Improvement of mechanical properties of acrylic bone cement by fiber reinforcement

IMPROVEMENT OF MECHANICAL PROPERTIES OF ACRYLIC BONE CEMENT BY FIBER REINFORCEMENT* SUBR,ATA S-\HA+ and SUBRATA Biomechanics

PAL:

Department c7i Orthopaedic Surgery. L.S.U. 33931. Shrr\eport. LA 71130, U.S.A.

Laboratory,

Medical

Center.

P. 0.

Box

Abstract-Acrylic bone cement is signiticantl) weaker and less still’than compact bone. Bone cement is also weaker in tension than in compression. This hmits its use in orthopaedics to areas where tensile stresses are minimum. We have attempted to improve the mechanical properties of PMMA by reinforcing it with metal wires. and graphite and aramid fibers. Normal. carbon fiber reinforced and aramid fiber reinforced bone cement specimens were tested in compression. Addition of a small percentage I l-2 “” by weight for carbon and up to 6”” for aramid) of these fibers improved the mechanical properties signiticantly. Due to the improved mechanical properties of tiber reiniorced bone cement, its clinical use may reduce the incidence of cement fracture and thus loosening of the prosthesis

bone, carbon

INTRODUCTIOS

or other fiber reinforcement

may be used

in those cases. Self curing joint

acrylic

replacement

gical fractures the

though

cement

is not

bone cement of the joint

without

replacement

proved

as compact

bone even ~1 trl..

improvement

in the

achieved Trent, The

Like weaker

other

brittle

in tension

Therefore,

than

could

been used clinically (Scoville

vestigated

concrete.

incorporating

et al.,

1967;

Dunn,

cement to match

(Table

1).

compact

wire

re-

reinforcement

significantly

1977).

and showed increased

annular

in total joint space

We

that

the failure

replacement

between

the

(Wright

and

was to compare bone

the

cement

fiber reinforced

if it is possible

with

PMMA.

to reinforce

bone

and elastic properties

of

bone.

LITERATURE

in-

Mechanical

(Saha

points anical

stress

of self-curing

properties

properties physical

properties

This

is not of

bone

reported surprising cement

pressure.

also affects due

are

investi-

atfected

by

many

like: (1) propor-

(Haas et al.. 1975) and time and speed (Lee er

of handiing

The strain

the strength to

this table

by different

variables,

(2) mixing

al.. 1977); (3) method

on the mech-

However,

since the mechanical

and monomer

their compositions:

cement

view

scatter in the data on specific

and environmental

tions of polymer

without

information

of bone cement.

a considerable

mechanical gators.

Receired April 1981: in recisedjhz February 1981. *Presented in part at the 7th and 8th Annual Meetings of the Society of Biomaterials, held at Troy, New York. on 28-31 March, 1981, and at Birmingham, Alabama. on 27 April-l May, 1983; and at the 29th Annual Meeting of the Orthooaedic Research Society. held at Anaheim. Califcrnia, on 7-i0 March, 1983. _ tAuthor to whom reprint requests should be addressed. ZCurrent address: Department of Mechanical Engineering. Jadavpur University. Calcutta 700032, India.

bone cement

from various

(Saha and Pal. 19S4). Tabie 2 shows a summary

indicates

and

REVIEH

by many authors

of some of the published

\vire

due to the

prosthesis

properties

have been studied

in all these modes. As use of metal wire is

not practicable

study

of normal

the strength

of the cervical

and shear (Saha and Warman,

of PMMA

in

has also been

metal wires have

and Saha, 1977). bending

1978,1979)

1978a) properties

reinforcement

capacity

the effect of metal wire on the tensile (Saha

and Kraay,

and

also be im-

improvement

reinforcement

and aramid

is

metal

fiber

of PMMA

of this

properties

cement

the load carrying

in the stabilization

er al., 1976; Taitsman

narrow

bone

in compression

improve

of bone cement. PMMA

of PMMA

materials,

as in reinforced

inforcement

spine

objective

We also investigated most

et al., 1981).

could

by graphite

propertics

those of carbon

future.

