Development of a Biomedical Polyurethane - ACS Symposium Series

31 Development of a Biomedical Polyurethane Orthopedic Implant Applications

Downloaded by KTH ROYAL INST OF TECHNOLOGY on August 24, 2015 | http://pubs.acs.org Publication Date: November 30, 1981 | doi: 10.1021/bk-1981-0172.ch031

EDWARD W. C. WONG

1

Biomaterials Group, Ontario Research Foundation, Sheridan Park, Mississauga, Ontario, Canada L5K 1B3

Polyurethanes are block copolymers containing blocks of low molecular weight polyesters or polyethers linked together by a urethane group:

These have the versatility of being either rigid, semi-rigid or flexible. These materials, in general, have excellent flex-life, strength and other mechanical properties. They have been described as resistant to gamma radiation, oils, acids and bases. The first generation of polyurethanes used for implant studies were industrial grade and are commercially available. Mirkovitch and Associates, (1) however, found the Estane (B.F. Goodrich, Company) polyester urethanes to degrade rapidly when implanted in the muscle of dogs or when used as monocusp valvular prosthese. Sharp(2) and co-workers observed thromboresistance of a polyester-polyether polyurethane (Goodyear Tire and Rubber Company) in intravascular replacenent. The first biomedical grade polyether polyurethane was synthesized by two groups. Boretos and Pierce (3) introduced the biomedical application of segmented polyether polyurethanes containing hard segments of urea and soft segments of polyether linked by the urethane group. These m a t e r i a l s sustained high R

modulus of e l a s t i c i t y , b i o c o m p a t i b i l i t y , r e s i s t a n c e to f l e x f a t i g u e and e x c e l l e n t s t a b i l i t y over long implant p e r i o d s . ( A ) From t h e i r previous experience i n the s y n t h e s i s of p o l y urethanes f o r d i a l y s i s membranes, Lyman and A s s o c i a t e s ( 5 ) a l s o introduced a segmented polyether polyurethane which has shown e x c e l l e n t thromboresistance.(6)

1

Current address: Avco Everett Research Laboratory, Inc., 2385 Revere Beach Parkway, Everett, Massachusetts 02149 0097-6156/81 /0172-0489$05.00/0 © 1981 American Chemical Society

In Urethane Chemistry and Applications; Edwards, K., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

490

URETHANE CHEMISTRY AND APPLICATIONS


K

N y i l a s , the developer of Avcothane , s n y t h e s i z e d a copolymer of polyurethane and p o l y d i m e t h y l siloxane(Z) which i s blood compatible and used i n the making of heart a s s i s t b a l l o o n pump s. Another b i o m e d i c a l grade segmented p o l y e t h e r urethane Biomer R which i s now a v a i l a b l e commercially from Ethnor, D i v i s i o n of E t h i c o n , Incorporated, i s based on p o l y e t h e r from polybutanediol-1,4. In 1968, Ontario Research Foundation developed a s e r i e s of segmented p o l y e t h e r polyurethanes as polymer membrane m a t e r i a l s f o r reverse cosmosis, u l t r a f i l t r a t i o n and h e m o d i a l y s i s . The elastomers of recent implant s t u d i e s are polyurea-urethanes w i t h m o d i f i c a t i o n of the s y n t h e s i s l i m i t e d to only one v a r i a b l e — the c h a i n l e n g t h of the p o l y e t h e r component. Synthesis and M o d i f i c a t i o n . A s e r i e s of polyurethanes and polyurethane-ureas of v a r y i n g degress of h y d r o p h i l i c i t y and h y d r o p h o b i c i t y and mechanical property were s y n t h e s i z e d . The polymers were prepared by a s o l u t i o n p o l y m e r i z a t i o n method and c o n s i s t e d of three components: a p o l y e t h e r , a d i i s o c y a n a t e , and a c h a i n extender. I n our s t u d i e s , polyurethanes (Table I) were based on a carbowax (polyoxyethylene g l y c o l ) , MDI (methylene bis-4-phenyl isocyanate) and 1,5-pentanediol. Polyurethane-ureas (Table I I ) were obtained by s u b s t i t u t i n g the c h a i n extender from a d i o l to a diamine. The polyurethane-ureas (Table I I ) were obtained by changing the c h a i n extender from a d i o l to a more r e a c t i v e diamine. The polyurea-urethanes (Table I I I ) were obt a i n e d by using a diamine terminated p o l y e t h e r i n s t e a d of the carbowax. The h y d r o p h i l i c i t y and h y d r o p h o b i c i t y of the polymer could be m o d i f i e d c h e m i c a l l y by changing the f o l l o w i n g v a r i a b l e s : •

