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Influence of ionizing irradiation in air and nitrogen for sterilization of surgical grade polyethylene for implants
Rad~at Phys Chem Vol 31, Nos 4-6, pp 693~98, 1988 Int J Radtat Appl Instrum Part C Pnnted m Great Britain
0146-5724/88 $ 3 0 0 + 0 0 0 Pergamon Journals Ltd
INFLUENCE OF IONIZING IRRADIATION IN AIR AND NITROGEN FOR STERILIZATION OF SURGICAL GRADE POLYETHYLENE FOR IMPLANTS
R. M. Strelcher Sulzer Brothers, Departement of Medical Engzneer~ng CH-8401W~nterthur, Switzerland
ABSTRACT The xnfluence of the atmosphere and the appl~ed dose during ~on~z~ng radmat~on treatment on selected properties of ultra hlgh molecular welght polyethylene (UHMWPE) have been mnvestxgated. A l~near correlation between extmnct~on coefflc~ent and applied dosxs ~n axr from 6 to 125 kGy was found, whlle oxidation was not llnear wxth irradiation in nmtrogen. Bacteria survival rate shows a necessary m~n~mum dose of 15 kGy for assured sterlllty of the product. Post reactlon of latent free radxcals in UHMWPE created durlng irradlatlon, whlch react or recombxne txme- and environment dependent, has also been mnvestlgated after storage of UHMWPE-fIlms xn air and nxtrogen at 21°C and mn water at body temperature 37°C for up to n~ne months. Results show that the propertmes of UHMWPE after radmat~on-sterml~zatxon change dependzng on t~me, the absorbed dose, the atmosphere where irrad~atlon took place and the environment of storage. UHMWPE, which maznly crossl~nks during mrrad~atmon degrades by an ox~datmon process after ster~l~zxng when stored ~n amr and even more in water at body temperature. So mrrad~atmon and storage zn nmtrogen before ~mplantatlon in the human body as benefxc~al.
KEYWORDS Polyethylene,
Stermllzatlon,
Irradxatmon,
Oxldatmon,
Storage, Aglng
INTRODUCTION Ultra hlgh molecular welght polyethylene (UHMWPE) as used as a materlal for endoprostheses for almost 25 years (Strelcher, 1986). Untll 1970 thls thermolablle materlal has been desmnflcated by chemmcal solutions. Since then 60Co-radlatlon zs used for stermlxzatxon purpose. Legxslatlon in almost all countrles requlres a mlnlmum absorbed dose of 25 kGy (Dorpema, 1983, Leemhorst, 1984, Ley, 1984). Because of oeconomlc aspects the prostheses are ster111zed an containers, therefore recelvlng doses up to 50 kGy (Moore, 1983) for reachlng the mlnlmum dose mn each posmtlon. Dependlng on the capacity of the source the irradlatlon tmme may vary from 1,5 to 18 hours for the same final dose. At the moment the mlnxmum dose required for a "securlty assurance level" (SAL) ms 1012 , whlch represents an absolute overkill (Dorpema, 1983) and there Is tendency to reduce the SAL to 106 and having almost the same relyab111ty when using low precontamlnatlon levels (Dorpema, 1983; Ley, 1984). This could reduce undeslred oxldatlon effects on the propertles of the polyethylene (Awatanx, 1960, Dole, 1979, Hemnze, 1966; Loy, 1960, Lue, 1981; Shaudy, 1978, W1nslow, 1965).
MATERIAL AND METHOD This investlgatlon was carrmed out on ultra hlgh molecular weight polyethylen CHIRULEN @ (UHMWPE) havln g an average molecular welght of 4 m1111ons/mol. Experlments were performed using mmcrotomed sllces of 300 um thickness. Irradiation of these f11ms together wlth lO0 artxflcmal hlp cups made of UHMWPE and contaminated wlth lO 9 bacterlas in a mlxture of S. Epmdermls, B. Subtllls, P. Aerugmnosa and S. Faecalms at a dose rate of 5 kGy/h took place in a SULZER 200 kCm plant to measured absorbed doeses of 6, 12, 18 and 25 kGy in air and nltrogenatmosphere. After irradlatlon the f11ms were characterized, stored in alr and nltrogen at 21°C and water at 37°C, and followed up by measurement of density, normalized IR-extlnctlon, soluble constituents and tensile propertles for nlne months.
693
694
R M STRmcmm
The zrradzsted precontamznated non growth crzterza.
cups were prepared in a CLAUSEN-medzum and jugded on a growth and
RESULTS
Irradlatzon to 25 kGy in sir and nitrogen Dependzng on the absorbed dose and the atmosphere durzng whzch zrradzatzon was performed several changes in structure and mschanlcal propertzes of UHMWPE occured, shown zn Table i.
