Characterizationof short-fibre reinforcedthermoplasticsfor fracture fixationdevices StanleyA. Brown,Robert S. Hastings+,Jeffrey J. Mason and AbdelsamieMoet* Depamnents of Biomedical Engineering and ‘Macromolecular OH 44 106, USA (Received 7 August 1989; accepted 19 October 1989)
Sciences. Case Western Reserve University, Cleveland,
This study focuses on determining the effects of clinically relevant procedures on the flexural and fracture toughness properties of three short-fibre thermoplastic composites for potential application as fracture fixation devices. The procedures included sterilization, heat contouring and saline soaking. The three materials tested were polysulphone, polybutylene terephthalate and polyetheretherketone. all reinforced with 30% short carbon fibres. The polysulphone composite showed significant degradation in mechanical properties due to saline soaking. The polybutylene terephthalate exhibited significant degradation of mechanical properties following both contouring and saline soaking. The polyetheretherketone composite, however, exhibited no degradation in mechanical properties. The results demonstrated that flexion and fracture toughness testing were effective for determining the response of the composites to different applied conditions and demonstrated the stability of polyetheretherketone subjected to these treatments. Scanning electron microscopy demonstrated the most effective fibre-matrix bonding to be in the polyetheretherketone. Keywords: Composites. fracture plates, mechanical properties
Although many bone fractures can be controlled by the application of external forces, as with a plaster or fibreglass cast, it is often necessary to apply some form of internal fixation device surgically. There are a number of methods for immobilizing and stabilizing a fracture, each with its special indication. The specific method to be addressed in this study was internal fixation with plates and screws. The standard plates are made of 316L stainless steel or titanium alloys. The selection of materials was based in part on their ductility; plates are usually contoured by bending and twisting to fit the bone. Much of the development of these rigid plates is the result of the evolution of the concepts of rigid fixation advocated by the AO/ASIF’ . The healing of fractures in the absence of rigid fixation is by rapid proliferation of fracture callus and subsequent remodelling 2*3. The use of rig id devices is associated with minimal callus formation; it is ‘as if the bone didn’t know it was broken’3. Healing is accomplished by a slow remodelling process called primary healing’. Another problem with using rigid fixation systems is the long-term stress4r5 or strain6 Correspondence to Professor S.A. Brown. 7 Present address: Office of Device Evaluation (HFZ-410). Ave., Silver Spring, MD 209 10, USA. @ 1990
Butteworth-Heinemann
8757
Georgia
shielding of the underlying bone. The response of bone to stress is referred to as Wolff’s Law’ which can be paraphrased as ‘the amount of growth in a bone depends on the need for it’. Lanyon and Rubin’ concluded that it was the functional or dynamic strain that controlled bone remodelling. This load sharing by the plate’ has been associated with cortical thinning4P’0,“, increased porosity4.‘0-‘2 and bone formation at the ends of the plate due to stress transfer and remodelling of the opposite cortex due to neutral axis shift5. Removal of plates to avoid long-term stress shielding effects has been associated with bone refracture13, 14. One solution to the problem of rigid fixation is the use of flexible or less rigid systems. A less rigid device allows for external callus formation resulting in a shorter healing time 2. 5. 15.Titanium has a modulus half that of stainless steel and its use has resulted in less stress shielding and bone resorption 4.6slo . The use of plastic materials with strengths comparable to that of bone, yet with one tenth the stiffness of bone, has demonstrated rapid healing with external callus and effective remodelling5* “. 16. However, in some instances, the callus formation may have been excessive, due to too much motion at the fracture site. The use of fibre-reinforced polymer composite materials offers the widest possible range of mechanical properties.
Ltd. 0142-9612/90/080541-07 8iomaterial.s
1990. Vol 11 October
541
Short carbon fibre reinforced thermoplastics: S.A Brown et al.
Long-fibre and fabric laminates have been studied by a number of groups 6,17-20. The flexural modulus of these materials ranged from 1.5 to 5 times that of bone. Tayton et a/.” observed rapid and effective healing in a clinical study using long carbon fibre-reinforced epoxy laminates, there was, however, some delamination of the plates22. Shortfibre-reinforced thermoplastics have been studied to a lesser extent, but showed excellent results in studies with canine femoral fractures fixed with nylon and pofybutylene terephthalate (PBT) composites reinforced with 30% carbon fibresz3. Short-fibre thermoplastic composites can be easily processed with injection moulding. Furthermore, they are thermoductile, in thattheycan be heated and contoured to fit the shape of the bone, without the delamination problems associated with long-fibre composites2*. This thermoductility was the focus of the present study. The present study involved the selection and evaluation of a few short-fibre-reinforced thermoplastics. These were selected to facilitate heat contouring of the devices. The selection criteria were that the flexural modulus should be less than bone to avoid stress shielding, and that the strength should be greater than bone for initial fracture stabilization24 and that the material fracture toughness should be greater than bone25 to guarantee satisfactory resistance to crack propagation from stress concentration sites. Three matrix materials were selected. Polysulphone (PS) was selected based on its demonstrated biocompatibility26 and its use as a long-fibre composite27.28. PBT was selected based on the clinical use of a similar polyester PETand on our previous studies5* 16. Polyetheretherketone (PEEK) was selected based on its chemical stability2s*30 and preliminary studies on its bi~om~tibili~31. The materials were reinforcad with polyac~lonitrile (PAN) fibres due to their known biocompatibility32-35. The objectives were to measure the effects of clinically relevant procedures on the mechanical properties. These procedures included steam sterilization, heat contouring and soaking in saline to simulate the in viva environment.
MATERIALS
AND METHODS
Materials The three materials selected were 30% chopped PAN carbon fibre composites, PS-30PANC, PBT-30PANC and PEEK-BOPANC. Samples were donated as flexural testing bars by LNP (PS and PBT) and Wilson Fiberfil (PEEK). All samples were 6.4 X 12.5 X 125 mm. Specimens were prepared using metallographic techniques to determine average fibre dimensions. The fibres ranged from 6 to 9 E.rrn in diameter with length in the range of 0.14-0.31, 0.240.54 and 0.18-0.40 mm for PS, PBTand PEEK, respectively.
Sterilization
(STZ)
Steam sterilization was performed using a standard autoclave at 90 KPa, 12 1 “C, for 20 min. The samples were placed on a metal basket to minimize contact with water and thus reduce any possible water absorption. The autoclave was set to exhaust rapidly and the procedure was repeated. All the samples went through two sterilization cycles, hence the designation ST2. All mechanical tests on ST2, sterilized-only samples, were performed under standard laboratory conditions at least 24 h after being sterilized.
542
3iomat~r~als f990,
Vol f f October
Contouring (CON) Heating the samples forcontouring was accomplished using a small electric oven (Proctor Silex toaster oven). Heating temperatures and times were selected based on the thermal properties of the matrix materials and the deformability of the heated specimens. The PS specimens were heated for 5 min at 220°C (Ts = l SOYI), the PBTspecimens for 6 min at 232°C (T, = 220°C) and the PEEK specimens for 5.5 min at 250°C (T, = 148”C, r,,., = 314°C). To avoid possible effects of gravity on the contouring, the plates were placed in the oven resting on their thin sides. The heated samples were immediately piaced in a surgical plate bending press (Zimmer). The normal fixture in the bending press, which is curved to fit the semitubular cross-section of metal plates, was replaced with an aluminium fixture with flat surfaces to allow for even distribution of stress across the surface of the fiat samples, reducing the possibility of cracking the plates during the bending operation. Pressure was applied to bend the plate and the position and pressure were maintained for a period of 30-45 s. This alfowed the material to cool to a point at which the deformation was permanent. The pressure applied to the plates was not quantified and was based on the subjective feeling of the resistance of the material to plastic deformation. The degree of contouring was measured by placing the samples on a flat surface and using a micrometer to measure the height. Preliminary studies demonstrated that the maximum contouring angles achievable without cracking for the PS, PBTand PEEKcomposites were 5.9”, 2.9’ and 5.8”, respectively. The total contouring procedure before mechanical testing involved heating the sample, bending it in the middle, heating the sample and bending one end, heating and bending the other end and then heating and reverse bending the whole sample to return it to its original shape. The samples were allowed to cool for approximately 20-30 min between heating cycles. The cross-sectional dimensions of each sample were measured in four places and averaged to check for any changes caused by the heating.
