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Wear 262 (2007) 13871398
Wear characteristic and biocompatibility of somehydroxyapatitecollagen composite acetabular cups
S.K. Roy Chowdhury a,, A.C. Kulkarni a, A. Basakb, S.K. Roy c
a Department of Mechanical Engineering, Indian Institute of Technology, Kharagpur, IndiabDepartment of Chemistry, Indian Institute of Technology, Kharagpur, India
cDepartment of Metallurgical and Materials Engineering, Indian Institute of Technology, Kharagpur, India
Received 28 March 2006; received in revised form 6 December 2006; accepted 9 January 2007
Available online 12 February 2007
Abstract
Both HDPE and UHMWPE have long been used successfully as socket materials in hip-joint replacements. Recently, however there are
concerns over the adverse biological responses due to the wear debris of these polymers. Although a good deal of work to improve the performance
of these polymers has been carried out a need still exists for an implant material with improved biocompatibility and mechanical properties.
Hydroxyapatitecollagen composites have been prepared by precipitation of calcium phosphate on collagen in the past but very few of these
attempts considered the mechanical strength of the composites that suits their realistic uses as implant material. Present work is an attempt to
develop hybrid composites of hydroxyapatitecollagenhyaluronic acid or gelatin with sufficient adherence to both hard and soft tissues and also
with good cohesive strength leading to improved mechanical and biological properties. It was possible to prepare acetabular cups of the newly
developed composites by compression moulding for tests on a hip-joint simulator. Pin specimens for tests on a pin-on-disc apparatus were also
moulded with these composites. Tests with the acetabular cups and pin specimens indicate that some of the newly developed materials offer wear
resistance comparable to those of the presently used socket materials. Biocompatibility tests with these materials show that their haemolysis counts
are well below the acceptable range. Hydroxyapatitecollagen composites with 10% hyaluronic acid offer suitable mechanical strengths, good
friction and wear characteristics and acceptable level of haemolysis and therefore the composite may be considered to be a potential socket material
of future generation. 2007 Elsevier B.V. All rights reserved.
Keywords: Hydroxyapatitecollagen composite; Hip-joint; Acetabular cups; Wear; Biocompatibility
1. Introduction
Since Sir John Charnley proposed metal to plastic pairing for
total hip replacements in early 1960s there has been a contin-
uous search for more suitable materials for the prosthetic pairs
[1,2].The initial choice of PTFE as a socket material was soon
abandoned due to its high wear rate despite its extremely low
frictional resistance. Subsequently, both HDPE and UHMWPE
have been used successfully as socket materials over the last
few decades [3,4]. Recently, however, there are concerns regard-
ing the adverse biological responses due to UHMWPE wear
debris. A good deal of work to improve the performance and
biocompatibility of these polymers hasbeen carried out in recent
Corresponding author. Tel.: +91 3222 282972; fax: +91 3222 282278.
E-mail address:[email protected](S.K. Roy Chowdhury).
years [5,6]. This also regenerated interestsin metals and ceramic
prosthetic pairs. Here too, it was soon realized that long-term
systematic problems might arise due to release of metal ions
and increase in cobalt and chromium concentration in blood
and urine[7].In view of this and also the need for bone aug-
mentations in several other applications, an acute need for an
artificial bone substitute with appropriate biocompatibility and
suitable mechanical properties was felt. Two basic approaches
were made to obtain such materials. One group of researchers
considered polymer composites with hydroxyapatite fillers or
thermally sprayed hydroxyapatite coatings on the metallic pros-
theses. These attempts were successful in improving the wear
resistance of metallic implants but the hydroxyapatitepolymer
composites could not yield the desired compatibility. Wear
debris at the contact between the prosthetic pairs of these com-
posites were often found to contain sizeable amountof polymers.
Another group of researchers considered that a composite of the
0043-1648/$ see front matter 2007 Elsevier B.V. All rights reserved.
doi:10.1016/j.wear.2007.01.023
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1388 S.K. Roy Chowdhury et al. / Wear 262 (2007) 13871398
two major solid phases of bone, namely the collagen and the cal-
cium phosphate would be a better choice to obtain near-natural
bone substitutes[8]. An added incentive for the development
of such collagencalcium phosphate composite was that the
collagen has been said to promote bone healing and its regener-
ation. This in turn promotes new bone growth and an eventual
replacement by natural bone[9].
Many attempts were made to develop such composites
mainly by precipitation of calcium phosphate on collagen
[8,10]. One of the earliest attempts to develop collagencalcium
phosphate composite was made by Mittlmier and Nizzard[11]
who mixed calcium phosphate granules with collagen web.
Many other researchers attempted to produce these composites
by mixing the preformed calcium phosphate granules in colla-
gen suspension[1014]. However, very few of these attempts
considered the mechanical strength of the composites that suits
their realistic uses. The main interest there was to reproduce the
physical properties of bone by nucleation and growth of calcium
phosphate crystals from solution on collagen fibrils [1518].
