* Correspondence address: Exponent, Inc., 2300 Chestnut St. Suite

150, Philadelphia, PA 19103, USA. Tel. : #[phone]; fax: #1-
215-751-0660.

E-mail address: [e-mail] (S.M. Kurtz).

Biomaterials 22 (2001) 1875}1881

A small punch test technique for characterizing the elastic

modulus and fracture behavior of PMMA bone cement

used in total joint replacement

V.L. Giddings , S.M. Kurtz

*, C.W. Jewett , J.R. Foulds , A.A. Edidin

Exponent, Inc., 149 Commonwealth Drive, Menlo Park, CA 94025, USA

Howmedica Osteonics, R&D Corporate, Allendale, NJ 07401, USA

Received 8 June 2000; received in revised form 28 September 2000; accepted 2 November 2000

Abstract

Polymethylmethacrylate (PMMA) bone cement is used in total joint replacements to anchor implants to the underlying bone.

Establishing and maintaining the integrity of bone cement is thus of critical importance to the long-term outcome of joint replacement
surgery. The goal of the present study was to evaluate the suitability of a novel testing technique, the small punch or miniaturized disk
bend test, to characterize the elastic modulus and fracture behavior of PMMA. We investigated the hypothesis that the crack
initiation behavior of PMMA during the small punch test was sensitive to the test temperature. Miniature disk-shaped specimens,
0.5 mm thick and 6.4 mm in diameter, were prepared from PMMA and Simplex-P bone cement according to manufacturers'
instructions. Testing was conducted at ambient and body temperatures, and the e!ect of test temperature on the elastic modulus and
fracture behavior was statistically evaluated using analysis of variance. For both PMMA materials, the test temperature had
a signi"cant e!ect on elastic modulus and crack initiation behavior. At body temperature, the specimens exhibited

`ductilea crack

initiation, whereas at room temperature

`brittlea crack initiation was observed. The small punch test was found to be a sensitive and

repeatable test method for evaluating the mechanical behavior of PMMA. In light of the results of this study, future small punch
testing should be conducted at body temperature.

2001 Elsevier Science Ltd. All rights reserved.

Keywords: Polymethylmethacrylate (PMMA); Bone cement; Small punch test; Temperature; Mechanical behavior; Elastic modulus; Fracture
toughness; Crack initiation; Body temperature

1. Introduction

The small punch test has been developed over the past

two decades to characterize the ductility and fracture
resistance of metals and polymers [1}11]. Also known as
the miniature disk bend test, the small punch test was

"rst developed for use in the electric power generation

industry [8}11]. Recently, the small punch testing tech-
nique has been used to evaluate the elastic and
large-deformation post-yield behavior of polymeric bio-
materials, focusing primarily on ultra-high molecular
weight polyethylene (UHMWPE) [1}7]. The miniature
specimen size has proven to be particularly useful when
investigating the local variations in mechanical behavior

that arise in UHMWPE total joint replacement compo-
nents after wear testing, long-term shelf aging, or im-
plantation. Although the small punch test has been used
to study a wide variety of semicrystalline polymers, such
as UHMWPE, HDPE, PTFE, and polyacetal, the mech-
anical behavior of polymethylmethacrylate (PMMA)
bone cement during small punch testing has yet to be
explored.

PMMA gained popularity in the early 1960s as a ma-

terial for securing orthopaedic implants to bone [12].
Although PMMA has been used successfully over the
past three decades, there is a concern that long-term
loosening of the prosthesis may be related to mechanical
failure of the underlying cement mantle [13}15]. Main-
taining the long-term integrity of bone cement, be it
through innovative preparation techniques or modi"ed
formulations, has received renewed interest, due in part
to its recent reclassi"cation by regulatory agencies in
Europe and United States [16]. As described in a recent

0142-9612/01/$ - see front matter

2001 Elsevier Science Ltd. All rights reserved.

PII: S 0 1 4 2 - 9 6 1 2 ( 0 0 ) 0 0 3 7 2 - 0

Fig. 1. Representative small punch test specimens prior to testing: (a)
PMMA; (b) Simplex-P. Microvoids 0.1}0.2 mm in diameter were ob-
served on the surface of the PMMA specimens, but not the Simplex-P
specimens.

monograph by KuKhn [16], over 60 di!erent types of
commercially available bone cement have been introduc-
ed into clinical practice worldwide.

