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CIS Lesson Plans provide members with ongoing education in the complex and ever-changing area of surgical instrument care and handling. These lessons are designed for CIS technicians, but can be of value to any CRCST technician who works with surgical instrumentation.

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Lesson Plan CIS 215
A Primer on Surgical Stainless Steel
[Reprinted from Communiqué: September/October 2009]

LEARNING OBJECTIVES: Present background information about surgical stainless steel. Explain the three major types of stainless steel and discuss common types of corrosion. Describe the mechanical, magnetic, and cryogenic characteristics of stainless steel. Review steps in the fabrication of stainless steel instruments and note important material selection concerns. Summarize concerns important in caring for stainless steel instruments.

Surgical instruments are manufactured in response to the need for an ever-increasing variety of devices required by surgeons. Designs are interpreted by bioengineers, refined by instrument makers, and adjusted to address constraints imposed by the materials used, along with their costs and fabrication methods. Surgical instrument manufacturing is a tradition passed through generations and involves time-consuming manual labor. Choosing the right instrument material is an important issue for the manufacturer, and reprocessing surgical instruments requires special considerations for Certified Instrument Specialist (CIS) technicians. Stainless steel is used to construct many instruments, and those with background information about this material will understand the “whys behind the hows” of many processing requirements.

Stainless steel is the most common alloy (a mixture or solid solution of two or more metals) used for surgical instruments. Alloys are used for manufacturing surgical instruments because they have specific properties that make them more useful than pure metals. There are several types of stainless steel alloys and understanding their characteristics is critical to determine how they can be used.

Historical Overview

Stainless steel was introduced commercially by Brearley of Sheffield in 1912 and was used for rifling (spiral grooves) in gun barrels. The alloy contained 13% chromium, 1% nickel and 0.2% carbon, and would now be classified as a “low” alloy stainless steel. Shortly thereafter (1916), Mayer and Company was commissioned to make otolaryngological instruments (those used for ear, nose, throat, and head/neck procedures), which appear to be the first non-rusting steel instruments.

Despite corrosion resistance, strength and suitability for thermal hardening and tempering, “low” alloy stainless steel provides an inferior cutting edge and requires constant sharpening. Also, when low-grade stainless steel is in temporary contact with human and other fluids, it can corrode. This promoted the development of “high” alloy (Martensitic and Austenitic) steels that are extremely inert within the body and can also be used to manufacture certain implants.

Stainless Steel and Corrosion

There are three main types of stainless steel.

• Austenitic – Austenitic steels are alloys containing chromium and nickel (and sometimes manganese and nitrogen). The most familiar stainless steel is Type 304, sometimes called T304. The high chromium content of austenitic stainless steel enables it to resist oxidation (scaling) and corrosion, and makes it more malleable and workable.
• Ferritic – Ferritic steels contain ferrite, iron and chromium. They are less ductile (able to be molded or shaped) than austenitic steels and cannot be hardened by heat.
• Martensitic – Martensitic steels contain a small amount of carbon and they can be tempered, hardened and sharpened. Martensite gives steel great hardness, but it also reduces its toughness and makes the steel brittle. This type of stainless steel is used when sharp cutting edges are required.

Resistance to corrosion is the primary reason stainless steels are used to manufacture medical instruments. However, they may corrode, and care is needed to select a grade suitable for a specific application. Corrosion can cause a variety of problems, including perforation (pitting), loss of strength, degradation of appearance, and deposits of scale or rust that can contaminate the material being handled.

Some types of stainless steel corrosion are caused by users:

• General corrosion – This is a uniform removal of material and is usually caused by a reaction to strong acid.
• Pitting corrosion – Under certain conditions, especially those involving high concentrations of chlorides, moderately high temperatures, and intensified by a low pH, localized corrosion that creates perforations can occur. Once a pit forms, it often continues to grow—even if the surrounding steel is still untouched.
• Crevice corrosion – The corrosion resistance of stainless steel depends on a protective oxide layer on its surface; however, this sometimes breaks down (often when instruments are improperly designed). Crevice corrosion is a severe form of pitting that occurs at significantly lower temperatures than pitting corrosion.
• Friction – Friction (rubbing of one object’s surface against that of another) occurs in articulated surgical instruments. Material fatigue can occur resulting in metal wear, cracks and breakage. Proper instrument lubrication may reduce friction.
• Stress corrosion – Cracking can occur from the combined influence of tensile stress (that caused by stretching or straining the metal) and a corrosive environment. The most damaging environment is a solution of chlorides and high levels of salts in water. Corrosion is accelerated because of the pressure applied.
• Sulphide stress corrosion cracking (SSC) — Stainless steel alloys can react with hydrogen sulfide when in contact with high sulfur materials and form metal sulfides and elementary atomic hydrogen. These, in turn, can diffuse into the stainless steel matrix and cause cracking.

Other types of corrosion may result from manufacturing defects. For example, intergranular corrosion is a form of relatively rapid and localized corrosion. It is caused by a defect in the stainless steel that allows carbide particles to form. These deplete the surrounding metal of chromium and reduce its corrosion resistance.

Corrosion is an electrochemical process that involves the flow of electric current, and galvanic corrosion can occur from the contact of dissimilar metals in an electrolyte (a substance capable of conducting electric current). This can be prevented by avoiding mixed metal fabrications or by coating or insulation.

Contact corrosion is a third example of a manufacturing defect. It occurs when small particles of foreign matter (especially carbon steel) remain on a stainless steel surface. They are likely to be quickly corroded away, but in severe cases a pit may also form in the stainless steel.

Characteristics of Stainless Steel

Stainless steel has special mechanical features. For example, it is strong and hard so it can absorb energy without breaking. As noted above, austenitic stainless steels are exceptionally ductile.