Lawson and Czernuszka [8] produced a collagencalcium phos-phate composite by precipitation method and concluded that the
mechanical and biological properties of their composite were
encouraging. However, such composites, in general, lack cohe-
sive strength and they are not entirely suitable for manufacturing
load-bearing implants. Attempts have been made to develop
hybrid composites of hydroxyapatitecollagenhyaluronic
acid with sufficient adherence to both hard and soft tissues
and also with good cohesive strength, improved mechani-
cal and biological properties [18]. The present work is an
attempt to develop such hybrid composites with improved
mechanical strengths to the extent that acetabular cups may
be manufactured for use in total hip-joint replacements. Themechanical, tribological and biological characteristics of the
cups made of the newly developed composites have also been
studied.
2. Materials and methods of preparation
2.1. Materials
In view of the present need for a near-natural prosthetic mate-
rial it was felt that the development of the following groups
of composites based on hydroxyapatite and collagen would be
useful.
2.1.1. Polymerhydroxyapatite composites with varying
percentages of hydroxyapatite in polymer matrix
Since HDPE and UHMWPE are widely used as socket
materials in hip-joint prostheses hydroxyapatite reinforced com-
posites of these polymers only were considered to be useful.
The fundamental argument for developing this class of com-
posites is that hydroxyapaptite being a natural body material
the above composites would necessarily be more biocompatible
and if wear debris contain more of filler material less harm to the
body system is expected. There is some evidence [19] to suggest
that the wear debris of these composites may contain low level
of polymer.
Table 1
Mechanical properties of hydroxyapatite[8]
Compressive strength (MPa) 310510
Tensile strength (MPa) 40
Vickers hardness (MPa) 4500
Elastic modulus (GPa) 4090
Fracture toughness K1c (MPa m1/2)
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Table 3
Properties of hydroxyapatite-filled polymer composites and the parent polymers
Specimen materials Tensile strength at break0 (MPa) Elongation at break0(%) HardnessH(MPa) Product parameter00H
HARHDPE (5%) 23.47 61.82 617.8 896,376
HARHDPE (10%) 20.7 55.3 657.1 752,189
HARUHMWPE (5%) 32.6 140 490.5 2,238,642
HARUHMWPE (10%) 26.57 101.75 578.8 1,564,784
HDPE 30 450 37 499,500UHMWPE 37 350 50.5 653,975
Table 4
Properties of composites using micro-mechanics approach
Properties HARHDPE (5%) HARHDPE (10%) HARUHMWPE (5%) HARUHMWPE (10%)
wf 0.05 0.1 0.05 0.1
wm 0.95 0.9 0.95 0.9
vf 0.135 0.249 0.135 0.248
vm 0.864 0.752 0.8644 0.751
pc (gm/cm3) 0.8556 0.7863 0.8578 0.7865
Ec(GPa) 12.546 22.097 2.547 22.011
c 0.3861 0.3755 0.4640 0.4424
reinforced with 5 and 10% hydroxyapatite by weight designated
as HARHDPE (5%), HARHDPE (10%), HARUHMWPE (5%)
and HARUHMWPE (10%), respectively. The tensile strength,
elongation at break and hardness were measured in an appro-
priate tensile testing machine and a standard Vickers hardness
tester respectively with the specimens made of all the four com-
posites and the parent polymer following the ASTM standard
[20,21].The results of these tests are given inTable 3.
The composites developed may be considered to be isotropic
and it is convenient to obtain some of the other relevant param-
eters by micromechanical approach using the standard relations[22].The properties so obtained are given in Table 4.
Here, suffixes m, c and f represent matrix, composite and
fiber, respectively, and w is the weight fraction, v the volume
fraction,p the density,Ethe elastic modulus and is the Pois-
sons ratio. General observation in Tables 3 and 4 is that the
elongation at break of thecomposites decrease whereas thehard-
ness rises significantly compared to the parent polymers. The
changes in tensile strength, Poisons ratio and density are not so
significant but the elastic constant increases many folds.
For tribological tests two types of specimens were necessary,
one for the tests under simulated conditions using a pin-on-disc
apparatus and the other for a hip-joint simulator under rela-
tively more realistic in vitro test conditions. The specimens for
the pin-on-disc tests are simply pins of 8 mm diameter for our
apparatus and those for the tests on a hip-joint simulator are
22 mm acetabular cups of standard dimensions. These speci-
mens were prepared by compression moulding using an existing
die described elsewhere [23]. The die allowsmoulding of 22 mm
standard acetabular cups along with a cylindricalextension at the
bottom part (Figs. 1a and 4).The objective of such a die design
is to produce specimens for both the hip-joint simulator and the
pin-on-disc apparatus within a single mould. Hydroxyapatite
and the polymer matrix materials were mixed in different weightpercentages to makea total mass of 25 gm. This value was calcu-
lated based on the volume of cup, pin and risers. The premixing
was done in a Brabender Plasticorder mixing machine. The pres-
sure and temperature were maintained at 0.6 MPa and 180 C,
respectively. Thereafter the premix was chopped and blended in
a mixer grinder. The prepared compound was then poured into
the mould cavity of the die. A silicon spray was used as a mould-
releasing agent. Pressure was then exerted on the die placed
between the heating plates of a compression-moulding machine
of 200 kN capacity. The load and the temperature were in the
range of 120140 kN and 160185 C, respectively, depending
on the composite being moulded. The die was then cooled in
Fig. 1. (a) A typical moulded acetabular cup of 10% hydroxyapatite-filled UHMWPE composite along with two pins cut out of the main mould. (b) A schematic
diagram of a typical acetabular cup.
