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Colloids and Surfaces B: Biointerfaces 39 (2004) 133142

Biomaterials in total joint replacement

Kalpana S. Katti

Department of Civil Engineering, North Dakota State University, CIE 201B, Fargo, ND 58105, USA

Available online 20 February 2004

Abstract

The current state of materials systems used in total hip replacement is presented in this paper. An overview of the various material systems

used in total hip replacement reported in literature is presented in this paper. Metals, polymers, ceramics and composites are used in the design

of the different components of hip replacement implants. The merits and demerits of these material systems are evaluated in the context ofmechanical properties most suitable for total joint replacement such as a hip implant. Current research on advanced polymeric nanocomposites

and biomimetic composites as novel materials systems for bone replacement is also discussed. This paper examines the current research in the

materials science and the critical issues and challenges in these materials systems that require further research before application in biomedical

industry.

2003 Elsevier B.V. All rights reserved.

Keywords:THR; Biomedical; Review; Bone; Mechanical properties

1. Introduction

1.1. Natural bone structure and mechanical properties

Natural bone is a composite material made up of col-

lagen fiber matrix stiffened by hydroxyapatite (HAP)

(Ca10(PO4)6(OH)2) crystals that account for 69% of the

weight of the bone [1]. The organic phase is composed

mainly of a protein, type I collagen. Like all collagens

the type I collagen is a triple helix. The protein molecule,

tropocollagen in the organic phase of bone is 260 nm long

and the molecules alongside each other are staggered by

about 1/4 of their length [2]. Histologically, the bone is

divided into immature woven bone and mature lamellar

bone. In woven bone, the collagen is fine fibered, 0.1m

in diameter, and is oriented almost randomly. The wovenbone consists of cells (osteocytes) and blood vessels. The

collagen in lamellar bone forms branching bundles, 23m

in diameter. Collagen-based biomaterials are routinely used

as sutures, blood vessels, heart valves and drug delivery

systems[3,4].Generally, elastic modulus, tensile and com-

pressive strengths are the mechanical properties investigated

to ensure suitability of the biomaterial. Mechanical proper-

ties of bone are shown in Table 1. The elastic modulus of

Tel.: +1-701-231-9504; fax: +1-701-231-6185.

E-mail address:[email protected] (K.S. Katti).

bone (17 GPa in tension in human femur) is intermediate

between that of apatite and collagen [5]. But its strength

is higher than that of both. The organic phase behaves asa compliant material with high toughness. The inorganic

phase, HAP, is present in the form of small crystallites of

dimensions 5 nm 20nm 40 nm. The stiffness of this

material is about two-thirds of steel. Also, it is quite brittle

and has poor impact resistance and fractures easily. The

properties of bone really arise from combination of the

high hardness (of HAP) and high fracture toughness (of or-

ganic). This superposition of two very dissimilar materials

with entirely different properties results in the formation of

a nanocomposite system of which the physical properties

surpass that of the individual components. Thus, bone rep-

resents a bio-nanocomposite system that has evolved over

millions of years and perfected with optimized properties.Bone can remodel and adapt itself to the applied mechani-

cal environment. This property of bone that is also called as

Wolffs law, results such that the new remodeled structure

is more suitably adapted to the applied load. Further the

application of higher stress results in a more dense bone.

1.2. Materials consideration for implants

Biomaterials were first defined as nonviable materials

used in a medical device, intended to interact with biologi-

cal systems[6]. Further, Black defined the term biomateri-

0927-7765/$ see front matter 2003 Elsevier B.V. All rights reserved.

doi:10.1016/j.colsurfb.2003.12.002


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134 K.S. Katti/ Colloids and Surfaces B: Biointerfaces 39 (2004) 133142

Nomenclature

BCP biphasic calcium phosphate

SEVA ethylene vinyl alcohol copolymer

HAP hydroxyapatite

HDPE high density polyethylene

UHMWPE ultrahigh molecular weight polyethylenePA polyacetal

PS polysulfone

PE polyethylene

PP polypropylene

PU polyurethane

PPh polyphosphosone

PEEK polyetheretherketone

PTFE polytetrafluoroethylene

PET polyethylene terepthalate

PMMA polymethylmethacrylate

PGA poly(glycolide)