Chan~e in Propertzes of UHMWPE one Day post Irradzatzon zn Azr or NztroKen
TABLE 1
(N2)
Change in Percent a f t e r Property
SI-Unlt
Normalized Extinction
%
Density So1
6 kGy
Reference
0,84
12 kGy
25 kGy
18 kGy Air
Air
N2
Air
N2
Alr
N2
+213
+187
+404
+292
+600
+461
N2
+851
+517 +0,23
gcm -3
0,9375
+0,2
+0,11
+0,35
g
71,7
-17,2
-15,5
-13,5
-41,3
YielO Strength
Nmm" 2
32,2
1,3
+1,6
0
UTS
Nmm"2
39,2 * 4,8
+3,1
+8,93
%
515 ± 8
-0,5
+8,93
Elongation at break
The most sensitive factor for zrradlatzon znduced changes zs the carbonyl extlnctzon detected by IR-spectroscopy. The normallzed extznction of the carbonylband at 1720 cm -I zncreases iznear with increased dose when UHMWPE is zrradzated zn alr whzle zncrease zs not iznesr when znert atmosphere is used, presented in F1 g. i. The effect of inert atmosphere becomes evldent for absorbed doses above i0 kay, below no slgnzfzcant dzfference to zn azr zrradzated UHMWPE could be detected. Denslty measurement wlth a denslty gradlent column showed slmllar trends.
tO
I
in air •
/./
/4 /
xuJ
/" j
•
a¢¢ SkSV/h
i°
ilh
N ).
%
/
/ / / ,//
1
[s] 70
% % % 50
% %
.I
/"
/
%
f
40
/
12 24 AmOn~O OOm (kSy] Flg. i.
in IAP-
%
C s r b o n y l ~ o u p extznction st 1720 cm -I for UHMWPE zrradlated in alr and in nitrogen (N2).
t2.5 FZE. 2.
l
Soluble constztuents of UHMWPE zrradzated zn azr and nztrogen (N2).
6th International M ~ t m g on Ra&ataon Processing
695
The comparison between nitrogen or alr presence during irradiation on the decrease of the soluble constituents is plotted in Fig. 2. Again up to a dose of i0 kGy no distinct change between those two environments used could be traced. Further irradiation to 25 kGy in a l p increases the sol while it zs substanclally reduced by using nltrogen, showlng a high degree of crosslznkzng. All other properties of UHMWPE change less than i0 percent due to the radlatlon-treatment. The Influence of lowerlng the sterilization dose for the contaminated hlp cups is presented in Fig. 3. Until 12 kGy bacteria growth was persisting, from 18 kGy on no growth could be detected.
100
i
I
doIa Pate: 5kGy~
75 iO' bit:tiP ! l
--
:
S. EP;DE~ZS S. FAECALZS
m
B. SUBTILUS
25
\ 1; 24 ABSORBED DOSE [kGy]
Fig.
3.
Surviving bacteria fraction as function of the absorbed dose.
Influence of storage Storage of pending on for UHMWPE irradiated
irradlated UHMWPE results zn postoxzdatlon (Dole, 1979, Helnze, 1966; Moore, 1983) deirradiation and storage condition. The change zn properties is always more pronounced i r r a d i a t e d in alr and stored in air 21°C or even more in water 37°C, than for samples or stored in nitrogen, presented zn Table 2.
TABLE 2
Chanse in Properties for UHMWPE~ or Water at 37°C
Storage time (month)
Density
0 g/cm 3 9
Normol|zed Extinction
%
0 9
25 kGy in air I water air J water 0,9375 0,9408 +0.03 Z I +0,15 % +0.07 % I +0.31% 0,84 7.99 +32,1% I +270 % +37,3 % I +79,1% 71.7 62,0 -9,4 % J -8,9 % +7,7 % I +18,4 Z 32,2 32,7 -5.9 % J -q.~ % +0.51% J +7,0 % 39,2 40.~ -6,6 % I -2,0 Z -2.5 % I -2.5 % 515 512 -3,9 % JI +0,8 % 24.0 % JI +6.3 % Reference
o|r
l
Sol
%
0 9
abed in Air and Nztrosen
B
(Nr) at 21°C
25 kGy Jn N2 I N2 J water 0,9397 +0,19 , J +0,0. % I +0,27% air
5,18
+46.5 % J +50.9 % J +i0, % 42.1 +12.6% I -i.0 % [ +31.8 %
m
o
Yield
Strength
Nlm~2
9
q
Ultimate Strength E}ongotlon at break
N/nl112
o g 0
%
g
m
)2,2 +1.9 % ] +0.3 % i +5.9 % 42.7 -9.6 % I -6.6 % J -7.7 % p
561 +4.8 % II +8.0 % I1 +5.5 %
Oxidation continues with storage time but major changes are completed after three months as shown in Flg. 4, 5a and 5b.
696
R M
STRmCHm~
stored
~5 V refm, ence
He0
• L:~key in e~r
I
• ~k6y tn H2
?
air Him
/-
1C
in:
mlr N2
60CO.5kSy/h I&r
0
Fzg. 4.
3
6 STORASE TIRE [month]
Increase zn carbonyl e x t z n c t z o n of UHMWPE as result of z r r a d z a t z o n atmosphere and agzng e n v l r o n m e n t (H20 = water 37°C, alr 21°C, N 2 = nltrogen 21°C).
low "
? ,
/
x w
0
3 6 STORA6E TIHE [month]
Fzg. 5.
Fzg.
5a.
0
9
3 6 STORA6E TII4E [month]
9
Increase zn carbonyl content of UHMWPE as a functlon of i r r a d z a t z o n and a g z n g tzme in azr at 21°C. UHMWPE,
zrradzated zn azr.
Fzg. 5b.
UHMWPE z r r a d z a t e d zn nztrogen.
