouldbe aware
39、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 (23 62C) and will be gradually raised to the aging temperature of70C. The initial he
40、ating 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 days) prior to subsequent testing. There are tobe no interruptions of the aging perio
41、d (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.9.7 Guide for Subsequent TestingSpecimens shall besubjected to further testing withi
42、n 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-ated aging, and the storage conditions for the test samples.10.2 Test Sample Preparatio
43、nThe 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 that were performed on the test articles after manu-facture, such as sterilization or h
44、igh 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 analysis or testing was performed on the ageditems.10.4 Test Sample Storage ConditionsIt
45、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 under which the specimens were stored.11. Keywords11.1 aging; oxidation; precondition
46、ing; 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 mechanical properties of UHM-WPE. (2, 3) Even under ambient conditions, oxidation ofirradi
47、ated 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 and therebyassess oxidative stability during a comparatively short timeperiod.X1.2 Ox
48、idation 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 severalstudies, post-irradiation aging has been simulated using acombination of thermal oxi
49、dation 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 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 t