PTC poly(trimethylene carbonate)

DLPLA poly(dl-lactide)

PDO poly(dioxanone)

DLPLLA poly(dl-lactide-co-l-lactide)

DLPLG poly(dl-lactide-co-glycolide)

PHA poly(-hydroxyalkanoates)

PGA-TMC poly(glycolide-co-trimethylene

carbonate)

SR silicone rubber

LPLG poly(l-lactide-co-glycolide)

PCL poly(-caprolactone)

THR total hip replacement

TCP tricalcium phosphate

als as materials of natural or manmade origin that are used

to direct, supplement, or replace the functions of living tis-

sues[7].Many synthetic materials are used in the medicine

for a variety of applications ranging from total replacement

of hard or soft tissues (such as bone plates, pins, total joint

replacement, dental implants, intra-ocular lenses, etc.), re-

pair, diagnostic or corrective devices (such as pacemakers,

catheters, heart valves, etc.). The two primary issues in mate-

rials science of new bone biomaterials are mechanical prop-

erties and biocompatibility. Although mechanical propertiesof biomaterials have been well characterized, the term bio-

Table 1

Mechanical properties of bones (adapted from [2,5,7])

Hard

tissues

Compressive

strength (MPa)

Tensile strength

(MPa)

Elastic modulus

(GPa)

Tibia 159 140 18.1

Femur 167 121 17.2

Radius 114 149 18.6

Humerus 132 130 17.2

Cervical 10 3.1 0.23

Lumbar 5 3.7 0.16

compatibility is only a qualitative description of how the

body tissues interact with the biomaterial within some ex-

pectations of certain implantation purpose and site [8].The

average load on a hip joint is estimated to be up to three

times body weight and the peak load during other strenuous

activities such as jumping can be as high as 10 times body

weight. In addition hip bones are subjected to cyclic loadingas high as 106 cycles in 1 year[9]. Materials scientists have

investigated metals, ceramics, polymers and composites as

biomaterials. The general criteria for materials selection for

bone implant materials are:

It is highly biocompatible and does not cause an inflam-

matory or toxic response beyond an acceptable tolerable

level.

It has appropriate mechanical properties, closest to bone.

Manufacturing and processing methods are economically

viable.

Ideally, a bone implant such as a hip implant should be

such that it exhibits an identical response to loading as realbone and is also biocompatible with existing tissue. The

compatibility issue involves surface compatibility, mechani-

cal compatibility and also osteocompatibility. These materi-

als are also classified as bioactive (illicit a favorable response

from tissue and bond well), bioinert and biodegradable. In

this paper, the different materials and composites used in

the fabrication of implants such as total hip replacement are

investigated for their merits and demerits. The purpose of

this paper is to provide a review of the various material sys-

tems currently being investigated as potential components

of the total hip replacement (THR) implants. Replacement

of joints such as a THR is a serious health concern. In theUSA alone, over 150,000 total hip replacement procedures

are undertaken annually. Most hip implants must be replaced

after 15 years. It is estimated that 20% of hip replacement

surgeries simply replace the original, failed implant. Of all

the joints in a human body, the hip and knee represents some

of the most important synovial joints. The hip joint consists

of two complementary articular surfaces separated by artic-

ular cartilage and the synovial fluid that has a pH between

7.29 and 7.45. Excessive wear of the interfaces due to de-

generative disease (such as osteoarthritis) or injury requires

a replacement of the entire hip joint. Historically, a total

hip replacement the articulation of a human hip is simu-

lated with the use of two components, a cup type and a long

femoral type element. A typical hip implant fabricated from

titanium is shown inFig. 1.The head of the femoral element

fits inside the cup to enable the articulation of human joint.

