JOURNAL OF THE MECHANICAL BEHAVIOR OF BJOMEDIC/¥L MllTERlllLS 5 1 (20 1 5 ) 1 3‑24
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ELSEVIER www.e l s e v i e r . c o m / l o c a t e / j m b b m
Research Paper
I n ‑ v i v o d e g r a d a t i o n o f m i d d l e ‑ t e r m h i g h l y c r o s s ‑ l i n k e d ( l ) c r o s s M a r k and r e m e l t e d p o l y e
由y l e n ec u p s : M o d i f i c a t i o n induced ‑
by c r e e p , wear and o x i d a t i o n
Y o s h i h i r o M i u r α
ぺMαsαhiroHαsegαωαa
ぺA k i h i r o Sudo a , Giuseppe P e z z o t t i b ,
Leon α rdo Puppulin '
a D e p a r t m e n t o f O r t h o p a e d i c S u r g e r y , Mie U n i v e r s i t y , G r a d u a t e S c h o o ! o f M e d i c i n e , 2 ‑ 1 7 4 E d o b
日s h i , T s u 514 ・ 8 5 0 7 ,M i e , J
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b c e r a m i c P h y s i c s L a b o r a t o r y , K y o t o I n s t i t u t e o f T e c h n o l o g y , S a k y o ‑ k u , M a t s u g a s a k i , 6 0 6 ‑ 8 5 8 5 K y o t o , J a p a n
< D e p a r t m e n t o f Mo l e c u l a r C e l l P h y s i o l o g y , K y o t o P r e f e c t u r a l U n i v e r s i t y o f M e d i c i n e , K a m i g y o ‑ k u H i r o k o j i A g a r u , K
日warama c h i ‑ d o r i , 602‑8566 K y o t o , J a p a n
ARTICLE INFO ABSTRACT
2 2 J u n e 2 0 1 5
I n t h i s s t u d y Raman { R S ) and F o u r i e r Transform I n
仕a r e d{ F T ‑ I R ) s p e c t r o s c o p i c t e c h n i q u e s were exp l o i t e d t o s t u d y 1 1 r e t r i e v e d l i n e r s made o f r e m e l t e d h i g h l y c r o s s ‑ l i n k e d p o l y e t h y l e n e { H X L P E ) , w i t h t h e i n t e n t t o e l u c i d a t e t h e i r i n ‑ v i v o mechanical and c h e m i c a l d e g r a d a t i o n . The r e t r i e v a l s had d i f f e r e n t f o l l o w ‑ u p s , r a n g i n g from a few months t o 7 y e a r s o f i m p l a n t a t i o n t i m e and b e l o n g t o t h e f i r s t g e n e r a t i o n o f h i g h l y c r o s s ‑ l i n k e d a nd r e m e l t e d p o l y e t h y l e n e c l i n i c a l l y i n t r o d u c e d i n 1 9 9 9 , b u t s t i l l c u r r e n t l y i m p l a n t e d . Raman a s s e s s ‑ ments enab l ed t o d i s c r i m i n a t e c o n t r i b u t e s o f wear and c r e e p on t h e t o t a l r e d u c t i o n o f t h i c k n e s s i n d i f f e r e n t l o c a t i o n s o f t h e c u p . A c c o r d i n g t o our r e s u l t s , a l t h o u g h t h e most o f t h e v i s c o e l a s t i c d e f o r m a t i o n o c c u r r e d d u r i n g t h e f i r s t y e a r ( b e d d i n g ‑ i n p e r i o d ) , i t p r o ‑ g r e s s e d d u r i n g t h e s t e a dy wear s t a t e up t o 7 y e a r s w i t h much l o w e r but not n e g l i g i b l e r a t e . O v e r a l l , t h e wear r a t e o f t h i s r e m e l t
巴dHXLPE l i n e r was l o w . P r e l i m i n a r y ana l y s i s o n microtomed s e c t i o n s o f t h e l i n e r s a f t e r i n ‑ v i v o and i n ‑ v i t r o a c c e l e r a t e d a g i n g {ASTM F 2 0 0 3 ‑ 0 2 ) ena b l e d t o o b t a i n a phenomenolo
肝心c o r r e l a t i o n between t h e o x i d a t i o n i n d e x { 0 1 ) and t h e amoun t o f orthorhombic phase f r a c t i o n （
吋，whichcan be e a s i l y n o n ‑ d e s t r u c t i v e l y measured by R S . P r o f i l e s o f α ， o b t a i n e d from d i f f e r e n t l o c a t i o n s o f t h e c u p s were used t o j u d g e t h e o x i d a t i v e d e g r a d a t i o n o f t h e 1 1 r e t r i e v a l s , c o n s i d e r i n g a l s o t h e ex ・ v i v ot i m e e l a p s e d from t h e r e v i s i o n s u r g e r y t o t h e s p e c t r o s c o p i c e x p e r i m e n t s . L ow but measurab l e l e v e l o f o x i d a t i o n was d e t e c t e d i n a l l t h e sho r t ‑ t e r m r e t r i e v a l s , w h i l e i n t he m i d d l e ‑ t e r m samples peaks o f 0 1 were o b s e r v e d i n t h e s u b s u r f a c e ( u p t o 0 1 = 4 . S ) , p r e sumably induced by t h e combined e f f e c t o f mechanical s t r e s s , l i p i d a b s o r p t i o n and p r o l o n g e d e x ‑ v i v o she l f ‑
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Accepted 2 3 J u n e 2 0 1 5 A v a i l a b l e o n l i n e 9 J u l y 2 0 1 5 K e γ w o r d s ・ ：
UHMWPE
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c 2 0 1 5 E l s e v i e r L t d . A l l r i g h t s r e s e r v e d .
* C o r r e s p o n d i n g a u t h o r
E ‑ m a i l a d d r e s s : masahase@clin . medic . m i e ‑ u . a c . j p {M . H a s e g a w a ) . h t t p : / / d x . d o i . o r g / 1 0 . 1 0 1 6 / j .jmbbm . 2 0 1 5 . 0 6 . 0 2 8
1 7 5 1 ‑ 6 1 6 1 / c 2 0 1 5 E l s e v i e r L t d . A l l r i g h t s r e s e r v e d .
