Biomedical Engineering Reference
In-Depth Information
Wear debris and consequent osteolysis and aseptic loosening are major
causes of implant revisions. This is partly a result of using UHMWPE for the
articulating bearing components. Some of the clinical studies have already
shown promising results of using second generation metal-on-metal bearing
systems rather than conventional metal-on-polymer systems. However, the
design of novel pure metal joint systems raises questions on their wear resistance
and metal ion release. A more detailed understanding of effects of metal
particles and ions on the human body is needed in the future. Meanwhile, some
methods, such as ion implantation and tantalum-enrichment, have demonstrated
promising results in improving titanium and cobalt±chromium alloy tribological
properties.
For titanium and cobalt±chromium alloys that are mainly used for stems and
metal backs that require tissue integration, quick and strong osseo- or
chondrointegration is a major goal. To meet this challenge, either bioactive
coatings, porous surface structures, or various surface modification methods
(such as anodization) have been employed and have demonstrated some success
in vitro and in vivo. Further investigations to optimize the modification
parameters and resulting surfaces are important to achieve more bioactive metal
implants.
Last, but not least, a metal hip implant with closer elasticity to bone will add
strength and a prolonged implant lifetime. This is partially achieved by selection
of metal alloys with new compositions. Some beta titanium alloys are of great
interest today due to their superior biocompatibility and low Young's modulus
compared with the current widely used Ti-6Al-4V.
In summary, titanium and cobalt±chromium alloys play an important role in
today's hip and knee replacements. The development of new metal prostheses
designs, new alloy compositions, new processing and heat treatments, new
implant surface coatings, and modifications will all contribute to improve the
efficacy of current metal joint prostheses and prolong their effective lifetimes.
However, in a field that requires maybe decades for a new standard to be
accepted, more laboratory work and clinical studies are needed for industrial
acceptance.
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2.6 References
AAOS (American Association of Orthopedic Surgeons) (2007a), Hip implants. Available
from: http://orthoinfo.aaos.org/topic.cfm?topic=A00355 (accessed 5 February
2010).
AAOS (American Association of Orthopedic Surgeons) (2007b), Knee implants.
Available from: http://orthoinfo.aaos.org/topic.cfm?topic=A00221 (accessed 5
February 2010).
Ahmed T, Long M, Silvestri J, Ruiz C and Rack HJ (1995), `A new low modulus,
biocompatible titanium alloy', presented at the 8th World Titanium Conference.
Birmingham, UK.
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