Biocompatibility and mechanical properti. pdf

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Biocompatibility and Mechanical Properties of a Totally
Absorbable Composite Material for Orthopaedic Fixation Devices
Kirk P. Andriano and A. U. Daniels

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Orthopedic Bioengineering Laboratory, Division of Orthopedic Surgery, University of Utah School of Medicine,
Salt Lake City, Utah

Jorge Heller
Controlled Release and Biomedical Polymers Department, SRI International, Menlo Park, California

Bioabsorbable polymer/inorganic phosphate fiber composites are prone to rapid degradation due to water sensitivity of the interface between the degradable polymer and the
degradable fiber. This article describes successful fabrication and laboratory evaluation
of a candidate bioabsorbable composite implant material with mechanical properties
similar to bone. The composite studied was poly(ortho ester) reinforced with randomlyoriented, crystalline microfibers of calcium-sodium-metaphosphate. The component
materials showed no acute cytotoxicity as determined by tissue culture agar overlay.
Treating the microfibers with a diamine-silane coupling agent improved mechanical
properties and slowed degradation in saline, byt strength still decreased 50% in 1 week.

When the composite material was then coated with a layer of matrix polymer alone it
retained 70% of its strength and 70% of its stiffness after 4 weeks exposure to 7.4 pH
Tris-buffered saline at body temperature. The marked improvement with the coating
can be attributed to the hydrophobicity of poly(ortho esters).

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INTRODUCTION
Background

The use of metal plates for internal fixation of bone often
prevents formation of the normal callus and also results in
incomplete healing.l,? A primary reason for this clinical
observation is that the plate bears a substantial portion of
the load normally seen by the bone. The elastic modulus
mismatch between bone (E = 6-20 GPa) and the metals
(E = 100-200 GPa) used for internal fracture fixation
devices means that a plate whose cross section is only
a fraction of the plated bone has a structural stiffness

approximating that of the bone. In isoelastic systems,
load bearing is in proportion to structural stiffness of the
components (e.g., bone and plate). According to Wolff's
Law, bone is a dynamic material and remodels itself in
accord to applied stresses to achieve and maintain density
and structural thickness. Rigid internal fixation of bone
thus shields healing bone from normal stresses, resulting
in below normal density and thickness. Also the eventual
surgical removal of metal plates and screws after they
have served their stabilization function leaves the bone at

least temporarily deficient in structural strength and prone
to refracture.
Lower-modulus, nonabsorbable fiber reinforced polymer composite bone plates have shown promise in reducing this stress shielding p h e n ~ m e n o n , ~but
- ~ these
plates must still be removed in many cases to prevent
complications. Totally absorbable composites&" with an
elastic modulus closer to that of bone have been proposed
as an alternative. Fixation devices of these materials would
provide less rigid fixation. This would allow some fracture

motion and resultant formation of normal callus not seen
with rigid fixation. However, rigidity must not be too low
or nonunion may result. Absorption of the device with
time would further decrease rigidity gradually, lessening
the effects of the stress-shielding and allowing the bone to
gain normal density and thickness. Also absorption would
eliminate the need for second surgeries.
Totally absorbable composites have been made using
absorbable polymers reinforced with higher strength absorbable organic polymer fibers. Both matrix and fiber are
typically poly(glyco1ic acid) (PGA) or poly(1actic acid)
(PLA), and when compared to unreinforced polymer, the
composites show large increases in strength but little increase in stiffness since the fibers are p ~ l y m e r i c . ' ~PLA
,~~
matrix composites reinforced with continuous absorbable,
calcium-metaphosphate glass fibers show a large increase
in both strength and rigidity.14,15
Unfortunately, composites of this type made to date
show a large loss in strength after short-term exposure
to an aqueous physiological environment, but do retain

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Requests for reprints should be sent to Dr. Kirk P. Andriano, Biomaterials
Laboratory, Institute of Plastics Technology, Tampere University of Technology,
P.O. Box 527, SF-33101 Tampere, Finland.

Journal of Applied Biomaterials, Vol. 3, 197-206 (1992)
CCC 1045-4861/92/030197- 10$4.00
0 1992 John Wiley & Sons, Inc.

