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BIOMAT., MED. DEV., ART. ORG., 7(2),
243-255 (1979)
IMPLANT FAILURES IN ORTHOPAEDIC SURGERY Leslie J. Harris, M.D. and Richard R. Tarr, M.S. Department of Orthopaedics University of Southern California Los Angeles, California ABSTRACT Common orthopaedic implant failures are reviewed in the areas of total joint replacement and fracture fixation. In particular total hip and total knee arthroplasty, intertrochanteric hip fractures and long bone fractures are discussed. Excessive motion of implant bone interfaces, stress concentrations within the implant and stress shielding of bone are implicated in implant failures. INTRODUCTION Orthopaedic implants fall conveniently into two major categories; joint replacement and fracture fixation devices. Some of the current ortho-biomaterials will be reviewed from the standpoint o f comnon implant failures. The scope of this paper allows a review of only a few highlights of both implant areas. Total Hip Arthroplasty Failures of total hip arthroplasty have become a fashionable topic for discussions in recent years, with femoral stem breakage Most of these fai-
attracting considerable
lures occur in fatigue, with the initial crack located at the an243 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 b y any information storage and retrieval system, without permission in writing from the publisher.
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terolateral surface of the stem. The crack progresses medially in a low cycle fatigue mode and then by rapid breakage.* The incidence of stem failure appears to be in the neighborhood of 0.5% and have occurred in stems manufactured from cast cobal t-chrome alloy, cast stainless steel, wrought stainless steel in the annealed and the fully cold worked states.
Stem breakages have stimu-
lated the study and use o f alloys with better fatigue properties. MP35N,
HIP cobalt-chrome and FHScobalt-chrome alloys have relative-
ly recently emerged and offer improved fatigue strengths and yield strengths.
Titanium 6-4 also has only recently been used for femo-
ral prosthetic components.
Femoral stem fractures using these
newer materials have not been reported to my knowledge, although the follow-up is relatively short. Several workers suggest that most, if not all stem breakages, are preceeded by cement failure and stem
X-ray evi-
dence of loosening occurs in 1.5% to 17% of patients. 6 926 3 2 ’
Many
of these patients are asymptomatic although the potential for progressive loosening and the development of pain definitely exists with longer term follow-up. The acrylic cement layer is sandwiched between two stiffer materials; its elastic modulus is ten times lower than cortical bone and one to two hundred times lower than the metal stem.
Since
the acrylic cement is relatively brittle, it is the weakest link in the system. There is little reason to expect that the use of stronger and stiffer alloys for prosthetic stems will reduce the incidence of cement failure and loosening.
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In general the f i r s t x-ray signs of cement f a i l u r e a r e f r a c -
ture of the d i s t a l p l u g of cement followed by, o r concurrent with, a radiolucent l i n e t h a t forms over t h e superior l a t e r a l p a r t of the The stem has, t h e r e f o r e , s e t t l e d w i t h i n the cone.
stem.
We know
from Niederer t h a t w i t h s e t t l i n g o f a s much as 0.5mm, hoop s t r e s s e s a r e s u f f i c i e n t t o rupture t h e wall of t h e cement. I / Presumably then, f o r looseninq t o occur, a longitudina rupture must t a k e place i n t h e cement cone which i s not appreciated on standard x-rays.
There a r e several f a c t o r s t h a t a r e imp icated in causing cement f a i l u r e . 1.
Cement Voids. a.
The removal of bone d e b r i s and blood from t h e femoral
canal i s important i n avoiding d i s c o n t i n u i t i e s in the cement.’
The
use of a d i s t a l canal plug and i n j e c t i o n of the cement under press u r e serve t o reduce t h e incidence of voids. b.
F i t of t h e stem within t h e canal.
The dimensions
and design of t h e stem influence t h e a b i l i t y t o obtain an i n t a c t sheath of cement around t h e stem.
In g e n e r a l , s m a l l e r , s t r a i g h t e r
stems a r e more e a s i l y encased i n a continuous l a y e r o f cement. There is an obvious trade-off necessary between the s i z e and strength of t h e stem and t h e a b i l i t y t o encase t h e stem i n a continuous i n t a c t layer of cement.
The proximal a n t e r i o r portion of
t h e cement wall i s p a r t i c u l a r l y prone t o voids due t o t h e a n t e 17 version of t h e proximal femur. c.
Orientation of t h e stem.
A varus position may r e -
s u l t i n voids medially a t t h e c a l c a r region and l a t e r a l l y a t t h e d i s t a l stern.
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2.
Excessive Localized Stresses on the Cement. a.
