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Biocompatible Alumina Ceramic for Total Hip Replacements

Peng Zeng

Department of Engineering Materials, University of Sheffield, Mappin Street, Sheffield S1 3JD, UK

Abstract:

Resistance to wear and biocompatibility make ceramics ideal materials for

medical applications, such as implants. For over 30 years, pure alumina hip prostheses

have dominated the history of ceramic hip prostheses and interest in alumina hip

prostheses continues to grow, due to the relatively short life of polymer/metal prostheses,

mainly resulting from osteolysis and aseptic loosening caused by polymer wear debris.

Introduced by Boutin in the 1970s, substantial improvements have been made in the

microstructure of medical-grade alumina by processing to give complete densificationand fine, uniform grain sizes. Additionally, a region of high wear, known as stripe wear,

is well known on the retrieved alumina hip prostheses, which can be replicated in vitro by

the introduction of microseparation. This paper reviews the aluminas used in total hip

replacement, the development of medical-grade alumina, methods of in-vivo and in-vitro

investigation of the alumina prostheses, and focuses on current knowledge about the

damage observed on alumina prostheses. The future of ceramic hip prostheses is also

addressed.

Keywords: Microstructure, alumina, wear, hip prostheses

1. Total Hip Replacements (THRs)

About 2% of people suffer hip problems and need to replace their natural hip

joints. In the UK at least 50, 000 hip replacement surgeries are done every year, and are

highly successful in reducing the pain and disability of worn or damaged hip joints1.

Total Hip Replacement (THR) is one of the most successful applications of biomaterials.

The basic design of a THR involves a femoral head, an acetabular cup and a

metallic stem (Fig.1). The review by Dowson3

summarised six alternative existing or

potential combinations, according to the different materials used for the femoral head and

acetabular cup, as: a) metal-on-polymer, such as Charnley prosthesis (Fig.1a); b) metal-

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on-metal, such as McKee-Farrar prosthesis (Fig.1b); c) ceramic-on-ceramic (Fig.1c); d)

ceramic-on-polymer (Fig.1d); e) ceramic-on-metal (Fig.1e); and f) metal-on-ceramic. The

former four combinations are already in the market. However, the latter two are potential

combinations and are still in the laboratory stage, such as ceramic-on-metal THR6.

The first metal-on-polymer THR, also the first true artificial hip joint, was

implanted by Charnley in 19597

which contained a stainless steel femoral head and a

PTFE (Polytetrafluoroethylene) acetabular cup. Because of the toxicity caused by PTFE

wear debris, Charnley changed PTFE to UHMWPE (Ultra-high molecular weight

polyethylene) shortly after8. Almost at the same time, McKee

9developed the metal-on-

metal THRs using a cobalt-chromium alloy (Fig.1b). The first ceramic-on-ceramic THR

was introduced by Boutin in 1970 and developed by Mittlemeier in 1974. Later in 1975,

alumina-on-polymer THRs were implanted in Switzerland 10. Since the first introduction,

metal-on-UHMWPE THR, namely the Charnley LFA (low-friction arthroplasty), has

dominated the THR markets for four decades and is widely referred to as the gold

standard3. Although achieving such a successful application, the survival rate of

Charnley LFA is not satisfactory in the long-term (> 15 years), dropping from 94.2%

after 10 years follow-up to 88.7% after 15 years follow-up11

. It is estimated that 15% of

hip replacement surgeries deal with second replacements1, which are also known as

revision surgeries, due to the failure of the first THR. Compared with the first

replacement, the revision operation always takes longer, and, is harder, resulting in an

even lower success rate. Therefore, some surgeons even reserve THRs for patients over

60 to avoid the failure of the THRs. However, with the increasing life expectancy of the

population, there is a requirement for long-term (> 15 years) performance THRs. In

addition, there is also a significant, and increasing, number of patients younger than 50

years old who have hip damage from severe sports injuries. It is important to reduce the

wear of THRs and improve the lifespan of the artificial hip prostheses.

The main reasons leading to failure of THRs are by loosening of the joint due to

wear, and inflammation caused by a reaction to particles that have worn away the

artificial joint surfaces and have been absorbed by surrounding tissue. Most importantly,

the osteolysis (resorption of periprosthetic bone) which is believed to be a result of

polymer wear debris has been widely observed and has become a major problem limiting

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the lifespan of the polymer THRs10, 12

. Therefore, there is renewed interest in hard-on-

hard THRs, such as ceramic-on-ceramic and metal-on-metal THRs.

