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BIOMAT.,
MED. DEV.,
ART. ORG.,
5(3), 267-276 (1977)
MECHANICAL PROPERTY CHANGES OF BARIUM TITANATE (CERAMIC) AFTER I N VIVO AND I N VITRO A G I N G
J.B.
P a r k , G.H.
Kenner, S.D.
Brown* and J . K .
Scott*
D i v i s i o n of I n t e r d i s c i p l i n a r y S t u d i e s Clemson U n i v e r s i t y Clemson, SC 29631 *Department of Ceramic E n g i n e e r i n g U n i v e r s i t y of I l l i n o i s Urbana, I L 61801 ABSTRACT
S i n c e barium t i t a n a t e (BaTiO ) can b e made p i e z o e l e c t r i c , i t 3 may b e used t o s u b s t i t u t e h a r d t i s s u e s d i r e c t l y . A s a f i r s t s t e p i n t e s t i n g t h i s c o n c e p t , a series of i n v i v o and i n v i t r o a g i n g and b i o c o m p a t i b i l i t y s t u d i e s were performed. The mean compressive s t r e n g t h of samples implanted s u b c u t a n e o u s l y i n t h e b a c k s of rabb i t s d e c r e a s e d t o 1 3 8 MPa a f t e r 20 weeks from a c o n t r o l v a l u e of 2 8 1 MPa. S i m i l a r , though l e s s d r a s t i c l o s s e s of s t r e n g t h w e r e s e e n when specimens were aged i n d i s t i l l e d water (182 MPa a t 2 8 weeks) and R i n g e r ' s s o l u t i o n (159 MPa a t 28 weeks). The most r a p i d d e c r e a s e of s t r e n g t h i n a l l c a s e s w a s s e e n p r i o r t o 4 weeks. T h e r e a f t e r , t h e d e c r e a s e w a s much s l o w e r . H i s t o l o g i c a l e v a l u a t i o n of t h e t i s s u e s u r r o u n d i n g t h e imp l a n t r e v e a l e d a t h i n f i b r o u s c a p s u l e and no e v i d e n c e of t i s s u e inflamtion.
INTRODUCTION
The f i x a t i o n o r a t t a c h m e n t of i m p l a n t s t o bone i s a major problem i n o r t h o p e d i c work (1-9).
C u r r e n t l y , t h r e e methods are
used t o f i x i m p l a n t s : ( a ) d i r e c t mechanical a t t a c h m e n t u s i n g
screws, n a i l s , e t c . ( 2 , 3 ) ; (b) i m p a c t i o n of t h e p r o s t h e s i s i n t o
267 Copyright 0 1977 by Marcel Dekker. Inc. All Rights Reserved. Neither this work nor any part may be reproduced or transmitted in any form or by any meahs, electronic or mechanical, including photocopying, microfilming, and recording, or by any information storage and retrieval system, without permission in writing from the publisher.
268
PARK ET AL.
bone [ e . g . ,
h i p j o i n t replacement i n which t h e stem o f t h e pros-
t h e s i s i s impacted i n t o t h e m e d u l l a r y c a v i t y of t h e femur ( 4 , 5 ) ] ; and ( c ) u s e of polymeric cements t o f i x t h e p r o s t h e s i s / t i s s u e
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i n t e r f a c e (e.g., etc.).
p o l y m e t h y l m e t h a c r y l a t e ( 6 ) , p o l y c a r b o x y l a t e (7),
None of t h e s e methods i s e n t i r e l y s a t i s f a c t o r y f o r a perma-
nent prosthetic fixation.
F a i l u r e s are l a r g e l y a t t r i b u t e d t o
stress c o n c e n t r a t i o n s i n t h e bone by t h e i m p l a n t s .
This r e s u l t s
i n t i s s u e n e c r o s i s and r e s o r p t i o n of bone (10) w i t h e v e n t u a l weakening and f a i l u r e (1,3) o r l o o s e n i n g of t h e i m p l a n t s , dependent d e t e r i o r a t i o n of t h e polymeric cement due t o
Time-
repeated
l o a d i n g and a t t a c k by t h e body environment ( l l ) , a c u t e r e a c t i o n s due t o release of t h e t o x i c monomer, and h i g h t e m p e r a t u r e due t o t h e h e a t of p o l y m e r i z a t i o n a l s o o c c u r ( 6 ) . The d i s c o v e r y t h a t t i s s u e ingrowth o c c u r s i n t o porous mater-
i a l s h a s provided a n a t t r a c t i v e a l t e r n a t i v e t o mechanical f i x a tion.
T i s s u e ingrowth h e l p s t o s p r e a d t h e l o a d over a l a r g e a r e a
t h e r e b y minimizing stress c o n c e n t r a t i o n s and f a c i l i t a t i n g a c o n t i n u o u s v i a b l e c o n n e c t i o n between t h e p r o s t h e s i s and t i s s u e (9,12-21).