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ANDRIANO, DANIELS, AND HELLER

a greater proportion of their stiffness. Reinforcement of
absorbable polymers with short nonabsorbable fibers may
result in better retention of mechanical strength after
aqueous e x p o s ~ r e , ' ~but

~ ~ ' the fiber residue could result
in an inflammatory response and mechanical irritation if
fibers enter the joint space.
Additionally, clinical studies of absorbable polymeric
fracture fixation devices have identified a serious problem
for devices made from PGA and PLA. Although fracture
healing occurred with these devices, there were reported
rates of between 7 and 48% of an associated noninfectious
foreign body inflammatory response requiring clinical
interventi~n.",'~Depending on the polymer, the time for
manifestation of this local toxic response was from 7
weeks to nearly 3 years after implantation.
This study describes a first effort to create a
completely absorbable composite implant material, using
an absorbable poly(ortho ester) (POE) matrix reinforced with absorbable, randomly oriented, calciumsodium-metaphosphate (CSM) microfibers. Such a
composite could be used in devices for low-load
mechanical stabilization of hard or soft tissues. A n
initial design goal was to produce a benignly absorbable
composite material with mechanical properties similar to
bone which would retain the majority of these properties

for the initial healing period of bone, approximately 4 to
6 weeks.
The methods used to develop and evaluate this totally
absorbable composite material included (a) biocompatibility testing of the reinforcing fibers and matrix polymer
to determine cytotoxicity, (b) characterization of POE
polymers with differential scanning calorimetry (DSC)
and dynamic rheometry to aid in composite fabrication,
(c) dissolution studies of the CSM fiber, (d) evaluation of
fabrication variables such as fiber content and pressing
temperature to optimize composite properties, and (e) an
in vitro wet strength retention study of the unaugmented
composite and of composites with fibers treated with a
silane coupling agent and with hot-film coating of the
entire specimen with unreinforced poly(ortho ester).
As a result of these studies, a CSM/POE composite
was fabricated successfully using silane treated fibers. The
component materials showed no acute cytotoxicity. The
composite material had mechanical properties similar to
cortical bone, and when coated with a layer of unreinforced POE retained 70% of its strength and 70% of its
stiffness after 4 weeks of exposure to Tris-buffered saline

at body temperature.

amorphous.21 Hydrolytic degradation occurs predominately by surface hydrolysis,22 in contrast to the bulk
hydrolysis of the more hydrophilic PGAs and P U S .
POEs have been successfully used as erodible matrices
for drug delivery of therapeutic agent^^^,^^ and explored
for potential use in fracture fixation device^.^' Polymer
degradation rates can be accelerated by adding acid
excipients to the bulk polymer or retarded by adding
basic excipients.26 Degradation rates also increase with
decreasing molecular weight.
The polymers investigated here were linear poly(ortho
esters) prepared by a condensation reaction of 3,9bis(ethy1ene 2,4,8,lO-tetraoxaspiro[5,5]-undecane)and a
60:40 and 90:lO mole ratio of the rigid diol, frans1,4-cyclohexanedimethanol and the flexible diol, 1,6hexanediol, respectively (Fig. 1). Polymers with a wide
range in glass transition temperatures (Tg),20 to 120 "C,
can be synthesized by varying the ratio of rigid to flexible
diols. For hot molding processes, molecular weights of
less than 100000 are best in order to avoid the necessity
of high fabrication temperatures required by high melt
viscosities.21

When a POE of this type is placed in an aqueous
environment, it undergoes an initial hydrolysis of the
ortho ester linkages to yield a mixture of the diols used
in the synthesis and the dipropionate of pentaerythritol.
This diester eventually hydrolyses to pentaerythritol and
propionic acid.24

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MATERIALS AND METHODS
Polymer Description and Preparation

Poly(ortho esters) are a family of synthetic absorbable
polymers that have been under development for medical applications for a number of years.20 This family of
highly hydrophobic absorbable polymers is completely

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POLY(ORTH0 ESTER)

LH3CH2

0-CH2

CH2-0

\ /
\ /
C
C
/ - \CH2-0 O-R

\ /
C

&d-'O-CH,

1

CH2CH3

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"R" CAN BE
RIGID

- trans-cyclohexanedimethanol
-CH2-

0

-CH2-

FLEXIBLE - 1,6-hexanediol

-(CH216Figure 1. Chemical structure of poly(ortho ester) repeat units.

ORTHOPAEDIC FIXATION DEVICES

Even though ortho ester linkages are much more labile
than the ester linkages, the polymer is highly hydrophobic,
so that the hydrolysis is largely confined to the outer
surface of a polymer specimen. Therefore, degradation
of this polymer may not generate acidic products rapidly
at the implantation site, provided that the water soluable,
low molecular weight products of the first hydrolysis at
the solidiliquid interface can diffuse away. This could
be a significant advantage over the bulk hydrolysis of
polyesters that hydrolyze directly to yield acidic compounds as the primary degradation products.
The 60:40 polymer used here had a weight average
molecular weight of 86000 and a Tg of 66 "C. The 90:10
material had a molecular weight of 73000 and a Tg of
approximately 100 "C. The polymers were dried in a
vacuum oven at 40 to 50 "C for 48 h and stored in dry
air sealed containers at ambient conditions until needed.
Because of extremely limited quantities of polymer and
labor intensive fabrication procedures, sample sizes reported in this research for mechanical testing are low, and
therefore no statistical analysis of the data was performed.