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Table 5
Dimensions of the moulded UHMWPE cup and a commercially available HDPE cup
Parent polymer D1(mm) D2(mm) d1(mm) Mass (gm)
UHMWPE (moulded) 49.42 43.35 22.18 18.9484
HDPE (commercially available) 49.37 43.33 22.17 19.2764
Table 6
Composition and mechanical properties of hydroxyapatitecollagenhyaluronic acid composites
Serial number Material composition Mouldability Hardness
(HV/20)
Elastic modulus
(MPa)
Tensile
strength (MPa)
1 Hydroxyapatite 90%, collagen 10% Poor: specimens could not be
prepared
2 Hydroxyapatite 90%, collagen 9.2%,
hyaluronic acid 0.8%
Poor: specimens could not be
prepared
3 Hydroxyapatite 80%, collagen 15%,
hyaluronic acid 5%
Acetabular cup and pins prepared 90 100 96
4 Hydroxyapatite 75%, collagen 15%,
hyaluronic acid 10%
Acetabular cup and pins prepared 117 350 25
5 Hydroxyapatite 90%, collagen 9.2%,
hyaluronic acid 0.8%, hydroxyapatitecollagenhyaluronic acid composite
60%, gelatin 40%
Acetabular cup and pins prepared 100 290 19
Symbol () in the table indicates that measurement could not be made due to lack of mouldability.
air for 3 h and finally the cup was ejected out of the mould
cavity. The pin specimens were then cut from the cup ends. A
typical moulded acetabular cup of a hydroxyapatite reinforced
UHMWPE composite along with two pins cut out of the main
mould is shown in Fig. 1(a). A schematic diagram of a typi-
cal acetabular cup is shown in Fig. 1(b) and the dimensions of
a moulded UHMWPE specimen are compared with those of a
commercially available HDPE specimen in Table 5.It can beseen that the dimensions matches reasonably well and the head
of a standard femoral implant fitted well into the hemispherical
cavity of a cup.
The preparation procedure for the specimens of
HARUHMWPE composites was almost the same as that
for HARHDPE except that UHMWPE was available in pow-
dered form rather than in granular form and the premixing
was not necessary. The pressure applied during compression
molding was maintained again at 0.6 MPa, but slightly higher
temperature of 200 C was necessary.
2.2.2. Preparation of hydroxyapatitecollagen composites
Hydroxyapatitecollagen composites have been prepared bymany workers in different forms [8,10,1315]. In our attempt
hydroxyapatite particles were gradually added to deionized
water and intensively mixed. Separately, the dispersion of very
fine collagen fibers in deionized water was also prepared. Nine
parts by weight of hydroxyapatite-in-dispersion and one part
by weight of collagen-in-dispersion were mixed intensively in
a magnetic stirrer for 24 h to form the complex precipitate. The
precipitate was filtered and dried at a temperature of 37 C for
72 h in a Petri dish. This forms the hydroxyapatitecollagen
composite. However, attempt thereafter to produce acetabular
cup and pin specimen by compression molding was not suc-
cessful. This was because the required degree of cross-linking
of collagen was not achieved [15]. The hydroxyapatitecollagen
composite was synthesized further using hyaluronic acid which
is known to provide binding capabilities. The samples con-
taining hyaluronic acid was prepared by adding nine parts
by weight of inorganic component, viz. hydroxyapatite-in-
dispersion and one part by weight of organic component that
contains 92% by weight of bovine collagen type-1 and 8% by
weight of hyaluronic acid. Samples with increasing percentageof hyaluronic acid were prepared by intensive mixing of the two
phases in a mechanical shaker. The samples with gelatin as a
binderwere also prepared by adding 40%by weightgelatin pow-
der and 60% by weight of the above composite with hyaluronic
acid. The detail composition of all the composites prepared and
their mechanical properties are shown in Table 6. The tensile
strengths and elastic moduli of the samples were determined
in a suitable tensile testing machine and the Vickers hardness
number was obtained in a standard hardness tester using 2 kgf
load.
The scanning electron micrographs of some of the compos-
ites are shown inFigs. 2 and 3and they are useful in studying
the morphology of the composites. The micrographs show thatthe particles of hydroxyapatite are anchored in the complex of
biopolymer matrix and a compact block structure is formed. At
higher magnifications the microphotographs show very intimate
contact of hydroxyapatite granules with the collagenhyaluronic
acid complex. The particles are seen to be completely covered
with the film of biopolymer conjugate and the structure of the
material is rather dense.