The chemical s t a b i l i t y of a n implant which i s t o b e
anchored by bone ingrowth i s i m p o r t a n t , s i n c e t h e removal of such a n implant r e s u l t s i n f a r g r e a t e r damage t h a n t h a t i n c u r r e d w i t h t h e removal of a m e c h a n i c a l l y a t t a c h e d implant. The need f o r e x t r e m e l y i n e r t b i o m a t e r i a l s h a s caused ceramics t o b e r e c o n s i d e r e d f o r u s e a s hard t i s s u e replacements.
Ceramic
materials were o r i g i n a l l y d i s m i s s e d a s p o t e n t i a l p r o s t h e t i c mat e r i a l s b e c a u s e of t h e i r poor impact r e s i s t a n c e , low t e n s i l e and f l e x u r a l s t r e n g t h s , and a s u s c e p t i b i l i t y t o b r i t t l e f a i l u r e . I f , however, a porous ceramic
could provide a scaffolding i n t o
which e x t e n s i v e t i s s u e ingrowth would o c c u r , the mechanical p r o p e r t i e s of t h e r e s u l t i n g composite would be f a v o r a b l y i n f l u e n c e d by t h e bone t i s s u e .
Although a minimum l e v e l of
mechanical s t a b i l i t y would b e r e q u i r e d , t h e c h e m i c a l c o m p a t i b i l i t y
AGING OF BARIUM TITANATE
269
o f t h e i m p l a n t would b e t h e more c r i t i c a l f a c t o r .
I n these c i r -
c u m s t a n c e s ¶ c e r a m i c s show g r e a t p o t e n t i a l .
Ever s i n c e t h e d e t e c t i o n o f s t r a i n i n d u c e d p o t e n t i a l s i n Artif Cells Blood Substit Immobil Biotechnol Downloaded from informahealthcare.com by University of British Columbia on 12/10/14 For personal use only.
bone and t e n d o n (22-24),
t h e p o s s i b i l i t y o f u s i n g t h i s phenomenon
t o c o n t r o l t h e o s t e o g e n i c and o s t e o c l a s t i c mechanisms of l i v i n g animals h a s been of i n t e n s e experimental i n t e r e s t .
W i l l i a m s (25)
r e c e n t l y c r i t i c a l l y reviewed t h e c o n t r o v e r s y o n t h e s o u r c e of the stress-generated
e l e c t r i c a l s i g n a l s of t e n d o n and bone
(wet o r d r y ) and c o n c l u d e d t h a t t h e y c a n b e i n t e r p r e t e d as piezoelectric. The a p p l i c a t i o n o f e l e c t r i c a l s t i m u l a t i o n around p o r o u s c a l c i u m a l u m i n a t e i m p l a n t s i n s e r t e d i n t o r a b b i t femurs i n c r e a s e d t h e t i s s u e ingrowth i n t o t h e implants.
I n t h i s work by P a r k and
Kenner ( 2 0 , 2 1 ) , a b a t t e r y and m e t a l l i c l e a d s were used t o p r o v i d e s t i m u l a t i o n during t h e course of t h e experiment. The u s e o f a p o r o u s p i e z o e l e c t r i c m a t e r i a l as a n i m p l a n t
seems t o b e t h e n e x t l o g i c a l s t e p i n t h e e x p l o r a t i o n o f t h e e f f e c t s of e l e c t r i c a l s t i m u l a t i o n on bone i n g r o w t h .
Hopefully,
s u c h a n i m p l a n t would e l i m i n a t e t h e need f o r t h e l e a d s and a n o u t s i d e s o u r c e o f power.
This r e p o r t e v a l u a t e s t h e biocompati-
b i l i t y a n d c o m p r e s s i v e m e c h a n i c a l p r o p e r t y of t h e p i e z o e l e c t r i c ceramic, barium t i t a n a t e .
EXPERIMENTAL PROCEDURE Reagent g r a d e b a r i u m t i t a n a t e powder (TAM D i v . , NL I n d u s -
t r i e s , P i t t s b u r g h , PA) was mixed w i t h p o l y v i n y l a l c o h o l ( 0 . 2 5 w e i g h t p e r c e n t ) and d i s t i l l e d water ( 5 w e i g h t p e r c e n t ) .
The
m i x t u r e w a s b l e n d e d w i t h a s p a t u l a and s t o r e d o v e r n i g h t i n a n air-tight
container t o allow the moisture t o e q u i l i b r a t e .
C y l i n d r i c a l s p e c i m e n s 16 mm i n h e i g h t were p r e s s e d u s i n g a Denison p r e s s w i t h a 16 mm d i a m e t e r d i e and 138 MPa p r e s s u r e .
2 70
PARK ET AL.
The specimens were a i r - d r i e d o v e r n i g h t . They were t h e n p l a c e d on platinum f o i l i n alumina s a g g e r s and f i r e d t o 1325°C i n a gasf i r e d furnace.
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The samples were d i v i d e d i n t o t h r e e groups: (1) aged i n v i t r o i n d i s t i l l e d water (2) aged i n v i t r o i n R i n g e r ' s s o l u t i o n ( b o t h a t room t e m p e r a t u r e ) and (3) aged i n v i v o .
