Stainless-Steel Options for Medical Instrument Tubing

William Fender and Robert S. Brown

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

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.

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.

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).

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