The micrographs of the other developed composites, such as
hydroxyapatitecollagen composites with 5 and 10% hyaluronic
acid show certain cross-linking between hydroxyapatite and
collagen (Fig. 3).Acetabular cups were prepared by compres-
sion moulding following the procedure described in Section
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Fig. 2. Scanning electron micrographs of hydroxyapatitecollagenhyaluronic acid composite at serial number 2 in Table 6with magnifications: (a) 1000 and (b)
5000.
Fig. 3. Scanning electron micrographs of: (a) hydroxyapatitecollagenhyaluronic acid composite at serial number 3 in Table 6and (b) hydroxyapatitecollagen
hyaluronic acid composite at serial number 4 in Table 6.
Table 7
Processing parameters for moulding acetabuylar cups of hydroxyapatitecollagenhyaluronic acid composites shown in Table 6
Material composition Temperature (C) Load (kN) Processing time (min)
Hydroxyapatite 90%, collagen 10% 185 110 15
Hydroxyapatite 80%, collagen 15%, hyaluronic acid 5% 200 115 20
Hydroxyapatite 75%, collagen 15%, hyaluronic acid 10% 210 115 25
2.2.1. The processing parameters for moulding are giveninTable 7.
Two such moulded specimens are shown inFig. 4.Attempts
to prepare the moulded specimen with the composite at serial
number l inTable 6(hydroxyapatite, 90%; collagen, 10%) were
not very successful. The mould was of brittle nature and could
not sustain sufficient compressive load for the purpose it was
prepared. However, the specimens with hydroxyapatitecollgen
composites with hyaluronic acid were suitable for the tests under
the simulated conditions.
2.2.3. Preparation of polymerhydroxyapatitecollagencomposites
Attempts were made to develop composites with varying per-
centage of hydroxyapatite and collagen in UHMWPE matrix. It
was observed that mouldability andnecessary strengths could be
obtained only with fairly large percentage of polymer content.
Two such polymers were prepared and the specimenswere made
of them. The processing parameters for moulding the compos-
ites and their mechanical properties are given in Table 8and the
dimensions of two typical pin specimens are given inTable 9.
Fig. 4. The integral moulds for the acetabular cup and pin specimens for tests on a hip-joint simulator and a pin-on-disc apparatus. The mould materials are
hydroxyapatitecollagen composites with (a) 5% hyaluronic acid and (b) 10% hyaluronic acid.
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Table 8
Processing parameters for moulding the polymerhydroxyapatitecolagen composites and their mechanical properties
Material composition UHMWPE (50%)hydroxyapatite
(40%)collagen (10%)
UHMWPE (70%)hydroxyapatite
(23%)collagen (7%)
UHMWPE
Mould processing temperature (C) 185 165 185
Mould processing load (kN) 110 115 105
Mould processing time (min) 1520 1520 1520
Tensile strength (MPa) 11 17 35Youngs modulus (MPa) 110 170 500
Hardness (MPa) 17 20 40
Table 9
Dimensions and weights of two typical pin specimens of the composites in Table 8
Material composition Dimensions (mm) Weight (gm)
DiameterD LengthL
50% UHMWPEhydroxyapatitecollagen 9.2 23.2 1.74514
70% UHMWPEhydroxyapatitecollagen 8.35 22.7 1.256
3. Friction and wear tests
As discussed earlier friction and wear tests of the new mate-
rials were carried out using a pin-on-disc apparatus and also
a hip-joint simulator. The tests are described in the following
sections.
3.1. Friction and wear tests on a pin-on-disc apparatus
A commercially available standard pin-on-disc apparatus
(Ducom, India TR-20) with a surgical grade stainless steel disc
of 230 mm diameter was used. The roughness of the disc surface
was maintained within a range of 0.030.05m cla by a com-bination of fine grinding and lapping. The pin specimens were
cut out from the main mould as demonstrated inFig. 1(a). The
pin ends were flattened for conformity and then cleaned in an
ultrasonic bath. The lubricant used was a solution of 1.5 gm of
carboxymethyl cellulose powder in 150 ml of distilled water.
Although bovine serum would be a better substitute for the
synovial fluid the carboxymethyle solution was used for the sim-
ilarity of its rheological properties with those of synovial fluid
[24]. The pin specimens were soaked in the carboxymethyle
solution for 48 h and then dipped in 4% formaldehyde solution
for 24 h for sterilisation. The disc was also cleaned thoroughly
before carrying out the experiments. Some experiments with
bovine serum as a lubricant were also carried out and the resultsare compared in Section3.1.2.
Tests for considerably long time were carried out under con-
stant loads with wear measurements made at regular intervals.
It is generally assumed that the load is distributed over the artic-
ular contact between the cup and the femoral implant as shown
inFig. 5[25].In order to estimate the appropriate load for the
pin-on-disc apparatus we refer to the load on the femoral head.