The compressive s t r e n g t h
of t h e p e l l e t s w a s determined a f t e r f o u r , e i g h t , twelve weeks i n
a l l cases and twenty weeks i n t h e i n v i v o s t u d y w h i l e t h e i n v i t r o specimens were t e s t e d a t 28 weeks.
-I n vivo
ageing of p e l l e t s w a s accomplished by t h e subcutane-
~ U Si m p l a n t a t i o n
of f i r e d p e l l e t s i n l a b o r a t o r y r a b b i t s .
The
p e l l e t s t o b e implanted were s t e r i l i z e d by h e a t i n g t o 200°C i n
a m u f f l e oven f o r two hours. t h e i r backs shaved.
The r a b b i t s were a n e s t h e t i z e d and
I n c i s i o n s were made w i t h a s c a l p e l , and f o u r
p e l l e t s p e r animal were i n s e r t e d j u s t under t h e s k i n of t h e back, The samples were s e p a r a t e d so a s t o avoid c o n t a c t i n s i d e t h e body. The i n c i s i o n s were t h e n c l o s e d . The compressive s t r e n g t h of e i g h t unaged c o n t r o l p e l l e t s
was determined u s i n g a Tinium O l s e n t e s t i n g machine.
The com-
p r e s s i v e s t r e n g t h s of t h e aged specimens were determined u s i n g t h e same machine.
E i g h t p e l l e t s were t e s t e d a f t e r each p e r i o d
of i n v i v o and i n v i t r o aging d e s c r i b e d above. RESULTS AND D I S C U S S I O N S
The body's r e a c t i o n t o t h e i m p l a n t a t i o n of barium t i t a n a t e
w a s unknown, s i n c e i t had never been used a s a b i o m a t e r i a l . S i n c e t h e element barium i s poisonous, t h e p o s s i b i l i t y of s e r i o u s problems e x i s t e d .
I f barium c o l l e c t e d i n t h e g r a i n b o u n d a r i e s
of barium t i t a n a t e , i t could b e l e a c h e d o u t by b i o l o g i c a l f l u i d s and r e l e a s e d i n t o t h e body.
I f t h i s occurred, the p o t e n t i a l
u t i l i z a t i o n of barium t i t a n a t e a s a b i o m a t e r i a l would b e d e s t r o y e d .
271
AGING OF BARIUM TITANATE
F o r t u n a t e l y , t h e r e s u l t s of t h e i n v i v o t e s t i n g demonstrated good t i s s u e c o m p a t i b i l i t y a s shown i n F i g u r e 1. s u r r o u n d i n g t h e i m p l a n t s showed no inflammation.
The t i s s u e In addition,
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o n l y a narrow membrane of f i b r o u s e n c a p s u l a t i o n surrounded each implant.
Barium t i t a n a t e i s h a r d enough s o t h a t when i t i s
p r e s s e d , a s l i g h t amount of metal i s abraded from t h e d i e and remains on t h e s u r f a c e of t h e specimens.
T h i s l a y e r of i m p u r i t y
metal was removed by t h e b i o l o g i c a l environment, b u t o t h e r w i s e , t h e r e was no v i s i b l e d e g r a d a t i o n of t h e sample.
A q u a n t i t a t i v e measurement of material d e g r a d a t i o n w a s determined by comparing t h e compressive s t r e n g t h of specimens aged b o t h i n v i v o and i n v i t r o w i t h each o t h e r and w i t h unaged c o n t r o l specimens.
Mean s t r e n g t h s a t each t e s t i n g p e r i o d are
summarized i n Table I , and are p l o t t e d a s a f u n c t i o n of a g i n g t i m e i n Figure 2 .
I n t h i s f i g u r e , t h e d a t a p o i n t s are t h e
computed a r i t h m e t i c mean v a l u e s .
I.. . . )
F i g u r e 1. O p t i c a l micrograph of s o f t c o n n e c t i v e t i s s u e a d j a c e n t t o t h e Barium T i t a n a t e i m p l a n t s a f t e r 20 weeks (H & E s t a i n , arrow (T) i n d i c a t e s e n c a p s u l a t i o n of dense connective t i s s u e ) .
PARK ET AL.
272 TABLE I .
Mean Compressive S t r e n g t h a t Each T e s t i n g P e r i o d
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Test Condition
Aging P e r i o d (Weeks)
Mean S t r e s s
S.D.
5
Unaged C o n t r o l
0
281
I n Vivo --
4
1 8 4 f. 2 0 . 4
8
168
I n -V i t r o _
in
Ringer's Solution
In _ V i_ tro _
in
D i s t i l l e d Water
t
35.9
23.7
12
1 6 0 f. 38.8
20
1 3 8 f. 38.7
4
209
8
176 f. 1 6 . 5
12
160
28
159
26.2
A A
24.5 40.6
4
222 f. 23.2
8
209 f. 2 2 . 1
12
194
28
182
("a)
5 5
17.9 68.7
The a v e r a g e compressive s t r e n g t h o f t h e unaged, c o n t r o l specimens w a s 2 8 1 MPa.