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199

calcium-metaphosphate
fibers (tensile strength =
0.35 GPa and tensile modulus = 28 GPa) which have
been used to reinforce poly(1actic acid).29
One kilogram of calcium-sodium-metaphosphate microfibers (T Grade, long fibers) was donated for this work
by Monsanto Chemical Co. (St. Louis, MO). Raw CSM
fibers contain about 8% glass which contains heavy metals
that must be removed before use in biological applications.
This was accomplished by boiling 25 g of CSM fibers in
500 mL of deionized water for 4 h, maintaining the water
level as needed. The hot suspension was then filtered
through a Buchner funnel under vacuum and washed
several times with deionized water at room temperature.
The fibers were dried in a vacuum oven at 90 to 100 "C
for 24 h and stored in a desiccator until needed.
The surface of washed CSM fibers is slightly acidic.
This is due to chain end hydrolysis of the inorganic
polymer, resulting in a small quantity of surface acidic
degradation products. This free hydrogen can react with
the methoxy -groups of commercially available silane
coupling agents to form silanol bonds between the
inorganic fiber surface and the coupling agent.30 Because
an acidic fiber surface will catalyze hydrolysis of
POEs, the fiber surface was made basic by treating
it with a diamine-silane, (N,beta-aminoethyl-gammaaminopropyl-trimethoxysilane;2-6020, Dow Corning,
Midland, MI).
Fifteen grams of washed CSM fibers were slowly added
to 100 mL of a 0.3% solution of silane coupling agent
in methanol (OmniSolv, EM Sciences, Gibbstown, NJ)
(v/v) and stirred using a magnetic stirrer until a slurry
was formed. The slurry was then fiitered using a Buchner
funnel, and the residue dried in a convection oven at 90 to
100 "C for 3.5 h. After cooling to room temperature, the
treated fibers were sieved through a 40 mesh Tyler sieve
using a Portable Rototap (W. S. Tyler Inc., Mentor, OH),
washed with excess methanol to remove any unbonded
coupling agent, and finally dried in a convection oven
at 90 to 100 "C for 1 h. Treated fibers were stored in a
desiccator at ambient conditions until needed.

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Fiber Description and Preparation

Calcium-sodium-metaphosphate
(CSM) mineral microfibers were developed as a nontoxic replacement for
asbestos but the material has not been used commercially
as yet. This degradable and bioabsorbable microfiber is
a crystalline, inorganic polymer of metaphosphate whose
negative charge is balanced by calcium and sodium ions
(Fig. 2).27,28The fiber exhibits an effective aspect ratio
of about 60:l and potentially can be used with a wide
range of coupling agents for reinforcing thermoplastic and
thermosetting polymers.
CSM microfibers are manufactured from pure elemental phosphorus using oxygen, sodium carbonate, and
lime.27 The microfibers cover a wide range of lengths
and diameters from submicron to hundreds of microns
depending on the type and degree of comminution to
which the material is exposed. These crystalline fibers possess the tensile strength and stiffness of Kevlar, 2.6 GPa
and 124 GPa, r e ~ p e c t i v e l y . ~In~ comparison, CSM
microfibers are much stronger and stiffer than absorbable

STRUCTURAL FORMULA OF CSM
*2

Ca

Na

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Biocompatibility Testing

Acute cytotoxicity of 60:40 POE, 90:lO POE, CSM fibers,
and silane-treated CSM fibers was evaluated by standard
tissue culture agar overlay
using the direct cell
contact method and L929 mouse fibroblast cells.
After sterilization with ethylene oxide, appropriate
amounts of each test material were aseptically placed
onto separate solidified agar, along with positive and
negative controls (natural black rubber and polyethylene,
respectively). Samples and controls were tested in
triplicate and incubated at 37 "C for 24 h. Neutral Red
was added for 2 to 3 h. The cells were then evaluated
using an inverted microscope.
The results were scored by adding the area of decoloration (zone index) and the area of lysis (lysis index). The

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- ~

I
I
(-o-P-o-P-o-P-o-)"

I

8 8 8
Figure 2. Chemical structure of the polymetaphosphate inorganic
polymeric backbone chain. The poly-metaphosphate polymers lie next
to each other in planes, with cations in the spaces in between and
ionically bonded to the polymer.