Varus orientation of the stem maximizes compressive
forces on the cement medially at the calcar and tensile forces over the lateral aspect of the stem. The use of relatively stiff materials may be responsible for concentrated stresses over the distal aspect of the cone of cement and the corollary to this may be stress shielding of proximal areas; notably the calcar region. Stress shielding has been implicated as a cause of calcar resorption, which was noted in 42% of Charnley’s patients followed ten years. I
With loss of medial support from calcar resorption medial
cement failure results in a cantilever type of loading which would lead to further rupture of the cement cone. Oh and Harris have recomnended a large stem collar to more effectively distribute the loading forces from the stem to the calcar region.18 Niederer has proposed the ECM structured Protasul stem. The interlocking between stem and cement would aid in distributing loading forces throughout the cement cone and resist settling and consequent rupture of the cone.”
Another possible solution to stress shielding
i s the use of a material with a lower modulus of elasticity such as
the titanium 6-4 alloy. A recent finite element study indicated that 30% higher normal stresses occur over the proximal medial femoral cortex using a titanium 6-4 component than when using a 22
cobalt-chrome stem of the same design. 3.
Poor Cement Ffxation Into Bone. Areas of cement cone
which do not have bony support will be more likely to develop critical hoop stresses and fail. Efforts to obtain a clean bony bed
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and i n j e c t the cement under pressure (perhaps a r a t h e r l i q u i d phase) a r e aimed a t forcing cement i n t o t h e i n t e r s t i c e s of t h e bone and obtain mechanical interlocking and continuous bony support about t h e e n t i r e cement cone. The incidence of acetabular cup loosening was found t o be about 9% i n p a t i e n t s followed a t l e a s t ten y e a r s by Charnley and about 70% of his p a t i e n t s show a radiolucent margin between t h e cement and bone."
The concern i s t h a t a g r e a t e r number of t h e s e
p a t i e n t s w i t h radiolucent l i n e s may have loosening w i t h longer follow-up.
One of t h e suggested reasons f o r t h e higher incidence
o f lucent demarcations about t h e cemented cup than around the ce-
mented stem i s the more e l a s t i c nature of t h e cancellous bone
-
cement - u l t r a high molecular weight polyethylene system, compared t o the l e s s e l a s t i c femoral component system.
The polyethylene cup
may be more subject t o micro motion w i t h loading f o r c e s than t h e femoral side.
Another d i f f e r e n c e is t h a t t h e cement cannot be
forced i n t o t h e acetabular bone w i t h nearly a s much pressure a s t h a t which can be obtained i n t h e intramedullary canal of the f e -
mur.
In addition t o the e l a s t i c i t y of cancellous bone and poly-
ethylene, technical f a c t o r s which appear r e l a t e d t o cup loosening a r e the location and o r i e n t a t i o n of the cup and t h e a b i l i t y t o s e a t t h e cup and cement t o t a l l y w i t h i n bone.
24
Since the use of various metals i n combination w i t h UHMW polyethylene component i n t o t a l h i p a r t h r o p l a s t y , wear and tissue t o x i c i t y have not been s i g n i f i c a n t problems f o r up t o ten year follow7 ups.
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Total Knee Arthropl asty There are currently several hundred different total knee replacements comnercially available.
Only a few will be mentioned
with particular reference to loosening. Unlike the total h i p replacements, wear of the tibial polyethylene component may prove to be a significant factor as longer follow-ups are achieved.
1.
Hinge Prostheses.
Five year follow-ups of the Waldius
and Shiers prostheses demonstrate a 67-92% incidence of loosening. 15
A loosening rate of 6% was reported for the Guepar prosthesis, although the average follow-up was only two years.15
Characteris-
tically, the hinge prostheses show metal to metal wear of the axis pin.
The high incidence of loosening and pin wear demonstrates
that normal knee motion cannot be well approximated by a hinge. 2.
Constrained Non-hinged Prostheses. The Herbert prosthe-
sis was a metal-on-metal design which had a high incidence of breakage and has been taken off the market.
The Sheehan prosthesis
suffered from a design flaw in that a single set screw was used to unite the polyethylene component to the tibial metal stem across an air gap. The set screw was seen to fatigue in bending in 17% of a recent series. 23 3.
Partially Constrained Prostheses.
The incidence of ti-
bial component loosening in various series using these prostheses is about 10%.15
The geomedic and ICLH designs do not allow for
axial r ~ t a t i o n . ~''Cold flow" of the polyethylene and loosening o f the tibial component may be related to this axial constraint.
3
Otherwise the critical factor in loosening appears to be the surgeon's ability in obtaining accurate alignment of the prosthesis.
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I n an e f f o r t t o reduce problems w i t h c o l d f l o w , some d e s i g n s i n c o r p o r a t e t h e p o l y e t h y l e n e components i n a m e t a l t r a y .
