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Invited Lecture IL9, Technical Session TS6, March 8th, 2013
C.Piconi :Alumina Ceramics for Biomedical Applications
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Alumina Ceramics for Biomedical Applications
Corrado Piconi
Catholic University “Sacro Cuore”, Orthopedics Institute, Rome, Italy.
Mailing Address: 240, via Pasquale II, 00168 Roma, Italy
1. Foreword
Hip arthroplasty is one of the most successful surgical procedures since ever. This procedure has a
clinical success in 95% of the patients at ten years postoperatively, and corresponding outcomes in
term of pain relieve and patient satisfaction [18]. However, the wear of the bearing surfaces is still
among the complications that limit the long term survival of arthroplasties. Namely the host reaction
to wear debris from the artificial joint – many of them less than one micrometer in size - starts a
cascade of cellular reactions leading to osteolysis and eventually to the loosening of the implant,
making mandatory a revision surgery.
Then, it is not surprising that surgeons and engineers devoted their efforts to the development of
suitable wear couples for hip replacements since the very beginning of arthroplasty, because of the
economic and social relevance of this problem, as revision arthroplasty is a surgery more difficult to
perform, costly, and has poorer outcomes than primary surgery.
Alumina ceramic is offering above other biomaterials the advantage of high chemical inertia and of
an outstanding hardness, making it the ideal material for the manufacture of components for
arthroprosteses bearings. Among the different bearing couples in clinical use today alumina-on-
alumina bearings are showing extremely low wear in standard walking conditions. Moreover
ceramic debris is less toxic than metallic wear debris, and less inflammatory than the ones from
polyethylene [9].
This justifies the success of alumina that in orthopedics is simply known as the “ceramic”. So far, in
some European countries (Austria, France, Germany, Italy, and Switzerland) more than 50% of the
hip implants are making use of ceramic ball heads, and in Asian Countries like e.g. Korea where
72% of THRs has an alumina ball. Also in countries where the introduction of ceramic bearing
components took place later (USA, China, India, Eastern European countries) the use of ceramic
heads in hip replacement bearings is spreading.
This paper traces in an historical perspective the development of alumina as a biomaterial, and gives
some hints of their future application in medical devices.
2. The Pioneers’ Age
The development of alumina as a biomaterial can be traced in the early 1930s, when the first patent
was issued by Rock in Germany [30] although there is was no report of clinical applications in these
years. First clinical applications of alumina in orthopedics have been reported during the 1960s. On
these years L. Smith developed Cerosium, a bone substitute formed by a porous aluminosilicate
matrix impregnated by epoxy resin [34] that was in clinical use up to the 1970s, while in the same
years Eyring and Campbell [8] developed alumina temporary osteotomy spacers that had a limited
clinical applications.
The first successful device made out alumina was a dental fixture – the Sinthodont implant -
developed in Switzerland by Dr. Sami Sandhaus during the first half of the 1960s [33].
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It could be hypothesized that the positive outcomes reported by these implants stimulated the
interest of Dr. Pierre Boutin, an orthopaedic surgeon in Pau (France). He had among his patients the
director of a CGE plant manufacturing alumina electric insulators. Based on his assistance, he
developed in the years 1969-1972 the first alumina-on-alumina bearing for a Total Hip Replacement
[1].
In the same years, pioneers like Langer in Keramed, Heimke in Friedrichsfeld, Dörre in Feldmüle,
and Maier in Rosenthal were working on the development of alumina for clinical application in the
framework of a German government funded research program. The best known result was the
Autophor THR, but also alumina resurfacing cups for the femoral head were developed especially
by Rosenthal [21]. The lists of devices developed shows how innovatively-minded were these
pioneers. While the step-shaped dental implant of the so-called Tübingen type had a good clinical
success, other alumina devices never left the development stage, like e.g. keratoprostheses that were
tested in the eye, subcutaneous injection ports, and all-ceramic hip implants. There are only a few
reports of the clinical use of monolithic hip either shoulder joint replacements [32] made out
alumina, shaped as conical sleeves that were implanted onto the bone stump shaped by a special
milling instrument.
However, the main results of the German project were the development of the BIOLOX® alumina,
that in orthopedics is “The Ceramic” so far, and introduction of the neck-head taper connection, a
technical solution that opened the way to the modularity in THR, replacing the epoxy fixation used
by the first Boutin’s series that led to unsatisfactory results for the in-vivo degradation of the glue.
