Implant retrieval: Material and biological analysis

National Bureau of Standards, Gaithersburg, Maryland, USA, l-2 May 1980

This meeting combined the experiences of scientists and

clinicians who have been examining retrieved implants in

the cardiovascular and orthopaedic fields. The materials

under consideration included metals, ceramics, plastics and

materials of natural origin. Twenty-seven papers were

presented over the four sessions of the meeting and very

adequate time was allowed for discussion of each paper.

BULK PHENOMENA

The in vivo performance of polyethylene components of

total joint replacements, by R.M. Rose

The need for a high level of investigation was emphasised

in order to validate laboratory research. One observation,

in a large number of the prostheses, was the presence of

acrylic cement debris with severe abrasion. Damage was not

always due to acrylic cement; bone fragments had also been

observed. SEM studies of the moulding powder and molecular

weight studies of soluble fractions of material indicated that

polyethylene is an aggregate of partly fused granules, held

together by fibrils and if these are degraded, the integrity of

the structure is impaired; localized fracture may occur. In

discussion, comment was made that the interpretation that

the fibrils had a high molecular weight was dubious. If there

is incomplete fusion, then the fibrils will be drawn out from

the material as it contracts on cooling. The very low M.W.

of extractable material reported was also queried.

A biological and structural evaluation of retrieved Dacron

arterial prostheses, by R. Guidoin

After the initial pathological examination the prostheses

were cleaned and fabric, fibre and polymer properties were

then examined. The reasons for re-operation were

summarized and in the case of the prostheses recovered

at autopsy the reasons were given for the initial surgery.

No cause of death was directly related to the graft. The

patient and the surgeon appear to be more important for

the long-term success of grafts. To examine the material,

standard tensile tests were used; physical factors examined

were loss of crimp, destruction of knitted loops, stretching

or swelling of filaments. After initial filament swelling, some

appeared to lose diameter, possibly due to the solubilisa-

tion of surface material or to stretching. Many filaments

show cracking and fibrillation.

Clinical histo-morphological material related observations

on removed alumina-ceramic hip joint components, by

W. Plitz

This group now has five years in vivo experience and fifteen

cases have undergone removal of the prosthesis, either due

to fracture of the component in connection with the

conical attachment or due to wear between surfaces.

Since 1974, 12 000 prostheses have been inserted with

approximately 17 breakages. The experience of broken

prostheses could be related to bad fitting of the cone or to

bad surgical technique. No sample had been received of

acetabular socket failure. Every wear particle has a detri-

mental effect; the form and size of the particles is important.

Extensive flooding of the joint interior over-burdens the

body’s capacity to deal with the material and leads to tissue

loss and loosening. Grain excavation is always the starting

point. The fragment between two surfaces avalanches

further grain excavation accelerated by mechanical overload

exceeding inter or intra grain strength. Despite the manu-

facturer’s claims, these joints are not without wear, although

the number of failures has so far been minimal.

Analysis of retrieved bone cement - the effect of load

transfer, by D.F. Gibbons

He used techniques akin to metallography to examine

Simplex P. Gross deformation is seen in instances where

there was a contained compressive stress, but not at the

cement/prosthesis interface. Samples taken along the

prosthesis border show evidence of spheres and a faceted

appearance. The bone interface is sometimes smooth,

sometimes rough and porous. In certain cases bone

trabeculae were found in the cement matrix, which questions

observations of thermal necrosis and fibrous capsule

formations. Either the capsule has been replaced by viable

bone or else completely destroyed.

Prosthetic heart valves: what have we learned from our

clinical pathologic experience, by A.P. Yoganathan et al.

They had seen gross pathological changes in 200 valves and

tried to relate this to in vitro studies. The paper discussed

characteristics required of valves and then described

observations seen on retrieved Bjork-Shiley tilting disc

valves of which 95000 had been implanted since 1969. He

also discussed flow characteristics of the Starr-Edwards,

Smeloff-Cutter and St. Jude (pyrolytic carbon) valves. In

porcine valve leaflets (Hancock valve) gross calcification is

seen and creep in the polypropylene valve stents over two

years. In the Kay-Shiley mitral valve, thrombus is seen at

the base of the struts. Excess tissue around the base ring

occludes poppet movement. Grooving of discs is seen. He

emphasized that in vitro studies of valves are hydro-

dynamic, not haemo-dynamic studies.