Graphite
f i b e r r e i n f o r c e d p o l y e t h y l e n e has been shown t o r e s i s t c o l d f l o w t o a g r e a t e r degree t h a n t h e standard UHMW p o l y e t h y l e n e . '
There
i s a l s o some suggestion t h a t r a d i a t i o n induced c r o s s l i n k i n g on t h e s u r f a c e o f UHMW p o l y e t h y l e n e may r e s t r i c t c o l d f l o w . S i g n i f i c a n t wear o f t h e UHMW p o l y e t h y l e n e component has been f r e q u e n t l y observed i n those i m p l a n t s which r e q u i r e r e v i s i o n .
Un-
l i k e t h e p o l y e t h y l e n e component o f t o t a l h i p replacements, t h e t i b i a 1 component i s an i n v e r t e d cup which may c o l e c t wear d e b r i s and m e t h a c r y l a t e fragments. wear r e s u l t s .
An a c c e l e r a t e d t h i r d body t y p e o f
Large p i t s and e x c o r i a t i o n s i n t h e p o l y e t h y l e n e sug-
g e s t t h i s t y p e o f wear a l t h o u g h f a t i g u e c r a c k i n g s i m p l i c a t e d where l o a d i n g s u r f a c e areas a r e r e l a t i v e l y small .*O
The r e s u l t i n g
p o l y e t h y l e n e d e b r i s has n o t been c l e a r l y shown t o i n i t i a t e a c e l l u l a r mechanism t o e x p l a i n l o o s e n i n g .
However, t h e f r i c t i o n a l f o r c e s
a s s o c i a t e d w i t h wear may i n c r e a s e cement-bone i n t e r f a c e s t r e s s e s .
4.
Minimal o r "Unconstrained" Prostheses. The i n c i d e n c e o f
l o o s e n i n g w i t h these prostheses appears t o be s i m i l a r t o t h e p r e v i o u s group o f p a t i e n t s and again i t appears t h a t o b t a i n i n g accur a t e a l i g n m e n t may be a major l i m i t i n g f a c t o r .
The r a d i o l u c e n t
margin which appears around t h e cement c o n t a i n i n g t h e p o l y e t h y l e n e components i n p a r t i a l l y as w e l l as u n c o n s t r a i n e d p r o s t h e s e s seems t o occur i n 50 t o 70% o f these p a t i e n t s and appears i d e n t i c a l t o t h e r a d i o l u c e n t area t h a t forms around t h e a c e t a b u l a r a c r y l i c cement o f t o t a l h i p replacements.
has shown w i t h f i n i t e Lewis l6
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element model that increasing the polyethylene tibial component thickness by a factor of two decreases cement-bone interfacial stresses significantly. In this study it was also shown that a metal tibial plate resulted in a three to five times lower interfacial stress than a polyethylene plate. The implication is that increasing the stiffness of the tibial component by using thick polyethylene components or by seating the polyethylene in a metal tray reduces cement-bone normal and shear stresses and hopefully will reduce the incidence of loosening. A similar type of analysis for the acetabular component of total hips has not been done to my knowledge. Orthopaedic Implants Used for Fracture Fixation The various stainless steels and cobal t-chromium-molybdium a1 loys have been the mainstay for the manufacturing of orthopaedic fracture fixation devices. A problem at the outset in discussing fracture fixation is that the ideal conditions for fracture healing are yet to be defined. Tn recent years there has been a mounting challenge to the dogma of anatomical reduction and rigid fixation. For the sake of discussion I would propose that ideal fracture fixation should include the following: 1) The implant should maintain alignment at the fracture site within variable tolerances, depending upon the location of the fracture. 2 ) The fixation should allow early mobilization of the patient. 3 ) Within these limits, physiologic forces should be transmitted acros; the fracture interfaces.
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Over the past fifty years a large number of devices have been designed for the fixation of intertrochanteric fractures and implant failures have been reported in up to 25% of cases.12 Most of these implants have been manufactured from forged stainless steel or cast cobalt-chrome alloys. Failure often occurs at the nail-plate junction. Bending or breakage of the nail or plate portions of the implant, failure o f the screws, or cutting out of the implant through the head and neck are also seen. The major problem with these devices appears to be one of design in that loading forces are concentrated over a small area of the implant and not transmitted across the fracture interfaces. The various sliding devices have attacked this problem somewhat although the implant may still cut-out through the osteoporotic femoral head. It does not seem likely that the use of stronger materials would overcome the design problems with these devices. A newer intramedullary implant design manufactured from the relatively elastic titanium 6-4 alloy shows some promise in fixation of unstable intertrochanteric fractures.13 Subtrochanteric fractures have presented even greater problems of fixation. The use of standard nail-plate devices has resulted in implant failure rates of 40 to 60%.19 Breakage of the side plate comnonly occurs with nonunited fractures. Although it appears unlikely that any device would survive indefinitely in the face of a nonunion, the device may be implicated as a cause of the nonunion in that it may shield the fracture area from stresses which appear to be beneficial to fracture healing.4 The use of a
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more e l a s t i c m a t e r i a l w i t h a broader e l a s t i c range m i g h t improve
I
A similar
t h e r e s u l t s w i t h t h i s p a r t i c u l a r design of implant.
