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A tribological study of a series of retrieved

Accord knee explants

e.EIII;;iez*, J. Fisher*, D. Dow son*, S.A. Sampath, R. Johnson* and

.

*Department of Mechanical Engineering, University of Leeds, Leeds LS2 9JT, UK;

University Departm ent of Orthopaedics and Accident Surgery, U niversity of Liverpool,

Liverpool, UK; *Knee Research U nit, Arrowepark Hospital, U pton, Wirral, Merseyside,

L49 5PE, UK;

* *DePuy International Ltd., St Anthony s Road, Leeds LSll SDT, UK

ABSTRACT

A tribological s y of a series of 27 retriev ed Accord meniscal bearing knee joints has been carried out. Ihe roughness of

the articulating sufaces of the men al and the femoral componen ts was investigated, and the penetration into the

polyethylene meniscal comp onent was determined. There was generally little damage to the highly polished metallic

femoral components. A general polishing and smoothing of the ultra high molecular weight polyethylene meniscal

compon ents was observed, although there was some subsequent deterioration of

the

olyethylene surfaces in certain cases.

h e penetration rate of thefemoral componen ts into the menisci was low, and was found to be similar to that reportedfor

other m eniscal k nee joints.

Keywords: Tribology, retrieval, knee prostheses

Med. Eng. Phys., 1994, Vol. 16, 223-228, May

INTRODUCTION

The natural kn ee offers a complex range of move-

ments with 150 of motion in flexion-extension,

rotation, and translation in the anterior-posterior

direction. In the more widely used total condylar

artificial knees this range of movem ents is achieved

by having relatively low conforming tibia1 and

femoral components . This produces high stresses in

the contact. In certain knee designs the high cyclic

contact stresses can cause sub-surface fatigue and

structural failure o f the ultra high molecular weight

polyethylene (U HM WPE) compo nent3. In some

cases inadequate

mechanical properties and

poor quality UHM WPE have been acknowledged

as accelerating factorsk7. Sub-surface fatigue can

initiate several millimetres into the UHM WPE at the

position of the maxim um shear stress, with cracks

propagating to the surface causing com plete failure

and break up of the UHMW PE component. This

type of failure has been widely described as delamin-

ation in the literature8~g. In addition structural failure

can also initiate from stress concentrations arising

from the metal tibia1 support or tray, which can lead

to the complete breaking up of the polyethylene

compo nent. It is important to emphasize that this

mode of failure and wear is not primarily a surface

wear process, as it is not caused by relative motion of

the surface asperities, and it initiates in the bulk of the

material. In some total condylar knees it has been

possible to reduce the levels of contact stress that

cause structural failure, by increasing the conformity

Correspondence and reprint requests: Dr J.L. Hailey

0 1994 Butterworth-Heinemann for BES

1350-4533/94/03223-06

of the compon ents, but this can restrict the range of

movem ents an d impose biomechanical constraints.

The meniscal bearing knee offers an alternative

approach to overcoming the desi

ated with total condylar knees

p problems associ-

.

Memscal bearing

knees achieve a wide range of motion by allowing

articulation at both the femoro-meniscal and the

tibio-meniscal surfaces. Figure 7 shows an Accord

meniscal knee. The femoro-meniscal bearing surfaces

are conforming and cylindrical in shape which gives

rise to a large area of contact on both cond yles in a

correctly aligned knee. The tibio-meniscal surfaces

are conical in shape and allow rotation of the

polyethylene meniscal comp onent with respect to the

tibial tra

.

In most components the translation of the

menisca ly omp onent is constrained by a pin through

the apex of the truncated conical surface. Thu s the

Accord meniscal knee permits flexion-extension at

the femoro-meniscal bearing surface, and rotation at

the tibio-meniscal surfaces. This allows more highly

conforming bearing surfaces to be used and it has

been postulated that reduced contact stresses may

reduce the incidence of early structural failure and

perhaps reduce the volume of wear debris generated

in the long term. The purpose of this study was to

investigate the tribological character istics of a series of

retrieved Accord meniscal knees, and in particular

focus on the condition of the UHM WPE and the

femoro-meniscal wear surfaces.

MATERIALS AND METHODS

A total of 768 Accord knees Figure

7)

have been

implanted at Arrowepa rk Hospital since a clinical

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Tribological tudy

o

retrieve knee expiaat s:J.L. Hai lq et al.

Figure Photograph of the Accord meniscal bearing knee

Figure

2 Photograph of an explant which suffered from metal-to-

metal contact during articulation

trial of the knee began in 1982, of which 73 have now

been revised. The majority of these were revised for

failure of cementless fixation and infection. None of

the explants were revised for structural failure of the

UHM WPE meniscus. Twenty-seven paired meniscal

and femoral com ponents were available for examina-

tion in this study.

