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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
displacements typically could have been
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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.
References:
1. BestTreatments and NHS Direct: Hip Replacement: an operation to replace your hip with an
artificial one, Oct. 2006, BMJ Publishing Group Limited. Available at:
http://www.bestreatments.co.uk/btuk/pdf/18618.pdf(accessed on 3rd June 2007)
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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
.
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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
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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 ()
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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
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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% --
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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.
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(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
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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
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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
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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
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(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
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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
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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
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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
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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
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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
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Fig.13 Schematic wear on the retrieved in vivo alumina hip prostheses. (a) pole zone
wear; (b) equatorial zone wear21
.
35
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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