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ANDRIANO, DANIELS, AND HELLER

final score was recorded as the response index, which is
the ratio of zone index to lysis index.

Characterization of Polymers

As an aid in the development of processing conditions for
POEs, the thermal and rheological properties of the 60:40
and 90: 10 POE polymers were determined. Differential
scanning calorimetry (DSC) was used to determine the
glass transition temperature of the polymer samples and
their propensities for thermal degradation at temperatures
above their Tgs. In addition rheometry was used to determine the visco-elastic properties of the polymer samples
in their melt phases. The instruments used were in Omnitherm DSC 700 using a heating rate of 10 "C/min and a
Rheometrics Inc. Model RDS-I1 Dynamic Spectrometer.

Characterization of Microfibers

In order to quantify both CSM microfiber degradation due
to solvation of polymer chains at the fiber surface and
water resistance of the fiber/silane coupling agent bond,
two degradation studies were performed. First, 50 mg of
untreated CSM fiber samples were placed into 25 mL
polystyrene vials containing 20 mL of normal saline while
another set of fibers samples were placed in vials containing 20 mL of distilled water. The sample filled vials were
sealed and placed in a shaker bath and maintained at a
temperature of 37 "C. Fiber samples immersed in normal
saline and distilled water were removed at 1, 2, 4, and 7
weeks and dried in a vacuum oven at 50 "C for 48 h. The
remaining fibrous residue was weighed using a standard
laboratory analytical balance and any change in weight
was recorded.
Second, the amount of coupling agent actually placed
onto the fibers can be semi-quantitatively determined by
titration of a fiber slurry in water, provided the coupling
agent has acidic or basic organo-functional groups." To
determine if a water resistant bond had been achieved
between the fiber surface and the coupling agent, 2.5 g of
CSM fibers sized with basic diamine-silane were boiled
in 100 mL of deionized water for 1 h. The slurry was then
filtered using a Buchner funnel under vacuum and dried in
a convection oven at 90 to 100 "C for 30 min. Titration of
a 2% slurry of the above treated fibers in deionized water
(2.0 g fiber/lOO g HzO), using 0.001N sulfuric acid was
performed until pH 4 was reached. Similar titrations were
performed with CSM fibers treated with diamine-silane
and untreated CSM fibers, followed by comparison of
the titration curves. The greater the volume of sulfuric
acid needed to reach the pH of 4, the greater the number
of basic sites on the fiber surface, hence the greater the
amount of coupling agent bonded to the fiber surface. This
was used as a semi-quantitative indicator for comparing
the amount of diamine-silane coupling agent bonded to
the fiber before and after boiling.

Fabrication Variables

Composite fabrication. Composite specimens measuring 38.1 X 12.7 X 1.60 mm were prepared in the following manner: first, 90:lO POE was reduced to a granular
form, particle diameter < 250 pm, by milling the polymer in a Thomas-Wiley Intermediate Mill (Arthur H.
Thomas Co., Philadelphia, PA), at room temperature in
open air, and using material which passed through a 40
mesh screen. Fiber and polymer were brought together by
a dry-mixing technique in air. CSM fibers were placed into
a narrow bottle (diameter = 5 cm and length = 15 cm)
fitted with a stirring rod assembly. Milled polymer was
sprinkled into the bottle while blending the fibers with
the impeller of the stirring rod. After final addition of the
polymer, blending was maintained for 15 min at low to
medium speed (10-30 rpm).
Finally, test specimens were prepared by hot compression molding in air of an appropriate amount of
dry-mixed composite in a three-piece stainless steel die
at selected temperatures (20-100 "C) above the Tg and
at a pressure of 2700 psi for 5 min using a modified
Carver Hydraulic Press (Fred S. Carver Inc., Menomonne,
WI) with heated and cooled platens. Composite specimens were then cooled to ambient temperature while
maintaining pressure at 2700 psi.