A f t e r o n l y f o u r weeks o f i m p l a n t a t i o n
i n_v_ i v o , t h e a v e r a g e s t r e n g t h dropped t o 1 8 4 MPa, a d e c r e a s e i n
s t r e n g t h of approximately t h i r t y - f i v e percent.
A f t e r e i g h t weeks
o f a g i n g , t h a t v a l u e d e c r e a s e d t o 1 6 8 MPa, a n a d d i t i o n a l d e c r e a s e of f i v e p e r c e n t .
A f t e r twenty weeks of a g i n g , t h e a v e r a g e
s t r e n g t h o f t h e b a r i u m t i t a n a t e had d e c r e a s e d t o 1 3 8 MPa, a t o t a l decrease i n s t r e n g t h of fifty-one percent.
The l a r g e s t d r o p i n
s t r e n g t h o c c u r r e d d u r i n g t h e f i r s t f o u r weeks of i m p l a n t a t i o n . A f t e r t w e n t y weeks of a g i n g , however, t h e specimens were s t i l l showing a s t e a d y d e c l i n e i n s t r e n g t h . The specimens aged i n v i t r o showed p a t t e r n s of s t r e n g t h r e d u c t i o n s i m i l a r t o t h a t of t h e i n v i v o specimens.
The d i s -
AGING OF BARIUM TITANATE
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+-
273
Control
I00
0
4
8
12
16
20
24
28
AGING TiMEheeks)
Figure 2 . The mean compressive strength o f barium titanate ceramic specimen in vitro and in vivo. tilled water specimens showed the least reduction in strength at each testing period, the Ringer's solution specimens were somewhat weaker, and the in vivo specimens showed the greatest reduction in strength similar to other ceramics (26-28).
h e
possible explanation is the possibility of occasional bumping of the specimens whenever the animal exercised although samples were separated. Also, the higher body temperature would be expected to lead to a more rapid weakening, other factors being equal. In summary, barium titanate demonstrated good tissue compatibility during the in vivo ageing experimentation; minimal encapsulation and no inflammation was observed. The strength decreased drastically in the first 4 weeks for both in vivo and
_ in _ vitro _
samples and the general trend of strength decrease was
similar Although the compressive strength decreased by over fifty percent, from 281 to 138 MPa, this lowest strength value was
PARK ET AL.
274
s t i l l c l o s e t o l i t e r a t u r e v a l u e s o f compressive s t r e n g t h f o r t h e
human humerus and femur.
The observed s t r e n g t h d e g r a d a t i o n was,
t h e r e f o r e , n o t s o s e v e r e a s t o d i s m i s s barium t i t a n a t e as a
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bioma t e r i a l .
ACKNOWLEDGMENT
T h i s work w a s s u p p o r t e d by B i o e n g i n e e r i n g F a c u l t y and Ceramic Engineering Department a t t h e U n i v e r s i t y of I l l i n o i s i n Urbana. The e x p e r t h e l p g i v e n by D r . R.L.
Cook i s g r a t e f u l l y acknowledged.
T h i s r e p o r t i s based on t h e M.S. T h e s i s of J . K .
S c o t t , Department
of Ceramic Engineering, U n i v e r s i t y o f I l l i n o i s (1976).
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1.
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2.
W i l l i a m s , D.F., London (1973).
3.
B e c h t o l , C.O., Metals and Engineering i n Bone and J o i n t Surgery, W i l l i a m s and W i l k i n s , Baltimore (1959).
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3
(2), 233 (1975).
21. Park, J. B., and Kenner, G. H., Biomat. Med. Dev. Art. Org. 4 (2), 225 (1976). 22. Fukada, E., J. Phys. SOC. (Jap.)
12, 1159
(1956).
23. ShamOs, M., and Lavine, L., Nature, 197, 81 (1963).
276
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24. B e c k e r , R. O . , Bassett, C. A . L . , and Backman, C . H . , "The B i o e l e c t r i c F a c t o r s C o n t r o l l i n g Bone S t r u c t u r e , " Bone Biodynamics, Ed. F r o s t , H. M . , L i t t l e Brown and Co., 209 ( 1 9 6 4 ) . 25. W i l l i a m s , W. S . , C r i t i c a l Rev. Bioeng. C l e v e l a n d , Ohio ( 1 9 7 4 ) .
2
(11, 9 5 , CRC,
26. S c h n i t t g r u n d , G. D . , Kenner, .G. H . , and Brown, S. D . , J. Biomed. Mater. R e s . Symp. No. 4 , 435 ( 1 9 7 3 ) .
2 7 . Kenner, G . H . , F r a k e s , J. T . , and Brown, S. D . , J . Biomed. M a t e r . Res. Symp. No. 6 , 6 3 ( 1 9 7 5 ) . 2 8 . Jecmen, R. M . , E g g e r d i n g , C. L . , Brown, S . D . , and S c h n i t t g r u n d , J. Biomed. Mater. R e s . 1, 369 ( 1 9 7 3 ) .