An initial visual examination and the clinical

history of the explants were used to determine

the modes of failure of the compo nents. Certain

mechanical modes of failure were then eliminated

from the tribological study. These included joints

where the meniscus had dislocated and led to the two

metal compo nents, the femoral compon ent and the

tibia1 tray, contacting. During articulation these two

metal compo nents maintained contact leading to the

generation of large amounts of metal w ear debris. A n

example of this type of failure is shown in Figure 2.

Sixteen of the 27 paired explants had failed in this

manner and were thus discarded from the rest of the

study, which has focused on the articulating surfaces.

The full clinical history, p atient details and regular

follow-up information

of the eleven remaining

explants (which were evaluated for wear) were

available from hospital records, and are summ arized

in Table 1. The average age of the patients was 60.0

years a nd their average weight was 71.1 kg. The

period of implantation for these compon ents ranged

from less than 1 year to over 5 ears with an average

lifetime of 33.4 months. In a dy ition to the revised

explants, four unwo rn p rostheses taken from clinical

stock prior to implantation were also evaluated as

control samples.

The articulating surfaces of the

menisci a nd femoral comp onents were characterized

by R, surface roughness measurem ents using a

Talysurf 5 machine (Rank Taylor Hobson , Leicester).

The R, was measured perpendicular to the direction

of sliding using a 0.8 mm cut off. Three readings were

obtained from each cond yle, giving a total of six

measurem ents per comp onent. The mean of these

readings was then calculated and used as the average

R, value for that particular compo nent.

The UHMW PE meniscal components were wider

than the femoral compon ents, and due to a close

fitting central locating step (see Figure 4b), the edges

of the menisci were not subjected to wear. This

resulted in the formation of a lip at the edges of

Table 1 Summ ary of clinical data, patient details and follow-up information

Case

Wear life

number (months)

Activity

capability

Age

(years)

Weight

(kgs)

Sex

Operation

side

Initial

diagnosis

Reason for

revision

I 8 A 73

72 F I. OA

I,, ST

2 9 - 62

66 F R RA

I

3 IO A 60

71 F L OA

I

4 I6 A 61

64 M L O A

I,

5 23 P 61

54 F R RA

I

6 26 G 67

79 M I* Other

I, L

7 43 G 46

73 F R RA

L, ST

8 45 G 63

61 F R RA

I, U

9 59 A 62

88 M L O A

I,, ST

IO 64 G 62 77 M R O A L

II 64 A 43

77 F L OA

L

G, good mobility; A, average mobility; P, poor mobility; OA, osteo arthritis; RA, rheumatoid arthritis; I, infection; L, loosening; ST, subsidence of

tibia; U, unstable

224 Med. Eng. Phys. 1994, Vol. 16, May

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the menisci. A Talycontor machine (Rank Taylor

Hobson, Leicester) was used to trace across the

surface o f the menisci (perpendicular to the sliding

direction) from one lip, over the worn region to the

worn area and unworn lip on the op osite side of the

comp onent. A rotating levelling ta 1 le was used to

ensure that the two unw orn lips were at the same

height, and that the menisci were orientated at

exactly 90 to the direction of sliding. This proce-

dure was repeated twice for each sample and the

average difference between the height of the unwo rn

lip and the height of the worn region was evaluated to

give the penetration depth for each polyethylene

comp onent. It should be pointed out that the

measured penetration into the menisci occurred as a

result of both creep and wear of the polyethylene.

Therefore the derivation of wear rates from penetra-

tion rates should be treated with caution, particularly

when penetration depths are small.

RESULTS

The surface roughnesses of the femoral compo nents

are shown in the graph in F igure 3. Th e number of

months that the joint was im lamed for is shown on

the x-axis, and the R, (in pm

P

It was found that general1

is shown on the y-axis.

were in very good cr

the femoral compo nents

con ition and had R, values

similar to the unw orn control. The av erage R, value

for the worn femoral com ponents was 0.055~m with

a standard deviation of 0.075p.m (average R= of

0.032pm if the one anomaly, 59 months, was not

included). This corn

P

ares with an Ra of 0.02p.m for

the unwo rn contra compo nent. There was one

sample (59 months) which ha d a surface roughn ess

an order of magnitude higher than the rest of the

samples. This comp onent had been badly scored

g

robably as a result of the entrapment of cement and

one debris. One other compo nent (64 months)

showed a slightly higher R, and a slight deterioration

of the surface.