I n c r e a s i n g h y d r o p h i l i c i t y by s e l e c t i n g p o l y e t h e r s longer carbon c h a i n : polyoxyethylene (2 C) < polyoxypropylene (3 C) < p o l y t e t r a h y d r o f u r a n e (4 C).

•

I n c r e a s i n g h y d r o p h i l i c i t y by r a i s i n g the molar r a t i o of p o l y e t h e r / c h a i n extruder.

•

I n c r e a s i n g h y d r o p h i l i c i t y by lengthening the c h a i n l e n g t h of p o l y e t h e r .

In most of our s t u d i e s , m o d i f i c a t i o n of the s y n t h e t i c route was l i m i t e d to only one v a r i a b l e , the c h a i n l e n g t h of the p o l y ether component. The same molar r a t i o of p o l y e t h e r / c h a i n extender disocyanate (MDI) and chain extender ( d i e t h l e n e g l y c o l ) were used. The molecular weight of the p o l y e t h e r component of J-20, J-9 and J-6 was 2,000, 1,000 and 600 r e s p e c t i v e l y . A wide range of p r o p e r t i e s was obtained from t h i s s e r i e s of polyurethanes. Prope r t i e s i n t e r m e d i a t e between J-20 and J-9 were a l s o obtained by



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URETHANE CHEMISTRY AND

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using a 1:1 molar r a t i o of J-20 and J-9 polyether without changing other v a r i a b l e s . Even though the polymer i s l i n e a r and has no chemical c r o s s l i n k i n g , i t behaves l i k e a rubber. There are p h y s i c a l c r o s s l i n k i n g from c r y s t a l l i n e packing of urethane hard segments. These c r y s t a l l i n e domain s t r u c t u r e s a l s o e x p l a i n why the longer the polyether c h a i n , the more f l e x i b l e the system, lower e l a s t i c modulus and more h y d r o p h i l i c i t y . The h y d r o p h i l i c property of these polymers was measured by the % moisture a b s o r p t i o n of dry polymer f i l m . ( 9 ) Films of a uniform s i z e and t h i c k n e s s (2 diame t e r d i s c , 1 m i l t h i c k n e s s ) were d r i e d under high vacuum and then exposed to 98% RH i n a d e s i c c a t o r c o n t a i n i n g a saturated s o l u t i o n of Pb(N03)2 u n t i l no more water was absorbed. The percent of water absorbed increased from J-20 > J-9 > J-6. R e s u l t s are shown i n Table IV. ff

TABLE IV % MOISTURE ABSORPTION OF DRY POLYMER FILMS (THICKNESS = 1 m i l , DIAMETER OF DISC =

2")

Film

Dry Wt. (gms)