BIOMAT.,
MED. DEV.,
ART. ORG.,
5(3), 267-276 (1977)
MECHANICAL PROPERTY CHANGES OF BARIUM TITANATE (CERAMIC) AFTER I N VIVO AND I N VITRO A G I N G
J.B.
P a r k , G.H.
Kenner, S.D.
Brown* and J . K .
Scott*
D i v i s i o n of I n t e r d i s c i p l i n a r y S t u d i e s Clemson U n i v e r s i t y Clemson, SC 29631 *Department of Ceramic E n g i n e e r i n g U n i v e r s i t y of I l l i n o i s Urbana, I L 61801 ABSTRACT
S i n c e barium t i t a n a t e (BaTiO ) can b e made p i e z o e l e c t r i c , i t 3 may b e used t o s u b s t i t u t e h a r d t i s s u e s d i r e c t l y . A s a f i r s t s t e p i n t e s t i n g t h i s c o n c e p t , a series of i n v i v o and i n v i t r o a g i n g and b i o c o m p a t i b i l i t y s t u d i e s were performed. The mean compressive s t r e n g t h of samples implanted s u b c u t a n e o u s l y i n t h e b a c k s of rabb i t s d e c r e a s e d t o 1 3 8 MPa a f t e r 20 weeks from a c o n t r o l v a l u e of 2 8 1 MPa. S i m i l a r , though l e s s d r a s t i c l o s s e s of s t r e n g t h w e r e s e e n when specimens were aged i n d i s t i l l e d water (182 MPa a t 2 8 weeks) and R i n g e r ' s s o l u t i o n (159 MPa a t 28 weeks). The most r a p i d d e c r e a s e of s t r e n g t h i n a l l c a s e s w a s s e e n p r i o r t o 4 weeks. T h e r e a f t e r , t h e d e c r e a s e w a s much s l o w e r . H i s t o l o g i c a l e v a l u a t i o n of t h e t i s s u e s u r r o u n d i n g t h e imp l a n t r e v e a l e d a t h i n f i b r o u s c a p s u l e and no e v i d e n c e of t i s s u e inflamtion.
INTRODUCTION
The f i x a t i o n o r a t t a c h m e n t of i m p l a n t s t o bone i s a major problem i n o r t h o p e d i c work (1-9).
C u r r e n t l y , t h r e e methods are
used t o f i x i m p l a n t s : ( a ) d i r e c t mechanical a t t a c h m e n t u s i n g
screws, n a i l s , e t c . ( 2 , 3 ) ; (b) i m p a c t i o n of t h e p r o s t h e s i s i n t o
267 Copyright 0 1977 by Marcel Dekker. Inc. All Rights Reserved. Neither this work nor any part may be reproduced or transmitted in any form or by any meahs, electronic or mechanical, including photocopying, microfilming, and recording, or by any information storage and retrieval system, without permission in writing from the publisher.
268
PARK ET AL.
bone [ e . g . ,
h i p j o i n t replacement i n which t h e stem o f t h e pros-
t h e s i s i s impacted i n t o t h e m e d u l l a r y c a v i t y of t h e femur ( 4 , 5 ) ] ; and ( c ) u s e of polymeric cements t o f i x t h e p r o s t h e s i s / t i s s u e
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i n t e r f a c e (e.g., etc.).
p o l y m e t h y l m e t h a c r y l a t e ( 6 ) , p o l y c a r b o x y l a t e (7),
None of t h e s e methods i s e n t i r e l y s a t i s f a c t o r y f o r a perma-
nent prosthetic fixation.
F a i l u r e s are l a r g e l y a t t r i b u t e d t o
stress c o n c e n t r a t i o n s i n t h e bone by t h e i m p l a n t s .
This r e s u l t s
i n t i s s u e n e c r o s i s and r e s o r p t i o n of bone (10) w i t h e v e n t u a l weakening and f a i l u r e (1,3) o r l o o s e n i n g of t h e i m p l a n t s , dependent d e t e r i o r a t i o n of t h e polymeric cement due t o
Time-
repeated
l o a d i n g and a t t a c k by t h e body environment ( l l ) , a c u t e r e a c t i o n s due t o release of t h e t o x i c monomer, and h i g h t e m p e r a t u r e due t o t h e h e a t of p o l y m e r i z a t i o n a l s o o c c u r ( 6 ) . The d i s c o v e r y t h a t t i s s u e ingrowth o c c u r s i n t o porous mater-
i a l s h a s provided a n a t t r a c t i v e a l t e r n a t i v e t o mechanical f i x a tion.
T i s s u e ingrowth h e l p s t o s p r e a d t h e l o a d over a l a r g e a r e a
t h e r e b y minimizing stress c o n c e n t r a t i o n s and f a c i l i t a t i n g a c o n t i n u o u s v i a b l e c o n n e c t i o n between t h e p r o s t h e s i s and t i s s u e (9,12-21).