Examination of orthopaedic implant failures, by

L M. Taussig

He summarized results of non-destructive testing of 110

cases and presented information on the types of implant

examined, the metal of which it was made and the type

of damage or failure seen. On hip stems, fluorescent pene-

trant examination was used and it was noted that the

Biomaterials 1980, Vol 1 July 169

possibility of litigation prevented destructive testing. He

differentiates between fatigue and corrosion fatigue and

emphasized that stress corrosion cannot occur in

stainless steel.

An analysis of inclusions in retrieval implants and their effect on corrosion, by R. Wilkinson

Corrosion was assessed by a standard series of SEM photos

and a type of corrosion index was used. Inclusions were

examined by energy dispersal analysis. Calcium and

aluminium inclusions were strongly related to corrosion,

but other sources of the inclusions were (a) retained

deoxidant, (b) refractory materials, (c) raw material

impurities, (d) pre-formed inclusions not dissolved in the

melt. Retrieval studies should show if it is worth improving

the quality of surgical steels with high technology methods.

RELEASE PHENOMENA

Biological response to wear debris in total joint replacement, by Professor H. Willert

552 tissue samples had been examined, taken mainly from

hip revisions and the implants included stainless steel, a

range of cobalt-based alloys and titanium alloy (AI-V),

polyethylene, PTFE, polyester, polyacetals, silicone rubber,

alumina and bone cement. The usual experience is that a new

capsule is formed, but this combined with foreign body

reactions. The tendency is towards fibrosis in the scar

tissues. Capsular tissue transports phagocytosed particles

away and establishes an equilibrium situation which is

optimized when wear is unavoidable. The bigger the granulosis,

the more likely is necrosis. The smallest particle sizes are

seen with metal/polyethylene components; more are seen

around metal bearings e.g. in knee and elbow prostheses.

Polyethylene, against cobalt/chromium/molybdenum cast

alloy and cobalt/chromium/molybdenum/nickel forged

alloy, show the slowest wear, even alumina is abraded and

particles are incorporated into the capsule and at the bone-

cement interface. He concluded that the intensity of the

reaction and the amount of foreign body incorporated is a

measure of the wear. The nature of the reaction indicates a

compensation. Necrosis is also a sequence of excessive foreign

body reactions. No changes were classified as characteristic of

allergic, Equilibrium achieved is optimum as long as it is not

possible to remove wear processes. Excess leads to de-

compensation and large areas of necrosis. These impede

phagocyte action and then tissue of bone becomes involved

leading to adsorption and loosening. Interference with

lubrication of the joint and with articulation occurs. Poly-

ethylene/metal combinations still seem to be the best.

Materials degeneration causing late failure of mechanical heart valve prostheses: problemsandpromises, by F.J. Schoen

He reported on the occurrence of degradation dysfunction

in valve patients. In a pathological series 19% of the deaths

had been due to prosthesis degeneration occuring 60+

days post-op. Before 1964 these were mainly silicone balls;

changes in the silicone rubber ball poppet had been observed,

but this was a patient factor not picked up in laboratory

testing. With disc valves, abrasive wear of the disc is seen and

fragments are found in the kidneys; the valve may be

incompetent. He reviewed the causes of late death, noting

that replacement of heart valve does not necessarily cure

the disease process. He emphasized the need for routine

examination of all valves, not just those which had failed,

because it yielded information which was not available by

laboratory tests. He considered that it was more likely that

one would see a patient dying with the prosthesis, rather

than due to the prosthesis.

Systemic effects of prosthetic valve wear debris, by J.M. J. M. Anderson

The author suggested that this was not a catastrophic

failure, but something that goes on over a long period.

Reviewing materials used for valve components, the features

observed included disc variability with strut wear, sewing

ring wear, fabric embolization, grooves worn in discs,

blocking up of the disc leading to mitral regurgitation

associated with fabric wear around valve seats. These were

for Beall valves. Wear particles are found in various parts;

foreign body granuloma cells in spleen and liver suggest the

possibility of liver biopsy to determine variability in heart

valves and the use of electrocardiography to detect changes.