These two parts of the hip implant have been made using

a variety of materials such as metals, ceramics, polymers

and composites. Typically polymeric materials alone tend to

be too weak to be suitable for meeting the requirement of

stress deformation responses in the THR components. Met-

als typically have good mechanical properties but show poor

biocompatibility, cause stress shielding and release of dan-

gerous metal ions causing eventual failure and removal of


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K.S. Katti / Colloids and Surfaces B: Biointerfaces 39 (2004) 133142 135

Fig. 1. A typical hip implant with titanium femoral (a) and polyethylene

acetabular cup (b) components.

implant. Ceramics generally have good biocompatibility but

poor fracture toughness and tend to be brittle. Composite

materials with engineered interfaces resulting in combina-

tion of biocompatibility, mechanical strength and toughness,

is the focus of many current studies.

Total joint replacements generally involve implantation

components held in place by a cement. Loosening of the

components often occurs at the interface between the cement

and bone due to failure of the fixation of the cement to the

bone. Although deeper penetration of the cement into the

interstices of cancellous bone should improve the mechan-

Table 2

Mechanical properties of alloys in total joint replacement (adapted from[10,13])

Alloy Microstructure Tensile strength (MPa) Modulus (GPa)

cpTi (pure titanium) {} 785 105TiZr Cast{/} 900CoCr alloys 6551896 210253

CoCrMo {Austenite (fcc) +hcp} 6001795 200230Ti6Al4V {/} 960970 110Ti6Al7Nb {/} 1024 105

Ti5Al2.5Fe {/} 1033 110Ti13Nb13Zr {/} 1030 79Ti15Mo5Zr3Al {Metastable} 882975 75

{Aged +} 10991312 88113Ti12Mo6Zr2Fe {Metastable} 10601100 74-85Ti15Mo5Zr3Al {Metastable} 882975 75

{Aged +} 10991312 88113Stainless steel 316 L {Austenite} 465950 200Ti15Mo2.8Nb3Al {Metastable} 812 82

{Aged +} 1310 100Ti35Nb5Ta7Zr (TNZT) {Metastable} 590 55Ti15Mo3Nb0.3O (21SRx) {Metastable} +silicides 1020 82Ti35Nb5Ta7Zr0.4O (TNZTO) {Metastable} 1010 66Ti0/20Zr0/20Sn4/8Nb2/4Ta+(Pd, N, O) {/} 7501200

ical interlock, subsequent bone resorption often results due

to the modulus mismatch between cancellous bone and ce-

ment. Fixation of the implants with polymethymethacrylate

(PMMA) allows patients to bear weight instantly as opposed

to a wait of about 12 weeks for implants that are attached by

only mechanical interlocking with bone ingrowth. Typically

implants are roughened and coated with PMMA before ap-plying bone cement (also PMMA). The bone cement inter-

face is highly dynamic with degradation of the polymer in

the cement and bone ingrowth. The nature of this interface

is specific to the materials used in implants. The following

sections evaluate the different materials systems used in or-

thopedic applications, particularly for total hip replacement.

2. Material systems in total hip replacement

2.1. Metals

The effort to find substitutions for repair of seriously dam-

aged human bones dates back to centuries. Metals have been

the primary materials in the past for this purpose due to

their superior mechanical properties[10], albeit dangerous

ions that are released in vivo from these alloys. Originally

femoral components of the THR were made of stainless

steel that was replaced by a cobalt-chromium-molybdenum

alloy (VitalliumTM)[11,12].Mechanical properties of com-

mon metallic THR materials are shown in Table 2. Metal-

lurgical heat treatments and resulting microstructures guide

the resulting mechanical properties in metallic implant ma-

terials [14]. Most commonly, the long femoral element ismade of stainless steel, CoCr alloys, or Ti alloys, and the

cup component is made up of alumina or zirconia ceramic,

polytertrafluoro ethylene (PTFE) or CoCr alloy.