2. Ceramic-on-ceramic Total Hip Replacements

Ceramics are regarded as favourable materials for THRs due to their wear

resistance and reduction of wear particles. Alumina is the most widely used ceramic for

hip prostheses. Compared with other combinations, such as metal-on-polymer and

alumina-on-polymer, alumina-on-alumina THRs show the lowest wear rate under

laboratory conditions13, 14

, as in Fig.2. Additionally, alumina is a bioinert material which

reduces the chance of osteolysis. However, early clinical performances of alumina-on-

alumina THRs were controversial. High fracture rates of alumina femoral heads from

6.9% to 13.4% were cited 15-17 during 1970 and 1976. Over a later period, fracture rate as

low as 0.4% was also observed18-20

. It is worth noting that the aluminas with high

fracture rate were based on aluminas that were developed for industrial applications15-17

,

while low fracture rates were observed from medical-grade alumina18-20

.

Apart from the material itself, poor hip design and operation skills were also

responsible for the early failure of alumina-on-alumina hip prostheses. Unlike polymers,

there is a high risk of fracture during an operation due to the brittleness of alumina. The

fracture of the alumina-on-alumina hip prostheses is a major problem for surgeons and

restricted the application of alumina-on-alumina THRs in the early years (see section 4.1).

Interestingly, research on alumina-on-alumina hip prostheses in the first thirty years was

mainly a European development, with some clinical research in Australia and Japan,

because the USA Food and Drug Administration (FDA) banned the use of alumina hip

prostheses before 2003 due to the high fracture rates of alumina-on-alumina THRs

encountered in 1970s.

Early alumina-on-alumina THRs used aluminas developed for industrial

applications, the microstructure of which is poor (Fig.3a): insufficient purity, low density

and coarse grain size (compared with latest alumina for hip prostheses, Fig.3b and c).

These were the main reasons causing the failure of the alumina-on-alumina THRs. Early

contributors realised the importance of developing alumina materials for medical use and

the first standard of alumina for hip prostheses, ISO 647422

, was set up in 1984 to guide

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the alumina hip prostheses market. ISO 6474 qualifies the medical-grade alumina and

decreases the risk of fracture of alumina. With the improvement of the medical-grade

alumina material and better hip design, alumina-on-alumina THRs achieved great success

in Europe. In addition, the work of Sedel et al.23

showed increasing survival rates of

alumina-on-alumina THRs for patients younger than 50 and it is now well accepted by

surgeons that alumina-on-alumina THRs are the first choice for the patients younger than

50. Based on the successes in Europe and good results of trials on modern alumina-on-

alumina hip prostheses in US24

, the FDA withdrew the ban on alumina-on-alumina hip

prostheses in 2003. The research on alumina-on-alumina hip prostheses is now a global

development and the likelihood of widespread adoption of ceramic-on-ceramic hip joints

is imminent.

3. Development of Medical-grade Alumina

The pioneering alumina materials for hip joints were based on ceramics that were

available, developed for completely different industrial applications. Some of the

materials had insufficient purity, low density, and a coarse grained microstructure

(Fig.3a), which led to a poor mechanical strength of the ceramic25

. Since mechanical

strength is correlated to reliability and fracture rate, these old aluminas were quickly

demonstrated to be inadequate for biomaterial applications. The ISO standard 647422

,

was set up to qualify the ceramics used in hip prostheses.

Significant improvement of medical-grade alumina was achieved in Germany in

1974, as part of the range of modern engineering ceramics26, 27

. There are three stages of

the development of medical-grade alumina, referred to as three generations, as in Table 1.

The developments have been concentrated on purity, grain size and density since a close

correlation exists between the mechanical strength and these aspects.

Glassy phases are commonly found on the grain boundaries of ceramic materials

as a result of impurities imported through the raw materials. The glassy phase tends to

degrade in the body and cause the material to age, i.e. to loose its mechanical strength28-

30. Therefore, efforts were made to increase the purity of the alumina. For the latest

generation of alumina, namely 3rd

generation alumina, the purity of alumina is as high as

99.9%26

and glassy phases are not observed even in TEM (Fig.3c). However, it is worth

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noting that some second phase is useful, such as magnesium oxide (MgO). It is believed

that the grain size of alumina can be minimised by doping with magnesium oxide (MgO)

26.

The grain size of the alumina plays an important role in the development of the

medical-grade alumina. Apart from mechanical strength, it is generally recognised thatthe wear rates of alumina decrease with decreasing grain size

28-33. Davidge et al.

28

reported results of a study of the wet erosion of a set of high purity polycrystalline

aluminas of similar hardness and fracture toughness. A relationship between wear rate

and grain size was seen (Fig. 4). They presented a simple model on wear rates of high

purity alumina of tailored grain size under the postulate that the critical wear process

involves the nucleation and propagation of grain boundary microcracks. The basic

features of the model are shown in Fig. 5a and b 28. The time t for the crack to reach a

certain length is given by

t = A + B/G (1)

where G is grain size, A and B are constants. Thus, the theoretical wear rate W is

W = C [G/ (AG +B)] (2)

where C is a constant can be incorporated into A and B.