Water Pickup

% Water Absorbed

J-6-1

0.1567

0.0155

9.9

J-9-1

0.1526

0.0409

26.8

J-20-1

0.0636

0.0397

62.4

JD-4-1

0.1591

0.0111

7.0

JD-6-1

0.2767

0.0115

4.2

B i o c o m p a t i b i l i t y and Mechanical P r o p e r t i e s . C u r r e n t l y , t h e i r are no s u i t a b l e a r t i f i c i a l m a t e r i a l s f o r the p r o s t h e t i c r e p l a c e ment of a r t i c u l a r c a r t i l a g e . The b i o c o m p a t i b i l i t y i s considered the primary c r i t e r i o n i n the s e l e c t i o n of such a m a t e r i a l . In a recent study, Furst and co-workers(10) compared the biocompatib i l i t y of the polyurethane to the w e l l known medical grade s i l i c o n e polymer. The t i s s u e r e a c t i o n s to small polymer d i s c s , i n s e r t e d i n an a r t i c u l a t i n g space—the s u p r a p a t e l l a r bursa of r a b b i t s , was examined. The f o r e i g n body r e a c t i o n of the t i s s u e at the i m p l a n t a t i o n s i t e was evaluated at i n t e r v a l s of 7 days, 21 days and 6 months. The t i s s u e r e a c t i o n was c h a r a c t e r i z e d by the t h i c k n e s s of the p a t e l l a r a r t i c u l a r c a r t i l a g e and the presence or absence of morphonuclear leukocytes, macrophages, and f o r e i g n body g i a n t c e l l s i n synovium and menisci t i s s u e samples.



31.

WONG

493

Biomedical Polyurethane

H i s t o l o g i c a l r e s u l t s i n d i c a t e d that the J-9 polyurethane produced l e s s f o r e i g n body t i s s u e r e a c t i o n than d i d the medical grade s i l i c o n e rubber c o n t r o l . The polyurethane a r t i c u l a t i n g p a t e l l a s showed normal a n t i c u l a r c a r t i l a g e and l i t t l e c a r t i l a g e appeared on the p a t e l l a s of the s i l i c o n e c o n t r o l knees. A l l menisci samples from both the experimental and c o n t r o l knees. A l l m i n i s c i samples from both the experimental and c o n t r o l knees appeared normal r e g a r d l e s s of the time i n t e r v a l . The synovium samples from the polyurethane knees appeared normal but the samples from the s i l i c o n e c o n t r o l knees contained many uniform holes that appeared to be l i p i d d e p o s i t s . Mechanical t e s t i n g of the polyurethanes was performed by a m o d i f i e d i d e n t a t i o n / s h e a r method d e s c r i b e d by Parsons and Black. (.11) Relaxed and unrelaxed shear moduli of these polymers were measured i n a simulated body f l u i d environment and compared to moduli of r a b b i t and human a r t i c u l a r c a r t i l a g e . This i s shown i n Table V. These t e s t s , i n a d d i t i o n to v i s c o e l a s t i c measurements of those polymers, i n d i c a t e that t h i s new polyurethane f o r m u l a t i o n i s an e x c e l l e n t candidate f o r s y n t h e t i c c a r t i l a g e fabrication. TABLE V VISCOELASTIC AND HYDROPHILIC PROPERTIES OF CARTILAGE AND SYNTHETIC MATERIALS (at 37°C i n S a l i n e S o l u t i o n )

Materials

Unrelaxed Shear Modulus (MPa)

Human C a r t i l a g e

1.04

Rabbit C a r t i l a g e (Alive)

0.53

Relaxed Shear Modulus (MPa)

Amount of Water (Weight %)

0.24

60-80

0.11

Polyurethane J-20

0.57

0.33

62

J-9

1.72

0.66

27

J-6

2.58

0.92

10

Wear and L u b r i c a t i o n I n - V i t r o Study. Polyurethane s u r f a c e l a y e r s w i t h v i s c o e l a s t i c p r o p e r t i e s s i m i l a r to n a t u r a l a r t i c u l a r c a r t i l a g e has been proposed f o r use w i t h h e m i a r t h r o p l a s t y , a s i n g l e component j o i n t replacement i n which the implant i s intended to bear a g a i n s t a n a t u r a l c a r t i l a g e s u r f a c e . Medley and