14
J O U R N A L 0 F T H E M E C H A N I C A L B E H A V I 0 R 0 F B l 0 M E D l C A L M AT E R I A L S 5 I ( 2 0 I 5 ) I 3 ‑2 41 . Introduction
B a l a n c e among w e a r , c r e e p and o x i d a t i o n r e s i s t a n c e o f UHMWPE e x p o s e d t o b i o m e c h a n i c a l l o a d s d u r i n g i m p l a n t a t i o n i n t h e human b o d y . A f t e r a l m o s t two d e c a d e s
仕om t h e development and l a u n c h o f t h e f i r s t g e n e r a t i o n o f h i g h l y c r o s s ‑ l i n k e d p o l y e t h y l e n e f o r t o t a l h i p a r t h r o p l a s t y σ H A } , t h i s has become t h e p r i m a r y i s s u e t o be a d d r e s s e d by s c i e n t i s t s and e n g i n e e r s i n o r d e r t o a c h i e v e h i g h e r s t a n d a r d s i n l o n g ‑ t e r m s u r v i v o r s h i p o f h i p j o i n t r e p l a c e m e n t s . The i n t r o d u c t i o n o f r a d i a t i on c r o s sl i n k i n g o f u l t r a ‑ h i g h m o l e c u l a r p o l y e t h y l e n e (UHMWPE} i n t h e l a t e 1 9 9 0 s i n d e e d r e p
陀s e n t s t h e major b r e a k t h r o u g h i n t h e r e s e a r c h and development o f o
口h o p e d i c b e a r i n g m a t e r i a l s w i t h s u p e r i o r wear and c r e e p r e s i s t a n c e ( Dumbleton e t a l . , 2 0 0 6 ; Gencur e t a l . , 2 0 0 6 ; K u r t z e t a l . , 2 0 1 1 } . I n f a c t , by i n c r e a s i n g t h e m o l e c u l a r w e i g h t t h r o u g h t h e f o r m a ‑ t i o n o f c r o s s ‑ l i n k s between a d j a c e n t m o l e c u l a r c h a i n s , t h e a b i l i t y o f t h e polymer t o undergo l a r g e d e f o r m a t i o n i s l a r g e l y d e c r e a s e d . As a m a t t e r o f f a c t , e x c e s s i v e d e f o r m a t i o n c a u s e d by t h e s l i d i n g o f t h e h a r d e r f e m o r a l head on t h e s u r f a c e o f t h e p o l
戸n e rmay i n d u c e m o l e c u l a r o r i e n t a t i o n a l o n g t h e d i r e c t i o n o f p l a s t i c f l o w ( n a m e l y t h e d i r e c t i o n o f d e f o r m a t i o n ) . H i g h m o l e c u l a r a l i g n m e n t a l o n g t h e p r i m a r y s l i d i n g d i r e c t i o n de t e ‑ r i o r a t e s t h e m e c h a n i c a l r e s i s t a n c e o f t h e p o l
戸ner a l o n g t h e o t h e r d i r e c t i o n s and such a s t r a i n ‑ s o f t e n i n g phenomenon l e a d t o t h e detachment from t h e s u r f a c e o f m i c r o m e t r i c de b r i s ( Wang e t a l . , 1 9 9 7 } , which may e v e n t u a l l y t r i g g e r an a c u t e immune r e s p o n s e t h a t r e s u l t s i n o s t e o l y s i s a n d , u l t i m a t e l y , i n j o i n t l o o s e n i n g O ames e t a l . , 1 9 9 3 ; Oparaugo e t a l . , 2 0 0 1 ; Zhu e t a l . , 2 0 0 1 } . I f from one hand wear i s a d e g r a d a t i o n o f t h e m a t e r i a l which o c c u r s on t h e s u r f a c e , on
せ1 eo t h e r h a n d , when we d e a l w i t h p o l y m e r s , a l s o t h e i n e v i t a b l e t e n d e n c y o f t h e b u l k m a t e r i a l t o deform when s u b j e c t e d t o l o a d f o r p r o l o n g e d t i m e must c a r e f u l l y t a k e n i n t o c o n s i d e r a t i o n d u r i n g t h e c o n c e p t i o n phase o f b e a r i n g s . The v i s c o e l a s t i c deforma
包on( i . e . , c r e e p ) might be d e l e t e r i o u s f o r t h e c o π e c t f u n c t i o n o f t h e p r o s t h e s i s ,
e s p e c i a l l y when i t m a n i f e s t s i n t h e form o f p i n c h d e f o r m a t i o n , which c h a n g e s t h e s p h e r i c i t y o f t h e cu p and i n d u c e s e q u a t o r i a l c o n t a c t and d e g r a d a t i o n o f t h e f l u i d ‑ f i l m l u b r i c a t i o n Q i n e t a l . , 2 0 0 6 ; Meding e t a l . , 2 0 1 3 ; Ong e t a l . , 2 0 0 9 } . I n o t h e r w o r d s , e x c e s s i v e c r e e p might a d v e r s e l y a f f e c t t h e wear r e s i s t a n c e and t h e s t a b i l i t y o f t h e j o i n t i n t h e l o n g t e r m . D e s p i t e t h e d i f f e r e n t p h y s i c a l o r i g i n o f wear and c r e e p , from a p h e n o m e n o l o g i c a l p o i n t o f view
，せ1e yboth m a n i f e s t t h e m s e l v e s w i t h a r e d u c t i o n o f cup t h i c k n e s s , whose t o t a l e x t e n t i s r o u t i n e l y measured both i n v i v o and on cup r e t r i e v a l s , and cumulat i v e l y r e c o r d e d i n c l i n i c a l d a t a . Moreover t h e y a r e b o t h e x a c e r b a t e d by o x i d a t i v e d e g r a d a t i o n , which i s p r o g r e s s i n g w i t h l o n g ‑ t e r m c l i n i c a l u s e , l e a d i n g t o d e b r i s f o r m a t i o n a n d , u l t i m a t e l y , t o j o i n t l o o s e n i n g , o r , i n t h e most u n f o r t u n a t e c a s e s , t o t h e p r e m a t u r e m e c h a n i c a l f a i l u r e o f t h e component ( S c h m a l z r i e d and C a l l a g h a n , 1 9 9 9 } . The f i r s t g e n e r a t i o n o f HXLPEs was m a n u f a c t u r e d w i t h i n c l u d ‑ i n g a me l t i n g o r annea l i n g s t e p i n o r d e r t o qu ench t h e e x c e s s o f f r e e r a d i c a l s g e n e r a t e d d u r i n g i r r a d i a t i o n , which may e v e n ‑ t u a l l y r e a c t m
自 由eoxygen d i s s o l v e d i n t h e po l ymer and l e a d t o o x i d a t i v e d e g r a d a t i o n ( Yeom e t a l . , 1 9 9 8 ) . H o w e v e r , t h e r e s u l t s o f t h e f i r s t s t u d i e s on r e t r i e v a l s made o f
白r s tg e n e r a t i o n HXLPE showed t h e u n e x p e c t e d a p p e a r a n c e o f m e c h a n i c a l and c h e m i c a l d e g r a d a t i o n ( C u r r i e r e t a l . , 2 0 0 7 , 2 0 1 0 ) . I f from one hand a s i m p l e p o s t ‑ i r r a d i a t i o n a n n e a l i n g s t e p has p r o v e d t o be i n s u f f i c i e n t t o remove a l l t h e f r e e r a d i c a l s g e n e r a t e d d u r i n g c r o s s ‑ l i n k i n g ( O r a l e t a l . , 2 0 1 1 , on t ) h e o t h e r hand p o s t ‑ i r r a d i a t i o n r e m e l t i n g r e d u c e s t h e c r y s t a l l i n i t y o f t h e f i n a l m i c r o s t r u c t u r e , which c o n s p i c u o u s l y d e c r e a s e s t h e m e c h a n i c a l s t r e n g t h o f t h e polymer ( B a k e r e t a l . , 2 0 0 3 ; G i l l i s e t a l . , 1 9 9 9 ; Gomoll e t a l . , 2 0 0 1 ; O r a l e t a l . , 2 0 1 1 } , and can n o t p r o t e c t t h e ma t e r i a l from p o s s i b l e g e n e r a t i o n o f f r e e ‑ r a d i c a l s d u r i n g i n ‑ v i v o s e r v i c e . I n t h e a t t empt t o enhance wear and ox i d a t i o n r e s i s t a n c e , a s e c o n d g e n e r a t i o n o f HXLPEs was e n g i n e e r e d by i n t r o d u c i n g two a l t e r n a t i v e m e t h o d s . The f i r s t one i s based on a 3 ‑ s t e p i r r a d i a t i o n and a n n e a l i n g p r o c e s s , u s i n g a l o w e r d o s e o f y ‑ r a y a t each s t e p ( i . e . , 3 0 k G y } , wh i ch e n a b l e s t o o p t i m i z e t h e e f f i c i e n c y o f t h e c r o s s ‑ l i n k i n g d u r i n g e a c h a n n e a l i n g , t o
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拘 ｜N o . I n ‑ v i v o t im e E x ‑ v i v o t i m e A g e S e x B M !