Thus:

W = G / (M + NG) (4)

Where M and N are the constants that are required. Therefore, reduction in grain size is

an essential requirement in reducing wear.

In addition, the appearance of the wear transition, which is believed to play an

important role for reducing wear rates30

, would delay with decreasing the grain size31

,

Fig.6. Therefore, minimizing the grain size of alumina is crucial and this has been done to

improve the medical-grade alumina (Table 1).

The porosity also plays an important role on wear of alumina32, 33

. Pores,

especially intergranular pores, can act as sources of slip or twinning and increase the

amount of wear32, 33

. Therefore, the density of alumina is also important to the quality of

medical-grade alumina. Modern alumina, i.e. 3rd

generation alumina, is hot isostatic

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pressed (HIPed), with increased density of 3.96 kgm-3

(Table 1). As the temperatures

used in HIP processing are lower than the temperature used for sintering, the speed of

grain growth is limited which also benefits the grain size.

The colour of medical grade alumina relates to the quality and chemical purity of

the material26

.The colour of alumina ceramics with low impurity levels, but which have

been doped with magnesium oxide (MgO) as specified by ISO 6474, is not white, but

ivory in the materials un-sterilized form. As soon as the ceramic ball heads are sterilized

using gamma rays, the ivory colour of the material turns to brown, which is explained by

the valences of the alumina ion (3 valences), the oxygen ion (2 valences) and the

magnesium ion (2 valences)24

.

4.In vivo wear performance of alumina-on-alumina THRs

In vivo study of hip prostheses, involving clinical evaluation of the retrieved hip

prostheses taken out of bodies, is the most direct method to investigate the wear of the

hip prostheses. Because of limited supply of retrieved hip prostheses and the protection of

patients, it is extremely difficult for researchers to access in-vivo materials. Most of the

alumina prostheses reviewed have been from 1st

or 2nd

generation of medical-grade

alumina, with limited information from retrieved 3rd

generation of alumina prostheses.

Also, it is worth noting that most observations have been made by surgeons who have

direct access to the first data.

4.1 Fracture rate of alumina components

The results of fracture analyses of the alumina hip prostheses are summarised in

Table 2 and Fig.7.

A fracture rate as high as 13.4% has been observed in alumina-on-alumina hip

prostheses in the early years, namely 1st

generation alumina16

. After 1980, there is a

dramatic decrease of fracture rates of alumina hip prostheses, several authors even

observed a fracture rate as low as 0%34-37

. No official data concerning 3rd

generation

alumina have been published; however, a lower fracture rate is suggested. It can be

concluded that fracture is no longer the problem for the surgeons that it was in 1970s and

modern alumina-on-alumina hip prostheses are unlikely to fail during hip operation.

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4.2 Wear rate/ survival rate of alumina-on-alumina THRs

The wear rate of alumina-on-alumina hip prostheses is the most direct way to

evaluate the wear performance of the prostheses. Alumina-on-alumina hip prostheses

exhibit the lower wear rates from in vivo investigation, than either metal-on-polymer or

metal-on-metal hip prostheses. The linear rates of wear ranged from less than 110-6

myr-1

to over 0.510-3

myr-1

with a mean value of 5.610-5

myr-1

25

. It can be claimed

that ceramic-on-ceramic prostheses could function in vivo without noticeable abrasion for

nearly 10 years47

.

Although the wear rate of alumina-on-alumina hip prostheses is relatively lower

than other combinations, the published survival rates of the prostheses are not satisfactory.

These are summarised in Table 3 and Fig.8. Table 3 shows that the survival rates of early

alumina hip prostheses, before ISO 6474, are even lower than Charnley LFA3. Limited

data on 2nd

generation alumina hip prostheses showed improved survival rates of 100%

on 5 years follow-up and 95.1% and 94.3% on 7 years follow-up for different hip design

44. However, there is a lack of data on 3

rdgeneration alumina hip prostheses as a result of

time.

It is worth noting the work of Sedel et al.23

which showed increasing survival

rates of alumina hip prostheses for patients younger than 50 (survival rate of 87% for

patients younger than 50 on 15 years follow-up despite overall survival rate of 70%).

Although there is uncertainty on the long-term performance of alumina-on-alumina hip

prostheses due to the limited data, alumina hip prostheses show better performance in the

patient group younger than 50. Suggestions23

are made by surgeons that for young and

more active patients, alumina-on-alumina hip prostheses are good choice, however,

standard Charnley LFA may still be most appropriate for the old patients8.

4.3 Reasons for revision

Table 4 summarises the published data on the reasons for revisions. It can be

concluded that fracture of alumina is no longer the main reason causing failure of the

alumina hip prostheses with the development of the medical-grade alumina, instead,

aseptic loosening becomes the main reason causing the revision surgeries. However, the

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mechanism leading to this loosening is not clear. There is no explanation on whether or

not wear is a factor in the loosening process. Therefore, studies on wear mechanisms of

alumina hip prostheses are required.