(Robinson

that the compressive cement

fiber

strength

life and the fracture

1979~ b). Similar

by aramid

or carbon

tensile

1979: Saha cr (11.. 19Slb).

mechanical

bone cement in the

of

significantly

the mechanical

but also open up greater

for new applications

of bone

(Saha and Warman.

of bone cement. This will not only failures.

cement

we have shown

shear properties

the cement is

1975). Moreover,

a need for

Previously,

sometimes

it is used to replace bone (Crobvninshield

possibilities

the

of bone

1974). This suggests strength

that graphite

increases

toughness

bone

in tension

reduce bone cement

have shown

et ul., 1975, 1976; Ray e’f al.. 1974).

and leads to failure

mechanical

authors

reinforcement

PI al., 1978), the fatigue

For example,

25 Y, as strong

fiber

(Litchman

(Weber and Charnley, only

Other

patholowith bone

use of

complications. fractures

used in artificial

1970). Likewise.

and bony defects are repaired

cement (Harrington However,

is widely

cement

(Charnley,

and molding

with or

rate used during

and

its viscoelastic

stiffness nature

testing

properties (Saha

of

el al..

19Sla). From Table 1, one can get an idea of the ranges rather than the exact value of each parameter 467

reported.

SUBRATA SAHA and SUBRATA PAL

465

Table

I. Mechanical

CTS (MN m-‘) Industrial

PMMA

and prosthesis

properties of human bone. PMMA

68.9

2.1

30.8 ‘7.6 ‘5

2.0

uss

LCS

E* IGN m-l)

(MN

material

m-:)

(MN

Reference

m-?)

103

(injection molded) Orthopaedic PMMA (hand mixed)

Wet femoral cortical bone

123

66.3 77 17.2

166

Wet lumbar vertebrae (adult av.)

3.7

0.3-l

4.6

Wet cervical

3.1

0.55

10.1

vertebrae

29.7 41

Litchman et al. (19781 Saha et 01. (1976.19791 Lee et al. (1977)

84

Yamada

(1970)

Yamada

(1970)

Yamada

(1970)

2.8

(adult av.)

Stainless steel

1724

UTS = u, = ultimate tensile strength; strength; E = modulus of elasticity.

199.96 UC’S = ultimate

Again. these ranges do not reflect actual differences in the bone cement, but rather dilrerences which depend on the testing methods, speed of loading and some of the other variables mentioned before. Different investigators have studied the effect of various inclusions, e.g. blood (Lee et al., 1977; Helm, 1977);additives like antibiotic mixtures (Lautenschlager or al., 1976); effect of irradiation (Greenwald et al., 1977; Murray et al., 1974) and the effect of in km environment (Rostoker et al., t979; Weinstein n al., 1976). Table 2 depicts some of the variables, the details of which have been reviewed in a separate paper (Saha and Pal, 1984). 11 may be mentioned here that bone cement is the weakest link in any orthopaedic prosthetic joint. In order to limit the wide difference in mechanical strengths of bone, bone cement and prosthesis, many authors utilized the idea of fiber reinforcement of bone cement with various biocompatible and chopped fibers, e.g. glass (Litchman et al., 1977), carbon (Knoell er al., 1975; Pilliar et al., 1976; Saha and Warman, 1978b, 1979a,b), stainless steel (Fishbane and Pond, 1977), and aramid (Wright and Trent, 1979; Saha et al., 1981b) fibers. Fiber concentration, i.e. percentage by weight or volume, was chosen somewhat arbitrarily or it was mostly guided by the ease of mixing and handling during rapid polymerization of the cement. Most of the authors kept it limited to 1 or 2 w/o of the cement. Out ofthe various fibers mentioned, randomly oriented carbon fibers (or graphite) were used by the majority of the researchers. Table 3 summarizes the mechanica properties of various fiber reinforced PMMA as reported by different authors. Here also those discrepancies in mixing and handling parameters, rate of straining, and method of testing resulted in wide variation in the results of their tests. Similarly. Table 4 shows the test results of carbon fiber reinforced PMMA. The carbon fibers or graphite fibers have been used by most of the investigators because of its better biocompatibility and high strength and stiffness qualities. Most previous investi-

compressive

strength;

USS = ultimate

shear

gators used hand mixing in preparing fiber reinforced bone cement specimens. Uniform dispersion of fibers is difficult to obtain by such mixing. Therefore, some specimens might have contained bundles of fibers, thus diminishing the ultimate strength (Table 3). MATERIALS AND METHODS