In our experiments on a hip-joint simulator muscle forces were
applied by equivalent springs and the resultant loadPwas of the
order of 2287 N over the contact region.
The maximum Hertzian contact pressure here is given by
Pmax= 3P/(2a2
) where a is the Hertzian contact radius and
P is the load. The contact radius a can be obtained using thevalues of elastic constants and the radii of the cups prepared
(Tables 4, 6 and 8). Matching the calculated maximum contact
pressurePmaxto the pin-on-disc configuration the pin load may
be obtained. Based on this pin-on-disc tests were conducted over
the load range between 10 and 70 N. The tests were carried out
at an exaggerated sliding velocity of 0.24 m/s in order to com-
pare the friction and wear characteristics of the newly developed
composites under rigorous conditions[23].
3.1.1. Friction and wear tests on a pin-on-disc apparatus
with pins made of hydroxyapatite-filled polymer composites
Two sets of tests with pins made of hydroxyapatite-filledpolymer composites (Table 3)were carried out in the presence
of carboxymethyl cellulose solution. In each set two pin load-
ings of 20 and 30 N were used. In the first set of experiments
the wear tests for the HARUHMWPE with 5 and 10% rein-
forcements were carried out over 100 min at a constant sliding
velocity of 0.24 m/s. Wear was measured by weight loss method
using a microbalance after each 20 min interval. Wear volumes
in mm3, calculated using the densities in Table 4 are plotted
against sliding distance inFig. 6for ease of comparison with
other results.
Fig. 5. Load distribution on a femoral implant head during articulation.
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Fig. 6. Plots of wear volume in mm3 against sliding distance for hydroxyapatite
reinforced UHMWPE composites at a constant sliding velocity of 0.24 m/s and
under two different loading conditions on a pin-on-disc apparatus.
Fig. 7. Plotsof wear volume inmm3 against loadat a constantslidingvelocity of
0.24 m/s for hydroxyapatite reinforced UHMWPE composite on a pin-on-disc
apparatus.
The second set of experiments were carried out with
HARUHMWPE of 5, 10 and 30% hydroxyapatite reinforce-
ments and also with an unfilled UHMWPE under pin loadings of
10, 30, 40, 50 and 70N at a constant sliding velocity of 0.24m/s
for 20 min. Wear and the steady value of frictional force during
sliding are plotted against load inFigs. 7 and 8,respectively.
Fig. 8. Variation of coefficient of friction with normal load for hydroxyapatite
reinforced UHMWPE composites on a pin-on-disc apparatus.
3.1.2. Friction and wear tests on a pin-on-disc apparatus
with pins made of hydroxyapatitecollagen-hyaluronic acid
composites
Pin specimens of only three composites in this class could be
prepared as shown inTable 6and the friction and wear tests of
these pins with both carboxymethyl solution and bovine serumas lubricants were carried out on the pin-on-disc apparatus at a
constant sliding velocity of 0.24 m/s and under a load of 20 N.
Variationsof wear volume andcoefficientof friction with sliding
distance are shown inFigs. 9 and 10,respectively.
A 6% by weight bovine serum solution in distilled water
was prepared and a 2% by weight of sodium azide in distilled
water was added as an antibacterial agent. Tests with this bovine
serum solution as a lubricant were conducted under conditions
identical to those with carboxymethyl solution as a lubricant and
the results are included in Figs. 9 and 10. In addition results with
UHMWPE pins are also shown for comparison.
In order to see the effect of change in sliding velocity on
the coefficient of friction some tests at sliding velocity rangingbetween 0.1 and 1 m/s were also carried out. Plots of coef-
ficient of friction against sliding velocity for the developed
hydroxyapatitecollagenhyaluronic acid composites under a
constant load of 20 N with carboxymethyl solution as a lubricant
is shown inFig. 11.
3.1.3. Wear tests on a pin-on-disc apparatus with pins
made of UHMWPEhydroxyapatitecollagen composites
Plots of wear volume against sliding distance for the two
UHMWPEhydroxyapatitecollagen composites (Table 8)are
Fig. 9. Plots of wear volume in mm3 against sliding distance of the hydroxyapatitecollagenhyaluronicacid composites lubricated withboth carboxymethyl solution
and bovine serum solution on a pin-on-disc apparatus.
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Fig. 10. Plots of coefficient of friction against sliding distance of the hydroxyapatitecollagenhyaluronic acid composites lubricated with carboxymethyl solution
and bovine serum solution on a pin-on-disc apparatus.
Fig. 11. Plots of coefficient of friction against sliding velocity for the
hydroxyapatitecollagenhyaluronic acid composites lubricated with car-
boxymethyl solution as a lubricant on a pin-on-disc apparatus.
shown inFig. 12.In these composites the UHMWPE contents
were reduced to only 50 and 70% compared to the earlier com-
posites (Table 3) with UHMWPE contents of 90% or more.These tests were carried out essentially to see the effect of the
presence of both hydroxyapatite and collagen in the polymer
matrix. These plots are also compared with UHMWPE results
inFig. 12.