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The commercial metallic THR implants are five to six

times stiffer than bone and result in significant problems

associated with stress shielding. Ti alloys in the femoral

elements of the THR have shown improvement in wear

properties. The regenerative and remodeling processes in

bone are directly triggered by loading, i.e. bone subjected

to loading or stress regenerates and bone not subjectedto loading results in atrophy. Thus, the effect of a much

stiffer bone implant is to reduce the loading on bone result-

ing in the phenomenon called as stress shielding. The key

problems associated with the use of these metallic femoral

stems are thus release of dangerous particles from wear

debris, detrimental effect on the bone remodeling process

due to stress shielding and also loosening of the implant

tissue interface. It has been shown that the degree of stress

shielding is directly related to the difference in stiffness of

bone and implant material [15,16]. Titanium alloys are fa-

vorable materials for orthopedic implants due to their good

mechanical properties. However, titanium does not bond

directly to bone resulting in loosening of the implant. Un-desirable movements at the implant-tissue interface results

in failure cracks of the implant.

One approach to improving implant lifetime is to coat

the metal surface with a bioactive material that can promote

the formation and adhesion of hydroxyapatite, the inorganic

component of natural bone. The application of bioactive

coatings to titanium-based alloys enhance the adhesion of

Ti-based implants to the existing bone, resulting in signif-

icantly better implant lifetimes than can be achieved with

materials in use today. Typically, several silicate glasses are

used as bioactive coatings. An ideal bioactive coating would

bond tightly both to the bone and the metal. Some ceramiccoatings are known to be bioactive and have been tested on

Ti implants. However, two problems arise when attempting

to coat metals with ceramics. For one, the thermal expansion

coefficients of the ceramic and metal are usually different,

and as a result, large thermal stresses are generated during

processing. These stresses lead to cracks at the interface and

compromise coating adhesion. In addition, chemical reac-

tions between the ceramic and metal can weaken the metal

in the vicinity of the interface, reducing the strength of the

coated system. This problem is particularly important when

coating Ti alloys, due to their high reactivity with most ox-

ide materials. Since the modulus of the Ti alloys is lower

than that of the CoCrMo alloys, they have been more

suitable for THR components. The elastic moduli of the Ti

alloys have been engineered to be more suitable by heat

treatments resulting in microstructures that have a reduced

elastic modulus. The fundamental wear mechanisms of the

Ti alloys is still not well understood. Bioglass coatings on

Ti implants further improves the biocompatibility of these

implants. The glasses are based on mixtures of the oxides

of silicon, sodium, potassium, calcium, and magnesium. By

adjusting the stoichiometry of the bioglass coating, the ther-

mal expansion coefficient of the glass is made to match that

of the Ti alloy, avoiding the generation of thermal stresses.

Also, the glasses become soft at the processing temperature,

which is well below the melting point of the Ti alloy. Thus,

they flow to uniformly coat the Ti surface. These coatings

develop a layer of HAP on their outer surface upon exposure

to simulated body fluid[17]. Metallic femoral head articu-

lating inside a polymeric (PTFE or UHMWPE) acetabular

cup has been one of the most favorable THR element struc-ture[18,19]. Clinical results show that excessive wear and

wear debris is the primary cause of failure of UHMWPE or

metal implants. Thus, the use of materials with lower mod-

ulus and strength such as polymers appear to be more useful

for use as bone biomaterials.

2.2. Polymers

For orthopedic applications such as fixation devices and

also use in THR, polymers of very high strength and stiff-

ness are required. The use of polymeric materials in bone

biomaterials research is extensive due to many useful prop-

erties of polymers. For orthopedic applications, common

polymers used are: acrylic, nylon, silicone, polyurethane,

ultra high molecular weight polyethylene (UHMWPE), and

polypropylene (PP) [20]. Mechanical properties of these

polymers are shown inTable 3.Highly stable polymeric sys-

tems such as PTFE, UHMWPE or poly(etheretherketone)

(PEEK) have been investigated due to their excellent me-

chanical properties. In the early 1960s, the stainless steel

femoral THR component was mated with a PTFE acetabu-

lar cup. Poor wearability and distortion in these components

prevented further use of PTFE as an important biomaterial

for acetabular cups. Acetabular cups made of ultra high

molecular weight polyethylene have shown to exhibit su-perior properties. In the use of acetabular cups made of

polyethylene, debris created by wear of polyethylene (PE)

articulating surfaces is attacked by the bodys immune sys-

tem. This leads to bone loss, also known as osteolysis. Since

the debris accumulates in the area close to the implant, the

bone loss leads to loosening of the implant stem. This results

in a repeat surgery. Thus, the main problems associated with

the use of PE as acetabular cups is not the wear of the cups

Table 3

Mechanical properties of polymers used in THR[3,23]