Saline soaking (SAL) The simulated in vim environment consisted of soaking in sterile physjol~jcal saline (0.9% NaCl) at 37°C for 3 wk. Samples were placed in individual test tubes and the saline added until each sample was covered. The tubes were then capped and placed in an incubator. The time span of 3 wk was selected based on preliminary water absorption studies which had shown that a significant portion of the total water absorption had occurred during this time. To prevent drying, mechanical tests performed after soaking were conducted in an environmental chamber filled with saline at 37” + 0.5”C.
Flexion tests Flexion tests were conducted according to ASTM standard D790M (Flexural Properties of Unreinforced and Reinforced Plastics and Electrical Insulation Materials) on a universal testing machine (Scott GRE/lOOO) equipped with a strip chart recorder to expand the deflection scale. The samples were tested in three-point bending with a span to a depth ratio of 16: 7 and a cross-head speed of 2.6 mm min-‘. The central nose-piece was positioned directly over the region that had been contoured, to determine the local effects of contouring. The flexion test of the contoured samples was set up so that the sample was loaded in the same direction in
Shon carbon fibre reinforced thermoplastics: S.A. Brown ef al.
sterilized, saline soaked and then tested (ST2SAL). A fourth group was sterilized, contoured, saline soaked and tested (STZ-CON-SAL). The prenotched ST2 and STZ-CON fracture toughness specimens were air-aged (AAG). while the ST2SAL and ST2-CON-SAL specimens were aged in saline (SAG). A one-way analysis of variance (ANOVA) was performed on the detailed study results. The differences between pairs of means was determined using multiple comparison. The least significance difference (LSD) was calculated using the standard error of the difference between the means and multiplying it by the 5% value oft with the degree of freedom equal to l(n - 1) where I was the number of groups and n was the number of samples in the group. Once the LSD was obtained, any difference between two groups greater than this value was considered significant to P < 0.05.
which it had been contoured. After contouring, most plates were not perfectly straight and had a very slight bow to them.
Fracture toughness testing From each flex bar tested, two fracture toughness samples obtained. The middle of these samples was located and a line was drawn perpendicular to the edge, to approximately the middle of the width dimension, as a guide for cutting the notch with a fine toothed (six teeth/cm) band saw blade. A razor blade held in the three-jaw chuck of a milling machine (Bridgeport) was used to create a sharp crack, approximately 0.70 mm deep. A new razor blade was used for each notch. There is no standard method for testing the fracture toughness of polymeric materials. The ASTM standard E399 (Plane-strain Fracture Toughness of Metallic Materials) was used, as it has been used as a common guideline for plastics and composites that exhibit linear elastic behaviour. The samples were tested in a three-point bending fixture at a cross-head speed of 12.7 mm min-‘, to achieve a rate of the range of increase of stress intensity within 0.55-2.75 MPa.m”‘/s. The sample was placed in the fixture so that the saw cut was directly underneath the loading nose-piece. After testing, a tangent line was drawn along the linear portion of the load-deflection curve on the strip chart. A secant line was then drawn on the plot through the origin of the tangent line, but with a slope of 95% of the tangent slope. The load (Pq) at the point where the secant line intersected the curve was used in the calculation of K,,. were
K,, = (P,S/f3W3’2).f(a/W)
(1)
where S is the span, B is the thickness, W is the depth, a is the crack length and f(a/W) is given by ASTM standard E399. The calculated stress intensity factor was then checked against the criteria given in ASTM E399, to determine if they represented a plane-strain fracture toughness value. Fracture energy values for each treatment were calculated from the average fracture toughness values using the following equation: G, = Kc2( 1 - v2)/E
(2)
where G, is the fracture energy, Kc is fracture toughness, v is Poisson’s ratio and E is the flexural modulus. Poisson’s ratio was assumed to be 0.40 in these calculations.
Ageing (AAG and SAG) Ageing studies involved the fracture toughness samples only. The samples were notched with a razor blade as described above and placed in individual test tubes. The airaged samples (AAG) were placed in an incubator at 37°C for 3 wk. Sterile saline (0.9% NaCI) was added to the test tubes of the saline-aged samples (SAG) before ageing for 3 wk in an incubator. The AAG samples were tested in air and the SAG samples were tested in saline at 37°C.
Experimental protocol All specimens were steam sterilized twice (ST2) at 121 “C for 20 min before any testing and divided into four groups of samples for testing for flexural strength and fracture toughness. One group of five samples per material was tested after sterilization (STZ). A second group was sterilized, contoured and tested (STZ-CON). A third group was
RESULTS The results for flexural testing are shown in Table 1 and Figures 1-3. The strength of the PS composite was significantly decreased after the saline soaking and increased by contouring. The strength of the PBT composite was significantly reduced by all treatments. The strain to failure of the PBT was reduced by contouring. The flexural modulus of the PBT composite was increased by contouring, but decreased by saline soaking. The flexural properties of the PEEK composite were not affected by any of these treatments. The effects of these treatments on the fracture toughness of the composites is shown in Table 2 and Figure 4. Contouring resulted in a small decrease in the fracture toughness of PS and a significant decrease in the fracture toughness of the PBT composite, whilst saline soaking and soaking after contouring resulted in a significant decrease in fracture toughness of the PS and PBTcomposites. The fracture toughness of PEEK was not affected by any treatments. The effects of the treatments on the flexural properties and fracture toughness were also determined from a statistical analysis similar to that used in factorial experimentation. The effects of contouring and soaking are summarized in Tab/e 3. Contouring significantly increased the flexural strength of the PS composite. Contouring significantly
Table 1 Flexural propeflies of PS-3UPAK 3OPANC composites after ST2 CON and SAL
PBT-3OPANC
and PEEK-
FleXUral strength (MPa)
Strain to failure (%)
Modulus (GPa)
178.4(9.4) 190.5(14.0)’ 149.9( 10.0)’ 164.7( 10.4)
1.98(0.3 1) 1.96jO.20) 1.86(0.15) 1.85(0.29)
10.64(1.70) 1 1.36( 1.40) 9.32( 1.39) 10.51(1.47)
ST2 ST2-CON ST2-SAL STZ-CON-SAL
167.9(5.7) 1 t9.8( 19.0)’ 145.Q3.7)’ 110.2(5.7)’
1.45(0.06) 0.93(0.14/ 1.39jO.03) 1 .OO(O.O6j*
13..?6(0.34) 14.16(0.39)* 11.98(0.14)’ 12.28(0.32)*
PEEK-BOPANC ST2 STZ-CON ST2-SAL STZ-CON-SAL
272.4(18.5) 277.2(10.6) 255.7(10.7] 271.9(5.0)
2.13(0.11) 2.27(0.18) Z.lO(O.25) 2.35(0.17)
15.56( 1.76) 15.07( 1.44) 15.21(1.04) 14.55(0.73)
Condition
PS-30PANC ST2 ST&CON STZ-SAL ST2-CON-SAL PET-30PANC
Numbers in parentheses are standard deviations. five samples per group. indicates significant differences (P < 0.05) from ST2. l
Biomaterials
1990. Vol 11 October
543
Short carbon fib? mk=hrced
rhemwpiastics: S.R 5mwn et at. Table 2 Fracture roughness of PS-JOPANG, PEEK4’OPANC composites afrer ST2 CON and SAL
PST-36 PANC
PS-30 PANC
PEEK-30 PANG
Figure 9 flexufal strength data for the three test materiafa after steam sterilization {ST2j* ccznrounrrg,saline soaking for 3 wk at 3 7°C jSALf/VE] end cantouring and then saline soaking (CONT-Sill]. (El} ST2 (5) contoured, (?Bj SALINE @I/ CONT-SAL
PBT-3C’PANc
and
Condition
n
Yield strength, K, (MW
Fracture toughness, G, fMPa’m”*)
Fracture energy &J/m21
PS-30PANC ST2 ST2-CON ST2-SAL STZ-CON-SAL
5 4 5 5
167.8(13.10) 176.8(17.50) 146.7f9.07) 157.5(6.48)
5.59(0.23) 5.47(0.22) 4.97(0.32)* 4.87(0.58)*
2.47 2.21 2.23 1.90
PBT-JOPANC ST2 ST2-CON ST&SAL STZ-CON-SAL
5 3 5 5
166.6(3.00) 11 S.S(l3.00) 145.6(3.73) 1 f 0.2f5.70)
4.7 l(O.44) 4.06(0,34) 4.35(0.12)* 3.86(0.t0)*
1.41 0.98 1.33 1.02
PEEK-30PANC ST2 STZ-CON SM-SAL STP-CON-SAL
2 5 5 5
269.4{3 1JO) 236.9(12.10) 224.1(7.53) 224.4(8.88)
7.38(0.05) 7x33(0.57) 7.45(0.35) 7*oa(o,t 7)
2.79 2.74 3.Q7 2.88
Numbers in parentheses are standard deviations. five samples per group. * Indicates significant differences (P < 0.05) from ST2.