3.2. Wear tests on a hip-joint simulator
Some tests with the acetabular cups made of the newly
developed composites were also carried out on an existing hip-
Fig. 12. Plots of wear volume against sliding distance for the polymerhydro-
xyapatitecollagen composites lubricatedwith carboxymethyl solutionon a pin-
on-disc apparatus.
joint-simulator described in details elsewhere [23]. Attempts
were made to reproduce the real situation in the simulator by
allowing two femurs with standard stainless steel implants to
articulate simultaneously within two standard 22 mm diameteracetabular cups fitted within the cavities on either side of a
pelvis. The pelvis is fixed in a structure but it is allowed to
rotate about superiorinferior axis and the appropriate motion
of the femurs and loading at joint was provided by a suitable
arrangement using pneumatic cylinders. In order to test the
newly developed cups two new cups were fitted within the
pelvic sockets after soaking and cleaning in the stipulated
manner described in Section 3.1. Tests with each pair were
conducted for up to 12,000 cycles at a rate of 25 cycles per
minute. The wear was measured, as before, by weight loss
method using a precision balance at an interval of 1000 cycles.
The load on the joints was maintained by a pressure regulator
connected to the pneumatic cylinder at the loading platform.
Efforts were made to determine the relative merits of the
developed composites in terms of their wear resistance.
Wear tests with only a few specimens could be carried
out on the simulators due to certain restrictions of the sim-
ulator and specimens. The results of hydroxyapatite-filled
UHMWPE (Table 3) are shown in Fig. 13 and those with
Fig.13. Variationof wearvolume against numberof walkingcyclesin a hip-joint
simulator for the hydroxyapatite-filled UHMWPE composite.
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Fig. 14. Plots of wear volume against number of walking cycles in a hip-joint
simulator for UHMWPEhydroxyapatitecollagen composites.
UHMWPEhydroxyapatitecollagen composites (Table 8)are
given inFig. 14.
4. Biocompatibility tests
Biocompatibility refers essentially to the compatibility of
materials with the biological systems. Since it is rarely pos-sible to find a fully biocompatible material, it is necessary to
identify the materials, which are physiologically tolerable. In
the current trend of biocompatibility studies, aspects of both
biosafety and biofunctionality are considered [29]. Biosafety
tests such as test on cytotoxicity and mutagenesis or car-
cinogenesis are aimed at excluding the severe harmful effects
of biomaterials on organisms. Among many the haemolysis
count is an important parameter in testing the biocompatibil-
ity of materials. In the present work, in vitro biocompatibility
tests were carried out broadly in line with the ASTM stan-
dards and the extent of haemolysis of the newly developed
implant materials were determined. Haemolysis indicates pre-
mature destruction of red blood cells when they come in contactwith water or other foreign elements. Haemolysis percentage is
given by
haemolysis(%) =OD (test) OD (negative)
OD (positive) OD (negative) 100
where the optical density OD is basically the light absorbency
and this is given by log (Io/I),IoandIbeing the intensities of the
original and transmitted lights respectively. The optical density
was measured in a standard UV/vis spectrometer. The haemoly-
sis counts of the developed composites were obtained following
Fig. 15. Haemolysis counts of the developed composites.
the standard procedure[23]using goats blood. The results are
shown inFig. 15.
5. Discussions
The three groups of composites prepared and tested are: (a)
polymerhydroxyapatite composites with varying percentage ofhydroxyapatite in polymer matrix, (b) hydroxyapatitecollagen
composites with varying percentage of hyaluronic acid or
gelatin and (c) polymerhydroxyapatitecollagen composites.
The method of preparation of the composites and specimens are
detailed in Section2 and the friction and wear test results are
given in Section3. The biocompatibihty results are described
in Section 4. All the results are compared with those of our
moulded UHMWPE specimens. We now discuss the results in
sequence.
Figs. 68show the friction and wear tests results of the com-
posites in group (a) above and it can be seen in Fig. 7 that
the wear resistance of the hydroxyapatite reinforced UHMWPE
composites arenoticeably higher than that of theparent polymer.Furthermore, the wear resistance of the composites increases
to an extent with the increase in hydroxyapatite content. The
increase in wearresistancecan be explainedusing Ratners equa-
tion which is widely used for predicting polymer wear with some
degree of success. The equation is given by:
V =wx
00H
whereVis the wear volume, wthe normal load andxis the slid-
ing distance. Table 3 shows that the product parameter00Hof
the composites are much higher than that of the parent polymer
and also the parameter increases with the increase in hydrox-yapatite content. Since the product parameter appears in the
denominator in the equation the experimental observation that
thewear resistance rises with theincreasein hydroxyapatite con-
tent is justified. FromFigs. 7 and 8,it can be seen that although
HARUHMWPE (30%) is most wear resistant its friction level
is higher than the other composites. It therefore seems that
HARUHMWPE (10%) offers an optimum tribo-performance
within this group of composites. However, it will be shown later
that the suitability of a new composite as a possible implant
material cannot be judged by its friction and wear characteristics
alone.