The list of the scientists that contributed by their work to the development of modern alumina must
also make mention of the pioneering work of Sam F. Hulbert in the USA [14] and of T.D. Drikell
whose alumina Sinthodont dental implant obtained a remarkable success in the USA between 1975
and 1980, and especially of the works of H. Oonishi and Kawahara in Japan. The original approach
of these Japanese scientists was initially based on the use of single-crystal alumina (synthetic
sapphire) to manufacture both dental implants both hip and knee joint replacements that were used
after bone tumours resections. Oonishi and Kawahara developed also the first successful ceramic
(polycrystalline alumina) knee replacement, implanted in Japan since the early 1980s [15, 19].
While in dentistry the alumina implants were soon replaced by the titanium fixtures, alumina was
finding its way in the arthroplasty of the hip: the review of the first series of implants made possible
to learn some interesting lessons that oriented the development of the material in the early 1980s.
Namely it was clear that there was a reduction in aseptic loosening using alumina-alumina joints,
and that the failures observed in some THR were due mainly to the design of the ceramic
components [29]. There were very high failure rates in some series [22], and some manufactures
withdrew from the field, but the ones committed to pursue the development of alumina components
introduced a series of improvements aimed to enhance the reliability of the alumina ceramic
components, leading to the widespread success of this technology in arthroplasty.
While the use of alumina was spreading in Europe an Japan, thanks to the combined efforts of
material scientists, engineers, surgeons and biologists, in the USA only THRs using ceramic femoral
heads coupled to polyethylene (Ceramic-on-Poly, CoP) have been in clinical use since 1989, under
the FDA clearance limited to these devices. But the experience cumulated by surgeons in non-US
countries was demonstrating the benefits brought in by modern alumina-on-alumina bearings for
young and active patients with a dynamic lifestyle. It was only on February 2003 that FDA granted
its approval for the use of BIOLOXforte alumina inlays (CeramTec GmbH, Plochingen, Germany)
in the United States of America in conjunction with the devices of two orthopaedic device
manufacturers.
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3. The Development of Alumina for Medical Devices
Many devices made out oxide ceramics were developed a tested, and ceramists - following the
theory of fracture mechanics - focused their efforts both on the reduction of the size of the defects,
both in enhancing the intrinsic toughness of the material.
The first approach can easily be traced in the development of high purity alumina the selection of
high purity precursors, the optimized batch and sintering cycle joined to the overall improvement
of the quality of the manufacturing process led to the so called “second generation alumina”. A
significant result achieved during 1980 was the definition of the international standard for alumina
for medical devices, ISO 6474.
Further significant steps were:
- the introduction of laser marking to replace the component identification marks that in the first
series were made by diamond engraving before firing, raising concerns about local stress
concentrations. This solution demonstrated in clinical use its positive effects on the reliability of
alumina ceramic components.
- Final densification by Hot Isostatic Pressing (HIP). This thermal treatment – performed slightly
below the sintering temperature under extremely high pressures (about 100 MPa) - allows obtaining
high density limiting the development of the grain size.
- final introduction of proof test in the testing protocol, a non-destructive test aimed to discard all
those components containing a critical size flaw that may shorten the expected lifetime.
All the process improvements in above led about 1995 to the introduction into the market of the
present “third generation alumina” (e.g. BIOLOX®forte).
The further approach – enhancing toughness - was focused on the development of toughened
ceramics, exploiting the transformation toughening behavior of zirconia. In the biomedical scene
this led to the development as biomaterials of YTZP then of Mg-PSZ during the mid-1980s, and to
the introduction in orthopedics of zirconia-toughened alumina matrix composite (BIOLOX®delta)
around 2002 [25].
The development on zirconia as a biomaterial was started in the mid-eighties in France to the aim of
taking advantage of the toughness and strength of this ceramic over alumina [5]. It was focused on
Yttria-stabilized Tetragonal Zirconia Polycrystal (Y-TZP), which soon becomes the standard
material for clinical applications [23].
Nevertheless, Y-TZP is a rather complex material that may exhibit different properties and varying
stability depending on the conditions of use that may trigger at the surface of the component
hydrothermal reactions that originate the degradation of the surface increasing its roughness, then
increasing the wear of UHMWPE cups that are usually coupled to Y-TZP heads in THRs [24]. These
concerns, joined to the unsuitability of zirconia for the manufacture of CoC bearings, and the
unexpected high rates of failure in some batches of Y-TZP since 2000 led to the practical abandon of
YTZP in orthopedics [27].
So far zirconia has found a new momentum in dentistry, where it is used for the construction of the
structures of bridges and crowns by CAD/CAM as well as of dental implants, endodontic pins and
orthodontic brackets [28], while it is exploited in orthopedics as reinforcing and toughening phase
in alumina, giving rise to the so called “Fourth Generation” of alumina ceramics.