Fiber content. To determine what effect fiber content
would have on initial mechanical properties, CSM/POE
composite specimens were fabricated from CSM fibers
treated with diamine-silane coupling agent and 90: 10
POE. Fiber content was 0, 10, 20, 30, 40, and 50%
by volume. This corresponds to fiber content by weight
of 0, 22, 38, 52, 63, and 72%, respectively (density
of POE = 1.14 g/cc and density of CSM = 2.86 g/cc).
Composites were fabricated as previously described with
the pressing temperature being 170 to 175 "C. Replicate
composite specimens were tested to failure in three-point
bending to determine flexural yield strength and stiffness.
Pressing temperature. A small study was carried out
to determine what effects pressing temperature would have
on initial flexural mechanical properties of CSM/POE
composites.
CSM/POE composite specimens were fabricated
from 30 volume percent CSM fibers treated with
diamine-silane coupling agent and 90:lO POE. Specimens were fabricated at pressing temperatures of 120, 170,
185, and 200 "C as previously described. Initial flexural
yield strength and flexural modulus were determined in
three-point bending.
Mechanical testing. The flexural mechanical properties of test specimens were determined by three-point
bending in accordance with ASTM Standard D 790-81
Method l.32The diameter of the load nose was 12.6 mm.
The diameter of supports was 6.4 mm and the distance
between the supports was 25.4 mm. Span-to-depth ratio

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201

ORTHOPAEDIC FIXATION DEVICES

was 16 to 1. An Instron Model 1125 materials testing
machine was used to load the specimens. Crosshead speed
was 5 mm/min.
Flexural yield strength and modulus of elasticity were
calculated using the following equations.

where

S

=

3PL/2bd2

S

=

stress in the outer fibers at midspan

P

=

load

L = support span
b = width of specimen
d = depth of specimen

and

E = L3rn/4bd3
E = modulus of elasticity in bending
L = support span
b = width of specimen
d = depth of specimen
m = slope of the load-deflection curve

Wet Strength Retention Experiments

The rapid strength loss of absorbable poly(1actic acid)
composites made with absorbable calcium-metaphosphate
fibers is due to the disruption of the polymer/fiber interface
by water,33 and not due to the degradation of either the
polymer or the reinforcing fibers. As fluids diffuse into
the polymer or wick along the fibedfiber interfaces, the
polymer/fiber interface is compromised and the fibers
lose their strength reinforcing effect. Since absorbable
phosphate fibers degrade through surface dissolution,8 the
adhesion between the fiber and the polymer matrix is
broken once water molecules reach the interface. To see if
an outer coating would slow the loss of composite strength
and stiffness by retarding the influx of fluids, a comparison
study was performed.
The first group of composite test specimens were
prepared by hot compression molding of 1.27 g of drymixed 90:lO POE polymer and untreated CSM microfibers
(Vf = 30%) and pressed at 180 to 185 "C as previously
described.
Additional specimens were prepared by the same
method using CSM fibers treated with the diamine-silane
coupling agent. Half of these CSM/POE composite
specimens were encapsulated in pure 60:40 POE by hotpressing the POE around the test specimens as described
below.
Covering composites with hot-pressed POE films.

Composite test specimens were machined to the following
dimensions: 36.5 X 11.1 X 1.00 mm. Milled 60:40 POE,
rather than 90:lO POE, was used to encapsulate the
composite specimens because of its lower glass transition
temperature. One fifth of a gram of 60:40 POE was

placed in the cavity of a rectangular stainless steel mold
(38.1 X 12.7 mm). Gentle tapping on the sides of the
mold helped to evenly distribute the powdered polymer at
the bottom of the mold. A composite test specimen was
very carefully placed on top of the powdered polymer
and centered. One quarter of a gram of milled 60:40 POE
was placed on top of the test specimen; gentle tapping
on the sides of the mold evenly distributed the powdered
polymer. The upper plunger of the mold was then fitted
into place and the polymer/composite system was further
processed by hot compression molding at 115 to 120 "C
for 5 min at 2700 psi. Encapsulated composite specimens
were cooled to ambient temperature while maintaining
pressure at 2700 psi. Reproducibility of the encapsulation
procedure was excellent resulting in a uniform coating
of polymer for all specimens fabricated. Examination
of specimen cross-sections by light microscopy (X60)
revealed the coating procedure left approximately a 300p m thick coating of polymer on the test specimens that
appeared free of voids and cracks. The thickness of the
coating was evaluated by measuring the thickness of the
specimens before and after the coating process, using a
micrometer accurate to +/-5 p m .
Initial flexural yield strength and modulus were
measured using three-point bending (ASTM D 790-81).
Specimens for in vitro degradation tests were immersed
in Tris-Buffered saline (200 mM Tris and 150 mM NaCl),
pH = 7.4 at 37 "C. Flexural mechanical properties were
determined in triplicate (except for specimens fabricated
with untreated CSM fibers where no replicate testing was
performed) at various times periods up to 6 weeks.
Fabrication of composite specimens with untreated
acidic CSM fibers and acid sensitive POE was extremely
difficult, frequently yielding specimens which could not
be removed from the mold and often demonstrating signs
of degradation. Consequently only a limited number of
usable specimens were obtained for this experiment.