Figure 4a shows the surface roughn esses of the

meniscal compon ents. The implant lifetime is shown

on the x-axis, and the R, is shown on the y-axis. From

this graph it can be seen that there was an initial

Lifetime months)

Figure 3 Histogram of the femoral surface roughness (R,) and

implant life

Tri bologicnl tudy of retri eved nee explant s:JL. Hai lq, et al.

a

Lifetime months)

Figure 4 a Histogram of the meniscal surface roughness ( ) and

implant life. 4b, Photograph of a worn (right) and an unworn (left)

meniscus

smoothing of the surfaces followed by some deteriora-

tion in certain cases. The average R, for the worn

menisci was 1.15 pm with a standard deviation of

1.03pm (using the eight polished com ponents the

average Ra was 0.60pm, using the three bad1

damaged compon ents the average R, was

2.63pm .

r

This compares with an avera e

(standard deviation 0.23 pm

Y

value of 1.04 pm

for the unworn

samples. The Ra values for the worn co mpon ents

ranged from 0.28 pm up to 3.27pm, whereas those

for the unwo rn con trols ranged from 0.76pm up to

1.33pm. Initially the polymer surface was quite

rough due to the machining marks. Initial wear and

creep produced a smoothing or polishing of the

UHM WPE meniscal compo nents. The polishing of

the worn menisci, compa red to an unwo rn control,

can be clearly seen in the

4b, where the sample on tK

hotograph shown in F igu re

e right is the worn sample.

Subsequently in certain cases the polymer surface

became damaged and the surface roughness

increased.

F igu re 5 shows the articulating surface of Case 10

(the first of the two samples implanted for 64 months

on the graph in

F igure 4a .

This sample showed severe

surface dama

explants, an B

e compa red with that seen in the other

failure of the surface can be seen in

F igure 5. This was the worst case of the 27 explants

which were studied. The Talycontor profile showed

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Tri bological tudy of etr ieved nee explunt xJ.1. Hai lq et al.

Figure 5 Photograph of the meniscus from Case 10 showing surface

micro-delamination

that this surface dam age and wear occurred within

0.3 mm of the meniscal bearing surface. Similar wear

features have been seen on acetabular cups from

simulator studies which we have performed at Leeds

University *. Howev er, there was also evidence of

gross misalignment and dislocation of the poly-

ethylene meniscal compo nent, and this surface

damage could have been caused by contact with the

edge of the femoral compo nent when grossly mis-

aligned. Local surface damage at the edge of the wear

scar was noted on several of the older meniscal

compo nents and this is shown in Figure 6. This is

believed to have occurred when the femoral com-

ponent and the meniscus became slightly misaligned.

The very small radii on the femoral compo nents led

to the development of high contact stresses a t the

edge contact with the meniscus. This in turn resulted

in local stru ctural failures, in the form of local severe

damage to the surface of the polyethylene.

The penetration de ths into the menisci are shown

in

Figure 7. The

imp ant life is shown on the x-axis

and the penetration (in pm) is shown on the y-axis.

There was a considerable variation in the penetration

depths of the explants, and no correlation could be

found with either wear life, patient weight, or patient

activity. It can also be seen from Figure 7 that four of

the menisci showed no detectable penetration, even

after implantation for over 5 years in one case (64

months on the graph in Figure 7). Two of the samples

were too badly deformed to obtain penetration

Figure 6 Photograph showing local structural failure of a meniscus

due to misalignment of the femoral component

i 23 26 45 59 64

Lifetime months)

Figure 7 Histogram of the meniscal penetration depth and implant

life

depths from. This was due to severe misalignment of

the meniscal compo nent and gross damage of the

polyethylene, and hence these samples were dis-

regarded from this part of the study. The average

penetration rate from the data shown in Figure 7 w as

found to be 40pmyea r-. In this study the

P

enetra-

tion depths an d the Ra values for the latera side of

both compon ents were found to be similar to those

obtained for the medial side.

DIS USSION

Recent reports13 of structural fatigue and early failure

in highly stressed nonconform ing total condylar

knees have caused considerable concern in the

surgical commu nity. This has promp ted a renewed

interest in the meniscal bearing knee w here it is

possible to increase contact area and reduce contact

stresses to levels m ore generally found in hip

prostheses. It is important to recognize that changes

in the design an d tribological cond itions in artificial

joints can cause marked changes in the wear and

failure processes in the UHM WPE compo nents. In all

polyethylene acetabular cups in hip prostheses, where

contact stresses are generally lower than in total

condylar knees, structural failure is extremely rare,

and wear particles are generated by surface wear

processes produced by asperity interactions14. In

Argenson and OConn ors long term study of

Oxford meniscal knees, surface wear and pentration

rates were reported, but no evidence of structural

failure was recorded, demonstrating the potential of

the meniscal bearing concept. In this study the

majority of the implants retrieved showed metal-to-

metal contact due to dislocation of the meniscal

compo nents. As this would have produced consider-

able metallic debris which could subsequently

damage the articulating surfaces, these prostheses

were excluded from the study.