5. Wear debris

Polymer wear debris is believed to be the main reason causing the osteolysis as a

result of biological reaction between tissues and polymer wear debris10, 12

. However, the

role of the ceramic wear debris is not very clear. Only limited data containing ceramic

wear debris are published.

Numerous particles with a mean size of 5 m were observed in the

pseudosynovial tissue obtained from revisions due to mechanical failure52

. In addition,

wear debris in the size range 5-90 nm (Fig.9) was revealed in the tissue from alumina-on-

alumina hip prostheses together with wear debris of .05-3.2 m53

. This was the first

description of nanometre sized alumina wear particles in retrieval tissues. Two

mechanisms were proposed to generate the two types of alumina wear debris53

: a) relief

polishing wear producing nanometre sized alumina wear debris under normal articulating

conditions; b) intergranular and intragranular fracture due to edge loading generating

larger wear particles under the condition of micro-separation of the head and cup on rim

contact.

It is worth noting that Yoon et al.54

reported the only case of osteolysis caused by

ceramic debris. However, some surgeons argued it as an occasional case mainly due to

the poor design of the THR8. Therefore, clinical results concerning osteolysis of alumina

hip prostheses are required to clarify the observation.

6.In vitro wear performance of alumina-on-alumina THRs

In vivo is the most direct and convincing method for long-term hip prostheses

research, however, studies are based on failure analysis of retrieved prostheses, largely

ignoring successful implants. In addition, supply of in vivo material is limited and is not

helpful to validate the new material. Therefore, in vitro studies are carried out as a

compliment of the in vivo studies.

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In vitro study refers to the technique of performing a given experiment outside a

living organism and is an effective way to investigate the new materials used in THRs.

Pin-on-disc machines, reciprocating pin-on-plate machines and joint simulators are the

most accepted in vitro studies for hip prostheses3. The first two are widely accepted in

materials science to study the tribology of materials, such as comparison of various

combinations and effect of different lubrications. A series of pin-on-disc tests indicated

that the coefficient of friction of alumina-on-alumina hip prostheses is different under

various lubrications55

. The coefficient of friction was much smaller when 1wt% water

solution of carboxymethyl cellulose sodium salt was used as lubricant instead of distilled

water. Therefore, alumina-on-alumina hip joints might benefit from full fluid film

lubrication with good machining, good fit and a proper lubricant. Additionally, in vitro

studies are used to analyze potential materials used for hip prostheses, such as

zirconia/alumina combination. Although zirconia/zirconia pairs showed poor wear

performance, zirconia/alumina and alumina/zirconia pairs exhibited similar wear

performance to alumina/alumina pairs. Potential combinations of zirconia/alumina or

alumna/zirconia for hip prostheses were proposed56

.

Although pin-on-disc studies have been proven as an effective way to

investigate the tribology of potential materials for hip prostheses, it is not enough for the

application due to the much more complicated environment and variable hip movements

in human bodies. From the 1970s onwards a more scientific approach to joint tribology

was adopted, called a joint simulator3. Most of the in vitro investigations are based on

use of joint simulators.

In a joint simulator alumina-on-alumina bearings show the lower wear rates than

metal-on-polymer or metal-on-metal bearings. However, the wear rates are significantly

lower than in vivo observations34, 50, 57-61

. The reason has been discovered by fluoroscopy

studies60

. Hip joint separation was determined to be present if the amount of separation

was > 0.75 mm. For gait, the maximum amount of separation was 2.8 mm, while the

minimum amount was 0.8 mm (average, 1.2 mm). For abduction/adduction leg lift, the

maximum amount of separation was 3.0 mm, while the minimal amount was 1.7 mm

(average, 2.4 mm). It is hypothesized that this micro-separation could occur with any hip

prosthesis and could be a factor in the initiation of fracture based wear, leading to the

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characteristic stripe in ceramic-on-ceramic hip prostheses61

. The small clearances of

the head and socket (typical radial clearances are 30 m) meant that it was possible that

the femoral head translated inferiorly and laterally if micro-separation occurred. These

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7. Microstructure of worn alumina-on-alumina THRs

Plastic deformation, cracking and chemical reaction are the main wear

mechanisms in the wear of alumina. It is well documented that microstructures, such as

grain size and pores, affect wear of ceramics30-33, 64

. In other words, understanding of the

wear mechanism of alumina-on-alumina hip prostheses requires research on material

microstructures. However, research to date on alumina-on-alumina hip prostheses has

concentrated on wear performance of the components rather than the wear mechanism of

the material. It is worth noting that most observations were made by surgeons, and there

is a lack of quantitative data on either microstructures or mechanical properties. Only

limited work concerning microstructures of worn alumina-on-alumina hip prostheses has

been published 21, 46, 49, 50, 62, 65-66. The wear performance of alumina-on-alumina hip

prostheses, such as wear rate and survival rate, may be sufficient for the surgeons or

patients to know the best of existing implant choice; however, it is not sufficient for the

fundamental understanding of wear mechanisms which is crucial to develop new ceramic

materials.