The bulk material mechanical behavior of bone ce-

ment as derived from tests using conventional, standard
specimens, is known to depend upon numerous factors,
such as the formulation of the cement, the test conditions,
and conditions used to prepare the cement [13}16]. Con-
ventional specimen testing techniques have also shown
substantial variability in results, which hinders, and in
some cases, precludes interlaboratory comparisons of
test data [15,16]. Additional limitations in conventional
testing techniques are due to the discrepancy in size
between the test specimens and the cement mantles
in vivo, which typically range in thickness between 1 and
5 mm. Because of the relatively large specimen sizes,
conventional test techniques cannot be used to evaluate
the mechanical behavior of retrieved bone cement sam-
ples, or to predict the response of bone cement in vivo.
Due to the numerous di$culties associated with conven-
tional, standard specimen testing, miniature specimen
testing techniques, if developed, could a!ord important
advantages over standard test methods.

The primary objective of the present study was to

evaluate the feasibility of using the small punch test to
characterize the mechanical behavior of PMMA bone
cement. Speci"cally, we used the small punch test to
measure elastic modulus and fracture initiation behavior
for a generic PMMA and widely used bone cement
(Simplex P). Our research addressed the following two
questions: Can the small punch test di!erentiate between
di!erent PMMA formulations/mixing methods? Is the
mechanical behavior determined by the small punch test
sensitive to the test temperature? The long-term goal of
this research is to develop a reliable, reproducible, and
sensitive miniature specimen testing technique for the
evaluation of retrieved bone cement samples.

2. Materials and methods

Selected for this study were two types of PMMA that

were expected to represent a wide range of mechanical
behavior, thereby providing a simple means of identify-
ing the potential usefulness of the small punch test for
characterizing this family of polymers. Simplex-P (How-
medica Osteonics, Rutherford, NJ) was selected for
evaluation to provide an example of a widely used, clinic-
ally relevant bone cement. For details regarding the for-
mulation of Simplex-P, refer to a recent review by Lewis
[15] or a monograph by KuKhn [16]. We also evaluated
an industrial

`generica PMMA (Seta-Tray, Accuarte Set

Inc., Newark, NJ), which is typically used for laboratory
potting applications. One dose of each PMMA was pre-
pared according to the manufacturer's instructions. In
the case of the generic PMMA, the unsterilized polymer

beads and liquid monomer were hand mixed. The
Simplex-P, on the other hand, was sterilized and pre-
pared using a commercially available vacuum mixer
(Stryker Vacuum Mixing system: Howmedica Osteonics,
Rutherford, NJ). Specimens were prepared by pouring
the doughy but still #uid PMMA into a UHMWPE
mold with multiple cavities measuring 6.5 mm in dia-
meter and 25.4 mm in length. Specimens were allowed to
cure for 36 h at 203C, 60% relative humidity, and atmo-
spheric pressure, prior to being machined into miniature
small punch test specimens, each with a thickness of
0.5 mm and a diameter of 6.4 mm.

Due to the di!erences in formulation and preparation,

we expected variations in the size and distribution in
microvoids between the two PMMA materials. Scanning
electron microscopy (SEM) was conducted on represen-
tative specimens to characterize the size and extent of
voids. Numerous microvoids, typically 0.1}0.2 mm in
diameter, were observed in the hand-mixed PMMA spec-
imens (Fig. 1a) whereas the vacuum-mixed Simplex-P

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V.L. Giddings et al. / Biomaterials 22 (2001) 1875} 1881

Fig. 2. Experimental setup for the small test for PMMA.