Material UCS (MPa) UTS (MPa) Modulus (GPa)

Polymers

HDPE 25 40 1.8

UHMWPE 28 21 1

PA 67 2.1

PS 75 2.65

PE 35 0.88

PU 35 0.02

SR 7.6 0.008

PEEK 139 8.3

PTFE 11.7 28 0.4

PET 61 2.85

PMMA 144 21 4.5


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themselves but wear of the interfacial adhesion between

tissue and implant. Polymers prepared from lactic acid and

glycolic acid have been used in the biomedical field since

the 1960s as sutures due to their highly unstable structure

leading to bio-degradability[21].Other biodegradable poly-

mers such as poly(dioxanone) (PDO), poly(trimethylene

carbonate) (PTC) copolymers, have been used in the medi-cal field[22]. As biodegradable polymers, besides PLA and

PGA, polycaprolactone (PCL), polyanhydrides (PA), poly-

orthoesters are also subject of current research. The use of

degradable polymers in THR is rather limited due to their

inadequate mechanical properties. Due to their degradation

properties these polymers have extensive application in

tissue engineering. The initial high strength of some degrad-

able polymers such as PLLA has spurred interest in use of

these polymers as composite systems with ceramic fillers.

The use of these materials for composites where stiffening

agents are used to enhance mechanical properties is the sub-

ject of several current studies. The mechanical properties

of polymers used in THR components is shown in Table 3.

2.3. Ceramics

As compared to metals, ceramics often cause reduced os-

teolysis and are regarded as favorable materials for joints or

joint surface materials. Several ceramics due to their ease of

processing and forming and superior mechanical properties

were investigated as bone substitute materials. Conventional

ceramics such as alumina were evaluated due to their excel-

lent properties of high strength, good biocompatibility and

stability in physiological environments [24]. Due to lack

of chemical bonding between sintered alumina and tissue,its applications as a potential bone substitute are limited.

Alumina, because of the ability to be polished to a high

surface finish and its excellent wear resistance, is often used

for wear surfaces in joint replacement prostheses. Femoral

heads for hip replacements and wear plates in knee replace-

ments have been fabricated using alumina. In hip replace-

ments, the alumina femoral head is used in conjunction with

a metallic femoral stem and an acetabular cup made from

UHMWPE for the opposing articulating surface. The wear

rates for alumina on UHMWPE have been reported to be

as much as 20 times less than that for metal on UHMWPE,

making this combination far superior and producing less

wear debris. Recently (February 2003) the United States

Food and Drug Administration (FDA) has approved alumina

ceramic-on-ceramic articulated hips for marketing in the

United States. Other ceramic materials have also been in-

vestigated for potential applications in orthopedics. The first

paper to report the use of zirconia in biomedical applications

was reported in 1969[25]and the first paper illustrating the

use of zirconia to manufacture ball heads for total hip re-

placement was reported in 1988[26].Considerable research

has focused on zirconia and yttria ceramics that are char-

acterized by fine grained microstructures. These ceramics

are known as tetragonal zirconia polycrystals (TZP). Zirco-

nia is the material of choice currently for ball heads. Over

300,000 TZP ball heads have been implanted[27]A better

match between the bulk material properties of the implant

and the bone it replaces can decrease some of the problems

associated with using coated metallic implants such as stress

shielding. This is often achieved with coatings on implants.

Since calcium phosphates are present as apatites in naturalbones, researchers have investigated calcium phosphates

extensively. Typically, the calcium phosphorus atomic ra-

tios range from 1.5 to 1.67. Tricalcium phosphate (TCP)

(Ca3(PO4)2) and HAP (Ca10(PO4)6(OH)2 are the two min-

erals at the extremes of this range of calciumphosphorus

ratios. Both TCP and HAP are biocompatible materials.