d i s c u s s i o n could be c a r r i e d o u t f o r t h e use o f r i g i d p l a t e s on femoral and t i b i a 1 s h a f t f r a c t u r e s . To some e x t e n t t h e use o f i n t r a m e d u l l a r y n a i l s f o r l o n g bone f r a c t u r e s avoids t h e problem o f s t r e s s s h i e l d i n g .
Despite t h e i r
r a t h e r massive s i z e and r i g i d i t y , t h e Z i c k e l and Sampson n a i l s a l l o w a x i a l impaction o f f r a c t u r e fragments w i t h p h y s i o l o g i c loadi n g forces,
However, r e c e n t animal e x p e r i m e n t a t i o n w i t h intramed-
u l l a r y n a i l s o f v a r y i n g e l a s t i c i t y suggests t h a t r e l a t i v e l y s t i f f devices may be r e s p o n s i b l e f o r s t r e s s s h i e l d i n g and l e s s e f f e c t i v e remodeling of bone.5
One wonders whether t h e use o f more e l a s t i c
m a t e r i a l s f o r i n t r a m e d u l l a r y i m p l a n t s m i g h t r e s u l t i n f a s t e r and mechanically s t r o n g e r bone heal i n g . R i g i d p l a t e s used on l o n g bone f r a c t u r e s have been shown t o r e s u l t i n marked osteoporosis and m e c h a n i c a l l y weak bone h e a l i n g .
4’30y14y27 A f t e r removal o f p l a t e devices many examples o f f r a c t u r e through t h e r e s u l t a n t o s t e o p o r o t i c bone have been seen. There i s mounting evidence t h a t t h e use o f more e l a s t i c m a t e r i a l s such as t h e t i t a n i u m 6-4 a l l o y o r t h e g r a p h i t e - m e t h a c r y l a t e comp o s i t e m a t e r i a l s may r e s u l t i n l e s s s t r e s s s h i e l d i n g and more e f f i c i e n t h e a l i n g o f bone. 4910914,27
A t t h e o p p o s i t e extreme t o o
l i t t l e f r a c t u r e f i x a t i o n may a l s o r e s u l t i n nonunion.
For ex-
ample, t h e use o f t h i n f l e x i b l e p i n s as i n t r a m e d u l l a r y devices f o r t h e f i x a t i o n of forearm f r a c t u r e s h i s t o r i c a l l y y i e l d e d a 14 t o 17% nonunion rate.21
The i d e a l i m p l a n t f o r f r a c t u r e f i x a t i o n would
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IMPLANT FAILURES I N ORTHOPAEDIC SURGERY
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appear t o be a happy compromise between r i g i d f i x a t i o n w i t h i t s consequent s t r e s s s h i e l d i n g and i n s u f f i c i e n t f i x a t i o n w i t h r e s u l t a n t nonunion.
SUMMARY The c h a l l e n g e i n o r t h o p a e d i c t o t a l j o i n t as w e l l as f r a c t u r e f i x a t i o n s u r g e r y appears t o be t h e development o f d e s i g n s and m a t e r i a l s which w i l l r e s u l t i n n e a r l y p h y s i o l o g i c a l t r a n s m i s s i o n o f s t r e s s e s across i m p l a n t t o bone i n t e r f a c e s , and bone t o bone fracture interfaces.
S t r e s s c o n c e n t r a t i o n s w i t h i n t h e i m p l a n t and
s t r e s s s h i e l d i n g of bone appear t o r e s u l t i n i m p l a n t f a i l u r e s and bony r e s o r p t i o n r e s p e c t i v e l y .
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T o t a l p r o s t h e t i c replacement o f t h e h i p 1977, I n Proceedings o f Workshop on T o t a l J o i n t Replacement, Northwestern U n i v e r s i t y , 46, 1977.
27.
S. Woo e t a l : A comparison o f c o r t i c a l bone a t r o p h y secondary t o f i x a t i o n w i t h p l a t e s w i t h l a r g e d i f f e r e n c e s i n bending s t i f f n e s s , J. Bone and J o i n t Surg., 50-A: 190, 1976.
