Biomedical Engineering Reference
In-Depth Information
of corrosion are the disintegration of the implant material
per se
, which will weaken the implant, and
the harmful effect of corrosion products on the surrounding tissues and organs.
1.2 Stainless Steels
The first stainless steel utilized for implant fabrication was the 18-8 (type 302 in modern classification),
which is stronger and more resistant to corrosion than the vanadium steel. Vanadium steel is no longer
used in implants because its corrosion resistance is inadequate
in vivo
. Later, 18-8sMo stainless steel was
introduced, which contains a small percentage of molybdenum to improve the corrosion resistance in
chloride solution (salt water). This alloy became known as
type 316 stainless steel
. In the 1950s, the car-
bon content of 316 stainless steel was reduced from 0.08 to a maximum amount of 0.03% (all are weight
percent unless specified) for better corrosion resistance to chloride solution and to minimize the sen-
sitization and hence became known as type
316L stainless steel
. The minimum effective concentration
of chromium is 11% to impart corrosion resistance in stainless steels. Despite being a reactive element,
chromium and its alloys can be
passivated
by 30% nitric acid to give excellent corrosion resistance.
he
austenitic stainless steels
, especially types 316 and 316L, are most widely used for implant fabri-
cation. These cannot be hardened by heat treatment but can be hardened by cold working. This group
of stainless steels is nonmagnetic and possesses better corrosion resistance than any other group. The
inclusion of molybdenum enhances resistance to
pitting corrosion
in salt water. The American Society
of Testing and Materials (ASTM) recommends type 316L rather than type 316 for implant fabrication.
The specifications for 316L stainless steel are given in Table 1.1. The only difference in composition
between the 316L and 316 stainless steels is the maximum content of carbon, that is, 0.03% and 0.08%,
respectively, as noted earlier.
Nickel stabilizes the austenitic phase [γ, face-centered cubic crystal (fcc) structure] at room tempera-
ture and enhances corrosion resistance. The austenitic phase formation can be influenced by both the
Ni and Cr contents as shown in Figure 1.1 for 0.10% carbon stainless steels. The minimum amount of Ni
for maintaining austenitic phase is approximately 10%.
Table 1.2 gives the mechanical properties of 316L stainless steel. A wide range of properties exist
depending on the heat treatment (annealing to obtain softer materials) or cold working (for greater
strength and hardness). Figure 1.2 shows the effect of cold working on the yield and ultimate tensile
strength of 18-8 stainless steels. The engineer must consequently be careful when selecting materials of
this type. Even the 316L stainless steels may corrode inside the body under certain circumstances in a
highly stressed and oxygen-depleted region, such as the contacts under the screws of the bone fracture
plate. Thus, these stainless steels are suitable for use only in temporary implant devices such as frac-
ture plates, screws, and hip nails. Surface modification methods such as anodization, passivation, and
TABLE 1.1
Compositions of 316L Stainless Steel
Element
Composition (%)
Carbon
0.03 max.
Manganese
2.00 max.
Phosphorus
0.03 max.
Sulfur
0.03 max.
Silicon
0.75 max.
Chromium
17.00-20.00
Nickel
12.00-14.00
Molybdenum
2.00-4.00
Source:
Adapted from ASTM. 1992.
Annual Book of
ASTM Standards
, Vol. 13,
Medical Devices and Services
,
F139-F86, p. 61. Philadelphia, PA: ASTM.