Calcium phosphate ceramics, especially HAP and -TCP

are widely used for hard tissue replacement due to their

biocompatibility and osteoconductive properties [28,29].

As bone defect fillers, these ceramics are utilized in powder

and block forms. Porous forms with 100300m pores are

preferred since they allow bone to grow into the implant,

promoting mechanical fixation with the natural bone. Theparticulate form lacks cohesive strength and lends to dis-

lodge and migrate under externally applied stresses during

healing period. In general, the applications of calcium phos-

phates in the body have been limited by the low strength

and low fracture toughness of the synthetic phosphates.

Synthetic HAP elicits a direct chemical response at the in-

terface and forms a very tight bond to tissue [30].Attempts

have been made to form high strength consolidated HAP

bodies [31,32]. However, its poor mechanical properties

such as low strength and limited fatigue resistance restrict

its applications. Bending strength as high as 90 MPa has

been achieved by colloidal processing of HAP [31]. Me-chanical properties of ceramic biomaterials are shown in

Table 4.

Alumina and titanium dioxide have been used as nanoce-

ramics separately or in nanocomposites with polymers

such as polylactic acid or polymethlyl methacrylate. The

nanoceramic formulations promote selectively enhanced

functions of osteoblasts (bone-forming cells). These func-

tions include cell adhesion, proliferation, and deposition

of calcium-containing minerals, an indication of new bone

formation in a laboratory setting (Table 5).

Ceramics that elicit a favorable bonding to bone tissue

are often called as bioactive ceramics. Some compositions

Table 4

Mechanical properties of ceramics used in THR[3,23]

Ceramic UCS (MPa) UTS (MPa) Modulus (GPa)

Zirconia 2000 820 220

Alumina 4000 300 380

Bioglass 1000 75

C(Graphite) 138 25

C(Vitreous) 172 31

HAP 600 50 117

C(LTI pyrolitic) 900 28

AW glassceramic 1080 118


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Table 5

Mechanical Properties of composites in comparison to bone

Materials UTS (MPa) Elastic modulus (GPa) Elongation at

break (%)

References

Functionally graded: HAP/Yttria, 040% yttria

content w/w

Bending strength:

160200

100160 [92]

PHB/HAP, 30% w/w 67 2.52 2.65 [41]

P(HB-co-824% HV)/HAP, 30%w/w 6223 2.750.47 2.255.42 [41]P(-hydroxy acids)/HAP 0.11 [93]

Chemically coupled HAP/PE, 740 vol% filler 18.3420.67 0.884.29 >500 to 2.6 [81]

Nano HAP, 3070 to 60 w/w 35.878.4 (Bending) 2.36.2 Elongation 12.8 [94]

BCP/PLLA, 025% v/v 3060 518 [80]

PAAC/HAP 15 [95]

PAAC/in situ nano HAP, 4070 w/w 2060 11.8 26 [96,97]

PLLA/hydroxyapatite powder, 1030 w/w 0.2962.48 (depending on

hot pressing parameters)

36.193.2 [76]

PLLA/HAP fiber, 070% w/w 3.511 0.0060.0375 [83]

Starch-EVOH (SEVA) blend/HAP, 1030% w/w 42.330.2 1.87.0 14.70.6% strain [44]

Starch-EVOH (SEVA) /10% HAP w/w 53.6 3.31 2.44% strain [45]

Starch-EVOH (SEVA) /10% HAP w/w with

1% coupling agents (zirconate, titanate and

silane)

43.349.9 3.754.3 1.331.99 [45]

of glasses containing SiO2, Na2O, CaO, and P2O5 bond to

soft tissues as well as bone [3335]. The practical use of

bioactive glass for THR components has been limited to

their use as bioglass coatings on the femoral and acetabular

THR components.

2.4. Composites

Generally the use of composites for bone biomaterials

have included three broad areas:

functionally graded composites, polymer-ceramic composites (with and without fiber re-

inforcements),

biomimetic composites or composites with biological

macromolecules.