The chemical s t a b i l i t y of a n implant which i s t o b e
anchored by bone ingrowth i s i m p o r t a n t , s i n c e t h e removal of such a n implant r e s u l t s i n f a r g r e a t e r damage t h a n t h a t i n c u r r e d w i t h t h e removal of a m e c h a n i c a l l y a t t a c h e d implant. The need f o r e x t r e m e l y i n e r t b i o m a t e r i a l s h a s caused ceramics t o b e r e c o n s i d e r e d f o r u s e a s hard t i s s u e replacements.
Ceramic
materials were o r i g i n a l l y d i s m i s s e d a s p o t e n t i a l p r o s t h e t i c mat e r i a l s b e c a u s e of t h e i r poor impact r e s i s t a n c e , low t e n s i l e and f l e x u r a l s t r e n g t h s , and a s u s c e p t i b i l i t y t o b r i t t l e f a i l u r e . I f , however, a porous ceramic
could provide a scaffolding i n t o
which e x t e n s i v e t i s s u e ingrowth would o c c u r , the mechanical p r o p e r t i e s of t h e r e s u l t i n g composite would be f a v o r a b l y i n f l u e n c e d by t h e bone t i s s u e .
Although a minimum l e v e l of
mechanical s t a b i l i t y would b e r e q u i r e d , t h e c h e m i c a l c o m p a t i b i l i t y
AGING OF BARIUM TITANATE
269
o f t h e i m p l a n t would b e t h e more c r i t i c a l f a c t o r .
I n these c i r -
c u m s t a n c e s ¶ c e r a m i c s show g r e a t p o t e n t i a l .
Ever s i n c e t h e d e t e c t i o n o f s t r a i n i n d u c e d p o t e n t i a l s i n Artif Cells Blood Substit Immobil Biotechnol Downloaded from informahealthcare.com by University of British Columbia on 12/10/14 For personal use only.
bone and t e n d o n (22-24),
t h e p o s s i b i l i t y o f u s i n g t h i s phenomenon
t o c o n t r o l t h e o s t e o g e n i c and o s t e o c l a s t i c mechanisms of l i v i n g animals h a s been of i n t e n s e experimental i n t e r e s t .
W i l l i a m s (25)
r e c e n t l y c r i t i c a l l y reviewed t h e c o n t r o v e r s y o n t h e s o u r c e of the stress-generated
e l e c t r i c a l s i g n a l s of t e n d o n and bone
(wet o r d r y ) and c o n c l u d e d t h a t t h e y c a n b e i n t e r p r e t e d as piezoelectric. The a p p l i c a t i o n o f e l e c t r i c a l s t i m u l a t i o n around p o r o u s c a l c i u m a l u m i n a t e i m p l a n t s i n s e r t e d i n t o r a b b i t femurs i n c r e a s e d t h e t i s s u e ingrowth i n t o t h e implants.
I n t h i s work by P a r k and
Kenner ( 2 0 , 2 1 ) , a b a t t e r y and m e t a l l i c l e a d s were used t o p r o v i d e s t i m u l a t i o n during t h e course of t h e experiment. The u s e o f a p o r o u s p i e z o e l e c t r i c m a t e r i a l as a n i m p l a n t
seems t o b e t h e n e x t l o g i c a l s t e p i n t h e e x p l o r a t i o n o f t h e e f f e c t s of e l e c t r i c a l s t i m u l a t i o n on bone i n g r o w t h .
Hopefully,
s u c h a n i m p l a n t would e l i m i n a t e t h e need f o r t h e l e a d s and a n o u t s i d e s o u r c e o f power.
This r e p o r t e v a l u a t e s t h e biocompati-
b i l i t y a n d c o m p r e s s i v e m e c h a n i c a l p r o p e r t y of t h e p i e z o e l e c t r i c ceramic, barium t i t a n a t e .
EXPERIMENTAL PROCEDURE Reagent g r a d e b a r i u m t i t a n a t e powder (TAM D i v . , NL I n d u s -
t r i e s , P i t t s b u r g h , PA) was mixed w i t h p o l y v i n y l a l c o h o l ( 0 . 2 5 w e i g h t p e r c e n t ) and d i s t i l l e d water ( 5 w e i g h t p e r c e n t ) .
The
m i x t u r e w a s b l e n d e d w i t h a s p a t u l a and s t o r e d o v e r n i g h t i n a n air-tight
container t o allow the moisture t o e q u i l i b r a t e .
C y l i n d r i c a l s p e c i m e n s 16 mm i n h e i g h t were p r e s s e d u s i n g a Denison p r e s s w i t h a 16 mm d i a m e t e r d i e and 138 MPa p r e s s u r e .
2 70
PARK ET AL.
The specimens were a i r - d r i e d o v e r n i g h t . They were t h e n p l a c e d on platinum f o i l i n alumina s a g g e r s and f i r e d t o 1325°C i n a gasf i r e d furnace.
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The samples were d i v i d e d i n t o t h r e e groups: (1) aged i n v i t r o i n d i s t i l l e d water (2) aged i n v i t r o i n R i n g e r ' s s o l u t i o n ( b o t h a t room t e m p e r a t u r e ) and (3) aged i n v i v o .
