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Replacement knee and hip joints bring relief from suffering and enable
patients to return to normal life. For our ageing population, and for younger
patients
 needing post trauma prosthetic surgery, the life span of an implanted
artificial joint will have a major impact on their quality of life. Implant
replacement or remedial surgery will at best be painful and expensive, and may
in some cases carry a mortality risk of its own.
Extensive research has been carried out to develop metal and ceramic
ball-joint prosthetics (implants) that are strong enough to perform well without
succumbing to corrosion in the hostile biochemical environment of the human
body. However, despite advances in these metal and ceramic components, much less
is known about the mechanisms responsible for wear of the associated plastic
'cup' component of artificial joints. The material of choice for these cup
components is ultra high molecular weight polyethylene. Clinical studies of
prostheses removed from patients have identified several failure mechanisms:
structural failure of the polyethylene due to high contact stresses; macroscopic
polymer asperity wear caused by plastic deformation; and microscopic surface
wear of the polymer cup surface. This latter process is accelerated by oxidative
degradation of the polymer and deterioration and roughening of the femoral
counterface (head). These are key factors in the increased wear and reduced
lifetime of both hip and knee prostheses.
With funding provided through the EU's Brite Euram programme, three academic
partners, the Universities of Leeds (UK) and INSA Lyon (France), and the
Portuguese research organisation Instituto de Engenharia Biomédica, set out to
investigate both the generation of polyethylene debris and the biological
reaction to it. The objective was to significantly increase the safe and useful
life of artificial joints.
Innovative anatomy
The researchers successfully developed new knowledge on ultra high molecular
weight polyethylene; in particular in wear simulation and particle analysis at
sub-micron levels, as well as on the biochemical reactivity of wear particles. A
novel six-station physiological anatomical hip joint simulator has been
developed with a single axis load and two motions. This produced physiological
wear rates and wear patterns consistent with clinical research. One patent was
filed during the project, and there have been some 120 publications related to
the research to date.
Impacting the joint industry
The University of Leeds considers this project as a cornerstone for its
current strong position in this technology. The project created the know-how and
infrastructure for an extensive range of follow-on projects. A new joint cap has
been designed using the project guidelines. Endorsers have improved their
positions in the materials supply market (Hoechst), and the artificial joint
market (HowMedica). The exploitation potential of the project's results has been
further realised in the artificial joint industry, where modified polyethylene,
such as the material developed in the project, has almost entirely replaced
traditional polyethylene. Additionally, the research arising after this project
has led to the development of other innovative joint replacement systems, which
have been licensed by the University of Leeds to another industrial partner.
Health benefits and economic impacts
The University of Leeds still has RTD agreements with endorsers, such as
Howmedica, and also with other companies involved in developing metal and
ceramic hip joint replacements. INSA has agreements with other industrial
partners for further research. The project improved the commercial prospects of
endorsing companies in the artificial joint market, which has a size of 800 000
joints per year worldwide, at an average price of €2000 each. The endorser
HowMedica has approximately 20% of that market, partly thanks to this project.
Hoechst, acting as a material supplier, has roughly a 40% share in the market
for highly cross-linked polyethylene for this specific application. The
project's results also influenced an ISO standard related to artificial joints.
The work significantly enhanced the research participants' skills, which have
been used in follow-up projects and attracted cumulative funding of
approximately €10 million in the five years following the end of the project.
(http://europa.eu.int/comm/research/
industrial_technologies/impacts/
article_3078_en.html
)
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