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T h i c k n e s s C a u s e o f F e m o r a l ( m o / y r ) ( m o / y r ) ( y r ) m 2 ) A n g l e （ 。 ） (mm) r e v i s i o n h e a d S h o r t t e r m
No 1 0 . 5 / 0 . 0 4 1 3 7 / 1 1 . 4
71F 2 9 4 5 1 0 . 3 Cup Z i r c o n i a l o o s e n i n g
No 2 1 . 0 / 0 . 0 8 6 . 0 / 0 . 5 0 5 8 F 3 5 4 6 6 . 3 I n f e c t i o n D e l t a No 3 1 . 5 / 0 . 1 3 9 2 / 7 . 7 0
71F 1 8 3 5 1 5 . 4 I n f e c t i o n Z i r c o n i a No 4 3 . 0 / 0 . 2 5 7 4 / 6 . 2 0 6 7 F 2 9 3 9 8 . 2 D i s l o c a t i o n Z 1 r c o m a No 5 6 . 0 / 0 . 5 0 3 9 / 3 . 3 0 7 8 F 2 0 3 0 7 . 2 Cup Co C r
l o o s e n i n g
No 6 6 . 5 / 0 . 5 4 9 4 / 7 . 8 0
71F 2 6 4 0 1 5 . 4 S t e m Z i r c o n i a l o o s e n i n g
No 7 8 . 0 / 0 . 6 7 8 9 / 7 . 4 0 3 2 M 2 6 4 7 1 2 . 4 N e u r o ‑ Co C r p a r a l y s i s
M i d d l e t e r m
No 8 4 9 / 4 . 1 0 5 0 / 4 . 2 0
72F 3 3 4 5 6 . 4 D i s l o c a t i
口n Co C r No 9 7 3 / 6 . 1 0 7 6 / 6 . 3 0 6 8 F 3 0 4 0 8 . 2 S t e m Z i r c o n i a
l o o s e n i n g
No 1 0 7 7 / 6 . 4 0 7 8 / 6 . 5 0 6 1 F 2 5 4 8 7 . 2 S t e
ロ1 Z i r c o n i a l o o s e n i n g
No 1 1 8 6 / 7 . 2 0 1 5 / 1 . 3 0 7 5 F 2 0 4 5 6 . 3 I n f e c t i o n Co C r
15
J O U R N A L O F T H E M E C H A N I C A L BEHAVIOR O F BIOMEDICAL MATERIALS
51 (20 1 5 ) 13‑24
2 0 0 7 , 2 0 1 1 ; P u p p u l i n e t a l . , 2 0 1 1 ) , w i t h t h e i n t e n t o f u n d e r ‑ s t a n d i n g how t h e s e phenome n a p r o g r e s s d u r i n g t h e i m p l a n t a ‑ t i o n t i m e and how t h e y a
仔e c teach o t h e r .
Materials and methods HXLPE r e t r i e v a l s
A t o t a l o f 1 1 r e t r i e v e d a c e t a b u l a r c u p s were o b t a i n e d a
氏e r p e r i o d s o f time from t h e r e s p e c t i v e h i p a r t h r o p l a s t y r e v i s i o n s u r g e r i e s r a n g i n g from 0 . 5 t o 1 1 . 4 y e a r s . Seven c u p s were s h o r t ‑ term i n ‑ v i v o exposed ( b e t w e e n 2 weeks and 8 months) and 4 c u p s were m i d d l e ‑ t e r m i n ‑ v i v o exposed ( b e t w e e n 4 . 1 and 7 . 2 y e a r s ) . Ta b l e 1 shows t h e l i s t o f t h e r e t r i e v a l s , whic h i n c l u d e s t h e c l i n i c a l d a t a o f t h e r e s p e c t i v e p a t i e n t , t h e i m p l a n t a t i o n t i m e ( h e r e a
氏e rr e f e r r e d a s i n ‑ v i v o t i m e ) , t h e s h e l f ‑ a g i n g t i m e i n a i r e l a p s e d from t h e r e v i s i o n s u r g e r y t o t h e s p e c t r o s c o p i c a n a l y s i s ( h e r e a f t e r r e f e r r e d a s e x ‑ v i v o t i m e ) , t h e c a u s e o f r e v i s i o n and t h e t y p e o f f e m o r a l head . A l l t h e cups were made o f t h e same brand o f HXLPE , L o n g e v i t y ( Z i m m e r , I n c . ; Warsaw , I n d i a n a , U S A ) , which b e l o n g s t o t h e f i r s t ‑ g e n e r a t i o n r e m e l t e d p o l y e t h y l ene c l i n i c a l l y i n t r o d u c e d i n t h e T r i l o g y a c
巴t a b u l a rcup de s i g n s i n c e 1 9 9 9 . The l i n e r s were ma c h i n e d
仕om molded s h e e t s , w h i c h were p r e v i o u s l y con ・ s o l i d a t e d from GUR 1 0 5 0 r e s i n ( 5 . 5 ‑ 6 m i l l i o n g l m o l ) , c r o s s ‑ l i n k e d by e l e c t r o n beam w i t h a t o t a l d o s e o f 1 0 0 kGy a t 40 。 c
and then r e m e l t ed ( > 1 3 5 ℃） t o quench r e s i d u a l f r e e r a d i c a l s . 2 .
2 . 1 . p r e s e r v e h i g h c r y s t a l l i n i t y and t o r e d u c e t h e number o f r e s i d u a l
f r e e ‑ r a d i c a l s , a l t h o u g h t h e y a r e n o t c o m p l e t e l y e l i m i n a t e d ( Ku
代z , 2 0 0 9 ) . The second one was c o n c e i v e d t o s t r o n g l y c o u n t e r a c t t h e l o n g ‑ t e
ロn o x i d a t i o n by i n c o r p o r a t i n g i n t h e r e s i n t h e a n t i o x i d a n t v i t a m i n E ( b l e n d e d o r i n f used a f t e r c r o s s ‑ l i n k i n g ) and t o i n c r e a s e f a t i g u e and wear r e s i s t a n c e ( B r a c c o and O r a l , 2 0 1 1 ; S l
曲a t ae t a l . , 2 0 0 6 ; Teramura e t a l . , 2 0 0 8 ) . N e v e r t h e l e s s , i m p l a n t a t i o n o f a c e t a b u l a r cups which b e l o n g t o t he f i r s t g e n e r a t i o n o f HXLPE i s s t i l l a r e c u
汀e n t p r o c e d u r e i n many o r t h o p e d i c c l i n i c s a l l around t h e w o r l d . I n t h e p r e s e n t p a p e r we show t h e r e s u l t s o f t h e a n a l y s e s on 1 1 r e t r i e v a l s made o f t h e same t y p e o f r e m e l t e d HXLPE ( L o n g e v i t y ; Zimme
功w i t hd i f f e r e n t f o l l o w ‑ u p s , r a n g i n g from a f ew months t o 7 y e a r s o f i m p l a n t a t i o n time . S e v e r a l c l i n i c a l s t u d i e s on reme l t e d HXLPE have been p u b l i s h e d d u r i n g t h e p a s t d e c a d e , which showed i n t e r e s t i n g d a t a on w e a r , f e m o r a l head p e n e t r a ‑ t i o n and o x i d a t i o n o c c u r r e d d u r i n g i n ‑ v i v o s e r v i c e ( Bragdon e t a l . , 2 0 0 7 ; Cu
ロi e re t a l . , 2 0 0 7 , 2 0 1 0 ; D i g a s e t a l . , 2 0 0 4 ; F u k u i e t a l . , 2 0 1 3 ; G e l l e r e t a l . , 2 0 0 6 ; Manning e t a l . , 2 0 0 5 ; M u r a t o g l u e t a l . , 2 0 1 0 ; R o w e l l e t a l . , 2 0 1 4 ; Thomas e t a l . , 2 0 1 1 ) , b u t on l y a few o f them r epo
口edf o l l o w ‑ u p s l o n g e r than 4 y e a r s ( F u k u i e t a l . , 2 0 1 3 ; M u r a t o g l u e t a l . , 2 0 1 0 ; Thomas e t a l . , 2 0 1 1 We ) . used and imp l emented an e x p e r i m e n t a l p r o t o c o l based on Raman ( R S ) and F o u r i e r t r a n s f o r m i n f r a r e d （
打ー問s p e c t r o s c o p i e s which we a l r e a d y i n t r o d u ced i n some o f our p r e v i o u s p a p e r s t o s y s t e m a t i c a l l y s t u d y t h e l e v e l o f d e g r a d a t i o n i n d u c e d by wear , c r e e p a nd o
ねi d a t i o non t h e m i c r o s t r u c t u r e o f p o l y e t h y ‑ l e n e ( Kumakura e t a l . , 2 0 0 9 ; O k i t a e t a . l , 2 0 1 4 ; P e z z o t t i e t a l . ,
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出emain c o n t a c t a r e a . ( b ) S c h e m a t i c s o f
t h e RS p r o t o c o l used t o analyze t h e i n ‑ v i v o r e t r i e v a l s a t d i f f e r e n t l o c a t i o n s ( L 1 ‑L 9 ) .