The compressive s t r e n g t h
of t h e p e l l e t s w a s determined a f t e r f o u r , e i g h t , twelve weeks i n
a l l cases and twenty weeks i n t h e i n v i v o s t u d y w h i l e t h e i n v i t r o specimens were t e s t e d a t 28 weeks.
-I n vivo
ageing of p e l l e t s w a s accomplished by t h e subcutane-
~ U Si m p l a n t a t i o n
of f i r e d p e l l e t s i n l a b o r a t o r y r a b b i t s .
The
p e l l e t s t o b e implanted were s t e r i l i z e d by h e a t i n g t o 200°C i n
a m u f f l e oven f o r two hours. t h e i r backs shaved.
The r a b b i t s were a n e s t h e t i z e d and
I n c i s i o n s were made w i t h a s c a l p e l , and f o u r
p e l l e t s p e r animal were i n s e r t e d j u s t under t h e s k i n of t h e back, The samples were s e p a r a t e d so a s t o avoid c o n t a c t i n s i d e t h e body. The i n c i s i o n s were t h e n c l o s e d . The compressive s t r e n g t h of e i g h t unaged c o n t r o l p e l l e t s
was determined u s i n g a Tinium O l s e n t e s t i n g machine.
The com-
p r e s s i v e s t r e n g t h s of t h e aged specimens were determined u s i n g t h e same machine.
E i g h t p e l l e t s were t e s t e d a f t e r each p e r i o d
of i n v i v o and i n v i t r o aging d e s c r i b e d above. RESULTS AND D I S C U S S I O N S
The body's r e a c t i o n t o t h e i m p l a n t a t i o n of barium t i t a n a t e
w a s unknown, s i n c e i t had never been used a s a b i o m a t e r i a l . S i n c e t h e element barium i s poisonous, t h e p o s s i b i l i t y of s e r i o u s problems e x i s t e d .
I f barium c o l l e c t e d i n t h e g r a i n b o u n d a r i e s
of barium t i t a n a t e , i t could b e l e a c h e d o u t by b i o l o g i c a l f l u i d s and r e l e a s e d i n t o t h e body.
I f t h i s occurred, the p o t e n t i a l
u t i l i z a t i o n of barium t i t a n a t e a s a b i o m a t e r i a l would b e d e s t r o y e d .
271
AGING OF BARIUM TITANATE
F o r t u n a t e l y , t h e r e s u l t s of t h e i n v i v o t e s t i n g demonstrated good t i s s u e c o m p a t i b i l i t y a s shown i n F i g u r e 1. s u r r o u n d i n g t h e i m p l a n t s showed no inflammation.
The t i s s u e In addition,
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o n l y a narrow membrane of f i b r o u s e n c a p s u l a t i o n surrounded each implant.
Barium t i t a n a t e i s h a r d enough s o t h a t when i t i s
p r e s s e d , a s l i g h t amount of metal i s abraded from t h e d i e and remains on t h e s u r f a c e of t h e specimens.
T h i s l a y e r of i m p u r i t y
metal was removed by t h e b i o l o g i c a l environment, b u t o t h e r w i s e , t h e r e was no v i s i b l e d e g r a d a t i o n of t h e sample.
A q u a n t i t a t i v e measurement of material d e g r a d a t i o n w a s determined by comparing t h e compressive s t r e n g t h of specimens aged b o t h i n v i v o and i n v i t r o w i t h each o t h e r and w i t h unaged c o n t r o l specimens.
Mean s t r e n g t h s a t each t e s t i n g p e r i o d are
summarized i n Table I , and are p l o t t e d a s a f u n c t i o n of a g i n g t i m e i n Figure 2 .
I n t h i s f i g u r e , t h e d a t a p o i n t s are t h e
computed a r i t h m e t i c mean v a l u e s .
I.. . . )
F i g u r e 1. O p t i c a l micrograph of s o f t c o n n e c t i v e t i s s u e a d j a c e n t t o t h e Barium T i t a n a t e i m p l a n t s a f t e r 20 weeks (H & E s t a i n , arrow (T) i n d i c a t e s e n c a p s u l a t i o n of dense connective t i s s u e ) .
PARK ET AL.
272 TABLE I .
Mean Compressive S t r e n g t h a t Each T e s t i n g P e r i o d
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Test Condition
Aging P e r i o d (Weeks)
Mean S t r e s s
S.D.
5
Unaged C o n t r o l
0
281
I n Vivo --
4
1 8 4 f. 2 0 . 4
8
168
I n -V i t r o _
in
Ringer's Solution
In _ V i_ tro _
in
D i s t i l l e d Water
t
35.9
23.7
12
1 6 0 f. 38.8
20
1 3 8 f. 38.7
4
209
8
176 f. 1 6 . 5
12
160
28
159
26.2
A A
24.5 40.6
4
222 f. 23.2
8
209 f. 2 2 . 1
12
194
28
182
("a)
5 5
17.9 68.7
The a v e r a g e compressive s t r e n g t h o f t h e unaged, c o n t r o l specimens w a s 2 8 1 MPa.