Metal allergy and Metallurgy, by S.A. Brown

Metal ions in vivo act as haptens. Skin testing is not

recommended to assess which patients are sensitized because

this itself can lead to sensitization. He then gave details of

the leucocyte migration inhibition test, which uses 10 cc

blood. The appropriate metal ion is introduced into the

incubation medium and migration is observed.

Techniques for biological assessment of implant retrieval analysis, by D. F. Williams

Dr. Williams said that the small number of failures indicates

the corresponding benefit to the patient. Retrieval analysis

is part of evaluating the cost effectiveness of the use of

implants but it must be of high scientific standard. An

examination must include the material, the pathology, the

clinical factors and biomechanical factors. Not only failure

examination but also the reasons for success are relevant. It

is necessary to look at the same material used in different

surgical applications and he compared the use of PTFE

in cardiovascular applications compared with its disastrous

use in joints. Tissue response is controlled by the rate of

release of wear particles rather than by their chemistry (see

paper by Willert). A complication is that pure metals are

not used. He then discussed histological techniques involving

the use of specific stains and masking techniques using

reflected as well as transmitted light. Highly pure metals

have been implanted into rat muscle and various tissue

structures have been identified in the surrounding capsule.

Enzyme histochemistry and neutron activation analysis

have been used. He emphasized that the aim of this work

was to improve patient care and not to apportion blame for

failure.

INTERFACE PHENOMENA

Retrieval analysis of calcific degeneration of prosthetic valves, by R.J. Levey

This study involved the determination of y-carboxyglutamic

acid, a calcium binding component in protein which is formed

from glutamic acid by the action of a vitamin K-dependent

carboxylase. Calcification of the valves appears to be an

age-related complication observed in children.

Prosthesis-related infections, by P.J. Parks

This paper dealt with vascular related infections and surveyed

the various cases that had been observed. In some cases

associated with vascular devices, unusual staphylococcal or

streptococcal organisms had been detected. It seems that

these are opportunistic organisms and also those which are

seen in patients undergoing immunosuppressive therapy.

170 Biomaterials 1980, Vol 1 July

He proposed that at the implant site there may be localized

immuno-suppressive reaction.

Study of 208 explanted bio prostheses: distribution of

dysfunction patterns and assessment of durability at ten

years of use, by R.C. Ouijano

He presented the protocol used for the examination of

removed Hancock valves. Reasons for removal include

endocarditis and extrinsic mineralization. In this process

the extension of the mineralized areas eventually disrupts

the valve leaflets. Intrinsic mineralization is also seen in

which calcium nodules are seen sub surface. This rises from

an infiltration of the leaflets by plasma proteins and

eventually these concentrates give fibrous forms and

mineralize. Examination of collagen taken from different

sites of the leaflets over periods of implantation show

changes in the collagen pattern. There is also change in

collagen shrinkage temperature.

Retrieved fracture plates; implant and tissue analysis, by

J. P. Simpson

Biopsies taken over the two proximal screws were used to

determine tissue concentrations of various metallic ions by

flame a.a.s. and the actual corrosion on the plate was

assessed in terms of extent and nature. He showed that steel

was more highly damaged than titanium, some cobalt based

alloys were also found to be severely attacked. Steel and

titanium in combination behave as two separate metals.

Clinically, fewer symptoms are seen from titanium, though

histologically, titanium produces more debris.

Corrosion, materials characteristics and local tissue reaction

associated with osteosynthesis devices, by H. Skinner

Mechanical tests and metallographic examination of more

than 100, mainly stainless steel, implants was reported.

With regard to corrosion, there was 100% evidence of

interfacial corrosion with steel, and intergranular corrosion

was seen even with low carbon steel. The severity of tissue

reaction was independent of material properties and time of

implantation, nor was it related to the reason for removal.

Evaluation of explanted vascular grafts, by S. Weinberg

Umbilical vein grafts were obtained over a 3-year period at

re-operation or following non-graft-related death. Surface,

mechanical, biochemical and morphological factors were

studied. He described the type of tissue reaction seen.

