Post-irradiation aging of ultra-high molecular weight polyethylene

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Post-irradiation aging of ultra-high molecular weight polyethylene
CM Rimnac, RW Klein, F Betts and TM Wright
J Bone Joint Surg Am. 1994;76:1052-1056.



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                                                                                                Post-Irradiation                                                                              Aging                                 of
                                               Ultra-High                                                 Molecular                                                     Weight                                     Polyethylene”t
                                                          BY        CLARE               M. RIMNAC.                            PH.D4.                ROBERT                 W.     KLEIN.               B.S.t.           FOSIER                  IJEITS.            PFI.D.:.
                                                                                            AND               TIMOTHY                        M.      WRIGHT.                 PH.D4.              NEW             YORK.                  N.Y.


                                                     IllVeStig(ItU)Il             J)erforlned                  at the               De/)artl?lellt              ()fBiol?zec/lallics.                       Tue       IIosjitalfor                  Special           Siir,t,’erv.
                                                                          The      New          York           Hospital-Cornell                                Univer.suv            Medical                (‘olle,ge.             Nett        York         (it



           ABSTRACT:                    A study                 was performed                                  to determine                               the               the         life        span              of joint                  replacements.                         Excessive                 wear         can
time-course                  of oxidative                         degradation                       and                     the extent                         to           lead          to gross                    mechanical                            failure,           in the        form           of fracture
which     the              degradation                          proceeded                       through                       the bulk                         of           and dissociation”’2’,                                         and to loosening       caused     by a bio-
ultra-high     molecular                                 weight     polyethylene     joint                                                        compo-                    logical  reaction                                     to particulate    polyethylene        debris’4’7.  It
nents     that had been                                  irradiated       and stored     on                                                       a shelf.                  has         been                shown                  that         stresses                that         occurred               in polyeth-
Standardized     cylindrical      samples,     taken   from    a single                                                                                                     ylene              components                              were             consistent                    with fracture                     mech-
batch of extruded      polyethylene,       were cleaned,    packaged,                                                                                                       anisms               that lead                          to wear              damage.                     The magnitudes                        and
and         sterilized                according                    to protocols                          used                 for commen-                                   distributions                              of these   stresses                                were         a function     both    of
cial joint-replacement          components.      After    sterilization,                                                                                                    the geometry                                of the component                                    and        of the elastic    mod-
the samples       were stored    in the packages     for time-periods                                                                                                       ulus      of              the polyethylene2.                                     Alterations,                    such     as those
of one day to more            than one year. At each               interval                                                                                                 resulting                   from oxidative                                  degradation.                     in physical.    chem-
studied, thin sections were cut as a function of depth                                                                                                   into               ical,        and,              hence.                mechanical                        properties               (including                  elastic
the bulk of the sample    and were used to determine                                                                                                      the               modulus)                        could.                 therefore,                     adversely              affect          the     wear         re-
density             and         the          infrared               spectra.               Marked                           alterations                         in          sistance                  of polyethylene                                   components.
the density     and the infrared     spectra    consistent     with con-                                                                                                               Polyethylene                                    components                       sterilized             with         gamma             ra-
tinuing    oxidative   degradation       occurred      throughout     the                                                                                                   diation     are known     to                                              undergo    oxidative                           degradation.
year of storage      on the shelf. The alterations            were most                                                                                                     both    while   being stored                                                on a shelf and                          after     implanta-
severe   near the surface       of the samples.                                                                                                                             tion’7”2”.                           Retrieved                       components                           and       stored             compo-
           CLINICAL                   RELEVANCE:                          The           results                ofthis                    study          sup-                nents              both            demonstrated                                   changes               in density              (a physical
 port    the hypothesis       that sterilization         of polyethylene                                                                                                    property)                        and                changes                in infrared                    spectra              (a chemical
joint     components      with gamma         irradiation     followed    by                                                                                                 property)       consistent       with oxidative     degradation.       While