BIOMAT., MED. DEV., ART. ORG., 7(2),
243-255 (1979)
IMPLANT FAILURES IN ORTHOPAEDIC SURGERY Leslie J. Harris, M.D. and Richard R. Tarr, M.S. Department of Orthopaedics University of Southern California Los Angeles, California ABSTRACT Common orthopaedic implant failures are reviewed in the areas of total joint replacement and fracture fixation. In particular total hip and total knee arthroplasty, intertrochanteric hip fractures and long bone fractures are discussed. Excessive motion of implant bone interfaces, stress concentrations within the implant and stress shielding of bone are implicated in implant failures. INTRODUCTION Orthopaedic implants fall conveniently into two major categories; joint replacement and fracture fixation devices. Some of the current ortho-biomaterials will be reviewed from the standpoint o f comnon implant failures. The scope of this paper allows a review of only a few highlights of both implant areas. Total Hip Arthroplasty Failures of total hip arthroplasty have become a fashionable topic for discussions in recent years, with femoral stem breakage Most of these fai-
attracting considerable
lures occur in fatigue, with the initial crack located at the an243 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 b y any information storage and retrieval system, without permission in writing from the publisher.
HARRIS AND TARR
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terolateral surface of the stem. The crack progresses medially in a low cycle fatigue mode and then by rapid breakage.* The incidence of stem failure appears to be in the neighborhood of 0.5% and have occurred in stems manufactured from cast cobal t-chrome alloy, cast stainless steel, wrought stainless steel in the annealed and the fully cold worked states.
Stem breakages have stimu-
lated the study and use o f alloys with better fatigue properties. MP35N,
HIP cobalt-chrome and FHScobalt-chrome alloys have relative-
ly recently emerged and offer improved fatigue strengths and yield strengths.
Titanium 6-4 also has only recently been used for femo-
ral prosthetic components.
Femoral stem fractures using these
newer materials have not been reported to my knowledge, although the follow-up is relatively short. Several workers suggest that most, if not all stem breakages, are preceeded by cement failure and stem
X-ray evi-
dence of loosening occurs in 1.5% to 17% of patients. 6 926 3 2 ’
Many
of these patients are asymptomatic although the potential for progressive loosening and the development of pain definitely exists with longer term follow-up. The acrylic cement layer is sandwiched between two stiffer materials; its elastic modulus is ten times lower than cortical bone and one to two hundred times lower than the metal stem.
Since
the acrylic cement is relatively brittle, it is the weakest link in the system. There is little reason to expect that the use of stronger and stiffer alloys for prosthetic stems will reduce the incidence of cement failure and loosening.
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In general the f i r s t x-ray signs of cement f a i l u r e a r e f r a c -
ture of the d i s t a l p l u g of cement followed by, o r concurrent with, a radiolucent l i n e t h a t forms over t h e superior l a t e r a l p a r t of the The stem has, t h e r e f o r e , s e t t l e d w i t h i n the cone.
stem.
We know
from Niederer t h a t w i t h s e t t l i n g o f a s much as 0.5mm, hoop s t r e s s e s a r e s u f f i c i e n t t o rupture t h e wall of t h e cement. I / Presumably then, f o r looseninq t o occur, a longitudina rupture must t a k e place i n t h e cement cone which i s not appreciated on standard x-rays.
There a r e several f a c t o r s t h a t a r e imp icated in causing cement f a i l u r e . 1.
Cement Voids. a.
The removal of bone d e b r i s and blood from t h e femoral
canal i s important i n avoiding d i s c o n t i n u i t i e s in the cement.’
The
use of a d i s t a l canal plug and i n j e c t i o n of the cement under press u r e serve t o reduce t h e incidence of voids. b.
F i t of t h e stem within t h e canal.
The dimensions
and design of t h e stem influence t h e a b i l i t y t o obtain an i n t a c t sheath of cement around t h e stem.
In g e n e r a l , s m a l l e r , s t r a i g h t e r
stems a r e more e a s i l y encased i n a continuous l a y e r o f cement. There is an obvious trade-off necessary between the s i z e and strength of t h e stem and t h e a b i l i t y t o encase t h e stem i n a continuous i n t a c t layer of cement.
The proximal a n t e r i o r portion of
t h e cement wall i s p a r t i c u l a r l y prone t o voids due t o t h e a n t e 17 version of t h e proximal femur. c.
Orientation of t h e stem.
A varus position may r e -
s u l t i n voids medially a t t h e c a l c a r region and l a t e r a l l y a t t h e d i s t a l stern.
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2.
Excessive Localized Stresses on the Cement. a.