494


co-workers(12) have c a r r i e d out i n v i t r o experiments to study the e f f e c t s of f l u i d f i l m l u b r i c a t i o n on the wear of polyurethane. The t e s t apparatus i s shown i n F i g u r e 1. The experimental model c o n s i s t s of a f l a t polyurethane block s l i d i n g over a s t a t i o n a r y l o a d i n g p i n w i t h a l i n e a r r e c i p r o c a t i n g motion. A v a r i a b l e speed motor was connected v i a a scotch yorke arrangement to produce a range of s i n u s o i d a l v e l o c i t i e s and a constant s t r o k e l e n g t h . A load was s e t by p l a c i n g the a p p r o p r i a t e amount of lead shot i n the cup. The p i n t i p could be detached thus a l l o w i n g e i t h e r a g l a s s or a metal sphere to be used against the p o l y urethane. In the wear t e s t s , the polyurethane under the t i p of the sphere was subjected t o a maximum s t r e s s of 2.4 MPa whereas a r t i c u l a r c a r t i l a g e ( 1 3 ) t y p i c a l l y i s subject to average contact s t r e s s e s of 2.75 MPa. A t y p i c a l "average" v e l o c i t y , i f only one s u r f a c e moves, f o r human h i p and knee(14) i s 0.05 m/s. The polyurethane was subject to v e l o c i t y from zero t o 0.126 m/s. Thus, except f o r higher deformation r a t e s , s p e c i f i c regions of the polyurethane were subject t o approximately p h y s i o l o g i c a l c o n d i t i o n s . F l u i d f i l m p r o t e c t i o n i s shown t o g r e a t l y reduce, i f not e l i m i n a t e , wear. The r e s u l t s of t h i s study suggest the p o s s i b i l i t y of designing a p r o s t h e s i s w i t h a polyurethane s u r f a c e l a y e r f o r h e m i a r t r o p l a s t y or a p r o s t h e t i c replacement f o r a r t i c u l a r c a r t i l a g e such that a f l u i d f i l m w i l l be formed t o p r o t e c t both s u r f a c e s during implant f u n c t i o n i n g . T o t a l Elbow J o i n t Replacement. A common problem encountered w i t h the hinge-type t o t a l elbow j o i n t replacement i s the loosen ing of the humeral component as a r e s u l t of l o a d i n g across the j o i n t . ( 1 5 , 1 6 ) A new concept f o r a t o t a l elbow replacement w i t h the c a p a b i l i t y t o absorb l o a d i n g and reduce peak f o r c e l e v e l s a t the humeral component/bone i n t e r f a c e has been i n v e s t i g a t e d . The design, Figure 2, i s based on two e l a s t o m e r i c spheres s e r v i n g as a p i v o t i n the replacement w i t h t h e i r e l a s t o m e r i c p r o p e r t i e s a l l o w i n g f o r the a b s o r p t i o n of l o a d i n g through deformation.(λ2) In t h i s study, the m o d i f i c a t i o n of s y n t h e t i c route i n r e l a t i o n t o the damping c o e f f i c i e n t has been examined. The method of measur ing the damping c o e f f i c i e n t of the polymer spheres and the impact t e s t i n g using the model elbow has been described p r e v i o u s l y by Wong and White. (17) The damping c o e f f i c i e n t ( q u a l i t y f a c t o r ) of the system can be r e a d i l y c a l c u l a t e d from the f o l l o w i n g impression: (18) Quality Factor, Q = - — f

where f ^ = frequency a t resonance (peak power) and f 2 and f± are frequencies r e s p e c t i v e l y above and below f R at the p o i n t of oneh a l f peak power. The damping c o e f f i c i e n t of the system i s equal to the r e c i p r o c a l of Q. F i g u r e 3 and 4 showed the measurement



31.

WONG


Figure 1.

495

Wear test apparatus.

Figure 2. Model elbow and new total elbow joint replacement design.