We used chopped graphite fibers (6mm long, 8/1 diameter. a, = 43 GN m-‘. E = 3380GN m-’ Hercules A-S Type) hand mixed with PMMA powder, to prepare the carbon fiber reinforced PMMA samples. Similarly. chopped aramid fibers (Kevlar-29, E. I. DuPont Nemours & Co. 12-13mm long, density 1.44gcm-‘, 6, = 2758 MNm-‘) were used for preparation of the aramid fiber reinforced samples. Surgical grade PMMA powder and the chopped fibers were well hand mixed prior lo the addition of liquid monomer. The cylindrical compression specimens were 18 mm in diameter and 33 mm long and were cast in a cylindrical Teflon@mold as shown in Fig. 1, and pressed by hand to ensure compactness. The specimens were kept at room temperature for several days and then stored in water for 48 hr before testing them in compression using a servohydraulic Instron testing machine at a crosshead speed of 4.5 mm min - ‘. Compression tests were performed on control specimens of cement along and on three groups of test specimens of cement reinforced with 1% by weight of carbon fibers and 2 % and 4 % by weight of aramid, respectively. The number of specimens tested in each group is shown in Table 5. Before the compression test, the end faces of the cylindrical samples were checked for parallelism and were greased to reduce the friction between the sample and the plantens of the Instron. RESULT’S

Typical compressive stress-strain curves of normal, carbon fiber reinforced (Hercules AS type chopped fibers) and aramid fiber reinforced bone cement

Improvement

of mechanical

properties

469

of acrylic bone cement

4 f ij

x

Fumich er al. (1979)

Sih er 01. (1980)

Rostoker YI al. (1979)

Kusy (1978)

Holm (1977)

Lautenschlager and Marshall (1976)

_-

Reference

1.21

21.1

46.2 + 3.5 46.2k4.1 48.2 f 2.8 33.8 f 3.5

42.1 f 5.5

29.18+

Tensile

Ultimate

98.5 95.3

80.66 + 5.10

Compressive

60.5 4-l. 1

48.3 48.8 50.9 40.6

58.2 53.0 62.6

+ + + +

3.8 4.9 5.7 .60

Flexural

stress (MNm-*)

Table

Shear

2. (continued)

2324

2756 k 261 X + 2549 f 2411+ 2549 f

2480 2428 2265

(MN

m-l)

138 (Tension) 69 (Tension) 275 (Tension) 69 (Tension) 275 (Tension)

Elastic modulus ____-.___-

5bcatmin’ 120beatmin-’

Surgical Simplex

_____

The bone cement were implamed and tested slier 1 day, 6, 12, and 24 monlhs

CMW Palaces R hktCOS RG Sulphix 6 Surgical Simplex

CM W ccmcnt Simplex Palaces R

Strain rate = O.OZ/mi.

Commcnl

___-

Surgical Simplex

Aramid Kevlar-29 El Dupoint

Aramid Kelvar-29 EI Dupoint

Glass fiber

Stainless steel AlSl-316

Type

*All length and diameters are in mm. tThese are fractures toughness in Mnm-“’ 0, = maximum stress in tension; ran = maximum v/o = percentage by volume.

Saha and Pal (1981)

rf al.

PMMA (KerrSybron Corp)

Simplex

Litchman (1978)

Wright (1979)

Surgical grade PMMA

Fisbane and Pond (1977)

er ul.

type

Author

Cement

-

13

stress in compression;

I=

r = maximum

2. w/o 4. w/o

PMMA

79.39 + 16.74 85.19+6.95

25:; increase

id

1.88t 2.3 1 2.85

Other

of elasticity;I =

(4

Strength (MPa) Compressive Shear

of reinforced

stress in shear; E = modulus

36. I 38.20 42.8

1. w/o 4 w/o 7 w/o

I=

13

41.7 54.4

(0,)

4.5, v/o 8.6 v/o

7, used

Tensile

properties

I = 12.5 I/d = 5000 bu =2Gnm-’

short fiber I=O.S to I* area = 31.75 x 10T4 mm*

Reinforcing material Prc&ties

Table 3. Mechanical

of Elasticity MPa _

length: tl = diameler;

Comment

percentage

~~_~...

-_

by weight;

- ~.

(r was measured a; 4” ,, strain

32 y,, increase in 6, 74 9” increase in fracture toughness

Ram Speed, 2.5 mm min _ ’

W/O =

______~___

l880+ 180(c) I556 _+240(c)

324 1 (T) 3482(T)

Modulus

P

Cement

Osteobond

CMW

CM W-Porous

Simplex P RG.

