Biomedical Engineering Reference
In-Depth Information
9.2 INTRODUCTION
Due to demographic changes, there is a worldwide increase in the average age of
the population, leading to a rapidly increasing number of surgical procedures
involving prosthesis implantation. Human joints are prone to degenerative and
infl ammatory diseases that result in pain and stiffness of joints [1]. Approximately
90% of the population over the age of 40 suffers from some degree of degenera-
tive joint disease [2]. Typically such premature degenerative diseases result in
degradation in the mechanical properties of the joint that has been subjected to
excessive loading conditions or from failure of normal biological repair processes.
Since the natural joints cannot function optimally, the joint surfaces are replaced
by artifi cial biomaterials through arthoplastic surgery [1]. This has resulted in an
urgent need for improved biomaterials and manufacturing technologies for
orthopedic implants such as hip, knee, and shoulder implants.
Furthermore, since such geometrically-complex implants have different
property requirements at different locations, their manufacturing becomes par-
ticularly challenging. The design and manufacture of these medical devices is
complicated by a host of inter-related factors, including regulatory requirements,
patient quality of life considerations, durability, weight of the device, cost, and
constraints of manufacturing. In order to achieve the best balance of all these
factors, compromises often have to be made in the design and development of
orthopedic implants. However, the advent of novel additive near-net shape manu-
facturing processes, such as the laser engineered net shaping (LENS™) process
[3,4] is expected to have a substantial impact on the design and development of a
new generation of orthopedic implants and other medical devices. Furthermore,
by employing such near-net shape processing technologies, it is possible to manu-
facture custom-designed implants with site-specifi c properties. Such a novel pro-
cessing approach is expected to have a substantial impact on the development
of next-generation orthopedic implants and is the primary motivation for this
chapter.
Typically the manufacturing of a hip implant is a multi-stepped process that
may take weeks [5-8]. The starting raw material is typically a casting or forging in
the rough shape of the femoral stem part of the hip implant. This casting or forging
is polished to remove scales and rough edges leftover from the casting or forging
process [9]. Usually, a skilled craftsman is required for carrying out the polishing
job. Subsequently, the stem of the implant is passed through a fl uorescent pene-
trate inspection (FPI) for surface defects such as cracks, porosity, and other
imperfections. Following the FPI test, the stem is exposed to sand-blasting in
order to give the implant an uneven surface fi nish for better bonding with the
porous coating that is subsequently sprayed on the surface of the implant. The
stem is initially sprayed with a binder followed by a layer of metal beads made of
either cobalt-chromium alloy or titanium. The stem is then sintered to allow for
the porous coating to adhere to the implant surface.
Subsequently, the taper or top of the stem is machined to precise measure-
ments using a lathe, followed by a fi nal polishing step. The stem is then put through
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