SEM studies of surfaces in the short term showed similarity

to that seen with other compatible materials. The structure

of the vessel did not appear to change after implantation

although histology showed compaction and some loss of

cellularity, and a benign reaction around the Dacron

sutures. Critical surface tension evaluation was carried out

and CST versus implant time plotted. 20-30 dynes/cm for

CST represents the zone of biocompatibility. Internal

reflection i.r. was used to identify the surface components

e.g. lipids, hydrocarbons and proteins. Lipid uptake is

patient- not implant-time-related. There was no neo-intima

build-up and the internal elastic laminar structure was

retained.

Prosthesis wear - clinical retrieval experience and laboratory

predictions, by 1. C. Clark

He reviewed the experience reported in the literature which

had indicated that wear of polyethylene was not a problem

in the hip. However, some centres now report wear and this

may be an emerging problem of longer term use. The

question of how to account for e.g. cup fracture, remains.

There are conflicting estimates on the effect of small and

large balls on the femoral component. The knee prostheses

appear to fail for reasons other than wear. Differences are

observed between conforming prostheses which have the

highest constraining force, and non-conforming which have

the highest contact stress. We have not really begun to

analyse the contact stresses involved. When polyethylene is

used as a convex component, the wear rate increases six

times, yet is acceptable when it is used as a patellar replace-

ment. He discussed future problems and the value of

composite materials; design should take into account the

relationship of conforming/loosening versus non-

conforming/wear-creep. He questioned the use of large

diameter hips and thin PE sockets.

INFORMATION UTILIZATION

An analysis of 130 removed total hips and knees, by R. Y. Hori

A bio-engineer was present at the time of surgery and

complete data on the patient and removal methods were

obtained. Cement fracture and bone PMMA interface failure

were the main factors with tibia1 components. Metal

components were usually firm. 3 out of 10 cracked cups in

for more than four and a half years were loose; three body

wear was associated with loose tibia1 components. Loose

acetabular cups were associated with bad wear. No

component failed due to wear alone. Loose geomedic knees

(tibia1 component) usually show anterior lip impingement.

Considerations concerning international standards for

accumulating and reporting of implant experience with

cardiac pacemakers, by R.C. Flink

Summarized the use of implant performance reports with

respect to patient management. An active system to get

information had been proposed using random samples.

This paper surveyed the progress being made in IS0

Committee.

The methods of implant retrieval analysis at the Hospital

for Special Surgery, by A. H. Burstein

This was an entertaining and informative lecture reviewing

their experience in examination of retrieved implants.

They were concerned as to how it affected patient handling

and implant improvement. Deeply grooved 32 mm cups are

design-deficient, lead to stress concentrations and are not

recommended for long term use. If a 32 mm cup is to be used,

then a thicker cup with lesser grooves should be employed.

Patella components have shown pits, burnishing and

flattening. No cases of excess wear or mechanical degradation

had been found. All are distorted into an ellipse, with the

major axis along the direction of the quadricept insertion.

Knee prostheses show surface degradation and craters

induced by service stresses exceeding the yield stress of

polyethylene. The thickness of the prosthesis is important:

it should be more than 7 mm thick. Where gross wear is

observed i.e. gross plastic deformation, particles are removed

and there is loosening. The primary mechanism is failure of

cancellous bone in the plateau, not an interface problem.

Excess stresses arise in cancellous bone due to the geometry

of the component and where it is loaded. The typical

example is a prosthesis well fixed that goes to failure. Pits

etc. are the symptoms of high loads. Eccentric loading may

increase stress in cancellous bone two and a half times.

Loads lead to bone remodelling and this then changes the

geometry. Plastic flow is also a problem and evidence is seen

of eccentric and unsymmetrical loadings. Changes in

Biomaterials 1980, Vol ? July 171

geometry lead to continuous changes in contact geometry

seen from wear pictures. Examples of distortions following

overload produced by mechanical factors are seen occuring

early on, before the bone adjusts to the new situation. It

is not just failure in the implant, this is too narrow a view,

but the whole system must be analysed. Hence, retrieval is

a step in the analysis of the whole performance of implants

and is very valuable for improvement of implant design.

The underlying concept for this meeting was presented

at the outset when it was pointed out that although

laboratory measuring methods can produce standardized data,

which are of value to the manufacturer of implant materials

and devices, the examination of retrieved implants gives the

ability to learn of the durability and performance of these

under actual service conditions. There is no other way by

which this information can be gained than by examination

of the consequences of human in vivo application, and this

message was constantly reiterated throughout the course of

the conference.