 storage     on a shelf contributes        to oxidative     degradation.                                                                                                    these     changes       occurred      for a considerable         depth   into
The degradative         changes     continue      with time and occur                                                                                                       the retrieved       components’,    the magnitude                                                                               of the deg-
not        only   on              the         surface    but also through                                                           the bulk of                             radative    changes     was unknown     because   the                                                                          initial prop-
the        component.                        Alterations     in the material                                                           properties                           erties             of the polyethylene                                                from    which     the                  components
associated                 with         the          degradation                        increase                       the           stresses                  on           were               fabricated    was                                      not          known.       While                    degradative
the        surface             of and                within               the          polyethylene                                  total          joint-                  changes   were documented                                                             for stored             polyethylene     sam-
replacement                      components                             once            these             components                                   have                 ples. the changes    were                                                  not           measured               as a function     of
been             implanted                   and         are            subjected                    to         loading                       in vivo.                      depth          from the                              surface     of the sample.
Thus, the wear resistance     of a polyethylene                                                                                     component                                          Alterations                               in material      properties                                caused             by degra-
that has been stored   on a shelf for an extended                                                                                      time may                             dation                of         polyethylene                               can          he        expected               to       affect         the
be diminished                          before              the          component                         is implanted.                                                     stresses                associated    with                                  wear            damage              observed   on                    and
                                                                                                                                                                            within                 the components.                                        To            assess           the magnitude                         of
           Wear          damage                  to ultra-high                          molecular                             weight                  poly-                 the degradative           effect.     the changes      in physical       and
ethylene                 (hereafter                     referred                  to       as        polyethylene)                                     total                chemical     properties          as a function     of both    time     after
joint        components                        is a serious                     clinical                problem                          that        limits                 irradiation     and location         within   the component        must be
                                                                                                                                                                            determined.       Therefore,        an in vitro experiment       was con-
           *No      benefits          in any         form          have         been       received                   or      will        be received                       ducted      in which   standardized      polyethylene     samples,                                                                                ap-
from    a commercial          party      related       directly     or indirectly      to the subject                                                                       proximately        the size of a condyle      from a total knee                                                                               tibial
of this article.      Funds         were       received         in total   or partial      support    of                                                                    component.        were irradiated     and were then analyzed                                                                                      for
the research       or clinical        study       presented        in this article.      The funding
sources     were     National         Institutes          of Health       Grants      AR40191      and
                                                                                                                                                                            changes                        in density                     and          infrared                spectra            as functions                    of
AR01876       and the Clark           Foundation.                                                                                                                           the time after   irradiation                                                     and the depth   below                                the sun-
           tRead         in part        at     the      Annual             Meeting              of      the           Society              for      Bioma-                  face of the hulk samples.                                                       Density was measured                                  because
terials.         Birmingham.                 Alabama.               May         2. 1993.
                                                                                                                                                                            increased                        density                      has         been            associated                  with          increased
           lDepartment                  of Biomechanics.                           iTie         Hospital                    for          Special          Sur-
gery,       535 East           70th     Street. New York.                           N.Y.        10021.                                                                      elastic               modulus”                              and,          thus,          can        he      used          to       assess         the


1052                                                                                                                                                                                                                            [TIE      JOURNAL                  OF      BONE          AND       JOINT          SURGERY
                                            POST.IRRADIATION                                AGING              OF      ULTRA-HIGH                 MOLECULAR                    WEIGHT               POLYETHYLENE                                                                                1053