Walter and Plizt21

investigated the wear surfaces of 29 retrieved in vivo 1st

generation alumina hip prostheses. Wear was observed in two different zones (Fig. 13): a)

a so called pole-zone (Fig. 13a), namely top of the femoral heads or bottom of the

acetabular cups; b) an equatorial arranged zone near the edges of the acetabular cups or

corresponding zones of the femoral heads (Fig. 13b). The results of these two wear zones

could be explained by edge loading due to the micro-separation, as Fig.10c for the pole

zone wear and Fig.10b for the equatorial zone. It was proposed that low wear began when

the surfaces first contact, which appeared to progress to severe wear in the contact areas

of the component associated with high stresses and hence poorer lubrication50

. It is worth

noting that the equatorial zone was widely observed by later authors46, 50, 62, 65, 66

.

However, no other pole zone wear was observed in the literature. Furthermore, some

authors66

argue that wear always occurred on the rim of the socket and never on the apex

(equal to the pole zone wear). Although the pole zone wear was possible according to the

theory of micro-separation, more observations of pole zone wear are required to support

the theory.

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Walter and Plizt21

also pointed out that there was not much difference in the

microstructure according to various wear patterns. A loss of agglomeration of grains

leads to severe wear either in the pole-zone or equatorial zone. However, outside the wear

areas, regions of loss of one or more grains were observed in the originally high-polished

bearing surfaces (occasionally grains pull out in the mild wear zone). They proposed that

the severe wear might not only be explained by the first-point-of-contact mechanism

but also by a dry-scratching-process of non-polished bearing surfaces21

.

Interestingly, a region of high alkaline earth and silicon concentrations was

observed in the grain boundaries after etching in energy dispersive micro-analysis21

,

Fig.3a. Those regions were sensitive to corrosion especially in zones of micro-cracks,

which might be the point of origin of a later fracture of the component. Note that Walter

and Plizt 21 investigated the 1st generation alumina hip prostheses implanted between

1975 -1978. Compared with modern alumina, namely 3rd

generation alumina, more

impurities were present in the 1st

generation alumina, which was why there were alkaline

earth and silicon concentrations present in the grain boundaries after etching. However,

for the modern alumina, highly purity alumina is chosen, so there is no possibility for the

concentration of impurities in the grain boundaries is minimised.

Further investigations were made on the worn surface of retrieved alumina-

alumina hip prostheses whose clinical observations had given a range of different reasons

for failure21

. Although different wear patterns were observed on the retrieved prostheses

failed by differing reasons, plastically deformed, agglomerated alumina wear debris was

believed to play an active role in the enhancement of the avalanche effect, associated

with exaggerated wear in certain alumina-alumina Autophor (Mitterlmeier) hip joints.

The group in Leeds50, 61

made a significant progress on characterising the

microstructures of worn alumina-on-alumina hip prostheses. They analyzed retrieved in

vivo 1st

and 2nd

generation alumina between 1977 and 1994. Although different materials,

not many differences between worn surfaces of different generation aluminas were seen.

Three regions according to the wear pattern are defined as: a) low wear; little visible wear

and the surface remained polished; b) stripe wear; an elliptical wear stripe visible on head,

with roughing of part of the cup surface sometimes visible; c) severe wear; both the head

and the cup showed large areas of wear and loss of sphericity. The unworn areas outside

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of the main contact area (low wear area) had a mean roughness of 0.005 m Ra. Wear

stripes had a typical roughness of 0.1-0.2 m and severely worn areas were usually in the

range 0.2 m-0.4 m Ra. Therefore, they proposed that low wear began when the

surfaces first contact, which appeared to progress to stripe wear in the contact areas of the

component possibly, associated with high stresses and hence poorer lubrication51

. During

low wear, the mechanism appears to be polishing with the occasional surface grain being

excavated due to a grain boundary fracture mechanism, Fig.14a. Stripe wear areas have

sharply defined edges with a low wear region outside the stripe and more severe surface

damage within, Fig.14b and c. Additionally, spherical wear debris (0.1-0.5 m in

diameter) seen in the pores left by grain removal was a result of a third-body mechanism.

Severe wear seemed similar to the areas of stripe wear but on a larger scale. However,

they failed to observed transgranular fracture. Some boundary lubrication (provided by

adsorbed proteins) which gave the surfaces a degree of protection was also proposed: for

short contact durations the proteins could have protected the surfaces; for longer and

harsher conditions such as rising form a seated position (squeeze lubrication effects) then

the boundary between would not have been sufficient to protect the surface.