PS-30
PI3 T -30 PANC
PANC
PEEK-30
PANC
PS-30 PANC
8
PBT-30
PS-30 PANC
PANC
PANC
PEEK-30
PBT-30
PANC
PEEK-30 PANC
decreased the strength, strain to failure and fracture toughness and increased the flexural modulus of the P5T c~mp~site. Contouring had no effect on PEEK. The main effects of saline soaking were to decrease the strength and fracture toughness of the PS and PBT composites and to decreasetheflexural modulusof the PBT. Salinesoaking had no effect on the PEEK composite. The results of fracture toughness testing after air and saline ageing are shown in Table 4 and presented graphically in Figure 5. The figure shows the pm-aged results as open bars, with their respective-aged results as cross-hatched bars. For PS, air ageing resulted in a small reduction in
feble 3 Main effact of contouring and saline soaking on the flexural properties and fracture toughness of the composite materials. One-way ANOVA with multiple comparison, significance at P < 45
Contouring (CON)
Material
Stress
544
Strain
MOD
%
Stress
a
I
D
D D
I
PS-JOPANC PBT-30PANC PEEK-SOPANC
I = increase, 0 =
Saline soaking (SAL)
a
decrease.
&?mafef&fs
1990, Vol 1 I October
Strain
MOD
Kit
D
D 0
Short carbon frbre remforced rhermoplastics: .%A ffrown et al.
Table 4
Fracrure toughness values af?er a,r and saline agetng n
Condmon
Yield strength
Fracture,tyghness. K,,
(MPat
(MPa‘m
167.8( 13 tot 176.U 17.50) 157.516.48)
5.16f0.48) 5.09f0.36) 4.8 l(O.42)’ 4.54(0.68)
2.10 2.34 1.71 1.65
Fracture energy, G,, (kJ/m2)
) _
PS-3OPANC STZ-AAG ST2-SAL-SAG STZ-CON-AAG ST2-CON-SAL-SAG
5 5 5 5
PBT-30PANC STZ-AAG STZ-SAL-SAG ST2-CON-AAG STZ-CON-SAL-SAG
5 5 5 5
166.6(3.00) 145.6(3.73) I 19.8(19.00) 110.2( 5.70)
5.07(0.17) 4.03(0.08)’ 3.90(0.47) 3.56jO.26)’
1.63 1.14 0.90 0.87
PEEK-30PANC STZ-AAG STZ-SAL-SAG ST2-CON-AAG STZ-CON-SAL-SAG
2 5 5 5
259 4(3 1.90) 224.1(7.53) 236.9(12.10) 224.4(8.88)
7.15jO.08) 7.36jO.33) 7.56(0.29) 7.40(0.23)**
2.79 2.99 3.19 3.16
l
**
746.7(9.07)
Indicates significant decrease (P < 0.05) due to ageing. lndxx%es a significant increase.
P
_..
PET-30
PS-30 PANC
PANC
PEEK-30
PANC
Figure 5 Effect of air ageing and saline ageing on test materials. (OJ ST2, ,%?.Iair aged, (E%Jsaline aged From ieft to right for each material each open and cross-hatched column pair represents: ST2, STZ-air aged: CONT. CONTairaged; saline, saline-saline aged; and CONT.SAL COOT-SA~~sai~ne aged
Figure 6 Typical SEM ph~tomjcrograph of PS-3OPANC STZ-CON fracture surface showing no fibre-matrix bo~d;ffg and fibre pu//ouf (original magnificarion X 2OOOJ.
fracture in
toughness
fracture
significant. changes.
of the ST2-AAG
toughness Saline
specimens;
ST2-CON-AAG
ageing
SAL-SAG
decrease
for both STZ-SAL-SAG
specimens.
resulted
Ageing
in no changes,
toughness
of ANOVA
ageing
caused
PS-30PANC SEM the three
a
and
PBT-30PANC.
data
decrease
in
Ageing
caused
had no effect
was used to characterize composites.
Some
circles
which
were
pulled out. The fractures appearance. X2000
are shown
of
fibre-matrix 6
in Figures interface minimal
shows
PS-30PANC.
were
Figure
bonding
between
contrast,
Figure
the carbon 8 shows
of
a decrease
for
failure
notably
surfaces
many small
fibres
had
rough and chunky
to demonstrate
samples.
bonding
of detectable,
PEEK
composite
covering
of matrix
for
micrographs
but poor,
microscopic
and the PBT matrix.
that the fibre-matrix
in
Figure 7 Typical SEM photomicrograph of PET-JOPANC STZ-CON fracture surface. showing minimal fibre-matrix bonding (original magnification X2000).
at
the nature
ST2-CON
fibre-matrix fibres
been
photomicrographs
in the
of
mechanisms
revealed
where
7 is an example
air
on PEEK-30PANC.
SEM 6-8
The that
toughness
the fracture
common
sockets
Representative
essentially
demonstrated fracture
ageing
in
in the fracture
ST2-CON-SAL-SAG.
on these saline
resulted
and ST2-CON-
an increase
were seen in all three. &lose inspection
Figure
significant
PEEK specimens
except
of the SAG specimens
results
the
statistically
of PS did not cause
The saline ageing of the PBTspecimens
a significant
dark
the decrease
was
bond
In
in the
PBT
is very
strong,
indicate
a difference
level in the regions
composites
indicated
by a complete
on most of the pulled-out
exhibited
whilst the PEEK composite
in failure
photographed.
fibre-matrix primarily
Bfomarerials
fibres.
These
modes
at the
The PS and
interface
showed
failure,
matrix failure.
1990, Vol 1 I October
545
Shon carbon fibre reinforced
thermopiastics:
5.A Brown et al
took place during the first 3 wk, after which the absorption
Figure 8 Ty~ic~i SEM ~hromicrugfaph of PEEiG3OPANC SIZ-CON fracture swface, showing extensive fibre-matrix banding (original magnification x2000/.