Varus orientation of the stem maximizes compressive
forces on the cement medially at the calcar and tensile forces over the lateral aspect of the stem. The use of relatively stiff materials may be responsible for concentrated stresses over the distal aspect of the cone of cement and the corollary to this may be stress shielding of proximal areas; notably the calcar region. Stress shielding has been implicated as a cause of calcar resorption, which was noted in 42% of Charnley’s patients followed ten years. I
With loss of medial support from calcar resorption medial
cement failure results in a cantilever type of loading which would lead to further rupture of the cement cone. Oh and Harris have recomnended a large stem collar to more effectively distribute the loading forces from the stem to the calcar region.18 Niederer has proposed the ECM structured Protasul stem. The interlocking between stem and cement would aid in distributing loading forces throughout the cement cone and resist settling and consequent rupture of the cone.”
Another possible solution to stress shielding
i s the use of a material with a lower modulus of elasticity such as
the titanium 6-4 alloy. A recent finite element study indicated that 30% higher normal stresses occur over the proximal medial femoral cortex using a titanium 6-4 component than when using a 22
cobalt-chrome stem of the same design. 3.
Poor Cement Ffxation Into Bone. Areas of cement cone
which do not have bony support will be more likely to develop critical hoop stresses and fail. Efforts to obtain a clean bony bed
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and i n j e c t the cement under pressure (perhaps a r a t h e r l i q u i d phase) a r e aimed a t forcing cement i n t o t h e i n t e r s t i c e s of t h e bone and obtain mechanical interlocking and continuous bony support about t h e e n t i r e cement cone. The incidence of acetabular cup loosening was found t o be about 9% i n p a t i e n t s followed a t l e a s t ten y e a r s by Charnley and about 70% of his p a t i e n t s show a radiolucent margin between t h e cement and bone."
The concern i s t h a t a g r e a t e r number of t h e s e
p a t i e n t s w i t h radiolucent l i n e s may have loosening w i t h longer follow-up.
One of t h e suggested reasons f o r t h e higher incidence
o f lucent demarcations about t h e cemented cup than around the ce-
mented stem i s the more e l a s t i c nature of t h e cancellous bone
-
cement - u l t r a high molecular weight polyethylene system, compared t o the l e s s e l a s t i c femoral component system.
The polyethylene cup
may be more subject t o micro motion w i t h loading f o r c e s than t h e femoral side.
Another d i f f e r e n c e is t h a t t h e cement cannot be
forced i n t o t h e acetabular bone w i t h nearly a s much pressure a s t h a t which can be obtained i n t h e intramedullary canal of the f e -
mur.
In addition t o the e l a s t i c i t y of cancellous bone and poly-
ethylene, technical f a c t o r s which appear r e l a t e d t o cup loosening a r e the location and o r i e n t a t i o n of the cup and t h e a b i l i t y t o s e a t t h e cup and cement t o t a l l y w i t h i n bone.
24
Since the use of various metals i n combination w i t h UHMW polyethylene component i n t o t a l h i p a r t h r o p l a s t y , wear and tissue t o x i c i t y have not been s i g n i f i c a n t problems f o r up t o ten year follow7 ups.
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Total Knee Arthropl asty There are currently several hundred different total knee replacements comnercially available.
Only a few will be mentioned
with particular reference to loosening. Unlike the total h i p replacements, wear of the tibial polyethylene component may prove to be a significant factor as longer follow-ups are achieved.
1.
Hinge Prostheses.
Five year follow-ups of the Waldius
and Shiers prostheses demonstrate a 67-92% incidence of loosening. 15
A loosening rate of 6% was reported for the Guepar prosthesis, although the average follow-up was only two years.15
Characteris-
tically, the hinge prostheses show metal to metal wear of the axis pin.
The high incidence of loosening and pin wear demonstrates
that normal knee motion cannot be well approximated by a hinge. 2.
Constrained Non-hinged Prostheses. The Herbert prosthe-
sis was a metal-on-metal design which had a high incidence of breakage and has been taken off the market.
The Sheehan prosthesis
suffered from a design flaw in that a single set screw was used to unite the polyethylene component to the tibial metal stem across an air gap. The set screw was seen to fatigue in bending in 17% of a recent series. 23 3.
Partially Constrained Prostheses.
The incidence of ti-
bial component loosening in various series using these prostheses is about 10%.15
The geomedic and ICLH designs do not allow for
axial r ~ t a t i o n . ~''Cold flow" of the polyethylene and loosening o f the tibial component may be related to this axial constraint.
3
Otherwise the critical factor in loosening appears to be the surgeon's ability in obtaining accurate alignment of the prosthesis.
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I n an e f f o r t t o reduce problems w i t h c o l d f l o w , some d e s i g n s i n c o r p o r a t e t h e p o l y e t h y l e n e components i n a m e t a l t r a y .