RESULTS
Biocompatibility Testing

60:40 POE, 90:lO POE, untreated CSM fibers, and CSM
fibers treated with the diamine-silane coupling agent all
were acutely noncytotoxic (Tissue Culture Agar Overlay).
Responses were comparable to negative controls (Table I).
Characterization of Polymers

The DSC scans of 90:lO POE revealed that its Tg was
about 98 "C and that it underwent some degree of thermal
degradation when heated above about 180 "C in air. The
rheological scans indicated that the viscosity of the melt
behaved normally between 110 and 180 "C. These results
indicate that the processing window for this polymer is
110 to 180 "C. Within this temperature range, the melt

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ANDRIANO, DANIELS, AND HELLER

TABLE 1. Results of Standard Tissue Culture Agar Overlay Response Index = Zone Index/Lysis Index for Direct Test

___

Sample
+ Control
- Control

60:40 POE

Replicate 2

Replicate 3

2.015.0"

2.015.0

o.o/o.o
o.o/o.o

o.o/o.o

2.015.0
0.0/0.0

Replicate 1

0.0/0.0

Average
2.015.0

o.o/o.o

o.o/o.o
o.o/o.o

Results: Noncytotoxic

+ Control

2.0/5.0

2.015.0

Control

o.o/o.o

90:lO POE

0.0/0.0

o.o/o.o
o.o/o.o

+ Control

2.015.0

-

2.015.0

2.015.0

o.o/o.o
o.o/o.o

o.o/o.o
o.o/o.o

2.015.0

2.015.0

0.0/0.0

o.o/o.o
o.o/o.o

Results: Noncytotoxic
Control
CSM Fibers
(Untreated)
-

o.o/o.o
o.o/o.o

2.0/5.0

o.o/o.o

o.o/o.o
o.o/o.o
Results: Noncytotoxic

+ Control
Control
CSM Fibers
(Treated)
-

2.015.0

2.015.0

2.015.0

2.015.0

o.o/o.o
o.o/o.o

o.o/o.o
o.o/o.o

o.o/o.o
o.o/o.o

o.o/o.o
o.o/o.o

Results: Noncytotoxic
"Zone/Lysis.

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viscosity of the polymer is rather high (lo4 to lo6 poises)
at low shear rates.
For the 60:40 POE, the DSC scans indicated that the
polymer had a Tg of about 70 "C,and it began to degrade
at about 130 "C. The rheological scans for this polymer
indicated that it behaves normally between 85 and 130 "C.
Within this temperature range, the melt viscosity of the
polymer is similar to that of the 90:lO POE.

Characterization of Microfibers

The degradation of polymetaphosphate fibers in the body
is a two-stage process: first, when exposed to an aqueous
media, polymer chains at the fiber surface dissolve and go
into solution; second, these solvated polymer chains then
undergo chain-end hydrolysis yielding metaphosphate
which is then removed by normal metabolic pathways8
Results of the fiber dissolution study are shown in
Figure 3. The CSM fibers lost about 10%of their mass
after 7 weeks exposure to distilled water and 15% in
normal saline at 37 "C.
Results of acid titration of 2% CSM fiber slurries
in deionized water with 0.OOLN sulfuric acid are shown
Figure 4. Twenty-two mL of acid was required to reach a
pH of 4.0 for fibers treated with diamine-silane coupling
agent while 10 mL of acid was needed for fibers subjected
to boiling water, compared to 5 mL of acid for untreated
fibers. Results suggest that the chemical bond between a
silane coupling agent and the CSM fiber surface resists
hydrolysis, even after being subjected to boiling water for
1 h.

Fabrication Variables

Fiber content. Composite specimens fabricated at
fiber contents of 30% or less by volume were uniform in
appearance. Those with higher fiber contents showed some
inconsistency in wetting out of the fiber surface by the
polymer. Flexural modulus increased in a linear fashion
with increasing fiber content for the entire composition
range studied ( r 2 = 0.9962). Maximum increase in
stiffness (525%) was observed at Vf = 50% (mean
strength = 103 MPa and mean modulus = 9.4 GPa).
Flexural yield strength also increased in a linear fashion
with increasing fiber content, peaking at Vf = 40% then
declining thereafter. Maximum increase in strength (77%)
was observed as Vf = 40% (mean strength = 115 MPa
and mean modulus = 8.2 GPa) (Fig. 5).

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WEIGHT LOSS

~

1

2

-A-

NORMAL SALINE

++

DISTILLED WATER

3
4
5
EXPOSURE TIME (WEEKS)

6

7

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Figure 3. Percent weight loss of CSM fibers after in vitro exposure
at 37 "C to distilled water and normal saline.