Storage zn water at 37°C causes increased postoxldatzon, storage zn N causes only mlnlmal oxzda. 2 tlon. Samples of UHMWPE whlch have been irradlated and stored zn n l t r o g e n for one month showed a s z g n z f l c a n t zncrease in carbonyl content when stored afterwards zn water at 37°C as can be seen in Flg. 6. After 3 month storage zn N 2 and then storage zn water p o s t o x z d a t z o n is neglzglble. Denslty increases a n a l o g wlth increaslng carbonyl groups but only to a m a x z m u m of 0,32 %; more p r o n o u n c e d w h e n UHMWPE zs stored in water. Fzg. 7 glves the v a r z a t z o n in soluble c o n s t l t u e n t s of UHMWPE as a functlon of storage d e p e n d e n t on the atmosphere zn whzch zrradzatzon took place the sol content zncreases c r e a s z n g storage tzme when water or alr as storage medzum are used. D u r z n g storage in the soluble c o n s t i t u e n t s decrease, z n d z c a t l n g further c r o s s l z n k z n g due to the absence All these structural m o d z f z c a t l o n s zndzcated zn Table 2.
tzme. Inwzth znnltrogen of oxygen.
of UHMWPE also znduce changes zn the m e c h a n z c a l p r o p e r t z e s as
6th Internauonal MeeUng on RadmUon Processing
697
SOL
[z]
7o
/
15
: m
o • •
•
stored in 1t20 37°C I month in I/2 then 3 month in N2 then s t o o d in ~ 21"C
I 1120 37"C ~
50
37"C
I
37"C
i
/-
qk1~ •
..~ - ~ ' -
{N22t'C /
60CO. 5kSy/h tO L~kSy I 9
Z
3
..--.--.--
Time dependent post aglng of UHMWPE in water 37°C after Irradiation to 25 kGy in nitrogen and prestorage in nitrogen at 21°C.
~
30
3
STORA6E TIME [month] Fig. 6.
in m~"
6
9
STORAGE TIME [month] Fig. 7.
Change in soluble constituents of UHMWPE, irradiated to 25 kGy in alr and nitrogen. Influence of the storing medium. (H~O = water 37°C, air = 21°C, N~ = nitrogen 21°C).
DISCUSSION Irradiation of UHMWPE wlth 60Co rays induces molecular chanEes (Lue, 1981; Moore, 1983; Wlnslow, 1965) of this polymer. Because of the long duration of the irradiation treatment the absorbed dose as well as the atmosphere used are of crucial importance (Glberson, 1962; Helnze, 1966; Wlnslow, 1965). In the presence of oxygen during radiation treatment carbonyl content of the UHMWPE increases linear with dose (Helnze, 1966; Schaudy, 1978). Thls oxidation inhlblts crosslinking (Lue, 1981; Schaudy, 1978) and causes density increase (Grood, 1982, Lue, 1981, Nussbaum, 1979; Roe, 1981, Wlnslow, 1965) which results from chain cleavage, mainly in the amorphous regions of the polymer (Eyerer, 1983, Shastrl, 1983; Wlnslow, 1965) The therefore increased mobllity of the molecular chains leads to recrystallization which enhances yield strength, hardness (Ungethuem, 1979) and creep resistance. The sol content increases (Lue, 1981, Schaudy, 1978; Shastrl, 1983; Ungethuem, 1979) and the elongation at break decreases. Due to incorporation of heteroatomlc oxygen in the carbon-hydrogen-polymer UHMWPE polarity and watersorptlon increase (Clarke, 1985, Nussbaum, 1979). All changes induced in UHMWPE become more pronounced wlth increasing dose, because of the longer exposition to alr under ionizing radiation. For thls reason resterzlizatlon should be abandoned, because polyethylene becomes more rigid and fracture of implant components could result (Eyerer, 1983, Roe, 1981, Shastrl, 1983; Ungethuem, 1979). Exluslon of oxygen by using inert gas during irradiation can also reduce the implantation of OXldative structures in UHMWPE markedly (Helnze, 1966). Until a dose of i0 kGy the effect is negllble due to the reaction of dissolved oxygen present in the polymer (Baeuerleln, 1983). Going to the minimal dose used for sterilization of medical products the beneficial effect of using inert gas during irradiation becomes evident. Crossllnklng can take place to a larger extend and the desired toughness of UHMWPE, which is responsible for its good wear performance (Rose, 1982) does not change, while increase in creepreslstance and crossllnking of the intercrystalline regions should be ideal for a long term trlbologlcal stressed plastlc component (Hornbogen, 1983). Irradiation induces latent radicals in UHMWPE (Helnze, 1966) which can react during storage, depending on the conditions used (Heznze, 1966; Moore, 1983, Nussbaum, 1979). Although UHMWPE irradiated in nitrogen and stored in alr or water exhibits a somewhat augmented postoxldatlon, the cumulatlve change remains less pronounced than for components irradiated in air. If nitrogen storage IS used the radicals can recombine and lead to enhanced crosslinking, while the presence of oxygen results in oxidation. Postoxldatlon of UHMWPE increases density (Grood, 1982, Roe, 1981, Shastrl, 1983), carbonyl- (Helnze, 1966, Moore, 1983) and sol content (Schaudy, 1978; Ungethuem, 1979). Tensile strength IS reduced and elongation increased. Below i0 kGy the observed changes are neEllglble and show no influence of the atmosphere in which irradlatlon took place.