A result on retrieved in vivo 3rd

generation alumina hip prostheses was also

published46

. Stripe wear was still visible due to edge loading. Grain pull-out with fine

scale debris trapped in the pits was revealed in the wear scar. In the centre of the scars,

evidence of repolishing of the surface was observed. Additionally, wider stripes were

seen on the femoral heads rather than acetabular cups.

8. The Future of ceramic hip prostheses

Pure alumina hip prostheses have been used for more than 30 years and have

dominated the history of ceramic hip implants. The latest high performance alumina

productprovides a competitive solution of high reliability and excellent wear resistance.

However, it is still a brittle material and subject to a small but persistent history of

fracture67

. The challenge for ceramic engineering is to develop an improved material

which maintains all advantageous properties of 3rd

generation alumina but allows new

applications which require high mechanical loading bearing capability68

. The answer to

this is a composite material based on an alumina matrix and a selection of ingredients

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which increase toughness and hardness, such as zirconia-toughened alumina (ZTA). In

2000, CeramTec AG launched a mixed alumina-zirconia ceramic under the trade name

Biolox-delta which contained Al2O3 (75 vol.%), ZrO2 (24 vol.%) and mixed oxides (1

vol.% CrO2 and SrO)68

. Compared with 3rd

generation alumina, this ZTA shows the

potential to double the strength of alumina while the hardness was slightly lower.

Therefore, the door is open for new ceramics based on alumina.

9. Conclusions

In summary, from the critical review of the present research on alumina hip

prostheses, conclusions can be made as following:

1) Due to osteolysis caused by polymer wear debris, interest in alumina-on-aluminaTHRs continues to grow.

2) The wear performance of alumina hip joints is related both to the alumina itself (grainsize, manufacturing process, etc.) and to the prosthesis design. Significant

improvement of medical-grade alumina has been achieved in Germany and the

modern alumina for hip prostheses is the 3rd

generation.

3) In-vivo investigations of alumina prostheses taken out from bodies show low wearrates of the hip prostheses compared with other material combinations, however,

occasional fractures still do happen.

4) In-vitro research on alumina prostheses in simulators show excellent performance of3

rdgeneration alumina prostheses compared with 1

stand 2

ndgeneration in vivo

alumina prostheses.

5) The introduction of a movement called micro-separation makes it possible toreplicate in vivoperformance of hip prostheses in-vitro.

6) Microstructure plays an important role in the wear mechanisms of alumina prosthesesand is a key issue in engineering new alumina ceramics for THRs. However, there is

a fundamental lack of research on microstructure of alumina prostheses, such as

quantitative dada of microstructure and mechanical properties, since most of

observations have been made by surgeons. Further investigations on hip prosthesis

microstructures are required.

7) Further ceramics for hip prostheses are based on alumina matrix with ingredients to

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increase toughness, such as ZTA.

Acknowledgement:

The author would like to thank the support, guidance and encouragement of Dr

Beverley J Inkson and Professor W Mark Rainforth. Studentship support from Overseas

Research Scholarship and EPSRC is also gratefully acknowledged.

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List of figure captions

Fig. 1 Existing or potential combinations for THRs according to the different

materials used for the femoral head and acetabular cup: a) a Charnley metal-on-

polymer hip prosthesis2; b) a McKee-Farrar metal-on-metal hip prosthesis

3; c) an

alumina-on-alumina hip prosthesis with metal shell

4

; d) a ceramic-on-polymer hipprosthesis3; e) a ceramic-on-metal hip prosthesis

5. Note that (a) ~ (d) are already in

the market, however, (e) is still in the laboratory stage.

Fig.2 The comparisons and volumetric wear rates (mm3million cycles

-1) between

different combinations under pin-on-disc wear test13

. Note that alumina-on-

alumina pairs show the lowest wear rates under laboratory conditions compared

with other combinations.

Fig. 3 Microstructures of alumina for THRs. (a) SEM image of early alumina for

THRs after thermal etching (4h, 1460 C, normal atmosphere)21

. (b) SEM image of

3

rd

generation alumina for THRs after thermal etching (15 mins, 1470 C, normalatmosphere). (c) TEM image of 3rd

generation alumina for THRs. Note the

enrichment of glass phases in the grain-boundaries and a coarse grain size in (a).

Fig. 4 Wear rate-grain size dependence in the wet erosive wear of pure

polycrystalline alumina28

. Note that wear rate decreases with decreased grain size.

Fig. 5 Schematic illustration of the intergranular micro-crack development process,

(a), and the wear process as the summation of crack progression and delay steps, (b)

28.

Fig.6 Wear data for nominally pure alumina ceramics of three grain sizes, l.Room-temperature data for rotating silicon nitride sphere, 12 mm in diameter, 450-

N load, on flat specimen, paraffin oil lubricant. Note initial slow, steady increase of

scar diameter with sliding time, followed by abrupt transition to severe wear at

critical sliding time. Sliding time for onset of transition diminishes significantly for

the larger grain-size materials. Vertical dashed lines are theoretical predictions of

the transition times31

.

