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BIOMAT., MED. DEV., ART. ORG., 7(2),
199-206 (1979)
PRE-CLINICAL TESTING OF ELASTOMERS AND ELASTOMERIC DEVICES by J.B. Koeneman, Materials Engineer LORD CORPORATION Erie, Pennsylvania
Introduction Prosthetic finger joint replacement has been studied since the mid-fifties.
The first finger
joint prosthesis widely used was that of Swanson in the late sixties.
Other early work was done by
Stefee with a sliding metal or plastic prosthesis with dacron-covered stems €or tissue attachment to the bone.
Recently, a number of sliding prostheses
have been introduced.
Relief of pain and correction
of deformities have usually been obtained with all of these devices; however, there has been a fracture problem with the silicone rubber devices and loosening and dislocation problems with the sliding p r o s theses.
Because of this apparent need, the technology
of the design and manufacture of elastomeric bearings'
199 Copyright 0 1979 hy Marcel Dekker, Inc All Rights Reserved Neither this work nor any part may be reproduced or transmitted in any form or by any means, electronic or mechanical. including photocopying, microfilming, and recording, or by any information storage and retrieval system, without permission in writing from the publisher
KOENEMAN
200
was applied to the development of a finger prosthesis.
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An elastomeric bearing used as an internal joint has the unique characteristics of stability and controlled motion without high restraining forces. It allows relatively rigid stems for fixation, controlled deformation to duplicate the natural biomechanics, controlled strains for higher reliability and predictability of joint fatigue characteristics, and no wear products.
Pre-clinical
testing lasted over three and a half years.
The
outline of the pre-clinical program is shown in Table I. Finger Biomechanics The forces existing across finger joints have been calculated by free body daigrams and compared to the more complicated calculations of Chao.* Listed in Table I1 are the loading requirements
TABLE I Pre-Clinical Test Program 'Design Specifications Kinematics Biomechanics 'Develop Design Concepts 'Design Analysis 'Materials Testing and Selection 'Prototype Construction ODevice Testing 'Cadaver Implantation
ELASTOMERS AND ELASTOMERIC DEVICES
201
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Load Design Criteria for Finger Prostheses* Load Type
Value 3 7 5 Newtons 150 Newtons 100 Newtons 0.1 Newton Meters
Compression Volar Subluxation Radial-Ulnar Dislocation Axial Twisting Moment *Pinch
chosen.
= 50
Newtons
The soft tissues around a joint will carry
some of these forces.
The normal flexion-extension
range of motion of a metacarpo-phalangeal (MCP) joint is 0 ' - 9 0 ° ,
with a range of 30'
in lateral flexion.
The range of motion of fingers with prostheses implanted is usually significantly less.
The maximum displace-
ment loading conditions which were used for design purposes are shown in Table 111. Design Characteristics The design shown in Figure 1 consists of metal stems sized to fit the medullary cavity of the phalange or metacarpal.
Connecting the stems is an elastomer
TABLE I11 Displacement Design Criteria for Finger Prostheses Moment Flexion Extension Radial-Ulnar
Range 80'
?loo
KOENEMAN
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202
Figure 1
which i s bonded t o t h e stems and t o t h e r i g i d c e n t r a l piece.
The purpose o f t h e i n s e r t i s t o p r o v i d e normal
f i n g e r kinematics, t o i p c r e a s e t h e a x i a l load-carryi n g c a p a b i l i t y , t o i n c r e a s e t h e l a t e r a l and s h e a r s t i f f n e s s e s , and t o p r o v i d e f o r a more even s t r a i n d i s t r i b u t i o n i n t h e elastomer.
F i n i t e element c a l c u l a -
t i o n s w e r e performed f o r v a r i o u s r u b b e r and m e t a l geo-
metries.
The f i n a l geometry f o r p r o t o t y p e t e s t i n g w a s
chosen based on t h e o v e r a l l s t i f f n e s s e s and s t r a i n d i s tribution results.
The f l e x i n g s t i f f n e s s was c a l c u l a t e d
t o be 0.025 Newton meters and measured t o be 0 . 0 2 0 7
203
ELASTOMERS AND ELASTOMERIC DEVICES
Newton meters.
Thus, the elastic joint is significantly
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less stiff than the natural passive joint (0.23 Newton meters).
The prosthesis is assembled in a mold which
locates the rigid pieces and has a cavity the s,hape of the desired rubber section.
The rubber is transferred
into the mold under pressure at the desired temperature by using a heated press.
The elastomer and adhesive are
cured for the required time in the press. Materials The rigid material selected for the prototype was annealed 6A1 4V Titanium alloy.
Numerous elastomers were
evaluated by coupon tensile and flex tests and flex tests of prototype finger prostheses.
The material with the
best characteristics was BIONTM, a synthetic polyolefin elastomer.3 The polymer backbone is a straight chain, saturated hydrocarbon totally resistant to oxidation and ozonation.
Thus, oxidative erosion, a major factor
in fatigue of unsaturated rubbers, is eliminated.
This,
in conjunction with its relatively low modulus, given it exceptional fatigue properties.
Table IV given some
comparative DeMattia flex life values.
Table V is a
summary of the biological test results which were performed at the Materials Science Toxicology Laboratories at the University of Tennessee.
Following molding, the
prosthesis is extracted to remove any extraneous sulfur and other curing agents from the elastomer.
KOENEMAN
204
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TABLE IV Flex Life of Various Polymers ASTM D430 DeMattia Test Machine Polymer Type
Cycles to Failure x
Oxypropylene Rubber Ethylene-Propylene-Diene Terpolymer Neoprene Styrene-Butadiene Rubber Natural Rubber BiomerR, Segmented Polyurethane BIONTM Silicone Rubber
lo6
10 14-15 6- 8 3-4 3- 4 18 300* 0.8
*No failure
TABLE V Summary of Biological Test Results BIONTM Rubber Tests Directly on Sample Tissue Culture-Agar Overlay Rabbit Muscle Implant Hemolysis Test
Very slight response Non-toxic Not significant
Tests on Extracts Tissue Culture-Agar Overlay Intracutaneous Test in Rabbits Systemic Toxicity in Mice Cell Growth Inhibition
Non-cytotoxic Non-irritating No deaths or adverse effects No significant growth inhibition
Testing A fixture was designed specifically for evaluating finger prostheses (Figure 2 ) .
Cables are threaded through
the blocks to flex the prosthesis in the same manner as
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ELASTOMERS AND ELASTOMERIC DEVICES
205
Figure 2
the tendons flex normal fingers.
Thus, the joint ex-
periences compression and subluxation forces as well as the strains due to rotation. The prostheses are immersed in a pseudo-extracellular fluid.' at 3 7 i ' C .
The solution is kept
The goal set for the prototypes was 10 x
cycles, with a flexing range of 80'.
lo6
One million of
those cycles were to be lateral flexing through 20'. Discussion One of the main causes of failure of internal prosthetic joints is loosening.
In general for sliding joints,
KOENEMAN
206
the greater the constraint, the greater the incidence
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of loosening.
The bonded elastomeric joint provides
stability and good kinematic function, but it is not
so precisely constrained to a prescribed locus so as to generate excessive forces in the fixation region.
In addition, the bonded elastomeric prosthesis produces no wear debris to accumulate over the years. has inherent shock absorption capabilities.
It also An ex-
tended pre-clinical testing program for this device has been described.
Controlled, limited clinical trials
are scheduled to begin soon.