Biomedical Engineering Reference
In-Depth Information
enhanced properties. These treatments are commonly
used to increase surface hardness and wear properties.
Finally, the manufacturer of a metallic implant device
will normally perform a set of finishing steps. These vary
with the metal and manufacturer, but typically include
chemical cleaning and passivation (i.e., rendering the
metal inactive) in appropriate acid, or electrolytically
controlled treatments to remove machining chips or im-
purities that may have become embedded in the implant 0 s
surface. As a rule, these steps are conducted according to
good manufacturing practice (GMP) and ASTM specifi-
cations for cleaning and finishing implants.
It is worth emphasizing that these steps can be ex-
tremely important to the overall biological performance
of the implant because they can affect the surface
properties of the medical device, which is the surface
that comes in direct contact with the blood and other
tissues at the implant site.
Fig. 3.2.9-2 Low-power view of the interface between a porous
coating and solid substrate in the ASTM F75 Co-Cr-Mo alloy
system. Note the structure and geometry of the necks joining the
beads to one another and to the substrate. Metallographic cross
section cut perpendicular to the interface; lightly etched to show
the microstructure. (Photo courtesy of Smith & Nephew Richards,
Inc. Memphis, TN.)
Microstructures and properties
of implant metals
In order to understand the properties of each alloy
system in terms of microstructure and processing history,
it is essential to know (1) the chemical and crystallo-
graphic identities of the phases present in the micro-
structure; (2) the relative amounts, distribution, and
orientation of these phases; and (3) the effects of the
phases on properties. This section of the chapter em-
phasizes mechanical properties of metals used in implant
devices even though other properties, such as surface
properties and wear properties, must also be considered
and may actually be more critical to control in certain
medical device applications. (Surface properties of ma-
terials are reviewed in more depth in Section 3.1.4 of this
topic.) The following discussion of implant alloys is di-
vided into the stainless steels, cobalt-based alloys, and
titanium-based alloys, since these are the most com-
monly used metals in medical devices.
Stainless steels
Composition
Although several types of stainless steels are available for
implant use ( Table 3.2.9-1 ), in practice the most
common is 316L (ASTM F138, F139), grade 2. This
steel has less than 0.030% (wt.%) carbon in order to
reduce the possibility of in vivo corrosion. The ''L'' in the
designation 316L denotes low carbon content. The 316L
alloy is predominantly iron (60-65%) with significant
alloying additions of chromium (17-20%) and nickel
(12-14%), plus minor amounts of nitrogen, manganese,
molybdenum, phosphorus, silicon, and sulfur.
Fig. 3.2.9-3 Scanning electron micrograph of a titanium plasma
spray coating on an oral implant. (Photo courtesy of A. Schroeder,
E. Van der Zypen, H. Stich, and F. Sutter, Int. J. Oral Maxillofacial
Surg. 9:15,1981.)
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