Metallic fracture fixation plates in current use need to be ductile, since they are frequently bent or twisted to fit the contours of the bone. It was this need for intra-operative formability that led to the selection of composite materials which demonstrate thermoductility, the capacity to be contoured with the application of heat. Thermoplastic matrix materials, reinforced with chopped carbon fibresfor increased strength and stiffness, show thermoductility. This led to the selection of three of this class of materials for evaluation. The three candidate materials also had good mechanical strength, with an elastic moduli less than that.of bone. The objective of these studies was to characterize the candidate materials for use as fracture fixation plates. F lexural strength testing was selected as an appropriate test method, since plate failure is usually associated with bending due to fracture instability’. Fracture toughness testing was also selected since it characterizes the resistance of a material to fracture in the presence of a sharp flaw. The fracture toughness of a composite material is dependent on the properties of the matrix and of the fibre reinforcement and the interface between the fibre and matrix. Addition of a rigid fibre to a ductile polymer matrix frequently reduces the fracture toughness of the composite whilst increasing the strength and modulus. The rigid fibres restrain the elongation of the matrix between fibres, producing a more brittle fracture36. For a brittle polymer matrix, however, the addition of rigid fibres frequently significantly increases the fracture toughness through crack blunting and branching, whilst increasing the strength. The applicability of linear elastic fracture mechanics to characterize the toughness of polymers has been verified by Nguyen and Moet3’ and Friedrich et a1.38. The conditions under which the specimens were tested were selected based on the clinical application. Since both sterilization by steam autoclave and repeated autoclaving are traditional for fracture fixation plates, all specimens were sterilized twice as a precondition. The heat contouring process was developed based on the thermal properties of the materials and on previous experience of bending without cracking the materials. Although the heat cycles were not optimized, they do provide a database for comparison. Saline soaking was conducted to simulate in viva exposure. An exposure time of 3 wk was selected, based on preliminary 18 wk studies which had shown that 70% of the absorption
546
Biomateriafs
1990, Vol I I October
rate was quite slow. Ageing was conducted to determine if there were any significant changes in fracture toughness after the precfacked samples had aged either in air or saline. Such changes might reflect plastic flow stress relieving and crack tip blunting or environmental effects on fibre-matrix bonding at the crack tip. PS is amorphous in nature and its glass transition temperature (T,} is 1 90”C3’. It possesses good thermal stability and rigidity up to temperatures of approximately 150°C. The ether linkages provide chain flexibility, imparting good impact strength. The polymer resists hydrolysis, but is susceptible to attack by organic solvents such as ketones and chlorinated or aromatic hydrocarbons. The PS-SOPANC composite demonstrated an increase in strength after heating and contouring. Robeson et a1.40 have also reported an increase in strength due to heating which may be indicative of a high-temperature annealing of the material. However, the increase in fracture toughness after contouring observed in the present study was not statistically significant. The saline soaking (STZ-SAL) and contouring (STZ-CON-SAL) both resulted in loss of flexurai strength, the former being significant, although saline ageing did not cause a decrease in fracture toughness. The lack of fibre-matrix bonding seen in the SEM may suggest that moisture could penetrate and interact with the interfacial bond. it is of interest that air ageing also caused a decrease in fracture toughness to a value close to that after saline soaking and ageing. ln this case, atmospheric moisture may have penetrated the fibre-matrix interface. it is also plausible that increased fibre debonding could occur, due to the mismatch of the thermal expansion coefficients of the fibre and the matrix. PBT is partially crystalline with regions of different degrees of order ranging from highly ordered crystallites to completely amorphous regions3’. Because of the two different components, it exhibits a 7, of 40°C and a melt temperature of 220°C. The material is resistant to water, weak acids and bases, ketones, alcohols and aliphatic and chlorinated aiiphatic hydrocarbons. The resin is sensitive to a~kal~es,‘oxidiz~ng acids and some phenols. The PBTcomposite was the most difficult to contour in this study. Heating to 10°C above the melt temperature for 6 min resulted in some superficial melting of the plates. Whilst this resulted in the surface becoming soft and flexible, the plates could only be contoured to a deflection angte half that of the other materials. Although the specimens were not visibly cracked after contouring, the treatment resulted in significant loss of mechanical properties. The increase in modulus may be an indication of a change in c~staliinity, resulting in a more brittle material. The SEM in Figure 7 suggests a moderate degree of matrix bonding to the fibres. The loss of strength due to saline soaking and saline ageing may be due to degradation or to hydrolysis. PEEK is a semicrystalline high-performance thermoplastic consisting of linear aromatic chains”‘. The r, is 143°C and the melt temperature is 334°C. Industrial grade PEEK can be obtained with crystallinity up to 48%42. There is sufficient chain mobility at a temperature of 170°C for the material to undergo nucleation and crystallization. Mouldings can be annealed for 2 min at 300°C or 1 h at 200°C to insure that a highly uniform crystallinity develops. It has outstanding resistance to abrasion and dynamic fatigue and is resistant to chemical attack over a wide range of pH levels, from 60% sulphuric acid to 40% sodium hydroxide. No organic solvent attack has been observed on moulded
Short car&on fibre reinforced the~maptastics: .%A Bmwn et ai.
partsz9. The material has been extensively tested with respect to aerospace applications3’. In this study, the PEEK-30PANC composite was the easiest to contour, despite having the highest thermal and best mechanical properties. The mechanical properties of the material were not significantly affected by any of the conditions studied. The SEM demonstrated extensive fibrematrix bonding. Preliminary biocompatibility test results have been encouraging3’. For these reasons, it was concluded that the PEEK-30PANC composite is an excellent candidate for further study for such applications.
ACKNOWLEDGEMENTS
18
methacylate
REFERENCES
19
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24 25 26
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10
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12
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15
16
17
Mueller, M.E., Allgower, M. and Willenegger. H.. Manual offntemaf Fixerion, Springer-Vertag, Berlin, 1970 McKibbin, 8.. The biology of fracture healing in long bones, J. Bone Joini Surg. 1978.808, 150-l 82 Ham, AW. and Cormack, D.H.. Histophysio/o~of Cartilage, Bone and Joints, Lippincott, Philadelphia, USA, 1979 Uhthoff, H.K. and Dubuc. F.L., Bone structure changes in the dog under rigid internal fixation. C/i/f. Onhop. 197 1, 81, 185- 170 Brown, S.A., Gillett, N.A. and Broaddus, T.W.. flexible versus nonflexible fracture fixation, in Fracture Healing (Ed. J.M. Lane), Churchill Livingstone. New York, USA, 1987, pp 19 l-202 Woo. S.L-Y.. Akeson, W.H., Coutts, RD., Rutherford, L., Doty, D., Jemmott, G.F. and Amiel. D.. A comparison of cortical bone atrophy secondary to fixation with plates with large differences in bending stiffness, J. Bone Joint Surg. 1976. 58A, 190-l 95 Wolff, J., Das Gesetz Der Transformation Der Knochen, Hirschwald, Berlin, 1892 Lanyon, L.E. and Rubin, CT.. Static vs dynamic toads as an influence on bone remodelfing 1984, .I. B~umech. 17. 897-905 Akeson. W.H., Woo. S.L-Y., Coutts, R.D.. Matthews, J.V., Gonsatves, M. and Amiel, 0.. Quantitative histological evaluation of early fracture healing of cortical bones immbolized by stainless steel and composite plates, Calcif. Tissue 1975, 19. 27-37 Uhthoff, H.K., Bardos, D.I. and Liskova-Kiar, M., The advantages of titanium alloy over stainless steel plates for internal fixation of fractures, an experimental study in dogs, .I. Bone Joint Surg. 198 1, 630.427-434 Tonino. A.J., Davidson, C.L., Klopper. P.J. and Linclau, L.A., Protection from stress in bone and its effects, ./. Bone Joint Surg. 1976. 586. 107-l 13 Baggon, D.G., Goodship, A.E. and Lanyon, L.E., A quantative assessment of compression plate fixation in vivo: an experimental study using the sheep radius, & Biomech. 198 1, 14, 701-7 11 Hidaka. S. and Gustilo, R.E., Refracture of bones of the forearm after plate removal, J. Bone Joint Surg. 1984, 66k t 241-1243 Duluca. P.A., Lindsey, R.W. and Ruwe, PA., Refracture of bones of the forearm after the removal of compression plates, .I. Bone Joint Surg. 1988,70A, 1372-l 376 Goodship, A.E. and Kenwright, J., The influence of induced micromovement upon the healing of experimental tibiai fractures, J. Bone Joinr Surg. 1985, 678. 650-655 Brown, S.A. and Vandergrift, J.. Healing of femoral osteotomies with plastic plate fixation, Biomater. Med. Dev. Artif Organs 198 1, 9. 27-35 Bradley, J.S.. Hastings, G.W. and Johnson-Nurse, C., Carbon fibre reinforced epoxy as a high strength. low modulus material for internal fixation plates, Biomaterials 1980. 1, 38-40
resin composites internal fixation plates. J. Biomed.
Mater. Res. 1974.
23
The authors wish to thank LNP and Wilson Fiberfil for donation of flex bar specimens used in this study.