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ORTHOPAEDIC FIXATION DEVICES

PH

10,

8

I

* TREATED
* TREATEDIBOILED

A

-

UNTREATED

401

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F
20

5

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10

15

20

25

-

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4 STRENQTH

4+ MODULUS

30

VOLUME OF 0.001 N SULFURIC ACID (ML)

Figure 4. Titration of 2% CSM fiber slurries using 0.001N sulfuric acid.

Pressing temperature. Figure 6 shows the effects
of increased fabrication temperature on the initial
mechanical properties of composites constructed with
diamine-silane treated fibers (Vf = 30%). As fabrication
temperatures were increased, composite flexural yield
MEAN FLEXURAL YIELD STRENQTH (MPa)

120 I

strength and modulus continued to increase until the
temperature reached 185 "C. With further increase in
temperature, strength declined while modulus continued
to increase. The flexural yield strength and stiffness at
185 "C were 127 MPa and 8.3 GPa, respectively.

A
Wet Strength Retention Experiments

40
6o

t
I

0

10

30
40
VOLUME FRACTION CSM FIBERS (%)
20

50

60

For comparison, Figure 7 shows the initial mean flexural
mechanical properties of 90:lO POE (strength = 65 MPa
and modulus = 1.5 GPa) and composites fabricated
with untreated CSM fibers (strength = 100 MPa and
modulus = 7.8 GPa) and treated CSM fibers (strength =
125 MPa and modulus = 8.3 GPa). Composite specimens
with diamine-silane treated CSM fibers had flexural
strength and stiffness similar to reported literature
values of cortical bone (strength = 150-185 MPa and
modulus = 10-15 GPa).34-36
Hot-film coated composite specimens after 4 weeks
immersion in the buffered saline showed no obvious
signs of delamination or degradation, by visual inspection.
Figure 8 shows percent retention of mean flexural yield

MEAN FLEXURAL MODULUS (QPa)

12

B
POE

n-6

UNTREATED-CSMlPOE

17.3

TREATED-CSMlPOE

n-3

CORTICAL BONE (MIN.)

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CORTICAL BONE (MAX.)

~

n
Y

0

10

20

30

40

VOLUME FRACTION CSM FIBERS (%)

50

60

Figure 5. Fiber content of diamine-silane treated CSM/POE composites (n = 2). (A) mean flexural yield strength (MPa) (mean 2 SD).
(B) mean flexural modulus (GPa) (mean 2 SD).

200

150

100

50

FLEX-STR (MPa)

0

_

5

0FLEX-MOD

10

I

_

15

(QPs)

Figure 7. Comparison of flexural mechanical properties for POE,
composites of untreated-CSM/POE, treated-CSM/POE, and the minimum and maximum reported literature values for cortical bone.

204

60

ANDRIANO, DANIELS, AND HELLER

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growth and spread. This may be because the build up
of acidic degradation products lowers the surrounding
tissue pH. This hypothesis seems likely since alphapolyesters show measurable mass loss only at the end
stages of polymer hydrolysis, and in light of the adverse
clinical observations, recently reported for PLA and PGA
fixation devices, of a late stage noninfectious foreign body
response requiring clinical intervention."J9 In contrast,
due to predominate surface hydrolysis, POE degrades into
a more balanced combination of nonacidic, alcoholic, and
acidic products gradually over time.24Therefore, it should
not alter the surrounding tissue pH as readily, suggesting
POE may be less toxic to cellular growth and spread
than lactide-glycolide. However, esterases may accelerate
POE hydrolysis. Preliminary results from our laboratory
of a comparison study of the in vitro cellular toxicity
of degradation products produced by poly(ortho esters),
poly(1actic acid), and poly(glyco1ic acid) as a function of
time has supported this hyp~thesis.~'
The CSM fiber may degrade somewhat more rapidly
in vivo than in vitro. Phosphatase enzymes are reported
to increase the hydrolytic decomposition rate of polymetaphosphate by a factor of about 1 million.42 Solubility
of CSM fibers exposed to media of epithelial cells showed
a 2 to 3 fold increase and a 7-fold increase in solubility
for media containing lung alveolar macrophages.28
Although the preliminary data showing a lack of
acute toxicity of poly(ortho esters) and calcium-sodiummetaphosphate microfibers are very promising, the
question of long-term toxicity cannot be answered until
composite implant studies in animals have been carried
out to complete absorption of the implant. There is
a preliminary report on the histological evaluation of
CSM/POE composites in a rabbit model with promising
results. Unfortunately the study was terminated before
complete absorption of the implant.38 This has been
a failing of many animal implant studies of the past
involving PLA and PGA implants also.11,43,44

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UNTREATED CSM FIBERS

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strength and modulus after in vitro exposure. Composite
specimens prepared with diamine-silane treated CSM
fibers and coated by hot-pressing a 60:40 POE film around
the specimen retained 70% of their strength and 70% of
their modulus after 4 weeks exposure.