Friction and wear test results of the composites in group (b)
above with both carboxylmethyl solution and bovine serum aslubricants are shown inFigs. 911.For all, the new materials
wear rises initially with sliding distance and then it levels off.
This is a typical behaviour of polymer wear. Hydroxyapatite
layer over the rough counterface is subjected to a small-scale
fracture process, which again is a common mechanism of wear
of brittle polycrystalline materials[26].Wear process here may
be considered to be a sequence of steps involving deformation
and fracture of hydroxyapatite layer at the asperity contact
regions followed by a fresh formation of debris layer which
gradually builds up to cause a decelerating wear pattern. The
results also indicate that with a higher percentage of hyaluronic
acid (10%) the wear is least among the present groups of
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composites. This may be because addition of hyaluronic
acid increases cohesion within a composite structure and
consequently the bonding strength rises. Gelatin also offers
bonding strength but fairly large proportion of gelatin is needed
to increase the wear resistance of a composite. It can be seen in
Fig. 9that the composite (serial number 5 inTable 6)with 40%
gelatin is more wear resistant than the composite with a lower
percentage (5%) of hyaluronic acid but the composite with 10%
hyaluronic acid (serial number 4, Table 6) is more wear resistant
than the composites with 5% hyaluronic acid, 40% gelatin and
also the unfilled UHMWPE. It can be seen in Fig. 10that the
coefficient of friction of hydroxyapatitecollagenhyaluronic
acid composites falls sharply initially and then reaches a steady
value with the further increase in sliding distance. This again
is a typical behaviour of polymer friction where transfer layer
builds up as sliding proceeds.Fig. 11shows that the coefficient
of friction of all the composites falls initially with increasing
sliding velocity and then reaches a steady value. The figure also
shows that the friction level of the hydroxyapatitecollagen
composite with 10% hyaluronic acid is the lowest among thecomposites tested. Considering that the composite is most wear
resistant among the present sets of new materials developed
and that its biocompatibility is likely to be of very high order
due to the presence of both hydroxyapatite and collagen it is
certainly a strong competitor as a future implant material The
results in Fig. 9 also show clearly that the wear levels of the
composites are higher in the presence of bovine serum solution
than carboxymethyl solution as a lubricant. It was shown by
several authors that its protein content plays a key role in
wear results. Scholes and Unsworth[27]observed that varying
degree of protein might be adsorbed on the surfaces of ceramic
and polymer specimens. They concluded that the adsorbedfilm of protein influenced the lubricating condition and wear
of the artificial joints. Another observation in Fig. 10 is that
the coefficient of friction is lower in the presence of bovine
serum than carboxymethyl solution as a lubricant. This is again
because the adsorbed protein layer on the metallic counterface
reduces the transfer layer formation and the counterface
roughness is retained to some extent leading to high wear level
but low friction. McKellop et al. [28] using a reciprocating
pin-on-plate rig investigated the effects of different lubricants on
the frictional resistance between UHMWPE and 316L stainless
steel. In the presence of bovine blood serum, the coefficient of
friction between the two surfaces (= 0.12) was lower than that
in the presence of either distilled water (= 0.18) or Ringerssolution (= 0.27). Heavy transfer film on the metal surface
was reported with Ringers solution and distilled water.
Fig. 12 shows the variation of wear volume against
sliding distance for the composites in group (c) above
(polymerhydroxyapatitecollagen composites) lubricated with
carboxymethyl solution on a pin-on-disc apparatus. The
wear levels of both the composites are seen to be higher
than the parent polymer. These two composites were devel-
oped primarily to compare their performance with that of
hydroxyapatite-filled UHMWPE composites without any col-
lagen (Figs. 6 and 7) because it was felt that if the wear
performance of polymerhydroxyapatitecollagen composites
compared well with those of hydroxyapatite-filled UHMWPE
then the former would score high as implant material because
of its improved biocompatibility. However, the results inFig. 12
did not support the hypothesis.
The wear test results for the composites in group (a)
on the hip-joint-simulator shown in Fig. 13 indicate that
HARUHMWPE-30% (Tables 3 and 4) is most wear resis-
tant and this is in agreement with the pin-on-disc results.
However, since the pin-on-disc results also indicate that the
coefficient of friction of HARUHMWPE-30% is high these
results confirm the previous conclusion that HARUHMWPE-
10% offers the best tribo-performance among the composites
in group (a). Wear test results for the composites in group (c)
(Fig. 14) show that none of the composites is suitable from
the wear point of view although they may have enhanced bio-
compatibility. Again this observation is in agreement with the
pin-on-disc results inFig. 12.The results inFig. 15are encour-
aging for all the hydroxyapatitecollagen based composites
because the accepted norm is that if the haemolysis percentage
is less than 10 the test material is taken as haemocompati-ble and if it is less than 5 the material is considered to be
highly haemocompatible. It can be seen in Fig. 15that all the
hydroxyapatitecollagen based composites are haemocompat-
ible. However, the haemocompatibility of the composites fell
with the increasing percentage of hyaluronic acid. The haemo-
compatibility also fell in the presence of gelatin and it seems
that gelatin affects the haemocompatibility more adversely than
hyaluronic acid. This probably indicates that haemocompati-
bility arises essentially due to the presence of the two major
solid phases of bone, namely the collagen and the calcium phos-
phate and the presence of any other binding element affects the
haemocompatibility adversely.Wear factors of some of the newly developed composites as
tested on a pin-on-disc apparatus and on a hip-joint simulator
along with their haemolysis counts are given inTable 10.