                                                                                                                                             on a shelf             in laboratory                    air at room                                temperature                              for one
                                                                                                                                             of         seven       time-periods                     (one,                       eight,          twenty-two,                               thirty-
                                                                                                                                             eight,         sixty-eight,               120,     and                 378             days          after                     sterilization).
         0.945                                                                                                                               The         time-periods                 were          chosen                         so that                most                of the       meas-
                                                                                                                                             urements       were made                          during     the                           first two months,                                     when
                                                                                                                                             more     rapid    degnadative                          changes                               were   expected.                                    After
  0
                                                                                                                                             storage,           the      samples             were           frozen                        at -20                     degrees             Celsius
  0
 .)                                                                                                                                          to         minimize            additional                 degradation                                              until           they        could
                                                                                                                                             be analyzed.                   Standard     principles                                        of first-order                                kinetics
         0.940
 C’)                                                                                                                                         suggest    that                 such    a reduction                                         in temperature                                      slows
 z
 w                                                                                                                                           degnadative                  reactions             by          more                     than                 an          order            of mag-
 0
                                                                                                                                             nitude.           Additional,          non-irradiated                                                samples                         from           the
                                                                                                                                             same           lot were       prepared        at the                                   start         of the                      experiment.
                                                                                                                                             These     samples                  were       also        kept                      frozen           until                     they       could         be
         0.935                                                                                                                               analyzed.
                                                                                                                                                   A sample                size of two was chosen                                               for each                      time-period
                                                                                                                                             on the             basis   of previous    experience                                                              with    density                  and
                                                                                                                                             infrared            measurements       on polyethylene                                                               material’523.                 For
                     0                                 1                                    2                                3               example.   thirty-six    density   measurements                                                                     made                  on sam-
                                            DEPTH            FROM              SURFACE                  (mm)                                 ples taken    at equally    spaced   distances                                                                throughout                     a nec-
                                                                                                                                             tangular              (sixty-three               by seventy-six                                      by 305-millimeter)
                                                             Fu;.         1
                                                                                                                                             extruded              rod      had        a mean               value                   of 0.933                         gram          per        cubic
    Graph          of density           plotted         against     depth      below                   the     surface       of     the
disk. Two          disks   were         evaluated           at each     time-period.                                                         centimeter                  (standard             deviation,                               0.0004                       gram          per        cubic
                                                                                                                                             centimeter)                 with     a range             of 0.932                          to 0934                       gram         per        cubic
severity               of the          effect          of degradative                            changes              on     wear            centimeter.
resistance.   Analysis      with                            infrared   spectroscopy                                  was used                           For each time-period,     density                                                     as a function                            of depth
to confirm     the presence                                   of chemical        products                               arising              from         the surface  was determined                                                         for each disk.                            A cylin-
from oxidative       degradation.                                                                                                            dnical    core (4.5 millimeters                                         in diameter)       was cut from
                                                                                                                                             the center         of each disk and                                     was cut into approximately
                                       Materials                 and          Methods                                                        0.2-millimeter-thick         sections                                    with a microtome.      The sec-
         Cylindrical                  disks,   25.4 millimeters                                   in diameter                     and
12.5      millimeters                   thick,   were   machined                                   from   one               lot     of
                                                                                                                                                           0.945
polyethylene                       extruded                nod       (GUR              415            resin;         Hoechst!
Celanese.                 Houston.                Texas).      The                  initial   properties                     pro-
                                                                                                                                                                                      0       0.5      mm                          DEPTH
vided     by             the vendor                  (Westlake                      Plastics,    Lenni,                    Penn-
sylvania)                were          a minimum                       molecular                      weight          of    three                                                     V       1.0      mm                          DEPTH

million,   a density    of 0.933                                 gram     per            cubic   centimeter,                             a                                             0      2.0      mm                          DEPTH
yield stress   of twenty-two                                     megapascals,                 an ultimate                         ten-                                                                                                                          0-     -‘
                                                                                                                                                   0
sile      stress          of forty-four                    megapascals,                         and         an elongation                          0

of 360
         The
                 per
                     sample
                           cent.
                                          thickness                 was         chosen                 to     be      approxi-
                                                                                                                                                                                                                                                          ,;7
                                                                                                                                                           0.940
mately            the          same        as that of a typical                                  polyethylene                  im-
                                                                                                                                                  (I,
plant.           The           same        machining   techniques                                     (cutting              tools,                z                                                                           p.     ..0’-
                                                                                                                                                  w
tool       speeds,              and        feed        rates)             used         to        machine                 implant                  0
                                                                                                                                                                                                                        ,l              ,,,                    ,,

components                  were    used to fabricate      the samples.    GUR                                                                                                                                  ,             .“                          II


415 resin                was chosen     because   it is used widely     by man-                                                                                                                             I                                         I