Fig. 3. Features of the small punch test load}displacement curve for
PMMA.

specimens were comparatively smooth and free of voids
at the surface of the specimens (Fig. 1b).

Mechanical tests were performed using a servohyd-

raulic testing machine. The small punch specimen was
held in place by a die and guide set-up, and deformed by
pushing the specimen against the die using a hemispher-
ical-head punch moving at a constant displacement rate
of 0.5 mm/min, as detailed previously [4,8] (Fig. 2). The
load and displacement were measured, and the back
(bulged) surface of the specimen was monitored by
a borescope and recorded such that the video was syn-
chronized with the measured load}displacement curve.
Thus, we were able to identify the onset or initiation of
cracking during the small punch test, with regard to both
point on the load}displacement curve and physical loca-
tion on the test specimen. Testing on both materials was
conducted at ambient and body temperatures (23.7$
0.63C and 37.3$0.23C, respectively).

The load}displacement curve was characterized by an

initial sti!ness, ultimate load, ultimate displacement, and
work to failure (Fig. 3). Based on previously validated

"nite element analyses of our small punch test apparatus

for a wide range of semicrystalline polymers, the initial
sti!ness of the load}displacement curve is proportional
to the elastic modulus of the material [2,4,7]

E"Ak,

(1)

where the elastic modulus (E) is in MPa when the initial
sti!ness (k) of the load}displacement curve is in N/mm.
The proportionality constant (A) is a function of Pois-
son's ratio and the frictional characteristics of the mater-
ial. The ultimate load and displacement were de"ned

based on the initiation of cracking in the specimen ob-
served via the synchronized video of the back surface of
the specimen. Work to failure, calculated from the area
under the load}displacement curve up to the point of
crack initiation, provided a measure of crack initiation
toughness.

After testing, selected specimens were examined using

a SEM to evaluate the fractured surfaces. The fractured
surfaces were examined and characterized using a vari-
able pressure SEM operating at 0.6}5.1 kV.

Previous "nite element simulations of the small punch

test, corroborated by small amplitude cyclic loading ex-
periments, have indicated that displacements in the range
of 0.064 mm are recoverable (i.e., elastic, representing the
range of applicability of Eq. (1)) [4]. Consequently, the
initial sti!ness of the load}displacement curves up to
a displacement of 0.064 mm was used to predict the
elastic modulus of the PMMA employing the "nite ele-
ment method according to our previously described ap-
proach [2,4,7]. A three-dimensional "nite element model
was created of the small punch test and validated in an
earlier study [2], so only the important highlights of the
technique are provided here. By parametrically varying
the elastic modulus in the "nite element model of the
small punch test, the relationship between elastic
modulus (E) and sti!ness (k), shown in Eq. (1), was ob-
tained [2,4]. Assuming a constant Poisson's ratio of 0.35
for PMMA [17], a range of elastic moduli were input
into the model, and the corresponding set of initial stif-
fnesses for the resulting load-de#ection curves was com-
puted for PMMA. Linear least-squares regression was
then used to obtain the material-speci"c relationship
between the elastic modulus and initial sti!ness of the
load-de#ection curves for PMMA (r

'0.99):

E"14.6k,

(2)

where, as in Eq. (1), the elastic modulus (E) is in MPa
when the initial sti!ness (k) of the load}displacement
curve is in N/mm.

V.L. Giddings et al. / Biomaterials 22 (2001) 1875} 1881

1877

Fig. 5. E!ect of test temperature on elastic modulus (a), ultimate load (b), ultimate displacement (c), and work to failure (d) for PMMA and Simplex-P.

Table 1
Mechanical properties from the small punch tests

n

Temp

Modulus

Ultimate load

Ultimate disp.