It was shown that, in general, the surface roughness

of the femoral comp onents did not change signifi-

cantly with implantation time. Some fine scratches

running parallel to the direction of sliding w ere

visible on the surfaces of the femoral compo nents,

although these did not appear to have a significant

226 Med. Eng. Phys. 1994, Vol. 16, May

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Tri boh@al study of etri eved nee explunt s:JL. Ha y et al.

reduced stresses have been demonstrated in this

study. However,

it is necessary to

em hasise

biomechanical difficulties of ensuring sta

the

meniscal knee design.

ility in the

influence on the R, values. Since the scratches were

orientate d in the direction of sliding, it is unlikely that

they would hav e had a significant effect on the wear

of the polyethylene, compa red to scratches which

were perpendicular to the sliding direction. In hip

joints this type of scratching has been related to

increased polyethylene wear as in the hip the

scratches tend to be more random ly orientated due to

the more complex movem ents generated.

The pattern of wear and changes in the surface

to ograph of the meniscal compo nent were gener-

al y ip ly similar to those found in acetabular

cr

where surface wear processes remove

EYsethylene particles within 10pm of the surface,

an dr severe plastic deformation of the polyethylene

has been noted up to a depth of 40pm below the

surface. T he initial roughness of the pol

marks was smoothed by creep

an Jrn

er machine

counterface as

microscopic

t

erity wear (Figure 4). Subsequent1 in

certain cases

e polymer surface deteriorated. Jh is

may be due to abrasive action by third body bone

cement particles, or alternatively the onset of macro-

scopic

1

olymer w ear processes which have been

reporte in explanted acetabular cups15. One

meniscal compo nent (Case 10) showed severe surface

damage, and the depth of this severe surface damage

was up to 300pm from the articulating surface.

Although similar surface wear patterns have been

seen in hip joint simulator studies in the laboratory,

there was also evidence of gross misalignment of this

compo nent and this severe surface damage ma

have

been caused by edge contact of the femora ly

com-

ponent. In the explants which d id not dislocate, or

show evidence of gross misalignment, gross structural

failure (characterized by delamination originating

from a depth of greater than 0.5mm into the

UHM WPE) was not found. However, the follow u

of 5 years is quite sh ort and direct comparison

wi

t

longer term follow u p studies of total condylar knees

should be approached with caution. W hen misalign-

ment of the Accord knee occurred, local stress

concentrations were formed at the edge of the

femoral co mpon ent, and local structural failure

resulted due to the abnormally high contact stresses.

The average penetration rate was also evaluated

from our data and compared with that obtained by

other workers. We calculated a penetration rate of

0.040 mm year- (4Opm ) for the Accord knee, w hich

compares well with the figure of 0.047 mm

year

obtained by Argenson and OConnor for the

Oxford m eniscal knee. In comparison Dow son et

a1.16 obtained significantly higher penetration rates

for conventional Geomedic and Freeman-Swan son

knees - 0.22 and 0.26mmyea r respectively. The

penetration rate in the meniscal knee was comparable

with that found with hip joints as shown by Dowling17

who obtained a penetration rate of 0.06 mm year- for

post mortem acetabular cups. How ever, care must be

taken when comparing penetration rates in different

types of knees and hips, as the different geometries

involved will produce different v olume

than es for

the same penetration rate. Also these volume c a anges

can include different contributions from creep and

wear. Des

mental tri

K

ite this, the authors feel that the funda-

ological

advantages of a meniscal knee

design, which include larger contact areas and

CONCLUSIONS

In conclusion, the tribological advantages of the

Accord meniscal bearing knee joint are as follows.

There was generally little damage of the articulating

surfaces. The wear and penetration rates were very

low. There was no evidence of any structural failures,

except in cases where the femoral corn

onent and the

meniscus were misaligned, and lot

ap

damage was

formed at the edge of the contact area. The p attern of

surface w ear in this knee was similar to that seen in

hips with comparable penetration rates for the two

types of joint.

In the 27 retrieved knees reported in this study

failure of the cementless fixation was the most

frequen t cause of revision. This led to instability of the

joint and edge contacts between the femoral com-

ponent and the meniscus, which due to the small radii

on the femoral compo nent led to the generation of

high contact stresses and local structural failure.

There were a number of cases where dislocation of

the meniscus which led to metal-on-metal contact

during articuation, and the potential for generation of

large quantities of metal wear debris.

AC KNOWLEDGFBIENTS

This work was funded by the Arthritis and Rheum at-

ism Council (ARC).

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1994 IEEE

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