Improved Stainless Steels for Medical Instrument Tubing
By William Fender, Medical Application Engineer. P.E.
Carpenter Special Products Corporation [now Veridiam]
El Cajon, CA, USA
Robert S. Brown, Principal Member
RSB Alloy Applications
Leesport, PA, USA
New Demands on Surgical Instruments
Continuing advances in surgical techniques are requiring more instruments that use metal tubular components. The requirements placed on these instruments by the surgeon have become more demanding as the surgical procedures become more complex and confined. As a result, selection of the proper material from which to make the tubular component has become more complex. Proper material selection is critical to the development of the most cost effective instrument; one that not only performs as intended, but that can be fabricated in an economical manner. The optimum material is rarely the least expensive material.
Historically, enhanced strength UNS S30400 (AISI Type 304) stainless steel has been the material of choice for the tubular components in dental and surgical instruments. While this alloy has worked well for the first generation of instruments designed for use in confined spaces, it has some drawbacks that limit its usefulness for instruments employed in the more aggressive surgical procedures performed today. These drawbacks include loss of strength during welding, poor edge retention, poor wear resistance and poor galling resistance, all of which are desired in today’s instruments.
Stainless steel alloy development has yielded a number of stainless steels with materials properties that make them worthy candidates for medical instrument tubing. Each of these alloys offers enhanced properties over those offered by UNS S30400.
The following review of desired material properties and the alloys that offer them is designed to help the instrument designer select the optimum material.
Corrosion Resistance (Bio-Compatibility) of Stainless Steels
The first attribute of a material used for a surgical instrument is its corrosion resistance. Historically, the inherent corrosion resistance of stainless steel has been deemed adequate for surgical instruments. In today’s environment improved corrosion resistance is considered a must. It is crucial to consider not only body fluids, but pre- and post- surgical instrument cleaning techniques when determining the level of corrosion resistance required. Table No.1 shows the relative corrosion resistance of alloys that are considered to have acceptable bio-compatibility for medical instruments. When more corrosion resistance is required because of cleaning or sterilizing solutions, an alloy further to the right should be considered.
Strength and Toughness of Stainless Steels
Medical instruments designed today frequently are required to be thinner and longer. This design requires the use of materials with increased toughness, fatigue strength and tensile strength. All of these properties are interrelated. Generally, as the strength and hardness increase, the ductility and toughness decrease. The degree to which this occurs differs among the alloy families and to some degree within a family. Table No. 2 illustrates the combinations of properties and the alloys available to achieve them.
Edge Retention, Wear and Galling Resistance of Stainless Steels
When an instrument is used for cutting or shaping, edge retention becomes one of the critical material properties. If a cutting edge becomes dull prematurely, the instrument becomes difficult and potentially dangerous to use.
Wear and galling resistance are crucial when metallic parts move in relation to each other in an instrument. If wear or galling occurs during use, not only will the instrument stop performing properly, but it may also introduce metallic debris into the wound.
The edge retention or wear resistance of a metal is determined by the material’s hardness and how that hardness is achieved. Generally, as the hardness of a metal increases,> so do the edge retention, wear resistance and galling resistance.
However, the method by which the hardness is obtained is also critical. The relationship between relative edge retention, wear resistance and galling resistance, how they are achieved and a ranking of selected alloys is shown in Table No. 3.
The edge retention of a martensitic stainless will be better than that of a precipitation hardening stainless or austenitic stainless at the same hardness due to the wear resistance of the hard carbides in the martensitic stainless.
Effect of Welding
Frequently it is necessary to weld one or more components during the fabrication of a medical instrument. Consider the effect of the welding operation on the material’s properties and the processing necessary to overcome any negative during the instrument design and original material selection phases.
The heat generated when a metal is welded causes metallurgical changes which differ with each alloy family. These changes range from softening the metal to making it very hard and brittle. While the welding method can influence these changes, all fusion (Metal Inert Gas - MIG, Tungsten Inert Gas -TIG, Laser, and Electron Beam - EB) welding processes cause them to occur. These effects tend to be less severe with Laser and EB welding than with MIG and TIG welding. Resistance and inertia welding minimize these changes. These changes and corrective heat treatments are summarized in Table No. 4.
The wide variety of material properties available in these alloys gives the design engineer the opportunity to develop instruments that are uniquely suited to the application.
In large part, as the demands placed on an instrument by the procedure and surgeon increase, so do the demands placed on the material from which that instrument is made.
Some instruments, such as trocars, that are not subjected to high stress or torsional loads and are not used for shaping, frequently may be made from UNS S30403 (Type 304L) stainless steel.
Long slender instruments, such as drivers or arthroscopic instruments, are likely to have high demands placed on them. The increased strength and toughness of UNS S46500 (Carpenter’s Custom 465 stainless) are put to good use in these types of instruments. The alloy’s high hardness and resulting edge retention, while not as good as a martensitic stainless of similar hardness, is more than adequate for many cutting and shaping applications.
Cutting and shaping instruments, such as shavers or samplers, require an alloy like UNS S42010 (BioDur TrimRite) or UNS S42000 (Type 420) that is hard and has good edge retention. The wear and galling resistance of these alloys is beneficial in insuring smooth operation when the instruments contain parts that move in relation to each other.
The information discussed above is intended to assist the instrument design engineer in identifying alloys to be consider for a particular application. A qualified metallurgist or materials engineer should be contacted for detailed information about material properties as they relate to the specific instrument being designed.
* * *
Note: Carpenter Special Products Corp. is now known as Veridiam and is not affiliated with Carpenter. Their website is www.veridiam.com.
Carpenter Special Products Corporation (CSPC) is a subsidiary of Carpenter Technology Corporation, based in Wyomissing, PA. CSPC is a major mill manufacturer of custom metal tubing and tubular components, including stainless steel and titanium tubing, and precision shape products used in medical tubing, aircraft tubing, nuclear tubing and industrial tubing applications. Carpenter Technology is a leading manufacturer and distributor of specialty alloys and various engineered products. More information about Carpenteris available at www.cartech.com
RSB Alloys Applications, LLC offers consulting and training in materials selection, and application and fabrication for medical devices and instruments and industrial applications. Extensive metallurgical experience with corrosion resistant alloys, combined with an understanding of mechanical design concepts and manufacturing technologies, ensure sensitivity to all aspects of device/instrument design.
Note: Custom 465, Custom 455, Custom 450, BioDur and TrimRite Reg. U.S. Pat. & Tm. Off. to CRS Holdings, Inc., a subsidiary of Carpenter Technology Corporation. 17-4PH is a registered trademark of Armco.