                                                                                                                                                                                                        I           /                             I
ufacturens                 of commercial                      implants.                                                                                                                 .....-d”                             Q--Q--13
         All       disks           were         cleaned,               sealed            in plastic                  pouches.                                                   ; ;    ;=:::
and       packaged                 in pairs.         The          packaged                  samples                 were         sten-
ilized           with          gamma              irradiation                  in     a single                 batch         by          a                 0.935
                                                                                                                                                                           1                           10                                         100                                    1000
commercial                      vendor    (Isomedix    Operations.      Whippany,
New Jersey).                      The packaging     and sterilizing    procedures                                                                                                                     TIME                         (DAYS)
were    those                  used in the manufacture         of commercial      im-                                                                                                                  Fio.                  2
plants.            The   estimated                     dose    of radiation      ranged       from                                               Graph    of density       plotted   against       time after         irradiation     for depths
28,300             to 42,000     gray,                 as determined        from     dosimetnic                                              of 0.5, 1.0, and 2.0 millimeters              below       the surface.           Each point      is the
                                                                                                                                             estimated      density      determined          from      linear       interpolation        between
measurements                          performed                  by the             vendor.                                                  adjacent    density       measurements           spanning         each       of the three      depths
         A package                    containing                 a pain          of samples                     was        stored            and represents         the average     of the results          from      the two disks.


VOL.       76.A,         NO.    7. JULY         1994
1054                                                           C. M.      RIMNAC.             R.     w.     KLEIN,         FOSTER           BETTS,        AND          T. M. WRIGHT


                                                                                                                                    0.936  gram  pen cubic  centimeter,        on approximately                                                          0.3
                                                                                   NON-IRRADIAT.
                                                                                   DAY 8                                            per cent. There    was a greater      increase    in density                                                       near
   I-      3                                                                       DAY 22                                           the surface   of the sample      than     within   the bulk.                                                      from
   I
                                                                                   DAY 38                                           approximately                      0.933    to 0.938 gram                    pen cubic             centimeter,
   w                                                                               DAY 120                                          or approximately                          0.5 per  cent.
   I
                                                                                                                                          In general,                   density     continued                    to     increase               with       the
                                                                                                                                    time     after   the irradiation.        By day 120, the degradative
   w       2
   0                                                                                                                                effects     were no longer        concentrated   near the surface     but
   -J                                                                                                                               extended        well into the bulk of the samples.       For example,
   >-
   z                                                                                                                                the density    from the                           surface    to a depth                    of one-half             mil-
   0                                                                                                                                limeter    was constant,                            at approximately                       0.942    gram            pen
                                                                                                                                    cubic        centimeter,                  after      120      days       of storage.               The       density
   0                                                                                                                                was       still    greater          (to      a depth          of two         millimeters)                  than       the
   0                                                                                                                                density  of 0.938 gram   pen cubic  centimeter                                                   at one             day
   I-
   w                                                                                                                                after the irradiation. The rate of increase                                                  in density             was
                                                                                                                                    a function                of depth.          For example.                 at a depth     of one-half
                                                                                                                                    millimeter,                the rate          of increase               in density    with time was
           0
                                                                                                                                    greater           than       that         at one           on two         millimeters                  below          the
                 0                            1                              2                             3
                                                                                                                                    surface           (Fig.      2).
                              DEPTH                FROM              SURFACE                   (mm)                                         In the        non-irradiated                       samples,          no appreciable                       keto-
                                                        FIG.     3                                                                  carbonyl    peak                   was noted   at any depth     (Fig.                                3). A peak
    Graph        of ketocarbonyl          peak      height    plotted       against      depth     below                            was observed,                      however,  in all of the irradiated                                    samples
the surface          of the disk. Two disks were evaluated                   at each time-period.                                   that were examined     with                            infrared          spectnoscopic     analysis.
There       was no detectable            absorption        in the carbonyl            region     for the
                                                                                                                                    As with density,   the peak                             height         of the ketocarbonyl         was
non-irradiated            material.  The non-irradiated               material      is plotted     at the
detection         limit (0.1 centimeter)           for peak     height      measured         at approx-                             more          pronounced                   near       the     surface             of the         disk.       In gen-
imately        1720 centimeters-’.                                                                                                  eral,      peak height     continued                          to increase    with time. By day
                                                                                                                                    120,       elevated    peak     heights                        were    found    well below  the
tions were obtained                       beginning                  near        the surface                and      con-           surface,          to a depth                of two          millimeters.
tinuing   to a depth                       of about                  three          millimeters                   below
the     surface.            The      actual         thickness                of the       section               was    de-                                                             Discussion