DISCUSSION
Biocompatibility and Absorption

The component materials showed no acute cytotoxicity
as determined by cell culture. This agrees with other
acute toxicity
which describe 60:40 and 90: 10
poly(ortho ester) and untreated CSM fibers as acutely
nontoxic in United States Pharmacopeia tests for systemic,
intracutaneous, and intramuscular implant toxicity.
A recent study4' suggests that the build up of
acidic degradation products from copolymers of lactideglycolide in confined spaces may be toxic to cellular

Composite Fabrication

Poly(ortho ester) is a completely amorphous polymer and
therefore viscous. Therefore, during composite fabrication, temperatures near the degradation temperature were
used to achieve low melt viscosities for adequate wetting
of the fiber surface. Because polymer melt viscosities are
a function of polymer molecular weight,45 POEs having
a molecular weight below 100000 were used. The DSC
scans of 60:40 and 90:lO POEs indicated a thermal
degradation averaging 70 "C above the polymer's Tg,
respectively. This represents a rather narrow temperature
range for processing these polymers. In addition, rheology
scans of the 60:40 and 90:lO POE indicated a rather
high melt viscosity near the degradation temperature for
each POE, even thought molecular weights were less than
100,000.

ORTHOPAEDIC FIXATION DEVICES

Therefore, the high melt viscosities necessitated by the
use of a narrow temperature processing window, may have
resulted in composites with incomplete wetting between
the fiber, which has a high surface area (1.0-1.8 m2/g)27
and the matrix polymer. Mechanical properties of
CSM/90: 10 POE composites improved with increasing
fabrication temperature, suggesting improved wetting was
achieved. Unfortunately, at temperatures above 185 "C
strength decreased, probably due to thermal degradation of
the polymer. However, a recent report on the physical and
mechanical properties of absorbable polymers, suggests
degradation temperatures of POEs can be extended to over
300 "C when heated in an inert atmosphere (nitr~gen).~'
Because surface acidity of the CSM fibers accelerates hydrolysis of the acid sensitive poly(ortho ester),
the fiber surface was made basic by treating it with
a diamine-silane coupling agent. When compared to
untreated fibers, use of the silane coupling agent modestly
improved initial mechanical properties of the composite
and markedly improved wet strength resistance after in
vitro exposure to Tris-buffered saline.
An outer polymer coating of hot-pressed 60:40 POE
sufficiently retarded influx of water to further slow the
degradation of the polymer/fiber interface. The coated
composite material retained 70% of strength and stiffness
for 4 weeks. For comparison, there is a report33 of
poly(DL-lactic acid) composites reinforced with continuous calcium-metaphosphate glass fibers and coated with
hot films of polycaprolactone to act as a water barrier.
When exposed to physiological saline, these coated composites retain about 50% of flexural modulus after 4 weeks
exposure. The ability of poly(ortho ester) to act as a superior water barrier when compared to polycaprolactone may
be due to the more hydrophobic nature of the polymer."
Whether the low stiffness of totally absorbable polymers and composites compared to steel will be a serious
limitation in their use for mechanical stabilization of
tissues is not known, since neither the optimal initial
stiffness of internal fixation devices nor the rate at which
stiffness can be decreased safely during healing has been
determined. In spite of these unknowns, clinical use of
absorbable polymers and polymer-fibedpolymer composites for fracture fixation has been successful~7~5'
except
for late complications. These complications are noninfective inflammatory foreign body responses as described
previously.
In summary, CSM/POE composites prepared with
diamine-silane treated fibers have initial mechanical
properties similar to cortical bone. The component
materials show no acute cytotoxicity. Hot POE film
coating slows loss of strength and stiffness enough to
suggest that this composite material is a viable candidate
for further consideration for fabrication of low-load bone
and soft tissue fixation devices.

zyxwv
205

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Funding for this work was provided by Baxter Health Care-Technology
and Ventures Division, SRI International, and the Orthopedic Bioengineering
Laboratory at the University of Utah School of Medicine.

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ANDRIANO, DANIELS, AND HELLER

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Received August 12, 1991
Accepted April 28, 1992