Graphite
f i b e r r e i n f o r c e d p o l y e t h y l e n e has been shown t o r e s i s t c o l d f l o w t o a g r e a t e r degree t h a n t h e standard UHMW p o l y e t h y l e n e . '
There
i s a l s o some suggestion t h a t r a d i a t i o n induced c r o s s l i n k i n g on t h e s u r f a c e o f UHMW p o l y e t h y l e n e may r e s t r i c t c o l d f l o w . S i g n i f i c a n t wear o f t h e UHMW p o l y e t h y l e n e component has been f r e q u e n t l y observed i n those i m p l a n t s which r e q u i r e r e v i s i o n .
Un-
l i k e t h e p o l y e t h y l e n e component o f t o t a l h i p replacements, t h e t i b i a 1 component i s an i n v e r t e d cup which may c o l e c t wear d e b r i s and m e t h a c r y l a t e fragments. wear r e s u l t s .
An a c c e l e r a t e d t h i r d body t y p e o f
Large p i t s and e x c o r i a t i o n s i n t h e p o l y e t h y l e n e sug-
g e s t t h i s t y p e o f wear a l t h o u g h f a t i g u e c r a c k i n g s i m p l i c a t e d where l o a d i n g s u r f a c e areas a r e r e l a t i v e l y small .*O
The r e s u l t i n g
p o l y e t h y l e n e d e b r i s has n o t been c l e a r l y shown t o i n i t i a t e a c e l l u l a r mechanism t o e x p l a i n l o o s e n i n g .
However, t h e f r i c t i o n a l f o r c e s
a s s o c i a t e d w i t h wear may i n c r e a s e cement-bone i n t e r f a c e s t r e s s e s .
4.
Minimal o r "Unconstrained" Prostheses. The i n c i d e n c e o f
l o o s e n i n g w i t h these prostheses appears t o be s i m i l a r t o t h e p r e v i o u s group o f p a t i e n t s and again i t appears t h a t o b t a i n i n g accur a t e a l i g n m e n t may be a major l i m i t i n g f a c t o r .
The r a d i o l u c e n t
margin which appears around t h e cement c o n t a i n i n g t h e p o l y e t h y l e n e components i n p a r t i a l l y as w e l l as u n c o n s t r a i n e d p r o s t h e s e s seems t o occur i n 50 t o 70% o f these p a t i e n t s and appears i d e n t i c a l t o t h e r a d i o l u c e n t area t h a t forms around t h e a c e t a b u l a r a c r y l i c cement o f t o t a l h i p replacements.
has shown w i t h f i n i t e Lewis l6
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element model that increasing the polyethylene tibial component thickness by a factor of two decreases cement-bone interfacial stresses significantly. In this study it was also shown that a metal tibial plate resulted in a three to five times lower interfacial stress than a polyethylene plate. The implication is that increasing the stiffness of the tibial component by using thick polyethylene components or by seating the polyethylene in a metal tray reduces cement-bone normal and shear stresses and hopefully will reduce the incidence of loosening. A similar type of analysis for the acetabular component of total hips has not been done to my knowledge. Orthopaedic Implants Used for Fracture Fixation The various stainless steels and cobal t-chromium-molybdium a1 loys have been the mainstay for the manufacturing of orthopaedic fracture fixation devices. A problem at the outset in discussing fracture fixation is that the ideal conditions for fracture healing are yet to be defined. Tn recent years there has been a mounting challenge to the dogma of anatomical reduction and rigid fixation. For the sake of discussion I would propose that ideal fracture fixation should include the following: 1) The implant should maintain alignment at the fracture site within variable tolerances, depending upon the location of the fracture. 2 ) The fixation should allow early mobilization of the patient. 3 ) Within these limits, physiologic forces should be transmitted acros; the fracture interfaces.
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Over the past fifty years a large number of devices have been designed for the fixation of intertrochanteric fractures and implant failures have been reported in up to 25% of cases.12 Most of these implants have been manufactured from forged stainless steel or cast cobalt-chrome alloys. Failure often occurs at the nail-plate junction. Bending or breakage of the nail or plate portions of the implant, failure o f the screws, or cutting out of the implant through the head and neck are also seen. The major problem with these devices appears to be one of design in that loading forces are concentrated over a small area of the implant and not transmitted across the fracture interfaces. The various sliding devices have attacked this problem somewhat although the implant may still cut-out through the osteoporotic femoral head. It does not seem likely that the use of stronger materials would overcome the design problems with these devices. A newer intramedullary implant design manufactured from the relatively elastic titanium 6-4 alloy shows some promise in fixation of unstable intertrochanteric fractures.13 Subtrochanteric fractures have presented even greater problems of fixation. The use of standard nail-plate devices has resulted in implant failure rates of 40 to 60%.19 Breakage of the side plate comnonly occurs with nonunited fractures. Although it appears unlikely that any device would survive indefinitely in the face of a nonunion, the device may be implicated as a cause of the nonunion in that it may shield the fracture area from stresses which appear to be beneficial to fracture healing.4 The use of a
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more e l a s t i c m a t e r i a l w i t h a broader e l a s t i c range m i g h t improve
I
A similar
t h e r e s u l t s w i t h t h i s p a r t i c u l a r design of implant.