termined      with a micrometer.                                   Section  thicknesses                             were                    Sterilization                with          gamma             irradiation,                conducted                in
added      to obtain    cumulative                                section  location                            with    re-          an ambient               oxygen      environment,                           has been     reported                         to
spect    to the surface      of the                              disk. The density                              of each             result  in            cross-linking,        chain                      scission,     and    oxidative
section         was         determined                  with      a gradient               column                 of iso-           degradation                  of      polyethylene9’6.                      Cross-linking                     is    sup-
propanol       and water’.                                                                                                          pressed         and chain       scission        and oxidative         degradation
      Infrared     spectroscopic                                analysis           was         conducted                   in       dominate          when     polyethylene            is irradiated      in the pres-
transmission     mode                     for        selected   time-periods          with                             use          ence      of oxygen22.       Other       investigators           have   shown     that
of a spectrophotometer                                   (model    1330;     Perkin-Elmer,                                          post-irradiation          effects       continue        with time.       Bhateja      et
Danbury,     Connecticut)                            on the same        sections      used                              for         al. exposed          polyethylene             to lange        doses    (as high      as
evaluation               of       the density.              The            chamber                 of the spec-                     1,200,000            gray) of beta radiation                             and        followed               the sam-
trophotometer,                      with the             section             in place,              was purged                      ples for            112 months.   Both     the                          heat        of fusion               (related
with      dry        air, free       of carbon                 dioxide,          before            collection              of       to the            amount             of      crystallinity)               and        the         melting           tem-
the spectra.                The peak      height                   at an absorbency      of ap-                                     penature            continued    to change      throughout                                    the test. Bin-
proximately                  1720 centimeterst                         (associated  with   C=O                                      kinshaw             et al.4 exposed      polyethylene                                      to doses    of as
ketocarbonyl                  groups    in the                    polyethylene)     was meas-                                       much          as 500,000              gray         of gamma              radiation               and       followed
ured.     A baseline       was drawn                              on the transmission        spec-                                  the samples                 for 335 days. They                          also found                that crystal-
trum,      and   the peak       height                             was   measured      from      the                                linity   and              mechanical    properties                          continued                 to change
baseline.      An increase     in peak                            height  reflects   an increase                                    throughout                 that time.
in the        concentration                   of   C=O ketocarbonyl                                groups          in the                   Streicher             exposed               polyethylene                    to 6000     to 25,000
polyethylene.                                                                                                                       gray      of gamma              radiation              and stored                 the polyethylene        in
                                                                                                                                    air.      Changes             in the          physical,            chemical,               and      mechanical
                                                     Results
                                                                                                                                    properties     due to the irradiation       occurred       only   for as
        The      density           of the          non-irradiated                    material               was       con-          long as three      months.  However,      these      tests  were     con-
stant     into        the     bulk       of the          disk        (Fig.       1). Sterilization                    with          ducted     on thin films (300 micrometers           thick)    for which
gamma            radiation             had         an     immediate                  effect           on        density,            the changes                  due to oxidative     degradation                                  would     be ex-
even at one day                    after the             sterilization,        when                   the       density             pected    to              saturate  in a relatively       short                             time.    As dem-
within the bulk                    increased               from       approximately                            0.933 to             onstrated                 in the present      study.     even                              small     doses   of