2.4.1. Functionally graded composites

Composites are fabricated of HAP and zirconia to en-

hance the mechanical properties of HAP while retaining

its bone bonding property. Functionally, graded com-

posites are an important area in composites research.

The main feature of a functionally graded composite

is the almost continuously graded composition of the

composite that results in two different properties at the

two ends of the composite. Powder metallurgy meth-

ods have been used to make HAP/titanium function-

ally graded composites offering the biocompatible HAP

on the tissue side and titanium for mechanical property

[36]. Functionally graded of tricalcium phosphate and

fluoroapatite composites combine the bioactive proper-

ties of fluoroapatite with the bioresorbable properties of

TCP [37]. The research in this field is quite promising

but currently, the mechanical properties of these com-

posites are clearly in excess of the properties of bone

(Table 5).

2.4.2. Polymer-ceramic composites

Ceramic polymer composites have superior properties

than either ceramics or polymers for use as THR materials

[38]. Typically the polymer components have included poly-

mers that have shown good biocompatibility and routinely

used in surgical applications. Many polymer composite

materials have used HAP as the ceramic filler component

[39,40]. Since the polymer materials such as PLA have very

low modulus (27 GPa) as compared to that of bone (330

GPa), the HAP needs to be loaded at a very high weight %

ratio in the composite. Composites mechanics suggests thata high aspect ratio particle such as a whisker or a fiber sig-

nificantly improves the modulus with a lower loading wt.%.

Thus, the attempts have also been made to prepare needle

like or whisker like or fibrous HAP. Some of these com-

posites such as composites of poly(-hydroxyalkanoates)

(PHA) with HAP have shown ultimate strength, elastic mod-

ulus and elongation at break similar to bone and are being

investigated as potential materials for THR [41]. Calcium

carbonate (vaterite) used as a reinforcing agent in poly(lactic

acid) composites has shown enhanced mechanical proper-

ties such as bending strength of 45 MPa and a modulus as

high as 7 GPa with a 050% vat rite loading [42].

Starch-based biodegradable polymers have recently

shown potential for applications for bone replacement [43].

Composites based on starch and ethylene vinyl alcohol

(EVOH) are known to show degradation when immersed

in a simulated body fluid. Recently, blends of EVOH

(SEVA-C) with starch filled with 1030% by weight of

HAP have been fabricated to yield composites with modulus

upto about 7 GPa with a 30% HAP loading[44]. Recently,

zirconate, titanate and silanes have been used as coupling

agents between EVOH and HAP [45]. Optimization of

properties with coupling agents is currently an important

area of research.


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The fibers used for toughening polymeric materials for

use in THR also need to be biocompatible. Carbon fibers

due to their good biocompatibility property have been

used to reinforce ultra high molecular weight polyethy-

lene in THR components. Carbon fiber-PMMA [46], car-

bon fiber-polypropylene and polysulphone [47,48], carbon

fiber polyethylene, polybutylene terephthalate, and PEEK[4951]have all been investigated for potential applications

for bone plates. The use of these composite materials in THR

components has been limited, by the mechanical property

mismatch between these composites and the femur bone.

Several composite systems such as poly(etheretherketone)

PEEK and glass fibers[5254]and carbon fiber carbon rein-

forced composites[55,56]have also been investigated as po-

tential bone replacement materials. Multilayered laminated

composites of carbon fibers and epoxy[57]and braided de-

signs of carbon fiber and glass fiber epoxy composite[54,58]

have been made. Hot pressing mixtures of polymers and

HAP fibers have also been attempted. HAP fibers are fabri-

cated from -Ca(PO3)2fibers [5961]. Needle like or fibrousHAP of lengths 1030m and 0.11m diameter have been

synthesized using hydrothermal synthesis using citric acid

[62],40150m length and 210 m diameter fibers using

a solid phase reaction[63].Bending strength is seen to be al-

most independent of fiber content improvement in modulus

from 3.5 to 11 GPa is observed with over 60 wt.% loading

of the polymer (polylactic acid) with HAP fibers [64].Al-

though polymeric fibers are used in biomedical applications

such as absorbable fracture fixation systems[65],and scaf-

folds for tissue engineering[66],the application of polymer

fibers as a reinforcement phase in THR components is lim-

ited due to the inadequate strength and stiffness of the fibers.Particulate reinforcement using ceramic phases offers a