Woo.S.L.-Y., Akeson, W.H., Levenetz, B., Couns. R.D., Manhews, J.V. and Amiel, D., Potential application of graphite fiber and methyl
27
28
29
30
31
32
33
34 35
36
37
38
39
40
41 42
8, 321-338
Bradley, G.W., McKenna. G.B., Dunn. H.K.. Daniels, A.U. and Statton. E.O., Effects of flexural rigidity of plates on bone healing, J. Bone Joint Surg. 1979,81A. 866-887 Claes. L., Huttner, W. and Weiss, R.. Mechanical propenies of carbon fibre reinforced polysulfone plates for internal fracture fixation. in Biological and Biomechamcaf Performance of Biomererials, (Eds A Christel. A. Meunier and A.J.C. Lee). Elsevier, Amsterdam, 1986. ~~81-86 Tayton. K.. Johnson-Nurse, C., McKibbin. B.. Bradley, J.and Hasttngs, G., The use of semi-rigid Cain-fibre-reinforced plastic plates for fixation of human fractures, J. Bane Joint Surg. 1982, 848, 105-l 11 Bradley, J.S., Cautionary note on the strength of materials for internal fixation devices, Biomateriak 198 1, 2. 124 Gillett, N.. Brown, S.A.. Dumbleton, J.H. and Pwl, R.P.. The use of short carbon fibre reinforced thermoplastic plates for fracture fixation, Biomaterials 1985, 6, 113-l 21 Gies, A.A. and Carter, D.R.. Experimental determination of whole long bone sectional properties, J. Biomech. 1982, 16, 297-305 Bonfield. W. and Datta, P.K., Fracture toughness of compact bone, J. Biomech. 1976, 9. 121-134 Behling, CA. and Spector. M., Ouantitative characterization of cells at the interface of long-term implants of selected polymers, J. Biomed. Mater Res. 1986, 20, 653-666 St. John, K.R., Applications of advanced composites in orthopaedic implants, in Biocompati&fe Polymers, Metals, and Composites, (Ed. M. Szycher), Technomic, Lancaster, PA, USA, 1983, pp 861-871 Magee, F.P.. Weinstein, A.M., Longo, J.A., Koeneman, J.B. and Yapp, R.A., A canine composite femoral stem: an in viva study, C/in. Qrtbop. 1988.235.237-252 Stober, E.J.. Seferis, J.C. and Keenan, J.D. Characterization and exposure of PEEK to fluid environments, Polymer 1984, 25, 1845-l 852 Searle. O.B. and Pfeiffer. R.H., Vitrex Poly(ethersulfone) (PES) and Victrex Poly(etheretherketone) (PEEK), Po/ymer. Eng. & Sci. 1985, 26.474-476 Williams, D.F., McNamara. A. and Turner, R.M., Potential of polyetheretherketone (PEEK) and carbon-fibre-reinforced PEEK in medical applications, .I. Mater. Sci. Let. 1987, 8. 188-l 90 Neugebauer. R., Helbing. G.. Wofter, 0.. Mohr, W. and Gistinger, G., The body reaction to carbon fibre particies implanted into the meduffary spaces of rabbits. Bjomate~~afs 198 1, 2. 182- 184 Jenkins, D.H.R. and McKibbin, 6.. The role of flexible carbon fibre implants as tendon and ligament substitutes in clinical practice, J. Bone Joint Surg. 1980, 828.497-499 Tayton, K., Phillips, G. and Ralis, 2.. long-term effects of carbon fibre on soft tissues, J. Bone Joint Surg. 1982.848, 1 12-l 14 Goodship, A.E., Wilcock, S.A and Shah, J.S., The development of tissue around various prosthetic implants used as replacements for ligaments and tendons, C/in. Orthop. 1985, 198, 6 l-68 Malzahn. J.C. and Friedrich, K.. Fracture resistance of short glass and carbon fibreipolyamide 8.6 composites, J. Mafer. Sci. Let. 1984, 3, 861-866 Nguyen, P.X. and Moet, A., The effects of fiber-matrix adhesion on the fatigue fracture of glass-fiber-reinforced polyfvinyl chloride), Polymer Composites 1987, 8, 298-306 Friedrich, K.. Walter, R., Voss, H. and Karger-Kocsis, J., Effects of shon fibre reinforcement on the fatigue crack propagation and fracture of PEEK-matrix composites, Composites 1986, f 7, 205-2 16 Fox. D.W. and Peters. E.N., Engineering thermoplastics: chemistry and technology, in Applied Polymer Science, (Eds R.W. Tess and G.W. Poehlein), ACS, Washington, USA, 1985, pp 495-5 15 Robeson, L.M., Dickinson, B.L. and Crisafulli, S.T., Hydrolytic stability of high Tg engineering polymers: relevance to steam sterilization, Polymer News 1986, 11, 359-365 Nguyen, H.X. and Ishida, H., Poly(aryl-ether-ether-ketone) and its advanced composites: a review, Poly Camp. 1987, 8, 57-73 Lee, Y. and Porter, R.S.. Crystallization of PEEK in carbon fiber composites, Polymer Eng. & Sci. 1986, 28, 633-639
Biomaterials
1990. Vol 11 October
547
Sciences. Case Western Reserve University, Cleveland,
This study focuses on determining the effects of clinically relevant procedures on the flexural and fracture toughness properties of three short-fibre thermoplastic composites for potential application as fracture fixation devices. The procedures included sterilization, heat contouring and saline soaking. The three materials tested were polysulphone, polybutylene terephthalate and polyetheretherketone. all reinforced with 30% short carbon fibres. The polysulphone composite showed significant degradation in mechanical properties due to saline soaking. The polybutylene terephthalate exhibited significant degradation of mechanical properties following both contouring and saline soaking. The polyetheretherketone composite, however, exhibited no degradation in mechanical properties. The results demonstrated that flexion and fracture toughness testing were effective for determining the response of the composites to different applied conditions and demonstrated the stability of polyetheretherketone subjected to these treatments. Scanning electron microscopy demonstrated the most effective fibre-matrix bonding to be in the polyetheretherketone. Keywords: Composites. fracture plates, mechanical properties
Although many bone fractures can be controlled by the application of external forces, as with a plaster or fibreglass cast, it is often necessary to apply some form of internal fixation device surgically. There are a number of methods for immobilizing and stabilizing a fracture, each with its special indication. The specific method to be addressed in this study was internal fixation with plates and screws. The standard plates are made of 316L stainless steel or titanium alloys. The selection of materials was based in part on their ductility; plates are usually contoured by bending and twisting to fit the bone. Much of the development of these rigid plates is the result of the evolution of the concepts of rigid fixation advocated by the AO/ASIF’ . The healing of fractures in the absence of rigid fixation is by rapid proliferation of fracture callus and subsequent remodelling 2*3. The use of rig id devices is associated with minimal callus formation; it is ‘as if the bone didn’t know it was broken’3. Healing is accomplished by a slow remodelling process called primary healing’. Another problem with using rigid fixation systems is the long-term stress4r5 or strain6 Correspondence to Professor S.A. Brown. 7 Present address: Office of Device Evaluation (HFZ-410). Ave., Silver Spring, MD 209 10, USA. @ 1990
Butteworth-Heinemann
8757
Georgia
shielding of the underlying bone. The response of bone to stress is referred to as Wolff’s Law’ which can be paraphrased as ‘the amount of growth in a bone depends on the need for it’. Lanyon and Rubin’ concluded that it was the functional or dynamic strain that controlled bone remodelling. This load sharing by the plate’ has been associated with cortical thinning4P’0,“, increased porosity4.‘0-‘2 and bone formation at the ends of the plate due to stress transfer and remodelling of the opposite cortex due to neutral axis shift5. Removal of plates to avoid long-term stress shielding effects has been associated with bone refracture13, 14. One solution to the problem of rigid fixation is the use of flexible or less rigid systems. A less rigid device allows for external callus formation resulting in a shorter healing time 2. 5. 15.Titanium has a modulus half that of stainless steel and its use has resulted in less stress shielding and bone resorption 4.6slo . The use of plastic materials with strengths comparable to that of bone, yet with one tenth the stiffness of bone, has demonstrated rapid healing with external callus and effective remodelling5* “. 16. However, in some instances, the callus formation may have been excessive, due to too much motion at the fracture site. The use of fibre-reinforced polymer composite materials offers the widest possible range of mechanical properties.
Ltd. 0142-9612/90/080541-07 8iomaterial.s
1990. Vol 11 October
541
Short carbon fibre reinforced thermoplastics: S.A Brown et al.