698
R M STI~Cm~
Also non zrradlated samples of UHMWPE show the effect of agznE due to storage (Grood, 1982; Roe, 1981; Shastrl, 1983). Three month after irradlatzon treatment the oxzdatlon induced changes a r e mostly terminated. The ~nfluence of sterlllzatzon Irradlatlon on the trlbologlcal propertles of UHMWPE has been reported by several authors. Although the results dxffer, all show the same (Ungethuem, 1979) or better wear reslstance (Lue, 1981; Rose, 1982) xf i r r a d i a t i o n was performed in inert atmosphere. The investlgatlon on a posslble reduction of the mlnlmal dose for sterzllzatlon s u ~ e s t s that at least 15 kGy for achlevlng an acceptable SAL could be used, whxch can reduce the oxldatlon effects xn UHMWPE.
CONCLUSION Ster111zatlon of UHMWPE wlth 60Co lonlzlng radlatlon in alr results in substantlal oxldatlon of this polymer, leadlng to a loss of ductzlzty. Irradlatzon in nltrogen enhances the crossllnklng of UHMWPE whlch shows beneflts in mechanlcal and trlbologzcal properties. The use of nltrogen durlng sterlllzatlon of surglcal xmplants manufactured from UHMWPE is beneflclal and therefore recommended.
REFERENCES Awatanz, J., and Mznegakz, M. (1960). Study on effects of x-ray xrradzated PE by a vzbratlon method. Pol. Jap. Soc. r 60, 1189-1196. Clarke, I. C., and coworkers (1985). Fluld sorptlon phenomena in sterzllzed PE acetabular prostheses. B1omaterlals r 6, 184-188. Dole, M. (1979). Crystalllnlty and crossllnkln E efflclency xn the zrradlatlon of PE. Radlat. Phys. Chem. r 14, 711-720. Dorpema, J. W., and van Asten, J. A. A. M. (1983). A new approach to sterxllzatlon Condltlons. Paper GPS 323. 3rd Gamma Processzn~ Semxnar, Ottawa. Eyerer, P. (1983). Werkstofflzche UntersuchunEen an explantzerten Hdftpfannen aus UHMWPE. Bzomed. Tech. 28, 297-309. Gzberson, R. C. (1962). Oxygen dlffuszon and reactzon durznE y-zrradzatlon of PE. J. Phys. Chem.~ 66, 463-486. Grood, E. S., Shastrl, R., Hopson, C. N. (1982). Analyszs of retrxeved zmplants" Crystalllnlty changes zn UHMWPE. J. Bzomed. Mat. Res. r 1G, 399-405. Handlos, V. (1981). Sterllxzatzon by electron beam. Radlat. Phys. Chem.~ 18/1-2, 175-182. Heznze, D. (1966). Das Verhalten yon Hochpolymeren Eegenuber enerEzerezcher StrahlunE. Kollozd Z. r 210/1, 45-54. Hornbosen, A. M. (1983). Mzkrostruktur und Verschlezss. In K. H. zum Gahr (Ed.), Rezbun~ und Verschlezss. DVM, Oberursel. 79-103. Leemhorst, J. G. (1984). Industrial appllcatzon of the gamma sterllzzatzon process. Paper, Sterzlzzatzon valzdatzon of medzcal devzces and sur~zcal products, Kopenhagen. Ley, F. J. (1984). Radlatzon sterzlxzation - Mzcrobzologzcal aspects. Paper, Sterxlxzatzon valzdatzon of medzcal devzces and sur~zcal products, KopenhaEen. Loy, B. R. (1960). ESR studles of free radzcal decay zn gamma-zrradzated polyethylene. J. Pol. Scz.~ 44, 311-847. Lue, C. T., Ellzs, E. J., Crugnola, A. (1981). Effects of gamma-zrradzatzon on UHMWPE. Abstracts 39th ANTEC, 246-248. Moore, E. P. (1983). Development zn radzatzon reszstant plastzcs. Paper GPS 315. 3rd Gamma Processzn~ Semznar, Ottawa. Nussbaum, H. J., and Rose, R. M. (1979). The effects of radzatzon sterzlzzatxon on the propertzes of UHMWPE. J. B~omed. Mat. Res.~ 13, 557-576. Roe, R.-J., and coworkers (1981). Effect of radzatzon sterzlzzatzon and ag~n E on UHMWPE. J. Bzomed. Mat. Res.~ 15, 209-230. Rose, R. M., and coworkers (1982). On the pressure dependence of the wear of UHMWPE. Wear~ 92, 99-iii. Schaudy, R. (1978). Ox~datlonsvorganEe bex der Strahlenvernetzung yon HDPE. Kunststoffe~ 68/3, 167-170. Shastrl, R., and coworkers (1983). Effect of aglng on UHMWPE. Abstracts 41st ANTEC, 17-19. Strexcher, R. M. (1986). Ultrahochmolekulares Polyethylen als Werkstoff fur Huftgelenkpfannen. In H. J. Reflor, M. H. Hackenbroch, C. J. W1rth (Ed.), Der alloplastxsche Ersatz der Hdftpfanne. G. Thleme, Stuttgart. 48-52. Un~ethuem, M., and Hxnterberger, J. (1979). Der Elnfluss der Strahlensterxlzsatlon auf das Verschlezssverhalten yon PE. Z. Orthop.~ 117, 790-79~. W1nslow, F. H., Matreyek, W., Stzlls, S. M. (1965). Oxzdatzve embrzttlement of PE. Trans. NY. Ac. Scl., 301-314.
