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Orthopaedic Surgery Knee
Zirconia. This process allows any cracks within the material to be blocked there are no differences between ceramic and metal implants.
by the conversion of the tetragonal particles of Zirconia to the monoclinic
system. This phase-changing is followed by the absorption of energy by At the moment, some recent studies looking at these polyethylene issues,
and subsequent increase in volume of the Zirconia particles; this stops the led by Ceramtec in collaboration with the Orthopaedic Institute of Rizzoli
progress of the crack by closing off its potential routes. In other words, it (Bologna), are under data review and will be published soon.
is as if the ceramic can self-repair, with the particles of Zirconia working
like an air bag to capture the energy of the cracks. Current Concepts of Ceramic in Total Knee Arthroplasty
Despite the undeniable advantages related to its tribological characteristics,
In order to improve the resistance and toughness of BIOLOX delta and at present ceramic has very few applications in the field of TKA. However,
increase its reliability in terms of the static distribution of resistance, the the improvements that have been made to ceramic materials, the expanding
introduction of strontium oxide (SrO) was assessed. SrO generates flat number of indications of this kind of surgery to include younger and
crystals of strontium aluminate (SrAl
12-x
CrxO
19
), which are uniformly younger patients and the growing number of people allergic to metal ions
26
distributed in the Alumina matrix during the sinterisation process. Due to mean that the opportunities for ceramic implants are increasing. In fact, the
their size, these flat, elongated crystals block any cracks by decreasing need for long-lasting implants and the necessity of not exposing patients to
their energy. the risk of developing allergies to metal ions or to metallosis phenomena
have led researchers to develop more reliable ceramic materials. With the
The Polyethylene Issue introduction of BIOLOX delta, most of the problems related to the structural
While ceramic has demonstrated its superiority over metal in total hip limits determined by the fragile nature of ceramic seem to have been solved.
arthroplasty (THA) – especially in cases of ceramic–ceramic implants, which However, there are still some firm rules regarding the use of these implants
from a tribological point of view last forever – the advantages of ceramic that have been derived from the experience with Alumina prostheses:
in TKA are yet to be determined. For example, in THA the difference chiefly, the need always to use cement.
17,18
between the diameter of the head and that of the acetabular liner is very
small and the effect of gravity prevents articular liquid coming between Finally, there is a need for large series of patients with long-term follow-
the two components; here, the better wettability of ceramic represents an up to evaluate long-term clinical and radiological results with ceramic
evident advantage in terms of lubrication. In TKA, on the other hand, the implants. For this reason, an international study group conducting a
difference between the curvature of the femoral component and that of prospective multicentre study in three European nations (Germany, Italy
the liner is very large, especially in flexion, and gravitational forces push and Spain) has been founded to evaluate the clinical, functional and
the articular liquid into the convex polyethylene. This improves lubrication, radiological outcome of TKAs using a ceramic femoral component – the
meaning that the wettability of the material is less important. Multigen Plus (Lima-Lto). This study is discussed in more detail below.
As regards wear, less abrasion of the polyethylene surface of the tibial Future Evolution of Ceramic Total Knee Arthroplasty
component can be expected if the femoral component is ceramic, but there The solution to the problem of polyethylene wear in TKA is prosthetic
is no difference in terms of pitting, which is caused mainly by the shear designs that do not contain polyethylene, instead using the ceramic–
forces generated under the polyethylene surface. In clinical series, ceramic ceramic coupling that is already used in many THA systems. As discussed
TKAs have not shown better results than metal implants, whose survival above, ceramic–ceramic coupling is possible in THA for two reasons: the
rate at 15 years is over 98%.
1,2
Because the issue of implant survival seems minimal dimensional difference between the head and the cup, and the
to be related principally to the release of polyethylene debris,
4,5,12,21,22
absence of translational movements. Neither of these applies to TKA. The
attention should be focused on the polyethylene and the processes it problem with TKA is that the kinematics of the knee is both rotational
undergoes, as well as on improving the material of the femoral component. and translational; a hypothetical solution to this is mobile bearing at the
femoral component–liner interface. However, although this solution may
In the majority of cases, the severe effects of wear and delamination theoretically be reliable according to the above-cited criteria, it cannot yet
experienced with polyethylene liners are due to oxidation phenomena, be put into practice due to inadequate lubrication between the liner and
most of which are related to the sterilisation process that is commonly the tibial baseplate, where the kinematics is translational. In a ceramic-
used. Polyethylene sterilisation with ethylene oxide (EtO), or with gamma on-ceramic joint, the radius of curvature of the coupled surfaces must be
rays in the absence of oxygen, can guarantee long-lasting implants different to allow the interposition of the lubricant, which is otherwise
regardless of the material used for the femoral component. impossible due to the very high stiffness of the ceramic. With no
lubrication, the joint is dry and very high localised contact pressures can
Yasuda
23
and Minoda
7
both demonstrated less wear of the polyethylene occur on the ceramic; this can cause the ceramic to fracture. In addition,
liner with the use of Alumina compared with CoCrMo femoral components, a dry ceramic–ceramic contact causes an audible high-frequency noise
although the sterilisation process performed on the polyethylene in the two (squeaking). In contrast, a flat-on-flat ultra-high-molecular-weight
groups of prostheses was not indicated. Ezzet
10
demonstrated the polyethylene (UHMWPE)–metal or UHMWPE–ceramic joint does not
advantages of using Oxinium compared with CoCrMo components in terms create any mechanical problem because UHMWPE is a self-lubricant
of wear on the polyethylene liner. The reduction of polyethylene wear when material and is much softer than ceramic, so it is slightly deformed under
using ceramic compared with CoCrMo implants could be explained by the load and the lubricant can therefore spread on the joint interface,
reduction of abrasive and adhesive wear processes and scratches; in in vitro reducing the friction between the counterparts. A theoretical solution to
studies this phenomenon was related to the superficial rugosity of the the problem with ceramic–ceramic joints could be the modification of the
femoral component.
24
However, according to a study conducted by articular kinematics; this was hypothesised at the end of the 1990s in a
Lancaster in 1997,
25
if the rugosity of the femoral component is the same, project based on a double rotation of the mobile bearings.
27
Despite
60 EUROPEAN MUSCULOSKELETAL REVIEW 2007
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