                                                                                                                                                                   THE         JOURNAL            OF      BONE        AND        JOINT         SURGERY
                                                   Posr-IRRADIATI0N                                             AGING             OF         ULTRAHIGH                                MOLECULAR                                   WEIGHT                    POLYETHYLENE                                                                        I 055

radiation,                    such           as are               used               commonly                        for        sterilization,                                  is         irradiated                             in         an       inert             environment,                               cross-linking,
are       sufficient                    to initiate                       degradative                             changes                   that        con-                    rather                   than               chain                 scission                 and            oxidative                   degradation,
tinue            to occur                  for         at least                  one            year            in bulk              samples                   of               dominates                               initially222.                        The             mechanical                           properties                         of
polyethylene.                             In       this            study.                 degradative                           changes                     oc-                 polyethylene  could                                              be substantially                             affected,                      depending
curned              as deep  as three  millimeters                                                                 into the                   samples                           on the extent    of                                             cross-linking.                             Additional                         studies                   to
(Figs.            1 and 3) at one year after      the                                                          irradiation.                    Thus.              a             measure                     the effect                              of oxidative   degradation                                                 on cross-
polyethylene                            joint             component                                  that         is six             millimeters                                linked                  polyethylene                                must be performed         before                                            it can be
thick             will          undergo                      degnadative                                changes                   through                      its              assumed       that gamma       irradiation      in an inert environment
thickness   before                               implantation                           if the time                        from              steniliza-                         would     be beneficial.
tion to implantation                                   is one                       year or more.                                                                                     Degradation        would       be expected      to affect   the wear
      Carhonyl        groups                                    are formed      by the reaction    of ox-                                                                       resistance   of polyethylene     adversely,      although     we know
ygen     with     long-lived                                      free radicals    that  are produced                                                                           of no direct    data from wear      tests or from clinical        stud-
during     irradiation”’.                                         At temperatures       well below    the                                                                       ies. The effect     of lange doses      of radiation      (more    than
melting               point      of the polyethylene.        the                                                           lives     of these                                   200,000                       gray)           on the                 wear             behavior                    has       been             studied’3’5,
radicals              are measured           in years”.     The                                                           breakdown           of                                but              the       polyethylene                               was           tested                shortly               after         the         stenil-
trapped               metastable       penoxy    on hydroperoxy                                                                  species   im-                                  ization. The changes     in density                                                                and infrared                         spectra   that
mediately                       after            irradiation                            may             provide                  one           oxygen                           were measured      in the present                                                                  study  were                        accompanied
source              for polyethylene                                   degradation5,          but                                diffusion      of                              by alterations       in mechanical                                                             properties                     that continued
oxygen               is required     for                            continued        degradation.                                       It is not                               to change      dramatically      with                                                         the time                    after   the inradia-
surprising.                     therefore,                      that,          consistent                       with           the          results            of               tion.             The           magnitude                            of the            density                  changes                 measured                     in
previous                  studies”,                the greatest                                 changes      in density                                  and                    the present     study    (for example.         0.932 to 0.942 gram    pen
infrared                 spectra                occurred     near                               the surface,     where                                  oxy-                    cubic centimeter)         is equated      with a 35 pen cent increase
gen        was           most            plentiful                   and               more             oxygen                 would               be       cx-                 in the elastic    modulus       of polyethylene       on the basis of an
pected              to he absorbed.                                                                                                                                             empirically                              derived                     relationship’.                               Such           an         increase                 in
      Sterilization      of polyethylene                                                               with    radiation      in the                                            elastic                  modulus                         near           the           articulating                         surface                  of      poly-
absence        of oxygen   might    diminish                                                           or eliminate      oxidative                                              ethylene                        joint components                                    would                        cause           substantial        in-
degradation.                          Sterilization                        in nitrogen’2’                             has        been              shown                        creases                       in the stresses                                  associated                          with           wean       damage
to diminish                        oxidative                      degradation                             during                the steniliza-                                  occurring                         on and                     near           these            surfaces2.
tion  process                        but, on                     subsequent                             exposure                   to oxygen,
                                                                                                                                                                                           N,,          ihe      authors          thank           N. Khanna           and J. Weintraub                  for their       assistance          with     the
degradation                          resumed2”.                    Furthermore,when                                            polyethylene                                     experimental                  measurements.


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VOL..      76.A,          NO.        7. JULY              1994
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