0146-5724/88 $ 3 0 0 + 0 0 0 Pergamon Journals Ltd
INFLUENCE OF IONIZING IRRADIATION IN AIR AND NITROGEN FOR STERILIZATION OF SURGICAL GRADE POLYETHYLENE FOR IMPLANTS
R. M. Strelcher Sulzer Brothers, Departement of Medical Engzneer~ng CH-8401W~nterthur, Switzerland
ABSTRACT The xnfluence of the atmosphere and the appl~ed dose during ~on~z~ng radmat~on treatment on selected properties of ultra hlgh molecular welght polyethylene (UHMWPE) have been mnvestxgated. A l~near correlation between extmnct~on coefflc~ent and applied dosxs ~n axr from 6 to 125 kGy was found, whlle oxidation was not llnear wxth irradiation in nmtrogen. Bacteria survival rate shows a necessary m~n~mum dose of 15 kGy for assured sterlllty of the product. Post reactlon of latent free radxcals in UHMWPE created durlng irradlatlon, whlch react or recombxne txme- and environment dependent, has also been mnvestlgated after storage of UHMWPE-fIlms xn air and nxtrogen at 21°C and mn water at body temperature 37°C for up to n~ne months. Results show that the propertmes of UHMWPE after radmat~on-sterml~zatxon change dependzng on t~me, the absorbed dose, the atmosphere where irrad~atlon took place and the environment of storage. UHMWPE, which maznly crossl~nks during mrrad~atmon degrades by an ox~datmon process after ster~l~zxng when stored ~n amr and even more in water at body temperature. So mrrad~atmon and storage zn nmtrogen before ~mplantatlon in the human body as benefxc~al.
KEYWORDS Polyethylene,
Stermllzatlon,
Irradxatmon,
Oxldatmon,
Storage, Aglng
INTRODUCTION Ultra hlgh molecular welght polyethylene (UHMWPE) as used as a materlal for endoprostheses for almost 25 years (Strelcher, 1986). Untll 1970 thls thermolablle materlal has been desmnflcated by chemmcal solutions. Since then 60Co-radlatlon zs used for stermlxzatxon purpose. Legxslatlon in almost all countrles requlres a mlnlmum absorbed dose of 25 kGy (Dorpema, 1983, Leemhorst, 1984, Ley, 1984). Because of oeconomlc aspects the prostheses are ster111zed an containers, therefore recelvlng doses up to 50 kGy (Moore, 1983) for reachlng the mlnlmum dose mn each posmtlon. Dependlng on the capacity of the source the irradlatlon tmme may vary from 1,5 to 18 hours for the same final dose. At the moment the mlnxmum dose required for a "securlty assurance level" (SAL) ms 1012 , whlch represents an absolute overkill (Dorpema, 1983) and there Is tendency to reduce the SAL to 106 and having almost the same relyab111ty when using low precontamlnatlon levels (Dorpema, 1983; Ley, 1984). This could reduce undeslred oxldatlon effects on the propertles of the polyethylene (Awatanx, 1960, Dole, 1979, Hemnze, 1966; Loy, 1960, Lue, 1981; Shaudy, 1978, W1nslow, 1965).
MATERIAL AND METHOD This investlgatlon was carrmed out on ultra hlgh molecular weight polyethylen CHIRULEN @ (UHMWPE) havln g an average molecular welght of 4 m1111ons/mol. Experlments were performed using mmcrotomed sllces of 300 um thickness. Irradiation of these f11ms together wlth lO0 artxflcmal hlp cups made of UHMWPE and contaminated wlth lO 9 bacterlas in a mlxture of S. Epmdermls, B. Subtllls, P. Aerugmnosa and S. Faecalms at a dose rate of 5 kGy/h took place in a SULZER 200 kCm plant to measured absorbed doeses of 6, 12, 18 and 25 kGy in air and nltrogenatmosphere. After irradlatlon the f11ms were characterized, stored in alr and nltrogen at 21°C and water at 37°C, and followed up by measurement of density, normalized IR-extlnctlon, soluble constituents and tensile propertles for nlne months.
693
694
R M STRmcmm
The zrradzsted precontamznated non growth crzterza.
cups were prepared in a CLAUSEN-medzum and jugded on a growth and
RESULTS
Irradlatzon to 25 kGy in sir and nitrogen Dependzng on the absorbed dose and the atmosphere durzng whzch zrradzatzon was performed several changes in structure and mschanlcal propertzes of UHMWPE occured, shown zn Table i.