]OURNAL OF THE MECHANICAL BEHAVIOR OF BIOMEDICAL MATERIALS SI (2015) 1 3‑ 24 16
c o n t a m i n a n t s was a c h i e v e d by s o a k i n g t h e s l i c e i n b o i l i n g hexanes ( 6 9 ℃ ） f o r 1 8 h . F i g . 2 ( a ) shows p i c t u
陀So f some o f t h e i n ‑ v i t r o and i n ‑ v i v o o x i d i z e d HXLPE s e c t i o n s .
Raman s p e c t r o s c o p y
Raman m i c r o p r o b e s p e c t r o m e t e r ( T
・6 4 0 0 0 ,H o r i b a / J o b i n ‑ Y v o n , K y o t o , J a p a n ) i n b a c k ‑ s c a t t e r i n g geometry was used t o c h a r ‑ a c t e r i z e t h e m i c r o s t r u c t u r e s o f t h e o x i d i z e d p o l y e t h y l e n e s l i c e s and t h e r e t r i e v a l s . The e x c i t a t i o n s o u r c e was a 5 3 2 nm d i o d e l a s e r ( S O C J u n o , Showa O p t r o n i c s C o . , L t d , T o k y o , J a p a n ) y i e l d i n g a power o f a p p r o x i m a t e l y 9 m W on t h e UHMWPE l i n e r s u r f a c e . The r e c o r d e d n o n ‑ p o l a
討z e ds p e c t r a were a v e r a g e d o v e r t h r e e s u c c e s s i v e measurements a t each s e l e c t e d l o c a t i o n . A s p e c t r a l r e s o l u t i o n b e t t e r t h a n 0 . 1 5 cm ‑ 1 was ach i e v ed by means o f an 1 8 0 0 Vmm g r a t i n g . S p e c t r a l deconvo l u t i o n h a s been p e r f o π n e d by means o f an au t o m a t i c f i t t i n g a l g o r i
出m e n c l o s e d i n a c o m m e r c i a l l y a v a i l a b l e c o m p u t a t i o n a l p a c k a g e ( L a b s p e c 5
，日o r i b a / J o b i n ‑ Y v o n ,K y o t o , J a p a n ) , u s i n g mixed G a u s s i a n / L o r e n t z i a n c u r v e s . The r e l a t i o n s h i p between t h e o b s e r v e d Raman ba nds and t h e v i b r a t i o n a l modes o f t h e p o l y e t h y l e n e m o l e c u l a r s t r u c t u r e h a s been amply documented i n t h e l i t e r a t u r e ( G l o t i n and Mandelkem, 1 9 8 2 ; M u t t e r e t a l . , 1 9 9 3 ; Nay l o r e t a l . , 1 9 9 5 ; R u l l e t a l . , 1 9 9 3 ; S t r o b l and H a g e d o r n , 1 9 7 8 . The ) r e d u c t i o n o f l i n e r t h i c k n e s s due t o wear and c r e e p was a s s e s s e d u s i n g t h e w i d t h o f t h e Raman band l o c a t e d a t 1 1 3 0 cm ‑ 1 ( A g + B 1 g mode o r symmetric s t r e t c h i ng o f C‑C b o n d s ) a s a s e n s o r f o r r e s i d u a l s t r a i n i n t h e p o l y m e r i c network b e c a u s e i t r e f l e c t s t h e d e g r e e o f d i s o r d e r o f t h e po
如ne r i c n e t w o r k , which i s d i r e c t l y a f f e c t e d by s t r a i n . C a l i b r a t i o n e x p e r i m e n t s o f b and b r o a d e n i n g a s a f u n c t i o n o f r e s i d u a l c o m p r e s s i v e s t r a i n f o r t h e L o n g e v i t y HXLPE we r e conducted a c c o r d i n g t o t h e e x p e r i m e n t a l p r o t o c o l a l r e a d y r e p o r t e d i n some o f o u r p r e v i o u s p a p e r s ( O k i t a e t a l . , 2 0 1 4 ; P e z z o t t i e t a l . , 2 . 3 .
The f i n a l s t e r i l i z a t i o n o f t h e a c e t a b u l a r component was made by g a s plasma a f t e r p a c k a g i n g i n a g a s ‑ p e π n e a b l e p o u c h . A l l t h e r e t r i e v a l s were c u t a l o n g t h e m e r i d i a n l i n e o f t h e cup i n t e r c e p t i ng
出ewear zone i n o r d e r p e r f o r m l i n e s c a n s o f Raman s p e c t r a a l ong t h e t h i c k n e s s . The s u r f a c e a f t e r c u t was c a r e f u l l y p o l i s h e d and s p e c t r a were c o l l e c t e d a t 1 0 0
~1mi n ‑ d e p t h t o n e g l e c t p o s s i b l e m i c r o s t r u c t u r a l m o d i f i c a t i o n s i n d u c e d by c u t t i n g . I n F i g . 1 a r e shown e x p l i c a t i v e s k e t c h e s o f t h e cup s e c t i o n i n g , t h e i n v e s t i g a t e d l o c a t i o n s o f each l i n e r and t h e p r o t o c o l o f s p e c t r a a c q u i s i t i o n .