A f t e r o n l y f o u r weeks o f i m p l a n t a t i o n
i n_v_ i v o , t h e a v e r a g e s t r e n g t h dropped t o 1 8 4 MPa, a d e c r e a s e i n
s t r e n g t h of approximately t h i r t y - f i v e percent.
A f t e r e i g h t weeks
o f a g i n g , t h a t v a l u e d e c r e a s e d t o 1 6 8 MPa, a n a d d i t i o n a l d e c r e a s e of f i v e p e r c e n t .
A f t e r twenty weeks of a g i n g , t h e a v e r a g e
s t r e n g t h o f t h e b a r i u m t i t a n a t e had d e c r e a s e d t o 1 3 8 MPa, a t o t a l decrease i n s t r e n g t h of fifty-one percent.
The l a r g e s t d r o p i n
s t r e n g t h o c c u r r e d d u r i n g t h e f i r s t f o u r weeks of i m p l a n t a t i o n . A f t e r t w e n t y weeks of a g i n g , however, t h e specimens were s t i l l showing a s t e a d y d e c l i n e i n s t r e n g t h . The specimens aged i n v i t r o showed p a t t e r n s of s t r e n g t h r e d u c t i o n s i m i l a r t o t h a t of t h e i n v i v o specimens.
The d i s -
AGING OF BARIUM TITANATE
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+-
273
Control
I00
0
4
8
12
16
20
24
28
AGING TiMEheeks)
Figure 2 . The mean compressive strength o f barium titanate ceramic specimen in vitro and in vivo. tilled water specimens showed the least reduction in strength at each testing period, the Ringer's solution specimens were somewhat weaker, and the in vivo specimens showed the greatest reduction in strength similar to other ceramics (26-28).
h e
possible explanation is the possibility of occasional bumping of the specimens whenever the animal exercised although samples were separated. Also, the higher body temperature would be expected to lead to a more rapid weakening, other factors being equal. In summary, barium titanate demonstrated good tissue compatibility during the in vivo ageing experimentation; minimal encapsulation and no inflammation was observed. The strength decreased drastically in the first 4 weeks for both in vivo and
_ in _ vitro _
samples and the general trend of strength decrease was
similar Although the compressive strength decreased by over fifty percent, from 281 to 138 MPa, this lowest strength value was
PARK ET AL.
274
s t i l l c l o s e t o l i t e r a t u r e v a l u e s o f compressive s t r e n g t h f o r t h e
human humerus and femur.
The observed s t r e n g t h d e g r a d a t i o n was,
t h e r e f o r e , n o t s o s e v e r e a s t o d i s m i s s barium t i t a n a t e as a
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bioma t e r i a l .
ACKNOWLEDGMENT
T h i s work w a s s u p p o r t e d by B i o e n g i n e e r i n g F a c u l t y and Ceramic Engineering Department a t t h e U n i v e r s i t y of I l l i n o i s i n Urbana. The e x p e r t h e l p g i v e n by D r . R.L.
Cook i s g r a t e f u l l y acknowledged.
T h i s r e p o r t i s based on t h e M.S. T h e s i s of J . K .
S c o t t , Department
of Ceramic Engineering, U n i v e r s i t y o f I l l i n o i s (1976).
REFERENCES
1.
Hench, L.L., and E t h r i d g e , E.C.,"Biomaterials-The I n t e r f a c i a l Problem," i n Advances i n Biomedical Engineering, v o l . 5 , 35, Brown, J . H . U . and Discosn, J . F . 111 (Ed.), (1975). Academic P r e s s , N.Y.
2.
W i l l i a m s , D.F., London (1973).
3.
B e c h t o l , C.O., Metals and Engineering i n Bone and J o i n t Surgery, W i l l i a m s and W i l k i n s , Baltimore (1959).
and Roaf, R.,
Implants i n Surgery, Saunders,
4. Sbarbaro, J. L . , J r . , " P r e s s F i t Implant A r t h r o p l a s t y of t h e Hip and Knee," 5 t h Ann. Biomater. Symp. S e s s i o n 8, Clemson U n i v e r s i t y , S.C., A p r i l 1973. 5. H i r s c h , C . , " C l i n i c a l Problems i n T o t a l Hip Replacement ,I' 5 t h Ann. Biomater. Symp. S e s s i o n 8 , Clemson U n i v e r s i t y , S.C. A p r i l , 1973.
6 . Homsy, C. A . , "Current Research on t h e I n V i t r o S t a b i l i z a t i o n of S k e l e t a l P r o s t h e t i c Elements," I n B i o m a t e r i a l s , Ed. Bement, A. L . , J r . , Univ. of Washington P r e s s , S e a t t l e and London, 133 (1971). 7. Smith, D. C . ,
8. Mooney, V . , J.
J . Biomed. Mater. Res. Symp. No. 1, 189 (1971).
P r e d e c k i , P. K . , Renning, J . , and Gray, J . , Biomed. Mater. R e s . Symp. No. 2 ( P a r t l ) , 143 (1971).
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AGING OF BARIUM TITANATE
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