d i s c u s s i o n could be c a r r i e d o u t f o r t h e use o f r i g i d p l a t e s on femoral and t i b i a 1 s h a f t f r a c t u r e s . To some e x t e n t t h e use o f i n t r a m e d u l l a r y n a i l s f o r l o n g bone f r a c t u r e s avoids t h e problem o f s t r e s s s h i e l d i n g .
Despite t h e i r
r a t h e r massive s i z e and r i g i d i t y , t h e Z i c k e l and Sampson n a i l s a l l o w a x i a l impaction o f f r a c t u r e fragments w i t h p h y s i o l o g i c loadi n g forces,
However, r e c e n t animal e x p e r i m e n t a t i o n w i t h intramed-
u l l a r y n a i l s o f v a r y i n g e l a s t i c i t y suggests t h a t r e l a t i v e l y s t i f f devices may be r e s p o n s i b l e f o r s t r e s s s h i e l d i n g and l e s s e f f e c t i v e remodeling of bone.5
One wonders whether t h e use o f more e l a s t i c
m a t e r i a l s f o r i n t r a m e d u l l a r y i m p l a n t s m i g h t r e s u l t i n f a s t e r and mechanically s t r o n g e r bone heal i n g . R i g i d p l a t e s used on l o n g bone f r a c t u r e s have been shown t o r e s u l t i n marked osteoporosis and m e c h a n i c a l l y weak bone h e a l i n g .
4’30y14y27 A f t e r removal o f p l a t e devices many examples o f f r a c t u r e through t h e r e s u l t a n t o s t e o p o r o t i c bone have been seen. There i s mounting evidence t h a t t h e use o f more e l a s t i c m a t e r i a l s such as t h e t i t a n i u m 6-4 a l l o y o r t h e g r a p h i t e - m e t h a c r y l a t e comp o s i t e m a t e r i a l s may r e s u l t i n l e s s s t r e s s s h i e l d i n g and more e f f i c i e n t h e a l i n g o f bone. 4910914,27
A t t h e o p p o s i t e extreme t o o
l i t t l e f r a c t u r e f i x a t i o n may a l s o r e s u l t i n nonunion.
For ex-
ample, t h e use o f t h i n f l e x i b l e p i n s as i n t r a m e d u l l a r y devices f o r t h e f i x a t i o n of forearm f r a c t u r e s h i s t o r i c a l l y y i e l d e d a 14 t o 17% nonunion rate.21
The i d e a l i m p l a n t f o r f r a c t u r e f i x a t i o n would
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IMPLANT FAILURES I N ORTHOPAEDIC SURGERY
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appear t o be a happy compromise between r i g i d f i x a t i o n w i t h i t s consequent s t r e s s s h i e l d i n g and i n s u f f i c i e n t f i x a t i o n w i t h r e s u l t a n t nonunion.
SUMMARY The c h a l l e n g e i n o r t h o p a e d i c t o t a l j o i n t as w e l l as f r a c t u r e f i x a t i o n s u r g e r y appears t o be t h e development o f d e s i g n s and m a t e r i a l s which w i l l r e s u l t i n n e a r l y p h y s i o l o g i c a l t r a n s m i s s i o n o f s t r e s s e s across i m p l a n t t o bone i n t e r f a c e s , and bone t o bone fracture interfaces.
S t r e s s c o n c e n t r a t i o n s w i t h i n t h e i m p l a n t and
s t r e s s s h i e l d i n g of bone appear t o r e s u l t i n i m p l a n t f a i l u r e s and bony r e s o r p t i o n r e s p e c t i v e l y .
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I. 6. Delce, 3. Charnley: Radiological demarcation of cemented sockets in total h i p replacement, Clin. Orthop., 121:20, 1976.
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L.J. Harris: Condylocephalic nailing of intra and subtrochanteric fractures of the femur, presented at the 1978 Annual Meeting of the A.A.O.S.
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D.B. KettelkamD: Knee imolants. In Froceedinss o f Wbrkshop on Total Joint' Replacement, Northwestern Univeisity, 62 , 1977. '
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F.A. Weber, J. Charnley: A r a d i o l o g i c a l s t u d y o f f r a c t u r e s o f a c r y l i c cement i n r e l a t i o n t o t h e stem o f a femoral head prost h e s i s , J. Bone and J o i n t Surq., 57-B:297, 1975.
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