I n ‑ v i t r o and i n ‑ v i v o o x i d i z e d HXLPE s e c t i o n s P r e l i m i n a r y s t u d y by F T ‑ I R and RS o f i n ‑ v i t r o and i n ‑ v i v o o x i d i z e d samp l e s e n a b l e d t o f i n d a p h e n o m e n o l o g i c a l c o r r e l a ‑ t i o n between o x i d a t i o n i n d e x ( O Q and o r t h o r h o m b i c pha s e
台a c t i o no f t h e HXLPE under i n v e s t i g a t i o n . One new L o n g e v i t y cup was c u t t h r o u g h i t s t h i c k n e s s t o o b t a i n a r e c t a n
思1 l a r p r i s m
合om t h e bottom o f t h e c u p . The p r i s m was c u t i n h a l f and 2 0 0
~1ms e c t i o n s were microtomed . One s l i c e was imm e ‑ d i a t e l y a n a l y z e d by I R and RS t o c h a r a c t e r i z e t h e a c e t a b u l a r cup a s r e c e i v e d . F o r t h e r e s t o f t h e s l i c e s , t h e a c c e l e r a t e d a g i n g t e s t was perfoηned a c c o r d i n g t o t h e s t a n d a r d s ASTM F 2 0 0 3
・0 2 ( r e a p p r o v e d i n 2 0 0 8 ) ( S t a n d a r d , 2 0 0 8 ) . The s l i c e s underwent t o a p r e l i m i n a r y c o n d i t i o n i n g s t e p which c o n s i s t e d o f i r r a d i a t i n g t h e specimens w i t h y ‑ r a y ( 2 5 kGy d o s e ) and m a i n t a i n i n g t h em a t 2 3 •c f o r 2 8 d a y s . S u b s e q u e n t l y , t h e s p e c t r o s c o p i c a n a l y s e s were c o n d u c t e d on one s l i c e , w h i l e t h e remnant was p l a c e d i n an oxygen bomb a t 7 0 ° C under 5 a t r n . o f p u r e oxygen f o r 3 , 1 4 , and 2 8 d a y s . The a g i n g t e s t on i r r a d i a t e d s l i c e s was p e r f o π n e d w i t h t h e aim o f c h a r a c t e r i z i n g t h e m i c r o s t r u c t u r a l modi
白c a ‑ t i o n s i n a wide r a n g e o f O l . I n a d d i t i o n , F T ‑ I R and RS a n a l y s e s were p e r f o r m e d on a microtomed s e c t i o n o b t a i n e d from one o f t h e r e t r i e v a l s ( s a m p l e n o . 8 ) . E x t r a c t i o n o f l i p i d s and 2 . 2 .
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F i g . 2 ‑ Thin s e c t i o n o b t a i n e d from R e t r i e v a l No.8 ( L S ) and from a new cup f o r a c c e l e r a t e d a g i n g . The r e t r i e v e d sample p r e s e n t s
a c l e a r white s t r i p e a t 1 m m i n ‑ d e p t h from t h e s l i d i n g s u r f a c e , due t o he avy o x i d a t i o n . ( b ) S c h e m a t i c s o f t h e s p e c t r o s c o p i c
p r o t o c o l u s e d t o ana l yze
出eo x i d i z e d UHMWPE s e c t i o n s .
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比h ‑ ampton, MA) and M i n i t a b 1 7 S t a t i s t i c a l So
仕ware( S t a t e C o l l e g e , P A : M i n i t a b , I n c . ) . F o r each r e t r i e v a l , a t d i f f e r e n t 9 p o l a r l o c a t i o n s ( s e e . F i g . 1 , h e r e a f t e r r e f e r r e d t o a s L l
・L 9 ) ,Raman s p e c t r a were c o l l
巴c t e da l o n g t h e t h i c k n e s s ( d a b s c i s s a ) , w i t h s t e p o f 1 0 0 μ m, from t h e i n n e r t o t h e o u t e r surfaαof t h e l i n e r . F o r each p o l a r l o c a t i o n , t h e i n ‑ d e p t h c r e e p p r o f i l e s were c a l c u l a t e d from t h e a v e r a g e o f two l i n e s c a n s . S t r a i n d a t a a t each p o l a r l o c a t i o n were t r a n s l a t e d i n t o t h i c k n e s s v a r i a t i o n s due t o C
児ep(M c ) by i n t e g r a t i n g t h e i n ‑ d e p t h s t r a i n p r o f i l e o v e r t h e i n ‑ d e p t h a b s c i s s a t h r o u g h t h e e n t i r e t h i c k n e s s o f t h e p o l y e t h y l e n e l i n e r . The t h i c k n e s s v a r i a t i o n due t o wear ( L l t w ) was t h e n c a l c u l a t e d by sub t r a c t i n g a t e t
コ＜t h e t o t a l r e d u c t i o n o f t h i c k n e s s c a l c u l a t e d by s t a n d a r d methods based on m i c r o ‑ m e t r i c m o t o r i z e d s t a g e c o u p l e d w i t h o p t i c a l m i c r o s c o p e . As f a r a s t h e i n v e s t i g a
包on o f o x i d i z e d t h i n s e c t i o n s i s c o n c e r n e d , RS was used t o c a l c u l a t e p r o
制e so f o r t h o r h o m b i c c r y s t a l l i n e phase f r a c t i o n
（叫fromt h e i n n e r t o t h e o u t e r s u r f a c e o f t h e cup . P h e n o m e n o l o g i c a l c o r r e l a t i o n betweenα c and OJ was o b t a i n e d by comparing RS and FT
・I Rd a t a ( s e e n e x t p a r a g r a ph 2 . 4 ) . While t h e t r a n s m i s s i o n i n f r a r e d s p e c t r o s c o p y has an i n ‑ p l a n e s p a t i a l r e s o l u t i o n o f 2 0 0 x 2 0 0
~1m2and t h e whole t h i c k ‑ n e s s o f t h e s e c t i o n i s p r o b e d by t h e i n f r a r e d beam, Raman p r o b e i s c o n f i n e d i n t e n s o f m i c r o n s around t h e f o c a l p l a n e . The c o n f i g u r a t i o n o f t h e p r o b e a d o p t e d t h r o u g h o u t t h e a n a l y ‑ s i s o f t h e o x i d i z e d s e c t i o n s c o r
陀sponded t o a 1 0 0 x o b j e c t i v e and a p i n h o l e d i a m e t e r f i x e d a t φ ＝ 1 mm, g i v i n g an i n ‑ depth p r o b e s i z e o f 〜 60 μm ( P e z z o t t i e t a . l , 2 0 0 7 ) . I n F i g . 2
例t h e mapping p r o c e d u r e f o l l o w e d i n t h e RS e x p e r i m e n t s i s s c h e ‑ m a t i c a l l y e x p l a i n e d i n comparison t o t h e FT ‑ I R sampling p r o c e d u r e . Raman maps on t h e s e c t i o n s w e r e c o l l e c t e d a t 3 d i f f e r e n t d e p t h s i n o r d e r t o c h a r a c t e r i z e t h e e n t i r e t h i c k n e s s and t o s t a t i s t i c a l l y c o r r e l a t e t h e r e s u l t s r e t r i e v e d on t h e same s a mple u s i n g two d i f f e r e n t e x p e r i m e n t a l t e c h n i q u e s .
I n t h e c a s e o f α c a s s e s s m e n t s , we u s e d t h e method i n t r o d u c e d by S
むo b land H a g e r d o n , which i s b a s e d on a s e t o f e q u a t i o n s t h a t i n c l u d e s t h e i n t e n s i t i e s o f v i b r a t i o n a l bands l o c a t e d a t 1 2 9 6 , 1 3 0 5 and 1 4 1 8 cm ‑ 1 , a s o b t a i n e d
仕omu n p o l a r i z e d Raman s p e c
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めI nd e t a i l , t h e f r a c t i o n o f o r t h o r h o m b i c c r y s t a l l i n e
（吋c a nbe c a l c u l a t e d f r o m t h e Raman s p e c t r u m o f p o l y e t h y l e n e , a c c o r d i n g t o t h e f o l l o w i n g e q u a t i o n
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F i g . 3 ‑ ( a ) I n ‑ d e p t h p r o f i l e s o f 0 1 c a l c u l a t e d from t h e s e c t i o n s o f p r i s t i n e , y ‑ i r r a d i a t e d / s h e l f a g e d , y ‑ i r r a d i a t e d / 28
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出es u b s c r i p t ( i . e . , a f t e r s p e c
凶ld e c o n v o l u t i o n ) . From t h e s p e c t r a c o l l e c t e d a t d i f f e r e n t p o l a r l o c a t i o n s o f t h e r e
出e v a l sf o r c r e e p a n a l y s i s we a l s o o b t a i n e d p r o f i l e s o f a c , which were c o n v e r t e d t o p r o f i l e o f
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O x i d a t i o n p r o f i l e s o f microtomed s e c t i o n s were d e t e r m i n e d u s i n g F T ‑ I R a s a f u n c t i o n o f d e p t h away
企omt h e i n n e r s u r f a c e o f t h e cup . F T ‑ I R m i c r o s c o p i c a n a l y s i s was c a
而edo u t u s i n g t h e i m a g i n g system S p o t l i g h t 2 0 0
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出ea r e a under
F o u r i e r t r a n s f o r m i n f r a r e d s p e c t r o s c o p y 2 . 4 .