Stainless steels have a high temperature resistance. The high chromium content that is so beneficial to the wet corrosion resistance of these metals also contributes to their high temperature strength and resistance to scaling at elevated temperatures. The high temperature strength of materials is generally expressed in terms of “creep strength” (the ability of the material to resist distortion over a long term).

Galling resistance is an important property of stainless steel instruments, especially those made from two parts that are in relative motion (like scissors). When in use under sufficient load, an instrument’s protective oxide layer is disrupted which permits metal-to-metal contact. Under high stress and poor lubrication conditions, stronger bonds may form over a large surface area and create galling symptoms, including fractures, alignment problems, scratches, and metal particles.

Magnetic permeability relates to the ability of a material to carry magnetism, indicated by the degree to which it is attracted to a magnet. All stainless steels—except the austenitic group—are strongly attracted to a magnet. Some procedures such as magnetic resonance imaging (MRI) interventions require non-magnetic surgical instruments, so material selection for these interventions is crucial.

Austenitic stainless steels also possess a unique combination of properties that makes them useful at cryogenic (very low) temperatures. These steels have low temperature tensile strengths that reduce strains and ruptures, while their toughness is only slightly degraded. This property is useful for surgical interventions that use low-temperature action on the tissue being dissected and for the forging stage of instrument manufacturing.

Instrument Fabrication

A major advantage of stainless steels and the austenitic grades, in particular, is their ability to be fabricated by all the standard manufacturing techniques. The common austenitic grades can be folded, bent, cold- and hot-forged, deep drawn, spun, and roll-formed. The first (forming) stage of instrument manufacture may be done manually or mechanically.

The weld ability of various grades of stainless steels varies considerably. Nearly all can be welded, and the austenitic grades are some of the most readily welded of all metals. Examples of welded instruments include Frazier suction tips, Deaver retractors with formed handles, and some orthopedic reamers. Inspection of all welded points is essential to ensure the weld is intact to prevent instrument failure during a surgical procedure.

During the manufacturing process, stainless steels are often heat-treated by methods that depend upon the type of stainless steel and the reason for the treatment. Annealing (to make the stainless steel less brittle), hardening and stress-relieving restore desirable features such as corrosion resistance and ductility to metal that were altered by fabrication. Note: while austenitic stainless steels cannot be hardened by thermal treatments, they do harden rapidly by cooling to sub-zero temperatures.

The desirable corrosion-resistant surfaces of stainless steel surgical instruments can only be achieved if proper cleaning and finishing operations are carried out after the fabrication process is completed. The presence of any iron, cast iron, mild steel, carbon steel, or low alloy steel particles on the surface of stainless steel will promote pitting corrosion at the points where the “free” iron and stainless steel meet. This potentially serious (and certainly unsightly) problem most often occurs from the contamination caused by scraping the instrument with carbon steel tools or fixtures, or from grinding and polishing tools.

Passivation creates a protective film on the surface of steel. This process involves the removal of free iron by immersing the steel in an oxidant such as nitric acid or citric acid solution.

Material Selection for Surgical Instruments

Proper material selection is critical for the most cost-effective instruments. The material must perform as intended, and it must also be one that can be fabricated economically.

Historically, enhanced-strength stainless steel (type 304) has been used for the tubular components of 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. These include loss of strength during welding and poor edge retention, wear resistance and galling resistance.

Some instruments, such as trocars, that are not subjected to high stress or loads, and are not used for cutting or shaping, can be made from a basic stainless steel. Many newer instruments, however, require that the selected material provide strength and wear resistance.

Long, slender instruments, such as drivers or arthroscopic instruments, will likely have high demands placed on them. Increased strength and toughness are necessary, and some alloys provide the required hardness and resulting edge retention. Cutting and shaping instruments, such as shavers and samplers (biopsy punches/forceps or rongeurs), require a hard alloy with good edge retention. The wear and galling resistance of some alloys best ensure the smooth operation of parts that move in relation to one another.

Instrument Care

Proper care and handling is essential for the satisfactory performance of surgical instruments. Water used for reprocessing is one of the most important concerns in preventing instrument damage. Natural water contains different kinds of salts that, at higher temperatures, are not soluble in water and tend to form a hard scale. Tap (treated) water may contain chlorides that can cause serious corrosion. The pH level of water is also a concern. Acid or base water may influence the properties of cleaning agents, and corrosion may result from instrument exposure to acid or basic water. Therefore, a neutral pH is recommended for cleaning and rinsing of surgical instruments.

Conclusion

Modern stainless steel alloys allow the manufacture of surgical instruments that can ideally perform tasks required by surgeons; however, these devices can be damaged if they are not handled correctly. CIS technicians who understand the basics of surgical steels can apply this knowledge to help ensure that processing procedures prolong rather than decrease the useful life of the instruments for which they are responsible.

Endnote

The author wishes to acknowledge and thank Zamal Jackobson, Aesculap, Israel, for use of the lesson photos.

References

Kirkup, J. From flint to stainless steel: observations on surgical instrument composition. Annals of the Royal College of Surgeons of England, v. 75, 365-374. 1993.

Technical Handbook of Stainless Steels. The Atlas Specialty Metals, www.atlasmetals.com.au.

Pagounis, E. and Lindroos, V. Processing and Properties of Particulate Reinforced Steel Matrix Composites. Materials Science & Engineering, 246, No. 1-2, pp.221-234, 1998.

To learn more about instrument cleaning, decontaminating and disinfecting procedures, see the International Association of Healthcare Central Service Materiel Management’s Central Service Technical Manual, Seventh Edition, 2007 (Chapters 9 and 10).

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