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of 1/Q. The damping c o e f f i c i e n t of J-9 polymer sphere was 0.7 which i s the optimum i n many v i b r a t i o n damping a p p l i c a t i o n s . Figure 5 of the impact t e s t showed that J-9 polyurethane was able to reduce the peak f o r c e v a l u e down t o 36% that of s t e e l b a l l s w i t h o u t s a c r i f i c i n g mechanical s t r e n g t h . The r e s u l t from the impact t e s t and the measurement damping c o e f f i c i e n t i n d i c a t e that polyurethane J-9 i s the optimum sphere m a t e r i a l i n the new t o t a l elbow j o i n t replacement design. In-Vivo Percutaneous Implant Experiment. The p r i n c i p l e of percutaneous attachment has e x t e n s i v e a p p l i c a t i o n i n many b i o medical areas, i n c l u d i n g the attachment of d e n t a l and o r t h o p e d i c prostheses d i r e c t l y t o s k e l e t a l s t r u c t u r e s , e x t e r n a l attachment f o r c a r d i a c pacer l e a d s , neuromuscular e l e c t r o d e s , energy t r a n s m i s s i o n t o a r t i f i c i a l heart and f o r h e m o d i a l y s i s . S e v e r a l attempts t o s o l v e the problem of f i x a t i o n and s t a b i l i z a t i o n of percutaneous implants(19) have been made. F a i l u r e s were a l s o a t t r i b u t e d t o the i n a b i l i t y of the s o f t t i s s u e i n t e r f a c e t o form an anatomic s e a l and a b a r r i e r t o b a c t e r i a . I n the c u r r e n t s t u d i e s , the e f f e c t of pore s i z e on s o f t t i s s u e ingrowth and attachment t o porous polyurethane (PU) surface and the e f f e c t of the f l a n g e t o stem r a t i o and biomechanical compliance on the f i x a t i o n and s t a b i l i z a t i o n of the percutaneous d e v i c e s have been investigated.(20) S o l i d d i s c s (2.5 cm diameter) were molded from J-9 p o l y urethane. A s e r i e s of porous J-9 PU surfaces w i t h two d i f f e r e n t t h i c k n e s s e s (1 mm and 10 mm) and two p o r o s i t i e s ( f i n e , 47-75 ym; coarse, 150-250 ym) was prepared and bonded on the s o l i d d i s c s . A s e r i e s of percutaneous implant d e v i c e s w i t h three stem/flange r a t i o s (1:1, 1:2 and 1:4) was f a b r i c a t e d from three m a t e r i a l s (Dacron v e l o u r - s i l i c o n e , porous PU and porous m e t a l ) . The implants were t o t a l l y porous (coarse, 150-250 ym) w i t h 1.5 mm diameter holes d r i l l e d throughout the device. F i g u r e 6 shows the mold and the polyurethane percutaneous devices w i t h three stem/ flange r a t i o s and 1.5 mm diameter d r i l l e d h o l e s . D i s c s were implanted subcutaneously i n the abdomen and i n the t h i g h a g a i n s t abraded muscle. Vigorous t i s s u e ingrowth i n t o the caorse porous PU was observed, as shown i n Figure 7. The muscle t i s s u e could be seen to be separated only by a t h i n f i b r o u s t i s s u e sheath ( ~ 30 ym), demonstrating i t s b i o c o m p a t i b i l i t y . The cross s e c t i o n of one of the d r i l l e d d i s c s showed how the f i b r o u s t i s s u e through-growth helped anchor the s o f t t i s s u e even t o the nonporous s i d e of the i m p l a n t , as shown i n Figure 8. Percutaneous d e v i c e s were implanted d o r s a l l y along the spine i n three p i g s (Figure 9 ) . Gross examination of six-month implants showed that the porous metal d e v i c e w i t h the 1:2 f l a n g e had e x c e l l e n t s t a b i l i z a t i o n and formed the best anatomic s e a l and b a r r i e r t o b a c t e r i a . Due to t h e i r weight, metal d e v i c e s w i t h a l a r g e f l a n g e o r no f l a n g e sank beneath the s k i n p u l l i n g the s k i n down. However, one-year implants showed signs of t i s s u e n e c r o s i s a f t e r 9 months which l e d to i n f e c t i o n and e v e n t u a l r e j e c t i o n . The