F i g . 4 ‑ Phenomenological c o r r e l a t i o n between c r y s t a l l i n e phase f r a c t i o n and O I a s o b t a i n e d from
出e e x p e r i m e n t a l r e s u l t s shown i n F i g . 3 .
2 0 1 1 ; P u p p u l i n e t a l . , 2 0 1 4 ) , which e n a b l e d u s t o o b t a i n t h e
p h e n o m e n o l o g i c a l e q u a t i o n t o quan t i
今t h e p e r c e n t a g e o f
c r e e p a l o n g t h e t h i c k n e s s o f t h e r e t r i e v a l s . The r e g r e s s i o n
18 JOURNAL OF THE MECHANICAL BEHAVIOR OF BIOM E DICAL M A TERIALS SI (2015) 13‑24
t h e c a r b o n y l peak a t a r o u n d 1 7 2 0 cm 1 t o t h e a r e a under t h e C‑H a b s o r p t i o n peak c e n t e r e d a r o und 1 3 7 0 cm ‑
t( S t a n d
釘d , 2 α活
）.I n o r d e r t o c
児a t eo x i d a t i o n p r o f i l e s a s a f u n c
包ono f d e p t h , t h e a d o p t e d e x p e r i m e n t a l r o u t i n e was c o n c e i v e d a s f o l l o w s : f i v e l i n e s c a n s were c o l l e c t e d f r o m t h e s l i d i n g s u r f a c e t o t h e b a c k ‑ s u r f a c e o f
せi el i n e r w i t h 2 0 0 μm s t e p s . The O I a t e a c h d e p t h was c a l c u l a t e d a s t h e a v e r a g e o f 5 m e a s u r e m e n t s . The i n v e s t i g a t e d a r e a was t h e same p r e v i o u s l y ana l y z e d by RS ( s e e F i g . 2 ( b ) )
3 . Results
3 . 1 . I n ‑ v i t r o and i n ‑ v i v o ox i d i z e d s l i c e s
F T ‑ I R a n a l y s i s and Raman maps c o l l e c t e d a t d i f f e r e n t d e p t h s
合omt h e p o l y e t h y l e n e s l i c e s a
丘e ra c c e l e r a t e d a g i n g t e s t v i s u a ‑ l i z e d t h e e v o l u t i o n o f
01an d c r y s t a l l i n e ph a s e c o n t e n t d u r i n g o x i d a t i o n . P r o f i l e s o f
01a r e shown i n F i g . 3 ( a ) a s c a l c u l a t e d
合om F T ‑ I R s p e c
廿a c o l l e c t e d
仕om t h e s l i c e s o f t h e p r i s t i n e
，せi e 2 8 d a y s s h e l f ‑ a g e d m a t e r i a l a f t e r i r r a d i a
包on , t h e 2 8 d a y s i r r a d i a t e d and i n ‑ v i t r o a g e d m a t e r i a l and t h e r e
凶e v a ln o . 8 O o c a t i o n L 5 ) . The p r i s t i n e showed v e r y low amoun t o f o x i d i z e d s p e c i e s , wi
せl 0 1 equa l t o O i n t h e most o f t h e a n a l y z e d l o c a t i o n s and a p e a k o f 0 . 0 1 . The o x i d a t i v e d e g r a d a t i o n o f t h e po
ゆner i n t h e s a m p l e s a f t e r i n
羽 田a g i n g c l e a r l y i n c r e a s e d w i t h t h e t i m e and i t was a l s o homogeneous t h r o u g h o u t t h e t h i c k n e s s o f t h e c u p . D i f f e r e n t outcomes were o b t a i n e d
仕om
出eF T ‑ I R a n a l y s i s o f t h e m i d d l e ‑ t e
ロnr e t r i e v a l . T h i s s l i c e was microtomed f r o m t h e l o c a t i o n 5 o f t h e c u p , which b e l o n g s t o
出e ma i n c o n t a c t a r e a w i t h t h e f e m o r a l h e a d . The o n s e t o f a h i g h peak o f o x i d a t i o n was o b s e r v e d a r o u n d 1 m m i n ‑ d e p t h f r o m
仕i es l i d i n g s u r f a c e o f t h e b e a r i n g , whic h was somewhat u n e x p e c t e d c o n s i d e r i n g
せi e m a g n i t u d e o f d e t e c t e d
01e q u a l t o 5 . 1 . S i m i l a r l y t o i n f r a r e d a n a l y s i s , p r o f i l e s ofαc were o b t a i n e d
合omせ i eRS i n v e s t i g a t i o n o f t h e same s l i c e s , w ho s e r e s u l t s a
陀showni n F i g . 3 φ
）.F o r e a c h s l i c e , t h e sim i l a r i t y b e t w e e n t h e t r e n d s o f c r y s t a l l i n i t y and 0 1 a l ong t h e t h i c k n e s s was e v i d e n t . F i g . 4 shows t h e p h enomeno l o g i c a l c o r r e l a t i o n b
巴tween a c and O I a s o b t a i n e d upon p l o t t i n g
せi ee x p e r i m e n t a l d a t a c o l l e c t e d from a l l t h e s l i c e s .
廿1 e d a t a i n
吐1 e p l o t r e p r e s e n t t h e a v e r a g e α c and O I c a l
四l a t ed
仕・ o mp o i n t s wi
出s i m i l a r O I ( i . e . , v a r i a t i o n o f 0 . 1 was s e t a s s i g n i f i c a n t ) . Co r r e l a t i o n s between c r y s t a l l i n i t y and
OIc o u l d be o b t a i ned by f i t t i n g t h e e x p e r i m e n t a l d a t a wi t h an expo n e n t i a l f u n c t i o n t h a t we a l r e a d y i n t r o d u c e d i n a p r e v i o u s s t u d y ( P e z z o t t i e t a . l , 2 0 0 7 A ) . c c o r d i n g t o t h e p a r t i a l ‑ l e a s t ‑ s q u a r e f i t t i n g meth od , t h e f o l l o w i n g e qu a t i on s c o u l d be o b t a i n e d f o r t h e L o n g e v i t y m a t e r i a l :
O l = e ( 1 2 5
回n ( 6 . 3 8
叫−4 . 2 3 ) + 0 . 6 9 }
(2) mせ1 c o e f f i c i e n t o f c o r r e l a t i o n , a
匂u s t e d ‑ R 2 ,e q u a l t o 0 . 9 8 . T h i s equ
油onwas u s e d t o a s s e s s t h e d e g r e e o f o x i d a t i o n i n d i f f e r e n t l o c a t i o n s o f t h e 1 1 r e t r i e va l s by means o f R S .
