Originally Published MDDI January
2005
Materials
Stainless-Steel Options for Medical Instrument Tubing
Understanding the properties of different alloys is crucial to selecting the
right material for a particular application.
William Fender and Robert S. Brown
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| Table I. Corrosion resistance of alloys with acceptable biocompatibility for medical instruments (click to enlarge). |
As surgical procedures have become more complex and confined, the requirements
for instruments have become more demanding. As a result, choosing the right
material to make tubular components has also become more difficult.
Proper material selection is critical to the development of the most cost-effective
instrument. A material must perform as intended, and it must also be one that
can be fabricated economically. The optimal material is rarely the least-expensive
material.
Historically, enhanced-strength UNS S30400 (AISI Type 304) stainless steel has
been used to make the tubular components for dental and surgical instruments.
This alloy has worked well for instruments designed for use in confined spaces;
however, it has some drawbacks that limit its usefulness. Its drawbacks include
loss of strength during welding, poor edge retention, poor wear resistance,
and poor galling resistance.
Some instruments, such as trocars, that are not subjected to high stress or
torsional loads and are not used for cutting or shaping can be made from
a basic stainless steel such as UNS S30403. Many newer instruments, however,
now also require that the selected material provide properties such as strength
and wear resistance.
Long, slender instruments, such as drivers or arthroscopic instruments are likely
to have high demands placed on them. Increased strength and toughness are necessary
for these types of instruments. An alloy such as UNS S46500 provides the high
hardness and resulting edge retention required. Although it is not as good as
a martensitic stainless of similar hardness, it is adequate for many cutting
and shaping applications.
Cutting and shaping instruments, such as shavers or samplers, require an alloy
that is hard and has good edge retention. The wear and galling resistance of
alloys such as UNS S42010 ensures smooth operation of parts that move in relation
to each other.
Corrosion Resistance of Stainless Steels
The most important attribute of a material used for a surgical instrument is
its corrosion resistance. In the past, the inherent corrosion resistance of
stainless steel has been deemed adequate for surgical instruments. Improved
corrosion resistance is considered critical to many instruments currently being
developed. In addition to body fluids, device manufacturers must also consider
pre- and postsurgical instrument cleaning techniques when determining the level
of corrosion resistance required. Table I shows the relative corrosion resistance
of alloys. When more corrosion resistance is required to withstand cleaning
or sterilizing solutions, an alloy farther to the right in the table should
be considered. All of the alloys listed are biocompatible.
Strength and Toughness
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| Table II. Steel property combinations and the alloys available to achieve a desired toughness and fatigue strength (click to enlarge). |
When new medical instruments are designed, they are often required to be thinner
or longer than their predecessors. These new designs require materials with
increased toughness, fatigue strength, and tensile strength. All of these properties
are interrelated.
As the strength and hardness increase, the ductility and toughness generally
decrease. The degree to which this occurs differs between the alloy families
and, to some extent, the difference varies within a family. Table II lists property
combinations and the alloys available to achieve a desired result.
Edge Retention, Wear, and Galling Resistance
When a medical device is used for cutting or shaping, edge retention becomes
a critical material property. If a cutting edge becomes prematurely dull, the
instrument becomes difficult to use and potentially dangerous.
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| Table III. Effects of hardening methods on selected properties with alloy rankings (click to enlarge). |
It is important to consider wear and galling resistance for designs in which
metallic parts will move in relation to each other in an instrument. If wear
or galling occurs during use, the instrument may not perform properly and could
introduce metallic debris into the patient’s wound.
A metal’s edge retention or wear resistance is determined by its hardness
and how that hardness is achieved. Generally, as the hardness of a metal increases,
its edge retention, wear resistance, and galling resistance also increase.
It is equally important to consider the method used to obtain the material’s
hardness. Table III shows the relationship between relative edge retention,
wear resistance, and galling resistance. The table also describes how hardening
is achieved.
The wear resistance of the hard carbides in the martensitic stainless steel
is excellent. Therefore, the edge retention of this material is better than
that of a precipitation hardening stainless or austenitic stainless at the same
hardness.
Welding
It is often necessary to weld one or more components during fabrication of a
medical instrument. When selecting a material, the effect of the welding operation
on the material’s properties must be considered. It is important to address
the effects of welding during the design stage. Any steps necessary to compensate
for processing must also be addressed.
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| Table IV. Welding effects on stainless steel and postweld corrective heat treatments (click to enlarge). |
When a metal is welded, the heat generated causes metallurgical changes that
differ for each alloy family. Such changes range from softening the metal to
making it very hard and brittle. The welding method can influence these changes,
but it is the fusion welding processes that cause the changes. These processes
include metal inert gas (MIG), tungsten inert gas (TIG), laser, and electron
beam (E-beam).
The effects tend to be less severe with laser and E-beam welding than with MIG
and TIG welding. Resistance and inertia welding cause minimal changes. Welding
effects and corrective heat treatments are summarized in Table IV.
Conclusion
The variety of material properties available in stainless-steel alloys enables
design engineers to develop medical instruments uniquely suited to the application.
Increased demands placed on medical instruments require materials that can provide
properties such as edge retention, wear resistance, and galling resistance.
Understanding a material’s properties is critical to identifying which
alloys should be considered for a particular application. A qualified metallurgist
or materials engineer can provide detailed information about material properties
as they relate to the specific instrument being designed.
William Fender is a medical applications engineer for Carpenter Special Products Corp. (El Cajon, CA). Robert S. Brown is a principal member of RSB Alloy Applications LLC (Leesport, PA).
Copyright ©2005 Medical Device & Diagnostic Industry