Long-fibre and fabric laminates have been studied by a number of groups 6,17-20. The flexural modulus of these materials ranged from 1.5 to 5 times that of bone. Tayton et a/.” observed rapid and effective healing in a clinical study using long carbon fibre-reinforced epoxy laminates, there was, however, some delamination of the plates22. Shortfibre-reinforced thermoplastics have been studied to a lesser extent, but showed excellent results in studies with canine femoral fractures fixed with nylon and pofybutylene terephthalate (PBT) composites reinforced with 30% carbon fibresz3. Short-fibre thermoplastic composites can be easily processed with injection moulding. Furthermore, they are thermoductile, in thattheycan be heated and contoured to fit the shape of the bone, without the delamination problems associated with long-fibre composites2*. This thermoductility was the focus of the present study. The present study involved the selection and evaluation of a few short-fibre-reinforced thermoplastics. These were selected to facilitate heat contouring of the devices. The selection criteria were that the flexural modulus should be less than bone to avoid stress shielding, and that the strength should be greater than bone for initial fracture stabilization24 and that the material fracture toughness should be greater than bone25 to guarantee satisfactory resistance to crack propagation from stress concentration sites. Three matrix materials were selected. Polysulphone (PS) was selected based on its demonstrated biocompatibility26 and its use as a long-fibre composite27.28. PBT was selected based on the clinical use of a similar polyester PETand on our previous studies5* 16. Polyetheretherketone (PEEK) was selected based on its chemical stability2s*30 and preliminary studies on its bi~om~tibili~31. The materials were reinforcad with polyac~lonitrile (PAN) fibres due to their known biocompatibility32-35. The objectives were to measure the effects of clinically relevant procedures on the mechanical properties. These procedures included steam sterilization, heat contouring and soaking in saline to simulate the in viva environment.
MATERIALS
AND METHODS
Materials The three materials selected were 30% chopped PAN carbon fibre composites, PS-30PANC, PBT-30PANC and PEEK-BOPANC. Samples were donated as flexural testing bars by LNP (PS and PBT) and Wilson Fiberfil (PEEK). All samples were 6.4 X 12.5 X 125 mm. Specimens were prepared using metallographic techniques to determine average fibre dimensions. The fibres ranged from 6 to 9 E.rrn in diameter with length in the range of 0.14-0.31, 0.240.54 and 0.18-0.40 mm for PS, PBTand PEEK, respectively.
Sterilization
(STZ)
Steam sterilization was performed using a standard autoclave at 90 KPa, 12 1 “C, for 20 min. The samples were placed on a metal basket to minimize contact with water and thus reduce any possible water absorption. The autoclave was set to exhaust rapidly and the procedure was repeated. All the samples went through two sterilization cycles, hence the designation ST2. All mechanical tests on ST2, sterilized-only samples, were performed under standard laboratory conditions at least 24 h after being sterilized.
542
3iomat~r~als f990,
Vol f f October
Contouring (CON) Heating the samples forcontouring was accomplished using a small electric oven (Proctor Silex toaster oven). Heating temperatures and times were selected based on the thermal properties of the matrix materials and the deformability of the heated specimens. The PS specimens were heated for 5 min at 220°C (Ts = l SOYI), the PBTspecimens for 6 min at 232°C (T, = 220°C) and the PEEK specimens for 5.5 min at 250°C (T, = 148”C, r,,., = 314°C). To avoid possible effects of gravity on the contouring, the plates were placed in the oven resting on their thin sides. The heated samples were immediately piaced in a surgical plate bending press (Zimmer). The normal fixture in the bending press, which is curved to fit the semitubular cross-section of metal plates, was replaced with an aluminium fixture with flat surfaces to allow for even distribution of stress across the surface of the fiat samples, reducing the possibility of cracking the plates during the bending operation. Pressure was applied to bend the plate and the position and pressure were maintained for a period of 30-45 s. This alfowed the material to cool to a point at which the deformation was permanent. The pressure applied to the plates was not quantified and was based on the subjective feeling of the resistance of the material to plastic deformation. The degree of contouring was measured by placing the samples on a flat surface and using a micrometer to measure the height. Preliminary studies demonstrated that the maximum contouring angles achievable without cracking for the PS, PBTand PEEKcomposites were 5.9”, 2.9’ and 5.8”, respectively. The total contouring procedure before mechanical testing involved heating the sample, bending it in the middle, heating the sample and bending one end, heating and bending the other end and then heating and reverse bending the whole sample to return it to its original shape. The samples were allowed to cool for approximately 20-30 min between heating cycles. The cross-sectional dimensions of each sample were measured in four places and averaged to check for any changes caused by the heating.
Saline soaking (SAL) The simulated in vim environment consisted of soaking in sterile physjol~jcal saline (0.9% NaCl) at 37°C for 3 wk. Samples were placed in individual test tubes and the saline added until each sample was covered. The tubes were then capped and placed in an incubator. The time span of 3 wk was selected based on preliminary water absorption studies which had shown that a significant portion of the total water absorption had occurred during this time. To prevent drying, mechanical tests performed after soaking were conducted in an environmental chamber filled with saline at 37” + 0.5”C.
Flexion tests Flexion tests were conducted according to ASTM standard D790M (Flexural Properties of Unreinforced and Reinforced Plastics and Electrical Insulation Materials) on a universal testing machine (Scott GRE/lOOO) equipped with a strip chart recorder to expand the deflection scale. The samples were tested in three-point bending with a span to a depth ratio of 16: 7 and a cross-head speed of 2.6 mm min-‘. The central nose-piece was positioned directly over the region that had been contoured, to determine the local effects of contouring. The flexion test of the contoured samples was set up so that the sample was loaded in the same direction in
Shon carbon fibre reinforced thermoplastics: S.A. Brown ef al.
sterilized, saline soaked and then tested (ST2SAL). A fourth group was sterilized, contoured, saline soaked and tested (STZ-CON-SAL). The prenotched ST2 and STZ-CON fracture toughness specimens were air-aged (AAG). while the ST2SAL and ST2-CON-SAL specimens were aged in saline (SAG). A one-way analysis of variance (ANOVA) was performed on the detailed study results. The differences between pairs of means was determined using multiple comparison. The least significance difference (LSD) was calculated using the standard error of the difference between the means and multiplying it by the 5% value oft with the degree of freedom equal to l(n - 1) where I was the number of groups and n was the number of samples in the group. Once the LSD was obtained, any difference between two groups greater than this value was considered significant to P < 0.05.
which it had been contoured. After contouring, most plates were not perfectly straight and had a very slight bow to them.
Fracture toughness testing From each flex bar tested, two fracture toughness samples obtained. The middle of these samples was located and a line was drawn perpendicular to the edge, to approximately the middle of the width dimension, as a guide for cutting the notch with a fine toothed (six teeth/cm) band saw blade. A razor blade held in the three-jaw chuck of a milling machine (Bridgeport) was used to create a sharp crack, approximately 0.70 mm deep. A new razor blade was used for each notch. There is no standard method for testing the fracture toughness of polymeric materials. The ASTM standard E399 (Plane-strain Fracture Toughness of Metallic Materials) was used, as it has been used as a common guideline for plastics and composites that exhibit linear elastic behaviour. The samples were tested in a three-point bending fixture at a cross-head speed of 12.7 mm min-‘, to achieve a rate of the range of increase of stress intensity within 0.55-2.75 MPa.m”‘/s. The sample was placed in the fixture so that the saw cut was directly underneath the loading nose-piece. After testing, a tangent line was drawn along the linear portion of the load-deflection curve on the strip chart. A secant line was then drawn on the plot through the origin of the tangent line, but with a slope of 95% of the tangent slope. The load (Pq) at the point where the secant line intersected the curve was used in the calculation of K,,. were
K,, = (P,S/f3W3’2).f(a/W)
(1)
where S is the span, B is the thickness, W is the depth, a is the crack length and f(a/W) is given by ASTM standard E399. The calculated stress intensity factor was then checked against the criteria given in ASTM E399, to determine if they represented a plane-strain fracture toughness value. Fracture energy values for each treatment were calculated from the average fracture toughness values using the following equation: G, = Kc2( 1 - v2)/E
(2)
where G, is the fracture energy, Kc is fracture toughness, v is Poisson’s ratio and E is the flexural modulus. Poisson’s ratio was assumed to be 0.40 in these calculations.
Ageing (AAG and SAG) Ageing studies involved the fracture toughness samples only. The samples were notched with a razor blade as described above and placed in individual test tubes. The airaged samples (AAG) were placed in an incubator at 37°C for 3 wk. Sterile saline (0.9% NaCI) was added to the test tubes of the saline-aged samples (SAG) before ageing for 3 wk in an incubator. The AAG samples were tested in air and the SAG samples were tested in saline at 37°C.