3 . 2 . C o r r e l a t i o n between c r e e p and Raman band b r o a d e n i n g
A c c o r d i n g t o t h e e xp e r i m e n t a l p r o c e d u r e s d e s c r i b e d i n some o f o u r p r e v i o u s p a p e r s ( O k i t a e t a l . , 2 0 1 4 ; P e z z o t t i e t a l . , 2 0 1 1 ; Pup p u l i n e t a l . , 2 0 1 4 ) , samples made o f L o n g e v i t y r e m e l t e d HXL P E were a n a l y z e d t o o b t a i n t h e p h e n o m e n o l o g i c a l c o r r e l a ‑ t i o n b etwe en c o m p r e s s i v e r e s i d u a l s t r a i n a f t e r r e c o v e r y an d
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0 . 4 0 . 3 0 . 2
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F i g . 5 ‑Phenomenological c o r r e l a t i o n between compressive r e s i d u a l s t r a i n measured a f t e r 24 h r e c o v e r y and
せl e experimental band broadening
（~FWH附.C u b i c r e g r e s s i o n model was used t o c a l c u l a t e t h e b e s t ‑ f i t t i n g c u r v e .
b r o a d e n i n g o f t h e Raman band l o c a t e d a t 1 1 3 0 cm ‑ 1 (sym ‑ m e t r i c s t r e t c h i n g v i b r a t i o n o f C‑C b o n d ) . F i g . 5 shows t h e c o r r e l a t i o n between t h e p e r c e n t a g e c o m p r e s s i v e s t r a i n mea‑
s u r e d a f t e r 24 h r e c o v e r y and t h e e x p e r i m e n t a l band b r o a d ‑ e n i n g , which was measured a s t h e v a r i a t i o n o f t h e f u l l w i d t h a t h a l f ma
氾mum(FWHM) . C u b i c r e g r e s s i o n model was used t o c a l c u l a t e t h e b e s t ‑ f i t t i n g c u r v e ( c f . b e s t ‑ f i t t i n g e q u a t i o n shown i n t h e i n s e t ) and t h e c o n f i d a n c e bands ( c o n f i d e n c e l e v e l o f 9 5 % ) . The s t a n d a r d eπor o f t h e r e g r e s s i o n ( S ) and t h e c o e f f i c i e n t o f c o r r e l a t i on ( R 2 ) were 0 . 2 5 % and 0 . 9 9 6 , r e s p e c t i v e l y .
3 . 3 . Wear, c r e e p and o x i d a t i o n o f r e t r i e v a l s
F i g . 6 shows t h e r e s u l t s o f c r e e p , wear and t o t a l p e n e t r a t i o n
a s c a l c u l a t e d from t h e s h o r t term r e t r i e v a l s u s i n g t h e me t h od
based on RS . F o r each c u p , t h e d a t a were p l o t t e d i n p o l a r
c o o r d i n a t e s a c c o r d i ng t o t h e a n g l e o f each o f t h e 9 l o c a t i o n s
i n wh i ch t h e t h i c k n e s s were measured by micro‑me t r i c
m o t o r i z e d s t a g e combined w i t h o p t i c a l microscope and
a n a l y z e d by R S . The p r o f i l e s were o b t a i n e d by B ‑ s p l i n e
i n t e r p o l a t i o n o f t h e e x p e r i m e n t a l p o i n t s . The polynomia l
f i t t i n g e q u a t i o n used t o a s s e s s t h e p e r c e n t a g e o f c r e e p
d e f o rm a t i o n h a s an e r r o r o f 0 . 5 % ( i . e . , 2 5 , 95% c o n f i d e n c e
l e v e l ) . I n a l l t h e seven s h o r t ‑ t e r m r e t r i e v a l s t h e r e d u c t i o n s o f
t h i c k n e s s due t o wear a s o b t a i n e d by sub t r a c t i n g c r e e p t o t h e
t o t a l p e n e t r a t i o n gave p o s i t i ve o r n e g a t i v e v a l u e s o f few t e n s
o f m i c r o n s , which were comparable t o t h e e r r o r i n t e g r a t e d
o v e r t h e e n t i r e t h i c k n e s s . I n f a c t , an eπor o f 0 . 5 % i n s t r a i n
assessment i n t e g r a t e d o v e r t he ma ximum t h i c k n e s s o f
1 5 . 4 m m y i e l d s a maximum e r r o r o f 77
~1m,wh i l e f o r t h e
6 . 3 m m cups t h e maximum eπor i s e s t i m a t e d a t 3 1 . 5 r i m . F o r
t h i s r e a s o n , t h e wear i n t h e s h o r t ‑ t erm cups was n e g l e c t e d
and t h
巴t o t a lc r e e p was c o n s i d e r e d a s e q u a l t o t h e t o t a l
p e n e t r a t i o n . I t w a s c l e a r t h a t sam p l e N o . l and 2 had t h e
l o w e s t r e s i d u a l s t r a i n amo n g t h e s h o r t
四term r e t r i e v a l s ,
mainly l o c a l i z e d between L 3 and L 6 , w h i l e t h e cups wi t h
f o l l o w‑ up l o n g e r than 6 months ( N o . 6 and 7 ) showe d peaks o f
p e n e t r a t i o n h i g h e r t h a n 1 0 0 r i m and d e t e c t a b l e deform a t i o n
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Wear
T o t a l p e n e t r a t i o n
F i g . 6 ‑ P o l a r graphs r e p r e s e n t i n g pro
創 出o fc r e e p , wear and t o t a l p e n e t r a t i o n as i n t e r p o l a t e d from t h e d a t a o f t h e s h o r t ‑ t e r m r e t r i e v a l s .
a l s o near t h e r i m , presumably due t o s t e m imp i n gement and h e e l s t r i k e . The h i g h e s t d e f o r m a t i o n n e a r t h e rim ( L l and L 9 ) was de t e c t e d i n sample N o . 7 , which had t h e l o n g e s t f o l l o w
u p and h i g h a b d u c t i o n a n g l e ( 4 °) 7 , which f a v o r s t h e impinge‑
ment o f t h e neck o f t h e s t
巴m a g a i n s t t h e rim and h e e l s t r i k e . I n sample N o . 5 t h e d e f o r m a t i o n was l o c a l i z e d around t h e t o p o f t h e cup ( L S ) , which might be c o r r e l a t e d t o t h e low a b d u c ‑ t i o n ang l e ( 3 0 。 ）
.Sim i l a r l y t o F i g . 6 , t h e r e s u l t s o f t h e f o u r middle ‑ term r e t r i e v a l s a r e r e p o r t e d i n t he p o l a r p l o t s o f F i g . 7 . The o n s e t o f wear c l e a r l y showed up i n a l l t h e middle‑term l i n e r s , man l y l o c a l i z e d between L 3 and L 6 , w i t h t h e excep t i o n o f l i n e r n o . 9 , i n which t h e r e d u c t i o n o f t h i c k n e s s was lower i n magnitude but widespread between L l and L 6 , p r o b a b l y due t o t h e low a b d u c t i o n a n g l e ( 4 0 ° ) . Sample No . 8 was c h a r a c t e r i z e d by t h e h i g h e s t c r e e p and t h e wear damage was s h a r p l y l o c a l i z e d i n L 4 , a s e x p e c t e d by t h e abduc t i on a n g l e o f 4 5 。 . I n t e r e s t i n g l y , t h e p a t i e n t o f t h e l a t t e r r e t r i e v a l had t h e h i g h e s t BM! among t h e f o u r ( B M ! = 3 3 ) and s u f f e r e d
d i s l o c a t i o n , which might e x p l a i n t h e magnitude o f r e s i d u a l
s t r a i n c a l c u l a t e d a l s o i n l o c a t i o n s t h a t a r e n o t s u pp osed t o be
l o a d e d . Moreover , a l l t h e mi d d l e ‑ t e r m r e t r i e v a l s showed
s i g n i f i c a n t c r e e p d e f o r m a t i o n i n b o t h l o c a t i o n s near t h e
r i m . The maximum c r e e p , wear an d t o t a l p e n e t r a t i o n s
c a l c u l a t e d from each r e t
巾v a l a s a f u n c t i o n o f i m p l a n t a t i o n
t i me were p l o t t e d i n F i g . 8 ( a
一（）c ) . The l i n e a r r e g r e s s i o n
a n a l y s i s o f t h e s e d a t a enabled t o c a l c u l a t e t h e c r eep , wear
and t o t a l p e n e t r a t i o n r a t e s d u r i n g t h e bedding ‑ i n p e r i o d , t h e
s t e a d y wear s t a t e p e r i o d and t h e o v e r a l l r a n g e o f f o l l o w ‑ u p s .