Chan~e in Propertzes of UHMWPE one Day post Irradzatzon zn Azr or NztroKen
TABLE 1
(N2)
Change in Percent a f t e r Property
SI-Unlt
Normalized Extinction
%
Density So1
6 kGy
Reference
0,84
12 kGy
25 kGy
18 kGy Air
Air
N2
Air
N2
Alr
N2
+213
+187
+404
+292
+600
+461
N2
+851
+517 +0,23
gcm -3
0,9375
+0,2
+0,11
+0,35
g
71,7
-17,2
-15,5
-13,5
-41,3
YielO Strength
Nmm" 2
32,2
1,3
+1,6
0
UTS
Nmm"2
39,2 * 4,8
+3,1
+8,93
%
515 ± 8
-0,5
+8,93
Elongation at break
The most sensitive factor for zrradlatzon znduced changes zs the carbonyl extlnctzon detected by IR-spectroscopy. The normallzed extznction of the carbonylband at 1720 cm -I zncreases iznear with increased dose when UHMWPE is zrradzated zn alr whzle zncrease zs not iznesr when znert atmosphere is used, presented in F1 g. i. The effect of inert atmosphere becomes evldent for absorbed doses above i0 kay, below no slgnzfzcant dzfference to zn azr zrradzated UHMWPE could be detected. Denslty measurement wlth a denslty gradlent column showed slmllar trends.
tO
I
in air •
/./
/4 /
xuJ
/" j
•
a¢¢ SkSV/h
i°
ilh
N ).
%
/
/ / / ,//
1
[s] 70
% % % 50
% %
.I
/"
/
%
f
40
/
12 24 AmOn~O OOm (kSy] Flg. i.
in IAP-
%
C s r b o n y l ~ o u p extznction st 1720 cm -I for UHMWPE zrradlated in alr and in nitrogen (N2).
t2.5 FZE. 2.
l
Soluble constztuents of UHMWPE zrradzated zn azr and nztrogen (N2).
6th International M ~ t m g on Ra&ataon Processing
695
The comparison between nitrogen or alr presence during irradiation on the decrease of the soluble constituents is plotted in Fig. 2. Again up to a dose of i0 kGy no distinct change between those two environments used could be traced. Further irradiation to 25 kGy in a l p increases the sol while it zs substanclally reduced by using nltrogen, showlng a high degree of crosslznkzng. All other properties of UHMWPE change less than i0 percent due to the radlatlon-treatment. The Influence of lowerlng the sterilization dose for the contaminated hlp cups is presented in Fig. 3. Until 12 kGy bacteria growth was persisting, from 18 kGy on no growth could be detected.
100
i
I
doIa Pate: 5kGy~
75 iO' bit:tiP ! l
--
:
S. EP;DE~ZS S. FAECALZS
m
B. SUBTILUS
25
\ 1; 24 ABSORBED DOSE [kGy]
Fig.
3.
Surviving bacteria fraction as function of the absorbed dose.
Influence of storage Storage of pending on for UHMWPE irradiated
irradlated UHMWPE results zn postoxzdatlon (Dole, 1979, Helnze, 1966; Moore, 1983) deirradiation and storage condition. The change zn properties is always more pronounced i r r a d i a t e d in alr and stored in air 21°C or even more in water 37°C, than for samples or stored in nitrogen, presented zn Table 2.
TABLE 2
Chanse in Properties for UHMWPE~ or Water at 37°C
Storage time (month)
Density
0 g/cm 3 9
Normol|zed Extinction
%
0 9
25 kGy in air I water air J water 0,9375 0,9408 +0.03 Z I +0,15 % +0.07 % I +0.31% 0,84 7.99 +32,1% I +270 % +37,3 % I +79,1% 71.7 62,0 -9,4 % J -8,9 % +7,7 % I +18,4 Z 32,2 32,7 -5.9 % J -q.~ % +0.51% J +7,0 % 39,2 40.~ -6,6 % I -2,0 Z -2.5 % I -2.5 % 515 512 -3,9 % JI +0,8 % 24.0 % JI +6.3 % Reference
o|r
l
Sol
%
0 9
abed in Air and Nztrosen
B
(Nr) at 21°C
25 kGy Jn N2 I N2 J water 0,9397 +0,19 , J +0,0. % I +0,27% air
5,18
+46.5 % J +50.9 % J +i0, % 42.1 +12.6% I -i.0 % [ +31.8 %
m
o
Yield
Strength
Nlm~2
9
q
Ultimate Strength E}ongotlon at break
N/nl112
o g 0
%
g
m
)2,2 +1.9 % ] +0.3 % i +5.9 % 42.7 -9.6 % I -6.6 % J -7.7 % p
561 +4.8 % II +8.0 % I1 +5.5 %
Oxidation continues with storage time but major changes are completed after three months as shown in Flg. 4, 5a and 5b.
696
R M
STRmCHm~
stored
~5 V refm, ence
He0
• L:~key in e~r
I
• ~k6y tn H2
?
air Him
/-
1C
in:
mlr N2
60CO.5kSy/h I&r
0
Fzg. 4.
3
6 STORASE TIRE [month]
Increase zn carbonyl e x t z n c t z o n of UHMWPE as result of z r r a d z a t z o n atmosphere and agzng e n v l r o n m e n t (H20 = water 37°C, alr 21°C, N 2 = nltrogen 21°C).
low "
? ,
/
x w
0
3 6 STORA6E TIHE [month]
Fzg. 5.
Fzg.
5a.
0
9
3 6 STORA6E TII4E [month]
9
Increase zn carbonyl content of UHMWPE as a functlon of i r r a d z a t z o n and a g z n g tzme in azr at 21°C. UHMWPE,
zrradzated zn azr.
Fzg. 5b.
UHMWPE z r r a d z a t e d zn nztrogen.