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f a i l u r e has been a t t r i b u t e d t o the i n c o m p a t i b i l i t y of i n t e r f a c e compliance l e a d i n g t o c y c l i c f a t i g u e of the t i s s u e a t t i s s u e / m e t a l i n t e r f a c e as load t r a n s f e r i s t a k i n g place. Percutaneous PU devices showed e x c e l l e n t r e s u l t s i n threemonth i m p l a n t a t i o n s . However, evidence of mechanical breakdown of the polymer due t o vigorous t i s s u e ingrowth and load t r a n s f e r appeared i n the six-month implants. The polymer percutaneous devices were t o t a l l y porous. The pores were i n t e r c o n n e c t i n g v o i d s and the devices were d r i l l e d w i t h 1.5 mm holes throughout. Therefore, s t r u c t u r a l l y t h i s porous m a t e r i a l was not strong although i t s compliance matched w e l l w i t h the s o f t t i s s u e . As the load t r a n s f e r t a k i n g p l a c e due t o vigorous t i s s u e ingrowth, the f a t i g u e f a i l u r e was a t the polymer i n t e r f a c e r a t h e r than t i s s u e i n the metal implants. E a r l y r e j e c t i o n of Dacron v e l o u r / s i l i c o n e rubber implants i n l e s s than 2 t o 4 weeks was caused by i n f e c t i o n s and d e l a m i n a t i o n o f Dacron v e l o u r / s i l i c o n e rubber interface. From these i n - v i v o implant s t u d i e s , we l e a r n that the f o l l o w i n g f a c t o r s may c o n t r i b u t e to the success of a percutaneous device : •

Porous i n t e r f a c e w i t h l a r g e p o r o s i t y t o provide vigorous ingrowth of s o f t t i s s u e t o form an anatomic s e a l and a b a r r i e r to bacteria.

•

Holes d r i l l e d throughout the device t o promote s o f t t i s s u e through growth f o r f i x a t i o n and to prevent m a r s u p e a l i z a t i o n of the epidermal c e l l s .

•

Importance of flange/stem r a t i o and the weight of the device i n s t a b i l i z i n g and anchoring the percutaneous device.

•

Biomechanical compliance o r impedance matching of the t i s s u e / m a t e r i a l i n t e r f a c e i s not important f o r short-term implant experiments. However, f o r long implanting p e r i o d s , c y c l i c f a t i g u e f a i l u r e of the t i s s u e / m a t e r i a l i n t e r f a c e i s caused by compliance mismatching.

Conclusions. R e s u l t s from the b i o c o m p a t i b i l i t y s t u d i e s i n r a b b i t s u p r a t e l l a r bursa, measurement of h y d r o p h i l i c p r o p e r t i e s , l u b r i c a t i o n and wear i n - v i t r o s t u d i e s , d e t e r m i n a t i o n of v i s c o e l a s t i c p r o p e r t i e s , measurement of damping c o e f f i c i e n t and impact t e s t , t o t a l elbow j o i n t replacement design and i n - v i v o percutaneous implant experiment, a l l i n d i c a t e that t h i s s e r i e s of polyurethanes i s an e x c e l l e n t candidate b i o m a t e r i a l f o r the p r o s t h e t i c replacement of a r t i c u l a r c a r t i l a g e , a r t i f i c i a l j o i n t prostheses and percutaneous implantable devices. Further t e s t i n g , i n v i v o f a t i g u e and wear t e s t s and c l i n i c a l e v a l u a t i o n , are recommended.


31. WONG



Literature Cited 1.

Mirkovitch; V., Akutsu, T. and Kolff, W.J.; Polyurethane Aortas in Dogs - Three-Year Results, Trans. Am. Soc. Artif. Intern. Organs, 1962, 8, 79 .

2.

Sharp, W.V.; Gardner, D.L.; Andresen, G.J. A Bioelectric Polyurethant Elastomer for Intravascular Replacement, Trans. Am. Soc. Artif. Intern. Organs, 1966, 12, 1979 .

3.