Work to failure

(3C)

(MPa)

(N)

(mm)

(mJ)

PMMA

4

24.2$0.3

2790$297

22.4$2.4

0.174$0.021

2.05$0.26

2

37.5$0.4

2222$42

33.3$2.9

0.382$0.006

7.77$0.85

Simplex-P

3

23.1$0.3

3191$231

25.7$1.4

0.174$0.049

2.34$1.28

4

37.2$0.0

2504$233

74.1$11.8

0.915$0.173

40.8$14.6

Fig. 4. Typical load}displacement curves for the Simplex-P specimens
at room temperature and body temperature.

Analysis of variance (ANOVA) was used to statistically

examine the e!ect on elastic modulus, ultimate displace-
ment, ultimate load, and work to failure of test temper-
ature and the type of PMMA material (generic vs.
Simplex-P). Statistical analysis was performed using
Statview 5.0.1 (SAS Institute, Cary, IN) and a p-value of
less than 0.05 was taken as signi"cant. Relative experi-
mental uncertainty was calculated for each of the metrics
of the small punch test by dividing the standard deviation
by the average value for each material group and temper-
ature condition [18].

3. Results

The mechanical behavior determined by the small

punch test was sensitive to the test temperature and type
of bone cement (Table 1). The room temperature small
punch test load}displacement curves showed limited in-
stability in the form of a load-drop coincident with ob-

served crack initiation. This instability was not present in
the body temperature tests (Fig. 4). These instabilities
were generally not large (limited load-drop) and
were followed by a monotonically rising load during
a crack propagation phase. On average, the experimental

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V.L. Giddings et al. / Biomaterials 22 (2001) 1875} 1881

Fig. 6. Representative Simplex-P specimens after testing at room tem-
perature (a) and body temperature (b).

Fig. 7. Surface of a fractured Simplex-P specimen after testing at room
temperature. The brittle fracture initiated at the center of the stellate
pattern and propagated between the polymer beads.

variability in the 13 tests we ran was found to be 7% for
elastic modulus, 10% for ultimate load, 15% for ultimate
displacement, and 29% for work to failure.

The elastic modulus was signi"cantly lower for both

cements when tested at body temperature as compared
with room temperature (Fig. 5a, p(0.002). The ultimate
load (Fig. 5b, p(0.0001), ultimate displacement (Fig. 5b,
p(0.0001), and work to failure (Fig. 5b, p(0.002) were
signi"cantly higher for both cements at body temper-
ature as compared with room temperature. Signi"cant
di!erences between the generic PMMA and the
Simplex-P were observed for the elastic modulus
(p(0.04), ultimate load (p(0.0005), ultimate displace-
ment (p(0.002), and work to failure (p(0.008). From
the videotapes of the deforming specimens, we observed
substantial di!erences in the mechanisms of crack initia-
tion as a function of temperature. At room temperature,
the specimens exhibited a radial crack initiation mode
(Fig. 6a), similar to brittle metal behavior [8]. At body

temperature, the specimens failed in a tearing mode
(Fig. 6b), previously observed in ductile metals behavior
and polymers [4,8].

Review of the videotapes of the back surface of the

specimens indicated that crack initiation did not occur at
the microvoids on the surface of the PMMA specimens.
Subsequent SEM of the fracture surfaces, conducted at
magni"cations of 500

;, con

"rmed that initiation occur-

red in the polymer matrix between the prepolymerized
beads (Fig. 7). Cracks were also observed to propagate
preferentially in the polymer matrix, rather than through
the prepolymerized beads (Fig. 7).

4. Discussion

We found a pronounced e!ect of temperature on elas-

tic behavior and crack initiation in the two bone cements
evaluated by the small punch test method. The bone
cements exhibited

`ductilea crack initiation patterns at

body temperature, while at room temperature the crack
initiation pattern was

`brittle,a suggesting that the

in vivo mechanism of cement fracture initiation may be
di!erent than at room temperature in vitro. The work to
initiate fracture in the specimens was signi"cantly higher
at body temperature, and this temperature sensitivity
was more pronounced for Simplex-P. Although, we did
not compute the fracture initiation toughness (J') from

these data, preliminary indications from these results
suggest that J' is temperature sensitive. The results of

this study highlight the importance of carrying out small
punch testing of bone cement at body temperature due to
the sensitivity of fracture initiation to temperature in this
material. The e!ect of temperature on crack propagation

V.L. Giddings et al. / Biomaterials 22 (2001) 1875} 1881

1879

resistance was not apparent from these data and will be
explored in future research.