The r e s u l t s a r e r e p o
口edi n T a b l e 2 , i n which a r e a l s o i n c l u d e d
t h e r e s p e c t i v e P e a r s o n ' s c o r r e l a t i o n i n d ex
（け.Accor d i n g t o E q .
( 2 ) , RS a n a l y s i s e n a b l e d t o s t u d y t h e ox i d a t i v e d e g r a d a t i o n o f
t h e r e t r i e v a l s a t d i f f e r e n t l o c a t i o n s . Peak o f O I a s we l l a s
a v e r a g e O I c a l c u l a t e d i n t h e wear ( L 3 ‑ L S ) and nonwear zone
( L 7 , L 8 ) a r e shown i n F i g . 9 ( a ) and ( b ) f o r t h e s h o r t and m i d d l e ‑
term r e t r i e v a l s , r e s p e c t i v e l y . I n t h e s h o r t ‑ t e r m r e t r i e v a l s t he
peak o f o x i d a t i o n a r e s i m i l a r ( O I〜0 . 2 ) , w h i l e t he a v e r a g e
20
J O U R N A L 0 F T H E M E C H A N I C A L B E H A V I 0 R 0 F B I 0 M E D I C A L M A T E R I A L S 5 I ( 2 0 I 5 ) I 3 ‑2 4折半1glc L.4
No8 No 9
.~b《lて~
~（が11
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<OU ~00 31~1 200 100 0 0 I 00 2＜山JOO‑1110 500 ~ilO 400 JOI) 2(1内Jl)I)υ 0I on 20(1 JOO 4111’~＼）υ
P e n e t r a t i o n [ pm]
園 田 園 ・
C 1 c c p
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T o t a l p e n e t r a t i o n
P e n l ! t
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No 1 0 No I l
八bd.弓Otl400ヌりIJ20υ1 1)0 0 (I 10υ200 .lOO .jl)(J号00 ベfill‑100 300 10υl 1)11り り 10り2111!3lHI 400号。。
P e n d r n t i o n [ f t m ) P e n e t r a t i o n [ f t m ・ I
F i g . 7 ‑ S i m i l a r l y t o F i g . 6 , p o l a r g r a phs r e p r e s e n t i ng p r o f i l e s o f c r e e p , wea r a nd t o t a l p e ne
位a t i o n o b t a i n e d f r o m t h e mi d d l e ‑ term r e t r i e v a l s .
o x i d a t i o n i n t he wear and non‑wear zones was v e r y l o w . I n t e r e s t i n g l y , t h e l o w e s t mean o x i d a t i o n was d e t e c t e d i n sample N o . 2 , which had t he s h o r t e s t e x ‑ v i v o t i m e . I n t h e midd l e ‑ t e r m r e t r i e v a l s t h e p e a k s o f o r were d e t e c t e d under t h e worn a r e a and t hey r a n g e d from 1 . 4 t o 4 . 5 . Moreover , i n s a
ロi p l e sNo . 8 and 9 t h e a v e r a g e o x i d a t i o n o f t h e wear zone was h i g h e r than t h a t c a l c u l a t e d from t h e non‑w
巴a rz o n e , which might s u g g e s t some i n f l u e n c e o f t h e m e c h a n i c a l l o a d on t h e o x i d a t i o n o f p o l y e t h y l e n e .
4 . Discussion
The o x i d a t i v e d e g r a d a t i o n o f r e m e l t e d H XLPE was s t u d i e d by a n a l y z i n g i n ‑ v i t r o a g e d s a m p l e s mi c r o t o m e d from new l i n e r s . A s shown i n F i g . 3 , t h e i n ‑ d e p t h p r o f i l e s o f o x i d a t i o n c l e a r l y s h i
仕edt o w a r d h i g h e r v a l u e s o f O I a f t e r s h e l f ‑and a c c e l e r a t e d a g i n g , a l t h o u g h t h e i r s h a p e d i d n o t c h a n g e . I f we c o n s i d e r t h a t t h e d i s t r i b u t i o n o f f r e e r a d i c a l s g e n e r a t e d by t h e p r e l i m i n a r y s t e p o f gamma i r r a d i a t i o n was u n i f o r m t h r o u g h o u t t h e i n t e r e s l i c e s , we c a n i n f e r t h a t t h e r e were no g r a d i e n t s o f r e s i d u a l f r e e r a d i c a l s a l o n g t h e t h i c k n e s s o f t h e v i r g i n l i n e r s . The m
句o r i n c r e a s e o f 0 1
（仕omOto a r o u n d 0 . 4 ) o c c u r r e d w i t h i n t h e 2 8 d a y s o f s h e l ι a g i n g i n a i r a f t e r i r r a d i a t i o n , w h i l e d u r i n g a c c e l e r a t e d a g i n g a t 7 0 ° c a l l t h e few r e s i d u a l
仕e e ‑ r a d i c a l sr e c o m b i n e d w i t h oxygen l e a d i ng t o t h e ma
氾mum 0 1 o f 0 . 5 1 . F o r e a c h s l i c e , t h e s i m i l a r i t y between t h e t r e n d s o f c r y s t a l l i n i t y and O I a l o n g t h e t h i c k n e s s was e v i d e n t and i t h i g h l i g h t e d t h e r e c r y s t a l l i z a t i o n p r o c e s s t r i g g e r e d by t h e f o r m a t i o n o f o x i d i z e d s p e c i e s . I n f a c t , i f f r e e r a d i c a l s a r e formed i n s i d e t h e amorphous phase o f p o l y ‑ e t h y l e n e , t h e y r e a c t w i t h o x y g
巴nl e a d i n g t o t h e f o r m a t i o n o f h y d r o p e r o x i d e s ( i . e
，，自r s tp r o d u c t o f o x i d a t i o n ) ( B r a c c o e t a l . ,
2 0 0 6 a , 2 0 0 6 b ; C o s t a e t a l . , 1 9 9 7 ) . H y d r o p e r o x i d e s c a n e v o l v e t o k e t o n e s , c a r b o x y l a c i d s and o t h e r o x i d i z e d s p e c i e s c o n t a i n i n g c a r b o n
折g r o u p s ,whose f o r m a t i o n i s r e s p o n s i b l e f o r t h e p o l y ‑ e t h y l e n e c h a i n b r e a k a g e , namely c h a i n s h o r t e n i n g , i n c r e a s e o f m o l e c u l a r m o b i l i t y and s u b s e q u e n t r e c r y s t a l l i z a t i o n . Compar‑
i s o n among s l i c e s a t d i f f e r e n t o x i d a t i v e c o n d i t i o n s showed
せi a t m
句o rr e c r y s t a l l i z a
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