Storage zn water at 37°C causes increased postoxldatzon, storage zn N causes only mlnlmal oxzda. 2 tlon. Samples of UHMWPE whlch have been irradlated and stored zn n l t r o g e n for one month showed a s z g n z f l c a n t zncrease in carbonyl content when stored afterwards zn water at 37°C as can be seen in Flg. 6. After 3 month storage zn N 2 and then storage zn water p o s t o x z d a t z o n is neglzglble. Denslty increases a n a l o g wlth increaslng carbonyl groups but only to a m a x z m u m of 0,32 %; more p r o n o u n c e d w h e n UHMWPE zs stored in water. Fzg. 7 glves the v a r z a t z o n in soluble c o n s t l t u e n t s of UHMWPE as a functlon of storage d e p e n d e n t on the atmosphere zn whzch zrradzatzon took place the sol content zncreases c r e a s z n g storage tzme when water or alr as storage medzum are used. D u r z n g storage in the soluble c o n s t i t u e n t s decrease, z n d z c a t l n g further c r o s s l z n k z n g due to the absence All these structural m o d z f z c a t l o n s zndzcated zn Table 2.
tzme. Inwzth znnltrogen of oxygen.
of UHMWPE also znduce changes zn the m e c h a n z c a l p r o p e r t z e s as
6th Internauonal MeeUng on RadmUon Processing
697
SOL
[z]
7o
/
15
: m
o • •
•
stored in 1t20 37°C I month in I/2 then 3 month in N2 then s t o o d in ~ 21"C
I 1120 37"C ~
50
37"C
I
37"C
i
/-
qk1~ •
..~ - ~ ' -
{N22t'C /
60CO. 5kSy/h tO L~kSy I 9
Z
3
..--.--.--
Time dependent post aglng of UHMWPE in water 37°C after Irradiation to 25 kGy in nitrogen and prestorage in nitrogen at 21°C.
~
30
3
STORA6E TIME [month] Fig. 6.
in m~"
6
9
STORAGE TIME [month] Fig. 7.
Change in soluble constituents of UHMWPE, irradiated to 25 kGy in alr and nitrogen. Influence of the storing medium. (H~O = water 37°C, air = 21°C, N~ = nitrogen 21°C).
DISCUSSION Irradiation of UHMWPE wlth 60Co rays induces molecular chanEes (Lue, 1981; Moore, 1983; Wlnslow, 1965) of this polymer. Because of the long duration of the irradiation treatment the absorbed dose as well as the atmosphere used are of crucial importance (Glberson, 1962; Helnze, 1966; Wlnslow, 1965). In the presence of oxygen during radiation treatment carbonyl content of the UHMWPE increases linear with dose (Helnze, 1966; Schaudy, 1978). Thls oxidation inhlblts crosslinking (Lue, 1981; Schaudy, 1978) and causes density increase (Grood, 1982, Lue, 1981, Nussbaum, 1979; Roe, 1981, Wlnslow, 1965) which results from chain cleavage, mainly in the amorphous regions of the polymer (Eyerer, 1983, Shastrl, 1983; Wlnslow, 1965) The therefore increased mobllity of the molecular chains leads to recrystallization which enhances yield strength, hardness (Ungethuem, 1979) and creep resistance. The sol content increases (Lue, 1981, Schaudy, 1978; Shastrl, 1983; Ungethuem, 1979) and the elongation at break decreases. Due to incorporation of heteroatomlc oxygen in the carbon-hydrogen-polymer UHMWPE polarity and watersorptlon increase (Clarke, 1985, Nussbaum, 1979). All changes induced in UHMWPE become more pronounced wlth increasing dose, because of the longer exposition to alr under ionizing radiation. For thls reason resterzlizatlon should be abandoned, because polyethylene becomes more rigid and fracture of implant components could result (Eyerer, 1983, Roe, 1981, Shastrl, 1983; Ungethuem, 1979). Exluslon of oxygen by using inert gas during irradiation can also reduce the implantation of OXldative structures in UHMWPE markedly (Helnze, 1966). Until a dose of i0 kGy the effect is negllble due to the reaction of dissolved oxygen present in the polymer (Baeuerleln, 1983). Going to the minimal dose used for sterilization of medical products the beneficial effect of using inert gas during irradiation becomes evident. Crossllnklng can take place to a larger extend and the desired toughness of UHMWPE, which is responsible for its good wear performance (Rose, 1982) does not change, while increase in creepreslstance and crossllnking of the intercrystalline regions should be ideal for a long term trlbologlcal stressed plastlc component (Hornbogen, 1983). Irradiation induces latent radicals in UHMWPE (Helnze, 1966) which can react during storage, depending on the conditions used (Heznze, 1966; Moore, 1983, Nussbaum, 1979). Although UHMWPE irradiated in nitrogen and stored in alr or water exhibits a somewhat augmented postoxldatlon, the cumulatlve change remains less pronounced than for components irradiated in air. If nitrogen storage IS used the radicals can recombine and lead to enhanced crosslinking, while the presence of oxygen results in oxidation. Postoxldatlon of UHMWPE increases density (Grood, 1982, Roe, 1981, Shastrl, 1983), carbonyl- (Helnze, 1966, Moore, 1983) and sol content (Schaudy, 1978; Ungethuem, 1979). Tensile strength IS reduced and elongation increased. Below i0 kGy the observed changes are neEllglble and show no influence of the atmosphere in which irradlatlon took place.