Surgical simplex (RO)

Author

Knoell rr al. (1975)

Pilliar et ul. (1976)

Pilliar cf crl. (1976)

Saha cr ul. (1979)

Litchman er ul. (1978)

Graphite ThorneI-300 Union (Carbide)

Chopped Carbon Hercules As-type

Carbon

Carbon

Graphite GY-70

Type

I. and 6.6, w/o

0.53, v/o 1.05, v/o 1.57, v/o

I = 12.5 d = 5.0

I and 2 v/o

2 v/o

1,2,3 and 10 by w/o

y0 used

i=6mm d = 8Itm

/=6mm d = lfkl5prn E = 380 to 460GPa

/=6mm d=7flm

/=6mm chopped

Fiber Property

34.1 37.8 42.2

38

Tensile (e,)

74 72

1.7

35 47

Strength (MPa) Compressive Shear (%) (r)

Table 4. Mechanical properties of carbon fiber reinforced PMMA

47

Flexure (er)

4008(T) 5730(T)

3241(T)

Static: 3700 5800 Cyclic: I:’ = 4000(7’) E = 375oc)

5560(T)

4600(c)

Modulus of Elasticity MPa

Crosshead speed = 2.5 mm min _ ‘.

a, t = 5 x lo-‘*s-’ at E = 2 x 10-zS~’

607; increase in u,, 100‘,, increase in E

Comment

Fig. 1. Normal (left) carbon fiber reinforced (right) bone cement specimens with their Teflon@ molds. The arrow indicates recessed aluminium bottom plate for facing the molds.

473

Fig. 5. Electron photomicrograph of fracture surface of a CFR-PMMA

474

sample failed in compression

Table 5 Fiber percentage Sample group

Type of fiber

Sum&r

thy weighti

rpximens

I

x

(normal bone cement) 7

I (’II

Graphite

;

Aramid

1 /b - ‘3

4

Aramid

-l “,,

COMPRESSIOR

TEST

s 6

6

spccimsw

are sho\
in Fig. 2. As portrayed

figure, the stress-strain 1oc) -

of tested

2 ‘t, ARAMID

relationships

in this

were linear up to

a fairly

high stress level and there were no sharp yield

points.

However.

with increasing strain, there was a

decrease in the load carrying capacity of normal

and

carbon fiber reinforced bone cement beyond the initial peaks. but this was not true reinforced

PMMA

for the aramid

different

failure

samples.

For instance. while carbon

mechanisms

of these three groups

specimens developed longitudinal aramid

fiber

(Fig. 2). This may be due to the

fibers increased

of

fiber reinforced

cracks, samples with

uniformly

in diameter

with

Increasing compression. The means and standard ive properties

0

,.

specimens

7

05

are compared

deviations and carbon

of thecompressfiber

reinforced

in Figs 3 and 4. Since there

was no sharp failure point. the ultimatestrength

0 10

stress)

STRAIN

of

aramid

calculatedat Fig. 2. Typical stress-strain behavior of normal, carbon fiber (1 w/o) and aramid tibcr (2 w/o) reinforced

of aramid

PMMA

4”,strain,asat

failed

reinforced

COMPRESSION PMMA

limit for I ‘I,, by

TEST

0

2% ARAMID

I

4% ARAMID

0 IJ 004

T

90 80

T

704

T

PROP

Fig. 3. Comparison

LIMIT

ULT

were

theCF-

in most of the cases.

0

1% CARBON

(proof

specimens

rhis level ofstrain,

As shown in Fig. 3, the proportional

PMMA.

NORMAL

fiber

STRENGTH

of (means and 1 S.D.) proporuonal limit and ultimatestrength PMMA and 2”, and 4”” AFR-PbfMA.

of normal.