G .W. Hastings

British Orthopaedic Association Spring Meeting Canterbury, UK, 1 O-l 2 April 1980

Over 300 delegates were assembled for the biannual meeting,

held on this occasion at the University of Kent. Owing to

the size of the audience, all presentations were transmitted

live by audio-visual aids to an overspill lecture theatre

showing identical slides. Microphones and public address

systems were installed in each theatre allowing free

disussion between both audiences and the speakers.

The meeting was opened by the Pro-Vice Chancellor

of the University, Professor Maurice Vile, who welcomed

delegates from all countries and wished every success for

the following three days.

A wide range of papers were presented throughout

the meeting and the standard of presentation was high

throughout. A selection of films were shown each morning

simultaneously with the lectures which necessitated the

sacrifice of one of these activities in favour of the higher

priority. Sadly further sacrifices were involved in the

interests of refreshment which was provided continuously

rather than in formal breaks.

Summary of the first day The morning opened with two papers on low modulus

materials for internal fixation devices; namely carbon fibre

reinforced epoxies and titanium alloy. Whilst both papers

concerned the use of less rigid fixation, the objectives of

each were significantly different. The former material,

described by Mr. K.J. Tayton (Cardiff) was employed to

give stable reduction of the fracture throughout the early

healing period by periosteal callus, whilst the latter,

described by Professor H.K. Uhthoff (Ottawa) was used to

stimulate a minimal amount of callus, merely as an indicator

of the progression of ‘primary bone healing’ (cortical

remodelling). Both types of plate were expected to alleviate

the late effects of plate induced osteopoenia.

The use of a unilaterally applied external fixator for

the treatment of tibia1 diaphyseal fractures was described

by Mr. J. Kenwright (Oxford). Early use of such a system

was recommended and delayed union necessitating early

supplementary bone grafting was cited as the main

disadvantage.

The indications and complications of open or closed

intramedullary nailing of the tibia were then presented by

Mr. I.W.L. Bintcliffe (London). In 36 cases of which

172 Biomaterials 1980, Vol 1 July

approximately one third were opened at operation, all

cases united giving good functional results. Infection was

the main complication which was exacerbated by open

reduction.

A novel method of reconstruction of compound skin

and bone defects of the tibia was presented by Mr. R.

Sanders (London). This technique involves free flap transfer

of iliac crest and skin on the deep circumflex iliac artery.

The value of lateral stress radiographs in the demon-

stration of anterior instability of the ankle joint was

described by Mr. M. Glasgow (London). This paper was

complemented by Mr. I.M. Sarsam’s presentation concerning

the role of the anterior tibio fibular ligament in talar

rotation and in ankle fractures.

The final scientific paper of the morning concerned

the orthopaedic complications of renal transplantation and

was given by Mr. J.E. Nixon (London). Thirty out of one

hundred and eighty one transplant patients developed

orthopaedic complications over the six year study period.

The commonest and most serious problem was avascular

necrosis of bone. The treatment and pathology of these

patients were discussed.

To close the first morning’s activities, the Presidential

Address was given by Mr. D.L. Evans who has taken over the

post from Mr. W.J.W. Sharrard. Mr. Evans paid tribute to

the late Reginald Watson-Jones and reminded the audience

of his contribution both to orthopaedic surgery and to the

B.O.A. Mr. Evans closed his address with a recording of the

late Sir Reginald Watson-Jones, the effect of which can be

measured by the silence in which it was received.

The afternoon of the 10th took the form of informal

instructional/discussion groups on the subjects of back

pain and sciatica, rigidity in internal fixation, Tharies hip

surface replacement and surgical approaches to the hip

joint.

The instructional group on the significance of internal

fixation of fractures was led by Professor B. McKibbin

(Cardiff) who gave an informative address on the biology

of fracture healing and the influence of the rigidity of

fixation on the processes involved therein. Whilst the

exact mechanisms of periosteal and medullary callus

formation are not known, a greater understanding of the

functional relationship between these healing processes

and so called ‘primary bone healing’ appears to be developing.

J.S. Bradley