The bulk mechanical behavior of bone cement is re-

ported to be sensitive to test temperature [13], but the
majority of recent studies have focused on mechanical
testing at either body or room temperature, and the
e!ects of temperature alone are typically confounded by
other changes in testing conditions (e.g., testing in air at
room temperature vs. in saline solution at body temper-
ature) [15]. When testing ordinary and antibiotic bone
cements in compression at room and body temperature,
Lee and colleagues noted an average reduction in elastic
modulus and ultimate compressive stress of about 4 and
10%, respectively [19]. In the present study, we observed
the average elastic modulus for both PMMA materials
decreased by 20}22% when the temperature increased
from 24 to 373C. The ultimate load, ultimate displace-
ment, and work to failure during the small punch test,
which were associated with crack initiation, were ob-
served to signi"cantly increase with increasing test tem-
perature. The temperature sensitivity of bone cement
mechanical behavior is likely due to the proximity to test
temperature of the glass transition temperature (T),

which in bone cement depends upon the details of the
formulation and storage condition [16]. In a recent com-
pilation of data for 26 di!erent bone cements, the
T ranged between 65 and 1003C after 24h under dry

storage conditions [16]. The temperature sensitivity of
bone cement mechanical behavior is hypothesized to be
speci"c to the formulation and storage conditions, due to
the sensitivity of T to these factors.

The presence of voids in bone cement has been asso-

ciated with deleterious consequences for fracture resist-
ance, due to the increased number of sites for crack
initiation [15,20]. Although the specimens prepared from
the generic PMMA had many more surface voids,
ranging in size from 0.1 to 0.2 mm in diameter, we ob-
served from the video record that cracks did not initiate
at these voids. Evidence of crack initiation and propaga-
tion was observed in the polymer matrix surrounding the
prepolymerized beads (Fig. 7). Recent research by
Topoleski and Vesnovsky suggests that the tendency for
cracks to propagate around polymer beads depends
upon both the formulation of the bone cement powder as
well as the radiopaci"er [21]. The crack propagation
initiation and propagation behavior observed in this
study may also be speci"c to the small punch test con"g-
uration.

The mechanical behavior of bone cement is well

known to be sensitive to formulation, preparation
method, and sterilization method [15,16]. In the present
study, the two test groups of PMMA materials had
di!erent formulations, di!erent mixing methods, and
were subjected to di!erent sterilization methods. Al-
though we observed signi"cant di!erences between the
di!erent test groups at body and room temperature, our

study was designed to evaluate the suitability of the small
punch test for evaluating bone cement. Our study was
not intended to evaluate the separate e!ects of formula-
tion, preparation method, or sterilization method. How-
ever, now that the suitability of the small punch test
technique has been established for PMMA, we expect to
use this technique in future experiments on di!erent bone
cement formulations and preparation conditions.

The small punch test was found to be a highly repro-

ducible and e!ective test method for evaluating PMMA
mechanical behavior. Research is ongoing to relate the
crack initiation behavior observed in the small punch test
with the fracture toughness parameters typically used in
conjunction with bulk (large) specimen testing tech-
niques. The ability to utilize small specimens makes the
small punch test method particularly attractive, as it can
be used to probe the elastic and fracture behavior of the
cement mantle around explanted devices. Future studies
will be directed to further exploring the formulation-
dependence, temperature-dependence, and preparation-
dependence of bone cements on the mechanical behavior
determined by the small punch test.

Acknowledgements

Special thanks to Jim Thompson, Howmedica Os-

teonics, for his assistance with specimen preparation.
Supported by a Research Grant from Howmedica Os-
teonics Corp.

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