HA Collogen Composite AC

8/13/2019 HA Collogen Composite AC

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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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S.K. Roy Chowdhury et al. / Wear 262 (2007) 13871398 1389

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%


5 Hydroxyapatite 90%, collagen 9.2%,

hyaluronic acid 0.8%, hydroxyapatitecollagenhyaluronic acid composite

60%, gelatin 40%


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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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


5% hyaluronic acid.

2.06E04 5.0


10% hyaluronic acid.

4.15E05 6.5


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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