Experimental protocol All specimens were steam sterilized twice (ST2) at 121 “C for 20 min before any testing and divided into four groups of samples for testing for flexural strength and fracture toughness. One group of five samples per material was tested after sterilization (STZ). A second group was sterilized, contoured and tested (STZ-CON). A third group was
RESULTS The results for flexural testing are shown in Table 1 and Figures 1-3. The strength of the PS composite was significantly decreased after the saline soaking and increased by contouring. The strength of the PBT composite was significantly reduced by all treatments. The strain to failure of the PBT was reduced by contouring. The flexural modulus of the PBT composite was increased by contouring, but decreased by saline soaking. The flexural properties of the PEEK composite were not affected by any of these treatments. The effects of these treatments on the fracture toughness of the composites is shown in Table 2 and Figure 4. Contouring resulted in a small decrease in the fracture toughness of PS and a significant decrease in the fracture toughness of the PBT composite, whilst saline soaking and soaking after contouring resulted in a significant decrease in fracture toughness of the PS and PBTcomposites. The fracture toughness of PEEK was not affected by any treatments. The effects of the treatments on the flexural properties and fracture toughness were also determined from a statistical analysis similar to that used in factorial experimentation. The effects of contouring and soaking are summarized in Tab/e 3. Contouring significantly increased the flexural strength of the PS composite. Contouring significantly
Table 1 Flexural propeflies of PS-3UPAK 3OPANC composites after ST2 CON and SAL
PBT-3OPANC
and PEEK-
FleXUral strength (MPa)
Strain to failure (%)
Modulus (GPa)
178.4(9.4) 190.5(14.0)’ 149.9( 10.0)’ 164.7( 10.4)
1.98(0.3 1) 1.96jO.20) 1.86(0.15) 1.85(0.29)
10.64(1.70) 1 1.36( 1.40) 9.32( 1.39) 10.51(1.47)
ST2 ST2-CON ST2-SAL STZ-CON-SAL
167.9(5.7) 1 t9.8( 19.0)’ 145.Q3.7)’ 110.2(5.7)’
1.45(0.06) 0.93(0.14/ 1.39jO.03) 1 .OO(O.O6j*
13..?6(0.34) 14.16(0.39)* 11.98(0.14)’ 12.28(0.32)*
PEEK-BOPANC ST2 STZ-CON ST2-SAL STZ-CON-SAL
272.4(18.5) 277.2(10.6) 255.7(10.7] 271.9(5.0)
2.13(0.11) 2.27(0.18) Z.lO(O.25) 2.35(0.17)
15.56( 1.76) 15.07( 1.44) 15.21(1.04) 14.55(0.73)
Condition
PS-30PANC ST2 ST&CON STZ-SAL ST2-CON-SAL PET-30PANC
Numbers in parentheses are standard deviations. five samples per group. indicates significant differences (P < 0.05) from ST2. l
Biomaterials
1990. Vol 11 October
543
Short carbon fib? mk=hrced
rhemwpiastics: S.R 5mwn et at. Table 2 Fracture roughness of PS-JOPANG, PEEK4’OPANC composites afrer ST2 CON and SAL
PST-36 PANC
PS-30 PANC
PEEK-30 PANG
Figure 9 flexufal strength data for the three test materiafa after steam sterilization {ST2j* ccznrounrrg,saline soaking for 3 wk at 3 7°C jSALf/VE] end cantouring and then saline soaking (CONT-Sill]. (El} ST2 (5) contoured, (?Bj SALINE @I/ CONT-SAL
PBT-3C’PANc
and
Condition
n
Yield strength, K, (MW
Fracture toughness, G, fMPa’m”*)
Fracture energy &J/m21
PS-30PANC ST2 ST2-CON ST2-SAL STZ-CON-SAL
5 4 5 5
167.8(13.10) 176.8(17.50) 146.7f9.07) 157.5(6.48)
5.59(0.23) 5.47(0.22) 4.97(0.32)* 4.87(0.58)*
2.47 2.21 2.23 1.90
PBT-JOPANC ST2 ST2-CON ST&SAL STZ-CON-SAL
5 3 5 5
166.6(3.00) 11 S.S(l3.00) 145.6(3.73) 1 f 0.2f5.70)
4.7 l(O.44) 4.06(0,34) 4.35(0.12)* 3.86(0.t0)*
1.41 0.98 1.33 1.02
PEEK-30PANC ST2 STZ-CON SM-SAL STP-CON-SAL
2 5 5 5
269.4{3 1JO) 236.9(12.10) 224.1(7.53) 224.4(8.88)
7.38(0.05) 7x33(0.57) 7.45(0.35) 7*oa(o,t 7)
2.79 2.74 3.Q7 2.88
Numbers in parentheses are standard deviations. five samples per group. * Indicates significant differences (P < 0.05) from ST2.
PS-30
PI3 T -30 PANC
PANC
PEEK-30
PANC
PS-30 PANC
8
PBT-30
PS-30 PANC
PANC
PANC
PEEK-30
PBT-30
PANC
PEEK-30 PANC
decreased the strength, strain to failure and fracture toughness and increased the flexural modulus of the P5T c~mp~site. Contouring had no effect on PEEK. The main effects of saline soaking were to decrease the strength and fracture toughness of the PS and PBT composites and to decreasetheflexural modulusof the PBT. Salinesoaking had no effect on the PEEK composite. The results of fracture toughness testing after air and saline ageing are shown in Table 4 and presented graphically in Figure 5. The figure shows the pm-aged results as open bars, with their respective-aged results as cross-hatched bars. For PS, air ageing resulted in a small reduction in
feble 3 Main effact of contouring and saline soaking on the flexural properties and fracture toughness of the composite materials. One-way ANOVA with multiple comparison, significance at P < 45
Contouring (CON)
Material
Stress
544
Strain
MOD
%
Stress
a
I
D
D D
I
PS-JOPANC PBT-30PANC PEEK-SOPANC
I = increase, 0 =
Saline soaking (SAL)
a
decrease.
&?mafef&fs
1990, Vol 1 I October
Strain
MOD
Kit
D
D 0
Short carbon frbre remforced rhermoplastics: .%A ffrown et al.
Table 4
Fracrure toughness values af?er a,r and saline agetng n
Condmon
Yield strength
Fracture,tyghness. K,,
(MPat
(MPa‘m
167.8( 13 tot 176.U 17.50) 157.516.48)
5.16f0.48) 5.09f0.36) 4.8 l(O.42)’ 4.54(0.68)
2.10 2.34 1.71 1.65
Fracture energy, G,, (kJ/m2)
) _
PS-3OPANC STZ-AAG ST2-SAL-SAG STZ-CON-AAG ST2-CON-SAL-SAG
5 5 5 5
PBT-30PANC STZ-AAG STZ-SAL-SAG ST2-CON-AAG STZ-CON-SAL-SAG
5 5 5 5
166.6(3.00) 145.6(3.73) I 19.8(19.00) 110.2( 5.70)
5.07(0.17) 4.03(0.08)’ 3.90(0.47) 3.56jO.26)’
1.63 1.14 0.90 0.87
PEEK-30PANC STZ-AAG STZ-SAL-SAG ST2-CON-AAG STZ-CON-SAL-SAG
2 5 5 5
259 4(3 1.90) 224.1(7.53) 236.9(12.10) 224.4(8.88)
7.15jO.08) 7.36jO.33) 7.56(0.29) 7.40(0.23)**
2.79 2.99 3.19 3.16
l
**
746.7(9.07)
Indicates significant decrease (P < 0.05) due to ageing. lndxx%es a significant increase.
P
_..
PET-30
PS-30 PANC
PANC
PEEK-30
PANC
Figure 5 Effect of air ageing and saline ageing on test materials. (OJ ST2, ,%?.Iair aged, (E%Jsaline aged From ieft to right for each material each open and cross-hatched column pair represents: ST2, STZ-air aged: CONT. CONTairaged; saline, saline-saline aged; and CONT.SAL COOT-SA~~sai~ne aged
Figure 6 Typical SEM ph~tomjcrograph of PS-3OPANC STZ-CON fracture surface showing no fibre-matrix bo~d;ffg and fibre pu//ouf (original magnificarion X 2OOOJ.
fracture in
toughness
fracture
significant. changes.
of the ST2-AAG
toughness Saline
specimens;
ST2-CON-AAG
ageing
SAL-SAG
decrease
for both STZ-SAL-SAG
specimens.
resulted
Ageing
in no changes,
toughness
of ANOVA
ageing
caused
PS-30PANC SEM the three
a
and
PBT-30PANC.
data
decrease
in
Ageing
caused
had no effect
was used to characterize composites.