Boretos, J.W.; Pierce, W.S. A Polyether Poylmer, J. Biomed. Mat. Res., 1968, 2, 121 .

4.

Boretos, J.W., "Conscise Guide to Biomedical Polymers, Their Design, Fabrication and Molding"; Charles C. Thomas Publisher, Springfield, IL, 1973; p. 10 .

5.

Lyman, D.J.; Loo, B.H. New Synthetic Membranes for Dialysis IV--A Copolyether Urethane Membrane System, J. Biomed. Mat. Res., 1967, 1, p. 17-26 .

6.

Lyman, D.J.; Brash, J.L.; Klein, K.G. The Effect of Chemical Structure and Surface Properties of Synthetic Polymers on Coagulation of Blood, Proceeding Artificial Heart Program USGPO, Washington, D.C., 1969, p. 113 .

7.

Nyilas, E. Development of Blood Compatible Elastomers, Theory, Practice and In-Vivo Performance, 23rd Conference on Engineering in Medicine and Biology, 1970, 12, 147 .

8.

Wong, E.W. Urethane-Polyether Block Copolymer Membranes for Reverse Osmosis, Ultrafiltration and Other Membrane Pro cesses, ORF Record of Invention No. 335, 1969.

9.

Pilliar, R.M.; Wong, W.E.; Black, J. Development of More Compatible Synthetic Articular Surfaces, Digest of 11th International Conference on Med. and Biol. Engineering, 1976, 492 .

10.

Furst, L.; Black, J.; Pilliar, R.M.; Wong, E.W. The Bio -Compatibility and Mechanical Properties of a Candidate for Articular Cartilage Replacement, Trans. of the 4th Annual Meeting Society of Biomaterials, 1978, 2, 159 .

11.

Parsons, J.R.; Black, J. The Viscoelastic Shear Behavior of Normal Rabbit Cartilage, J. Biomechanics, 1977, 10, 21 .

12.

Medley, J.B.; Strong, A.B.; Pilliar, R.M.; Wong, E.W. The Breakdown of Fluid Film Lubrication in Elastic Isoviscous Point Contacts, Wear, 1980, 63, 1 , 25 .


503


504


13.

Keinpson, G.E. Mechanical Properties of Articular Cartilage, "Adult Articular Cartilage" Edited by M.A.R. Freeman, Pitman, London, 1973.

14.

Dowson, D, Modes of Lubrication in Human Joints, Proc. Inst. Mech. Engrs., 1966, 181, 3J, 45.

15.

Joyce, G.C.; Rack, P.M. The Effects of Load and Force on Tremor at the Normal Human Elbow Joint, J. Physiol. London, 1974, 240, 2, 375 .

16.

Ewald, F.C. Total Elbow Replacement Orthop. Clinics, North Am., 1975, 6, 3, 685 .

17.

Wong, E.W.; White, R.C. Development of a Stock Absorbing Biomedical Elastomer for a New Total Elbow Replacement Design, Biomat. Med. Dev., Art. Org., 1979, 1, 2, 283 .

18.

Kinsler, L.E. Fundamentals of Accoustics, 2nd Ed., Wiley & Sons, New York, 1962 (p. 43).

19.

Fernie, G.R.; Kostwik, J.P.; Lobb, R.J.; Pilliar, R.M.; Wong, E.W.; Bennington, A.G. A Percutaneous Implant Using a Porous Metal Surface Coating for Adhesions to Bone and a Velour Covering for Soft Tissue Attachment, Results of Trials in Pigs, J. Biomed. Mat. Res., 1977, 11, 883 .

20.

Wong, E.W.; Fernie, G.R.; Pilliar, R.M.; Bennington, A.G. A Percutaneous Implant Experiment Using Porous Coatings for Soft Tissue Attachment: A Preliminary Report, accepted for presentation at International Congress of Implantology and Biomaterials in Stomatology, June 9-12, 1980, in Kyoto, Japan.

RECEIVED April 30,

1981.


Development of a Biomedical Polyurethane - ACS Symposium Series

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