Fig.7 Summarised fracture rates of alumina-on-alumina hip prostheses as a function

of ceramic brand and implantation period. Note the dramatic decrease of the

fracture rates in later 1970s and early 1980s15-18, 24, 27, 34-46

.

Fig.8 Summarised survival rates of alumina-on-alumina hip prostheses in literature

as a function of implantation period23, 34, 39, 44, 48

.

Fig. 9 Wear debris in the size range 5-90nm in the tissue from worn alumina-on-

alumina hip prosthesis50

.

19


20/36

Fig. 10 Schematic of relative micro-separation during gait cycle25

.

Fig.11In-vitro micro-separation schematic63

.

Fig. 12 Stripe wear of HIPed alumina components (marked)

63

. Left hand is a pair ofalumina hip prostheses explanted after one year and right hand is a pair of alumina

hip prostheses following in-vitro micro-separation.

Fig.13 Schematic wear on the retrieved in vivo alumina hip prostheses. (a) pole zone

wear; (b) equatorial zone wear21

.

Fig. 14 SEM of stripe wear (a) Low wear area; (b) Worn/ low wear boundary of

wear stripe; (c) Near edge of wear stripe.61

20


21/36

Table 1 Mechanical Properties of Medical-grade Alumina27

.

1970s alumina 1980s alumina 1990s alumina

Strength (MPa) min. 400 500 580

Hardness HV min. 1,800 1,900 2,000Bending Strength (MPa) >450 >500 >550

Wetting angle ()


22/36

22

Table 2 Some fracture rates of alumina-alumina hip prostheses.

Authors Implantation

period

No. of

implantations

Fracture

rate (%)

Griss et al.16

1974-1978 130 6.9

Trepte et al.18 1977-? 41 0

Knahret al.17

1976-1979 67 13.4

Boutin et al.15

1977-1985 560 0.54

OLeary et al.35

1982-1983 69 0

Higgs38

1980-1987 337 0.29

Hoffingeret al.36

1983-1984 119 0

Nizard et al.39

1977-1979 187 1.6

Winteret al.40

1974-1979 100 8Burckerd et al.

411978-1992 > 1200 0.16

Fritsch et al.42

1974-1982 1069 0.4

Fritsch et al.42

1982-1994 1763 0.06

Boehleret al.43

1976-1979 243 0

Bizot et al.44

1990-1992 234 0.002

Garino39

1997-1998 333 0

Willmann27

1974-1982 0.026

Willmann27

1982-1994 0.004

Hamadouche et al.34

1979-1980 118 0

DAntonio et al.24

Bierbaum et al.45

1996-1998 514 0.002

Walteret al.46

1, 588 0


23/36

Table 3 Survival rates of alumina-on-alumina hip prostheses in literatures.

Authors Implantation period 5 years 7 years 10 years 15 years

Nizard et al.39

19771979 89.5% 87.1% 82.5% --

Sedel et al.23 1977? -- -- 83% 70%

86% for patients

younger than 50

Bizot et al.44

19781994 97.3% 94.1% 90.4% 78.9%

94.7% 88.8% 88.8% --

100% 95.1% -- --

100% 94.3% -- --

Hamadouche et al.34 19791980 -- -- -- --

-- -- -- --

Nizard et al.48

19771984 -- -- 88.6% --


24/36

Authors Implantation

period

No. of

revisions

Reasons for revisions (N

Walteret al.21

1975-1978 28 Loosening of stems and sockets (10), looseni

loosening (5), loosening of sockets (3), ball frac

Nevelos et al.49

1980-1989 10 Displacement of acetabular cup (4), femoral

(2), femoral stem too small (2), failure of bo

dislocation of prosthesis (1).

Fritsh et al.42

1974-1994 6 Direct trauma (4), recurrent neck impingement

Nevelos et al.50

1980-1994 11 Aseptic loosening of acetabulum (2), loosen

loosening of both components (4) and looseni

stable (1).

Bizot et al.44

1990-1992 11 Aseptic loosening (6), recurrent dislocation

fracture of head (1) and persistent hip pain (1)

Bierbaum et al.45

DAntonio et al.24

1996-1998 4 A periprosthetic femur fracture (1) recurrent in

suspected, but not confirmed sepsis (1).

Garino et al.51

4 Migration of cup (1), deep infection (1),

malplacement (1).

Walter et al. 46 1997-2002 21 Periprosthetic fracture (6), psoas tendonitis

loosening (3), dislocation (2), and heterotopic o

Table 4 Published date of revisions and reasons.