Starting from the second half of the eighties, a consistent amount of research work has been
performed in France and in Italy, focused on the development as a biomaterial of Zirconia-
Toughened Alumina (ZTA) but the results of these studies [17,31] did not led to clinical applications.
The first alumina matrix composite for THR bearings was introduced by CeramTec GmbH under
the trademark BIOLOXdelta. This ceramic composite contains a fine and homogeneous dispersion
of Y-TZP grains, and platelets hat are acting as short fibres dispersed in the matrix. This is obtained
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by adding e.g. strontium oxide to the batch before firing, thus obtaining the formation of SrAl12O19-
platelets by solid state reaction during sintering [3]. The presence into the system of Chromia (Cr2O3)
giving to BIOLOXdelta its characteristic mauve colour, and of Yttria (Y2O3) that acts as stabilizer of
the t-phase of zirconia, leads to a material toughened both by phase transformation, both toughened
and reinforced by the presence of platelets. Hardness is maintained high, close to the alumina one
thanks to chromia that compensate the reduction in hardness due to zirconia [16].
<Table 1, Near Here>
4. Why Ceramic in Hip Replacement Bearings?
4.1. Functional Biocompatibility
Biocompatibility is the “The ability of a material to perform with an appropriate host response in a
specific application” [37]. For alumina used in THR bearings then biocompatibility is then expressed
by a) low wear, b) biological reactions to wear debris.
There is general agreement in the literature that volumetric wear of CoC bearings much less than
the one of metal-on-polyethylene bearings. This is due to the extremely high hardness of alumina
that assure its low wear and the resistance of the surface to scratching by third bodies. Furthermore,
the hydrophilic character of the alumina surface allows establishing a fluid film at the ball-cup
interface that provides the lubrication of the joint.
Moreover, there is no ion release from alumina wear debris in the biologic environment. The low
cellular reactivity of alumina has been demonstrated in a number tests performed in-vitro and in-
vivo, whatever the experimental conditions and the physical forms of the samples tested [26] while
the studies performed to characterize the tissue reactions to ceramic wear debris show advantages
on polyethylene and metals, and there are indications of lower systemic toxicity of ceramic debris in
comparison to alternative bearing materials [38].
The high biological safety of alumina, joined to the low wear volume of CoC alumina joints make
the functional biocompatibility of this materials much higher than the one of metals or polymers
used in joint replacement bearings [10].
4.2. Clinical Wear of Alumina THR bearings
The tissue reactions to wear debris from the implant joints may trigger the cascade of events leading
to osteolysis and to implant loosening, making mandatory a revision surgery. Although the wear of
bearings has been widely studied on simulators in laboratory conditions, the wear observed in
patients with well functioning implants either on implants retrieved at revision surgery are giving
more conclusive information.
The Implant Register of the Health Service Agency of the Emilia-Romagna region (Italy) is giving
an interesting insight on CoC THRs [36]. In the period from 1990 to 2001 2984 alumina-on-alumina
and 3432 metal/polyethylene were observed. The analysis at 7 years follow-up shows 96,07%
survival for THRs with alumina CoC bearings Vs. 92,61% of the THRs with metal/polyethylene
joint. The global incidence of aseptic loosening in THRs with CoC alumina bearings resulted to be
less than one half (45,5%) of the one observed in metal/polyethylene bearings. These data are
showing that the use of CoC bearings decrease the risk of revision of in comparison to the
metal/polyethylene bearings, leading the Author to the conclusion that to use implants with ceramic
bearings is fully justified on an economical basis, notwithstanding the higher costs of these devices.
Namely, the comparison of different bearings studied by Greenwald and Garino [12] is showing that
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using alumina CoC bearings the wear was reduced 40 times in comparison to metal on polyethylene
bearings.
The results of the prospective randomized study carried out in the USA from 1996 to 1998 under the
control of FDA that put in comparison modern alumina-on-alumina bearings (BIOLOX®forte,
CeramTec AG, Plochingen, Germany) with metal-on-UHMWPE bearings in the same THR system
has particular significance as it characterize the performance of a so-called “Third Generation”
alumina. This study [7] involved 349 alumina CoC bearings and 165 metal-on-UHMWPE bearings.
Revisions concerned four THR with alumina CoC bearings and eight with metal-on-UHMWPE; no
ceramic fracture was observed in the follow-up. The Author’s remarked that two fractures of the
ceramic inlay were observed in more than 7000 alumina CoC joints implanted in the same THR
system in EU and Australia. Both were due to the non correct positioning into the metal back during
surgery, and that the results obtained confirm the appropriateness of the indication of alumina CoC
joints in hip arthroplasty for younger and more active patients.