698
R M STI~Cm~
Also non zrradlated samples of UHMWPE show the effect of agznE due to storage (Grood, 1982; Roe, 1981; Shastrl, 1983). Three month after irradlatzon treatment the oxzdatlon induced changes a r e mostly terminated. The ~nfluence of sterlllzatzon Irradlatlon on the trlbologlcal propertles of UHMWPE has been reported by several authors. Although the results dxffer, all show the same (Ungethuem, 1979) or better wear reslstance (Lue, 1981; Rose, 1982) xf i r r a d i a t i o n was performed in inert atmosphere. The investlgatlon on a posslble reduction of the mlnlmal dose for sterzllzatlon s u ~ e s t s that at least 15 kGy for achlevlng an acceptable SAL could be used, whxch can reduce the oxldatlon effects xn UHMWPE.
CONCLUSION Ster111zatlon of UHMWPE wlth 60Co lonlzlng radlatlon in alr results in substantlal oxldatlon of this polymer, leadlng to a loss of ductzlzty. Irradlatzon in nltrogen enhances the crossllnklng of UHMWPE whlch shows beneflts in mechanlcal and trlbologzcal properties. The use of nltrogen durlng sterlllzatlon of surglcal xmplants manufactured from UHMWPE is beneflclal and therefore recommended.
REFERENCES Awatanz, J., and Mznegakz, M. (1960). Study on effects of x-ray xrradzated PE by a vzbratlon method. Pol. Jap. Soc. r 60, 1189-1196. Clarke, I. C., and coworkers (1985). Fluld sorptlon phenomena in sterzllzed PE acetabular prostheses. B1omaterlals r 6, 184-188. Dole, M. (1979). Crystalllnlty and crossllnkln E efflclency xn the zrradlatlon of PE. Radlat. Phys. Chem. r 14, 711-720. Dorpema, J. W., and van Asten, J. A. A. M. (1983). A new approach to sterxllzatlon Condltlons. Paper GPS 323. 3rd Gamma Processzn~ Semxnar, Ottawa. Eyerer, P. (1983). Werkstofflzche UntersuchunEen an explantzerten Hdftpfannen aus UHMWPE. Bzomed. Tech. 28, 297-309. Gzberson, R. C. (1962). Oxygen dlffuszon and reactzon durznE y-zrradzatlon of PE. J. Phys. Chem.~ 66, 463-486. Grood, E. S., Shastrl, R., Hopson, C. N. (1982). Analyszs of retrxeved zmplants" Crystalllnlty changes zn UHMWPE. J. Bzomed. Mat. Res. r 1G, 399-405. Handlos, V. (1981). Sterllxzatzon by electron beam. Radlat. Phys. Chem.~ 18/1-2, 175-182. Heznze, D. (1966). Das Verhalten yon Hochpolymeren Eegenuber enerEzerezcher StrahlunE. Kollozd Z. r 210/1, 45-54. Hornbosen, A. M. (1983). Mzkrostruktur und Verschlezss. In K. H. zum Gahr (Ed.), Rezbun~ und Verschlezss. DVM, Oberursel. 79-103. Leemhorst, J. G. (1984). Industrial appllcatzon of the gamma sterllzzatzon process. Paper, Sterzlzzatzon valzdatzon of medzcal devzces and sur~zcal products, Kopenhagen. Ley, F. J. (1984). Radlatzon sterzlxzation - Mzcrobzologzcal aspects. Paper, Sterxlxzatzon valzdatzon of medzcal devzces and sur~zcal products, KopenhaEen. Loy, B. R. (1960). ESR studles of free radzcal decay zn gamma-zrradzated polyethylene. J. Pol. Scz.~ 44, 311-847. Lue, C. T., Ellzs, E. J., Crugnola, A. (1981). Effects of gamma-zrradzatzon on UHMWPE. Abstracts 39th ANTEC, 246-248. Moore, E. P. (1983). Development zn radzatzon reszstant plastzcs. Paper GPS 315. 3rd Gamma Processzn~ Semznar, Ottawa. Nussbaum, H. J., and Rose, R. M. (1979). The effects of radzatzon sterzlzzatxon on the propertzes of UHMWPE. J. B~omed. Mat. Res.~ 13, 557-576. Roe, R.-J., and coworkers (1981). Effect of radzatzon sterzlzzatzon and ag~n E on UHMWPE. J. Bzomed. Mat. Res.~ 15, 209-230. Rose, R. M., and coworkers (1982). On the pressure dependence of the wear of UHMWPE. Wear~ 92, 99-iii. Schaudy, R. (1978). Ox~datlonsvorganEe bex der Strahlenvernetzung yon HDPE. Kunststoffe~ 68/3, 167-170. Shastrl, R., and coworkers (1983). Effect of aglng on UHMWPE. Abstracts 41st ANTEC, 17-19. Strexcher, R. M. (1986). Ultrahochmolekulares Polyethylen als Werkstoff fur Huftgelenkpfannen. In H. J. Reflor, M. H. Hackenbroch, C. J. W1rth (Ed.), Der alloplastxsche Ersatz der Hdftpfanne. G. Thleme, Stuttgart. 48-52. Un~ethuem, M., and Hxnterberger, J. (1979). Der Elnfluss der Strahlensterxlzsatlon auf das Verschlezssverhalten yon PE. Z. Orthop.~ 117, 790-79~. W1nslow, F. H., Matreyek, W., Stzlls, S. M. (1965). Oxzdatzve embrzttlement of PE. Trans. NY. Ac. Scl., 301-314.