25/36

(a) (b)

(c) (d)

(e)

Fig. 1 Existing or potential combinations for THRs according to the different

materials used for the femoral head and acetabular cup: a) a Charnley metal-on-

polymer hip prosthesis2; b) a McKee-Farrar metal-on-metal hip prosthesis

3; c) an

alumina-on-alumina hip prosthesis with metal shell4; d) a ceramic-on-polymer hip

prosthesis3; e) a ceramic-on-metal hip prosthesis

5. Note that (a) ~ (d) are already in

the market, however, (e) is still in the laboratory stage.

25


26/36

1

2

3

4

1 2 3 4

Fig.2 The comparisons and volumetric wear rates (mm3million cycles

-1) between

different combinations under pin-on-disc wear test13

. Note that alumina-on-

alumina pairs show the lowest wear rates under laboratory conditions compared

with other combinations.

26


27/36

Fig. 3 Microstructures of alumina for THRs. (a) SEM image of early alumina for

THRs after thermal etching (4h, 1460 C, normal atmosphere)21

. (b) SEM image of

3rd

generation alumina for THRs after thermal etching (15 mins, 1470 C, normal

atmosphere). (c) TEM image of 3rd

generation alumina for THRs. Note the

enrichment of glassy phases (white contrast) in the grain-boundaries and a coarse

grain size in (a).

27


28/36

Fig. 4 Wear rate-grain size dependence in the wet erosive wear of pure

polycrystalline alumina28

. Note that wear rate decreases with decreased grain size.

28


29/36

(a)

(b)

Fig. 5 Schematic illustration of the intergranular micro-crack development process,

(a), and the wear process as the summation of crack progression and delay steps, (b)

28.

29


30/36

Fig.6 Wear data for nominally pure alumina ceramics of three grain sizes, l.Room-temperature data for rotating silicon nitride sphere, 12 mm in diameter, 450-

N load, on flat specimen, paraffin oil lubricant. Note initial slow, steady increase of

scar diameter with sliding time, followed by abrupt transition to severe wear at

critical sliding time. Sliding time for onset of transition diminishes significantly for

the larger grain-size materials. Vertical dashed lines are theoretical predictions of

the transition times31

.

30


31/36

Fracture rate of alumina THRs

0

2

4

6

8

10

12

14

16

Frialit

(197

4-1978

)

Frialit

(197

4-1979

)

Biolo

x1

(197

4-1982

)

1stB

iolox

(197

4-1982

)

Rose

ntha

l(1976

-197

9)

Rose

ntha

l(1976

-197

9)

Biolo

x(197

7-?)

Cera

ver-O

steal(

1977

-197

9)

Cera

ver-O

steal(

1977

-198

5)

Biolo

x(197

8-1992

)

Cerave

r-Oste

al(197

9-1980

)

Biolo

x(198

0-1987

)

Biolo

x(198

2-1983

)

Biolo

x2(198

2-1994

)

2ndBi

olox

(198

2-199

4)

Biolo

x(198

3-1984

)

Cera

ver-O

steal(

1990

-199

2)

Tran

scen

d(199

7-1998

)

ABC(199

6-1998

)

Oste

onics

Sec

urfit,

ABC

,SRO

M,ABG

IIan

dLi

neag

e(199

6-?)

Ceramic Brand (implantation period)

Fracturerate(%)

Fracture rate (%)

Fig.7 Summarised fracture rates of alumina-on-alumina hip prostheses as a function

of ceramic brand and implantation period. Note the dramatic decrease of the

fracture rates in later 1970s and early 1980s15-18, 24, 27, 34-46

.

31


32/36

survival rate

0.00%

20.00%

40.00%

60.00%

80.00%

100.00%

120.00%

1977

1979

1977

?

1978

19

94

1978

-1994

1978

-1994

1978

-1994

1977

19

84

1979

19

80

1979

-1980

implantation period

5years

7years

10years

15years20years

Fig.8 Summarised survival rates of alumina-on-alumina hip prostheses in literature

as a function of implantation period23, 34, 39, 44, 48

.

32


33/36

Fig. 9 Wear debris in the size range 5-90nm in the tissue from worn alumina-on-

alumina hip prosthesis50

.

Fig. 10 Schematic of relative micro-separation during gait cycle25

.

33


34/36

Flexion/extension

Loa

Internal/external

lrotatio

Lubrican

bat

holde

Head/steholde

Cu

Fig.11In-vitro micro-separation schematic63

.

Fig. 12 Stripe wear of HIPed alumina components (marked)63

. Left hand is a pair of

alumina hip prostheses explanted after one year and right hand is a pair of aluminahip prostheses following in-vitro micro-separation.

34


35/36

Fig.13 Schematic wear on the retrieved in vivo alumina hip prostheses. (a) pole zone

wear; (b) equatorial zone wear21

.

35


36/36

Fig. 14 SEM of stripe wear (a) Low wear area; (b) Worn/ low wear boundary of

wear stripe; (c) Near edge of wear stripe.61

al medical grade

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