I I’,,CFR-

SUBRATA

476

SAHA

and SLBRATA PAL

possible with normal bone cement. In order to examine the mechanism of strength improvement. the fracture surfaces of CFR-PMMA were examined in a scanning U 4% ARAMID 1800 electron microscope. 1600 Figure 5 shows the electron photomicrograph of a fracture surface of a specimen failed in compression. It 1400 reveals that even after failure of the cement, some of F1 E 1200 the fibers are intact and still can carry some load. The 2 failure mechanism was the interracial shear of the fiber I 1000 and cement. This shows that the energy absorption 800 capacity of CFR-PM MA could be increased due to the 600 fiber pull outs. A high rise in temperature during the setting of bone 400 cement may cause tissue necrosis (Jefferiss et al., 1975) 200 and thus reduction in peak temperature is highly desirable in the clinical use of bone cement. We moniMOO. ELASTICITY tored the temperature rise in similar sized cylindrical IN COMPRESSION specimens for normal, carbon fiber reinforced and aramid fiber reinforced specimens and found that the Fig. 4. Comparison of modulus of elasticity of I r, CFRPMMA and Z”,, and 4’” AFR-PMMA. peak temperature of bone cement could be significantly reduced by the use of fiber reinforcements. This may be a highly beneficial side effect of using fiber weight of carbon fiber reinforced samples is comreinforcement in the orthopaedic use of bone cement. parable to 21:” by weight of aramid fiber reinforceThe improved mechanical properties of PMMA ment. However, the workability of 29, aramid fiber reinforced with ultra high strength graphite fiber, reinforced PMMA was much better than similar 1To Thornel-300 (Union Carbide, Inc.), or aramid fiber, carbon reinforced cement. When normal bone cement Kevlar-29, are compared with those of human compact (radiopaque) specimens were tested in compression, bone in Fig. 6. About 2 ‘:I,,Thornel-300 fibers improved they failed at an ultimate strength of 66.3 + 6.6 MPa. the tensile strength of PMMA to about 50% and Thus, 1 Oncarbon and 2 Y,,aramid fiber reinforcement modulus of elasticity to about 40”, that of compact improved the ultimate strength by 20.5 “” and 19.5 ‘:,, human bone. These data on AFR-PMMA were obrespectively. Four percent by weight of aramid fibers tained from Wright and Trent (1979) and on CFRimproved the ultimate strength by 28.7”“. The moPMMA from Litchman er al. (1978). dulus of elasticity obtained by 2 I:,, aramid fibers was Our result suggests that although significant imI1 Y0greater than that obtained by 1 O0carbon fiber provement in the mechanical properties of bone reinforcement (Fig. 4). cement can be achieved by fiber reinforcement, it may not be possible to match the properties of compact bone by such fiber reinforcement. Further improveDISCUSSIOS AKD CONCLUSIOS Cl 1% CARBON

zoo0 -

12%

ARAMID

The results of mechanical testing of graphite and aramid fiber reinforced specimens show that incorporation of such fibers produces a significant increase in the compressive strength of bone cement. As shown in Table 1, we have also shown previously that the shear strength of PMMA could be significantly improved by graphite fiber reinforcement (Saha and Warman, 1978a, b, 1979a, b). The modulus of elasticity of fiber reinforced PMMA also increases with increased fiber content when the volume percent is kept small. Other investigators have shown before that the tensile and fatigue strength, and fracture toughness of bone cement can be improved by graphite and aramid fiber reinforcements (Litchman rr al., 1978; Pilliar et al., 1976, and Wright and Trent, 1979). Recently we have also demonstrated that creep deformation of carbon fiber reinforced bone cement was significantly less than that of normal bone cement (Saha and Pal, 1982). The improved mechanical properties of graphite fiber reinforced PMMA would allow the clinical use of this material in more diverse applications than is presently

TENSILE

TEST

150I-

140

120

16000

v/o 0 THORNEL-300.1.05 I THORNEL-300, 2.08 v,c 0 ARAMIDI w/o 0 ARAMID- 4 w/o 0 Human Bone

NE z‘ L 100 E

14000

12000,

80

8000

SO

6000

40

4000

20

2000

6 E

E

9 10000 E

ULT. STRENGTH

z 0 f u) 5 =

MODULI ELASTI

Fig. 6. Comparison of ultimate tensile strengths and mod&i of elasticity of CFR-PMMA, AFR-PMMA and human compact bone.

Improrement of mschamcal properties of acrylic bone cement ment in the mechanical properties of bonecement may be achieved by the use of different types of fibers and by a more uniform mixing of the fiber and cement. A machine-mixed carbon fiber reinforced bone cement (Zimmer, Inc.) in which the fiber cross-sections are dog bone shaped for better bonding, is presently being used for limited clinical trial. We are in the process of testing this CFR-PMMA mechanically and our preliminary result indicates somewhat higher strength values compared to those reported in this paper.

Mechanical properties of bone cements conrammg large doses of antibiotic powders. J. bwmrd. .!far. Rrs. 10, 929-938.

Lautenschlager. E. P. and Marshall, G. W. (19761 bfschanical strength of acrylic bone cements impregnated Hith antlbioti&. J. biomid. Mat. Rrs. 10. 837184% Lee. A. J. C.. Line. R. S. M. and Vaneala. S. S. 119771 The _ mechanical properties of bone cements. J. .ift~l. Etlyny Tech. 2. 137-140. Lee A. J. C., Ling, R. S. M. and Wrighton, J. D. (19’3) Some properties Of polymethlmethdcrylatc with reference to Its use in orthopaedic surgery. C/in. Or~hop. 95. 2s I -287. Lida. M.. Furuya, K., Kawachi. S., Masuhara, E. and Tarumi. J. (1974) New improved bone cement (MMA-TBB). Cfin. Orrhop.

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