More recently Buchanan [2] reviewed the results of a prospective study on ceramic-on-ceramic THR
bearings extended over 19 years, involving 467 alumina-on-alumina bearings and 169 ceramic-on-
ceramic bearings with the new BIOLOX®delta composite. Aseptic loosening was observed in three
components only (3/1252) but in absence of osteolysis and debris-related diseases. Also the short
term results (3 years) of the prospective, randomized, controlled trial designed to make the
comparison between CoC (BIOLOX®delta) and CoP bearings [4] are showing absence of osteolysis
in the CoC group, while it was radiographic evidence of osteolysis in 3 patients of the CoP group.
5. Complications in Alumina bearings
5.1. Fractures
During the first years of clinical use there were many component fractures, especially in some series,
leading several manufacturers to abandon the production of alumina ball heads and inlays for hip
replacements, as well as some design – e.g. skirted heads - due to the unacceptable complication
rates [29] . A retrospective analysis of the literature on ball head fractures published before 1995 [22]
shows the effectiveness of the improvements introduced in processing alumina after the
introduction of the international standard ISO 6474 on 1980. The reference standard allowed
screening out of market all the products that did not meet the strict requirements of a medical grade
product, leading to dramatic decrease of the fracture rate of ball heads and inlays. Also the size of
the study appears a biasing factor as numerous series are showing no or little fractures in
comparison with the smaller one.
It is noted that the fracture of ceramic components for THR is a rare event, less frequent than the
fracture of the metallic components of the implant, especially the stem or the neck. The study
performed by Heck, DA et al. [13] that involved about one half of the members of the American
Association for Hip and Knee Surgeons shows that taking as reference the observed rate of head
fractures (0,22%), the probability of fracture of the metallic stem either of catastrophic failure of the
PE insert are respectively about 22% and 32% much higher. It is noted that three out the eleven
ceramic fractures reported were related to femoral heads implanted on suboptimal quality tapers
[11]. Then the adjusted rate of head fractures (second generation, non HIP-treated alumina) results
0,15%. The low incidence of fractures of ceramic components is put in evidence also in the analysis
by Castro, F.P. et al [6] of 1717 adverse events related to THRs reported to the FDA. The results
shows that only 1% of adverse events were related to ceramic failures, while 18% are related to the
failure of the femoral component (stem fracture, head dissociation, neck fracture, and head wear)
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and 38% were failures of the acetabular cup. It is noted however that these last figures may be biased
by the low number of ceramic heads in use in the USA on that time.
The data released by CeramTec in 2011 [35] show that the overall in-vivo fracture rate of
BIOLOX®forte heads was 21/100’000, while the fracture rate of BIOLOX®delta heads is one tenth of
it (0,002%). It is noted that the most of the fractures occurred in 28 mm heads, mostly in long and
short neck designs.
5.2. Squeaking
From 2006 several papers were reporting variable incidence of noises from ceramic bearings. Most
of the papers were related to implants performed in the USA, using implants characterized by
peculiar design either new materials [20]. So far, there is agreement in the literature on the
multifactorial origin of this phenomenon. The comprehensive analysis performed by Pokorny and
Knahr [29] enlists among the factors influencing acoustic emission from hip replacements a) the
design and the metal alloy used for the stem and for the shell of the cup, b) the presence of metal
contamination at the bearing interface, c) joint laxity due to surgery, leading to component relocation
during gait and edge loading of bearing components, d) malpositioning of the acetabular component
(anteversion and inclination), f) patient-related factors, e.g. overweight.
5. What Future for Oxide Ceramics in Arthroprostheses?
The high mechanical performances of the new ceramic composites are making feasible a number of
innovative ceramic devices. In hip replacements, thinner ceramic acetabular cups to be pre-
assembled in the metallic shell allowed the reduction of the overall diameter of the implant, saving
in this way the precious patient’s bone stock. Moreover, the development of thin ceramic cups, with
a porous osteoconductive outer surface will allow limiting further the implant size.
Also the development of tri-polar joints for hip replacements and of ceramic heads especially
conceived for revision surgery had been developed thanks to the behavior of the new composite
material, as well as composite femoral components for knee replacements [21].
Other developments are in progress, based on alumina-zirconia composites, and new devices based
on these ceramics can be foreseen in the short term for applications in spine and in shoulder surgery.
It could also be expected the new breakthroughs could come from the exploitation in medical
devices of the behavior of oxide ceramics processed by nanometric precursors.
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