First observation to be made here is that the wear factors
of the in-house moulded UHMWPE specimens tested both
on a pin-on-disc and a hip-joint simulator are higher than
the range of values (0.8E06 to 1.1E06mm3/Nm) reported
in literatures [3032]. It must be emphasised here that the
UHMWPE specimens were moulded with ultra-high molecular
weight polyethylene powders (Aldrich Chemical Company,
Inc., India; density: 0.84) in the same manner as the specimens
for the new composites. This was done for consistency in the
method of manufacturing of all the specimens. However, someinaccuracy in our moulding procedure is expected and therefore
the friction and wear characteristics of these specimens are not
likely to match those of the commercially available UHMWPE
specimens. Nevertheless, the present results are believed to
be quite satisfactory for a comparative assessment of the new
composites with reference to the parent polymer. The second
important observation here is that the wear factors of the new
composites lie within the range of 4.15E05 to 1.80E03 for
the pin-on-disc tests and 2.67E5 to 1.70E3 for the hip-joint
simulator. Within this range the hydroxyapatite reinforced
UHMWPE composites (group (a), Tables 3 and 4) seem to
be relatively more wear resistant. However, these composites
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S.K. Roy Chowdhury et al. / Wear 262 (2007) 13871398 1397
Table 10
Wear factors and haemolysis counts of some of the composites developed
Composites Wear factor (mm3/Nm) Haemolysis count
Pin-on-disca tests Hip-jointa simulator tests
HARUHMWPE (5%) 2.02E04 4.68E05
HARUHMWPE (10%) 6.58E05 2.67E05
HARUHMWPE (30%) 9.42E05 1.43E05 Hydroxyapatitecollagen composite with no
binding agent
Poor mouldability; specimens
could not be prepared
2.2
Hydroxyapatitecollagen composite with
0.8% hyaluronic acid.
Poor mouldability; specimens
could not be prepared
3.8
Hydroxyapatitecollagen composite with
5% hyaluronic acid.
2.06E04 5.0
Hydroxyapatitecollagen composite with
10% hyaluronic acid.
4.15E05 6.5
Hydroxyapatitecollagen composite with
gelatin.
1.41E04 9.0
50%UHMWPEhydroxyapatitecollagen
composite
1.80E03 1.70E03
70%UHMWPEhydroxyapatitecollagen
composite
1.09E04 1.03E03
UHMWPE 4.86E04b 5.63E05c 7.0a Tests on both the pin-on-disc and hip-joint simulator tests were earned out with carboxymethyl solution as a lubricant.b Average value fromFigs. 7, 9 and 12.c Average value fromFigs. 13 and 14.
are not likely to have biocompatibility of high order due
to the large polymer content. The hydroxyapatitecollagen
composites with varying percentage of hyaluronic acid and
gelatin (group (b)) probably come next in order of their wear
resistance. Composites in group (c) do not offer any meaningful
wear resistance. If we consider both the wear resistance and
biocompatibility hydroxyapatitecollagen composite with
10% hyaluronic acid probably offers the best combination.
Although the hydroxyapatitecollagen composites with no
binding agent are extremely biocompatible they do not offer
sufficient mechanical strength to be used as a socket material.
Finally, considering both friction and wear results along with
the haemolysis count it seems that for higher biocompatibil-
ity some compromise on the wear performance of materials is
necessary.
6. Conclusions
A number of hydroxyapatitecollagen based compositeshave
been developed using a chemical synthesis route and their
mechanical properties, such as hardness, tensile strength andmouldability could be improved when they are synthesized fur-
ther using hyaluronic acid or gelatin. It was possible to prepare
acetabular cups of some of the new composites by compres-
sion moulding for tests on a hip-joint simulator. Pin specimens
for pin-on-disc tests were also prepared. Tests with both the
acetabular cups and pin specimens indicate that some of the
newly developed materials offer wear resistance comparable to
moulded UHMWPE specimens. Following standard test meth-
ods the biocompatibilities of the composites were determined
and the haemolysis counts for all the materials were well below
the acceptable range of 10. Considering both the mechanical and
tribological characteristics hydroxyapatitecollagencomposites
with 10% hyaluronic acid seems to be a potential candidate for
implant materials of future generation with higher biocompati-
bility. Finally, it must be mentioned that this is the first attempt
to establish the possible future use of the new composites as
socket materials and certainly more work is needed to bring it
to the stage of commercial acceptance.
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