methodology for improvement in mechanical properties of

biomaterials for THR. HAP containing composites retain

their useful bioactive properties as well as provide some im-

provement in mechanical properties. The composites include

fiber reinforcement of HAP [67,68], HAP/polyethylene

[6971], HAP/polyethyl ester [72], HAP/polyphosphasone

[73], HAP/polylactide[7476] and HAP/alumina compos-

ites[77]. A swelling type biocompatible structural material

for bone implants has been investigated recently[78]where

the swelling strains are controlled by using a copolymer

poly(methyl methacrylate-acrylic acid). In order to improve

mechanical properties of such expansion-fit materials rein-

forcement of such copolymers with carbon and Kevlar fibers

were attempted[56].Fiber matrix debonding and fibrillation

was observed for Kevlar fibers resulting in low modulus and

yield strength of Kevlar reinforced composites and a loss

in modulus occurred with increasing swelling for the car-

bon fiber reinforced composites. Carbon fiber-polysulphone

composite has been used for the design of a press-fit device

for a femoral component of a THR [79]. A self-reinforced

polylactide/biphasic calcium phosphate composite has re-

cently been fabricated primarily for use for fracture fixation

plates[80]. The phosphate content is varied upto 25% by

volume resulting in 515% failure strains and 6030MPa

ultimate tensile stresses.

Chemically modified reinforcement phase-matrix inter-

face results in improvement in mechanical properties of

composites. Examples of such interface modified composite

biomaterials include chemically coupled HAP-polyethylene

composites [81], chemically formed HAP-Ca poly(vinylphosphonate) composites [82] and polylactic acid HAP

fiber composites[83].

HAP along with bioceramics and bioglasses have been

studied extensively as bone repairing material and is used as

a coating for implanted prostheses to enhance direct adhe-

sion to bone tissue[84,85]. Bone cements based on PMMA

are used to secure orthopedic implants to bone. Due to lim-

ited mechanical properties of PMMA, incorporation of HAP

in PMMA has been investigated. In addition, enhanced os-

teogenic properties of the implants is observed with incor-

poration of HAP in PMMA.[8689]. It has been shown that

not only are the mechanical properties of PMMA improved

but the osteoblast response of PMMA is also enhanced withaddition of HAP[86].Biosorbable devices made of forged

composites of HAP particles and poly l-lactide have shown

improved fatigue properties over metallic implants in addi-

tion to superior biocompatibility. A new injectable compos-

ite for bone repair: poly(-caprolactone) microparticles with

biphasic calcium phosphate granules shown some promise

[90]. In general the polymer/HAP interfaces are known to

have an important role on the resulting mechanical prop-

erties[91]. The mechanical properties of various compos-

ites investigated in literature for THR materials is shown in

Table 4.

2.4.3. Biomimetic composites or composites with

biological macromolecules

Bone is a nanocomposite of HAP and type I collagen.

The HAP-polymer composites are typically simple mixtures

fabricated to give a combination of properties of biocompat-

ibility and mechanical strength. Methods to mimic biologi-

cal processes with synthetic and biological macromolecules

has been the focus of recent research. Composites fabricated

using co-precipitation of HAP nanocrystals with soluble

collagen have been attempted [98100]. Although nanos-

tructure of bone is partially achieved in the HAP/collagen

composites, the high cost of type I collagen is an impor-

tant deterrent in future research in these composites unless

less expensive sources of type I collagen are available.

HAPgelatin composites are being currently studied for

potential bone replacement materials[101].In addition the

biomimetic HAP-embedded collagen nanostructure has in-

adequate mechanical properties and the proper pore sizes

compared to biological bone are not achieved. Attempts

are being made in literature to simulate the collagenHAP

interfacial behavior in real bone with crosslinking agents

such as glutaldehyde[100]with the purpose of potentially

improving the mechanical properties of these composites.

Other biomimetic routes include in situ mineralization of


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