Some
circles
which
were
pulled out. The fractures appearance. X2000
are shown
of
fibre-matrix 6
in Figures interface minimal
shows
PS-30PANC.
were
Figure
bonding
between
contrast,
Figure
the carbon 8 shows
of
a decrease
for
failure
notably
surfaces
many small
fibres
had
rough and chunky
to demonstrate
samples.
bonding
of detectable,
PEEK
composite
covering
of matrix
for
micrographs
but poor,
microscopic
and the PBT matrix.
that the fibre-matrix
in
Figure 7 Typical SEM photomicrograph of PET-JOPANC STZ-CON fracture surface. showing minimal fibre-matrix bonding (original magnification X2000).
at
the nature
ST2-CON
fibre-matrix fibres
been
photomicrographs
in the
of
mechanisms
revealed
where
7 is an example
air
on PEEK-30PANC.
SEM 6-8
The that
toughness
the fracture
common
sockets
Representative
essentially
demonstrated fracture
ageing
in
in the fracture
ST2-CON-SAL-SAG.
on these saline
resulted
and ST2-CON-
an increase
were seen in all three. &lose inspection
Figure
significant
PEEK specimens
except
of the SAG specimens
results
the
statistically
of PS did not cause
The saline ageing of the PBTspecimens
a significant
dark
the decrease
was
bond
In
in the
PBT
is very
strong,
indicate
a difference
level in the regions
composites
indicated
by a complete
on most of the pulled-out
exhibited
whilst the PEEK composite
in failure
photographed.
fibre-matrix primarily
Bfomarerials
fibres.
These
modes
at the
The PS and
interface
showed
failure,
matrix failure.
1990, Vol 1 I October
545
Shon carbon fibre reinforced
thermopiastics:
5.A Brown et al
took place during the first 3 wk, after which the absorption
Figure 8 Ty~ic~i SEM ~hromicrugfaph of PEEiG3OPANC SIZ-CON fracture swface, showing extensive fibre-matrix banding (original magnification x2000/.
Metallic fracture fixation plates in current use need to be ductile, since they are frequently bent or twisted to fit the contours of the bone. It was this need for intra-operative formability that led to the selection of composite materials which demonstrate thermoductility, the capacity to be contoured with the application of heat. Thermoplastic matrix materials, reinforced with chopped carbon fibresfor increased strength and stiffness, show thermoductility. This led to the selection of three of this class of materials for evaluation. The three candidate materials also had good mechanical strength, with an elastic moduli less than that.of bone. The objective of these studies was to characterize the candidate materials for use as fracture fixation plates. F lexural strength testing was selected as an appropriate test method, since plate failure is usually associated with bending due to fracture instability’. Fracture toughness testing was also selected since it characterizes the resistance of a material to fracture in the presence of a sharp flaw. The fracture toughness of a composite material is dependent on the properties of the matrix and of the fibre reinforcement and the interface between the fibre and matrix. Addition of a rigid fibre to a ductile polymer matrix frequently reduces the fracture toughness of the composite whilst increasing the strength and modulus. The rigid fibres restrain the elongation of the matrix between fibres, producing a more brittle fracture36. For a brittle polymer matrix, however, the addition of rigid fibres frequently significantly increases the fracture toughness through crack blunting and branching, whilst increasing the strength. The applicability of linear elastic fracture mechanics to characterize the toughness of polymers has been verified by Nguyen and Moet3’ and Friedrich et a1.38. The conditions under which the specimens were tested were selected based on the clinical application. Since both sterilization by steam autoclave and repeated autoclaving are traditional for fracture fixation plates, all specimens were sterilized twice as a precondition. The heat contouring process was developed based on the thermal properties of the materials and on previous experience of bending without cracking the materials. Although the heat cycles were not optimized, they do provide a database for comparison. Saline soaking was conducted to simulate in viva exposure. An exposure time of 3 wk was selected, based on preliminary 18 wk studies which had shown that 70% of the absorption
546
Biomateriafs
1990, Vol I I October
rate was quite slow. Ageing was conducted to determine if there were any significant changes in fracture toughness after the precfacked samples had aged either in air or saline. Such changes might reflect plastic flow stress relieving and crack tip blunting or environmental effects on fibre-matrix bonding at the crack tip. PS is amorphous in nature and its glass transition temperature (T,} is 1 90”C3’. It possesses good thermal stability and rigidity up to temperatures of approximately 150°C. The ether linkages provide chain flexibility, imparting good impact strength. The polymer resists hydrolysis, but is susceptible to attack by organic solvents such as ketones and chlorinated or aromatic hydrocarbons. The PS-SOPANC composite demonstrated an increase in strength after heating and contouring. Robeson et a1.40 have also reported an increase in strength due to heating which may be indicative of a high-temperature annealing of the material. However, the increase in fracture toughness after contouring observed in the present study was not statistically significant. The saline soaking (STZ-SAL) and contouring (STZ-CON-SAL) both resulted in loss of flexurai strength, the former being significant, although saline ageing did not cause a decrease in fracture toughness. The lack of fibre-matrix bonding seen in the SEM may suggest that moisture could penetrate and interact with the interfacial bond. it is of interest that air ageing also caused a decrease in fracture toughness to a value close to that after saline soaking and ageing. ln this case, atmospheric moisture may have penetrated the fibre-matrix interface. it is also plausible that increased fibre debonding could occur, due to the mismatch of the thermal expansion coefficients of the fibre and the matrix. PBT is partially crystalline with regions of different degrees of order ranging from highly ordered crystallites to completely amorphous regions3’. Because of the two different components, it exhibits a 7, of 40°C and a melt temperature of 220°C. The material is resistant to water, weak acids and bases, ketones, alcohols and aliphatic and chlorinated aiiphatic hydrocarbons. The resin is sensitive to a~kal~es,‘oxidiz~ng acids and some phenols. The PBTcomposite was the most difficult to contour in this study. Heating to 10°C above the melt temperature for 6 min resulted in some superficial melting of the plates. Whilst this resulted in the surface becoming soft and flexible, the plates could only be contoured to a deflection angte half that of the other materials. Although the specimens were not visibly cracked after contouring, the treatment resulted in significant loss of mechanical properties. The increase in modulus may be an indication of a change in c~staliinity, resulting in a more brittle material. The SEM in Figure 7 suggests a moderate degree of matrix bonding to the fibres. The loss of strength due to saline soaking and saline ageing may be due to degradation or to hydrolysis. PEEK is a semicrystalline high-performance thermoplastic consisting of linear aromatic chains”‘. The r, is 143°C and the melt temperature is 334°C. Industrial grade PEEK can be obtained with crystallinity up to 48%42. There is sufficient chain mobility at a temperature of 170°C for the material to undergo nucleation and crystallization. Mouldings can be annealed for 2 min at 300°C or 1 h at 200°C to insure that a highly uniform crystallinity develops. It has outstanding resistance to abrasion and dynamic fatigue and is resistant to chemical attack over a wide range of pH levels, from 60% sulphuric acid to 40% sodium hydroxide. No organic solvent attack has been observed on moulded
Short car&on fibre reinforced the~maptastics: .%A Bmwn et ai.
partsz9. The material has been extensively tested with respect to aerospace applications3’. In this study, the PEEK-30PANC composite was the easiest to contour, despite having the highest thermal and best mechanical properties. The mechanical properties of the material were not significantly affected by any of the conditions studied. The SEM demonstrated extensive fibrematrix bonding. Preliminary biocompatibility test results have been encouraging3’. For these reasons, it was concluded that the PEEK-30PANC composite is an excellent candidate for further study for such applications.
ACKNOWLEDGEMENTS
18
methacylate
REFERENCES
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20
21
22
24 25 26
1 2 3 4 5
6
7 8 9
10
11
12
13 14
15
16
17
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23
The authors wish to thank LNP and Wilson Fiberfil for donation of flex bar specimens used in this study.
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Biomaterials
1990. Vol 11 October
547
