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Stainless Steel Podiatry Instruments

The ideal material for surgical instruments would be shock resistant, economical, readily available in a wide range of thickness and finishes, able to be keenly sharpened to a fine edge, possess outstanding corrosion resistance, have no distortion after heat treatment and have high wear capability.
Unfortunately, it is not possible to have all these qualities in one material. Therefore a compromise has to be reached. Different instruments require different steel grades. A cutting edge would be made from softer steel than an instrument used for grasping.
Although other grades of stainless steel are sometimes used, 420 grade stainless steel provides a good compromise for surgical instruments. The comparable grade to 420 Stainless in German Steel is X20Cr13. 420 grade stainless steel is more corrosion resistant than carbon steel and can be sharpened to a keen edge. It is resistant to the atmosphere, fresh water, dilute acids and alkalis, and fruit and vegetable juices. 420 stainless steel needs to be heated above 1600F to avoid cracking when forging the steel and be continually reheated above this when working with it. As a result, 420 grade stainless steel will not withstand robust work at colder temperatures, as it is likely to crack.
It is worthwhile for a medical practitioner to understand what makes stainless steel corrosion resistant. Note: it is “corrosion resistant” not “corrosion proof” – i.e. stainless not stain proof. The alloy elements in stainless steel form a thin transparent passive layer on the surface of the metal. Although it is only a few atoms thick, it instantaneously reforms in the presence of oxygen from air or water, so even if the material is scratched or damaged the passive layer continues protecting the surface from corrosion. However, it must have time to form this new layer over the scratch. If the scratch is contaminated by another substance such as a chemical before it has had time to “heal” itself, the area will begin to corrode. Some chemicals will actually attack the passive layer itself if given a foothold. This explains why stainless steel instruments do not require any other coating or corrosion protection and why they are able to remain bright and shiny even after decades of use. In order to provide this layer of atoms the steel must go through a passivation process. When the parts of the surgical instrument are first made they do not have a thick passive layer. However, after exposure to the air, the chromium and iron present in the stainless steel are oxidized and this forms a slightly passive layer. By further treating with chemicals, usually nitric acid, a thicker passive layer is formed that also removes some of the iron particles from the surface but leaves the chromium behind. Chromium is the principal metal responsible for passivation. After this process the instrument parts are ready to be assembled. The amount of chromium and iron oxide varies depending on the type of stainless steel. Layers with more chromium are generally more passive, that is more resistant to corrosion.
Scratches to instruments can occur through improper handling or by using unsuitable cleaners with a high acidic or high alkaline content. Destainers if misused will eventually erode the passive layer and soil residues that are allowed to dry on the instrument can cause damage. When instruments are left overnight in disinfectant this can cause damage. Chlorine compounds and sodium chloride (saline solutions) are very damaging to stainless steel and instruments should not be exposed to chlorine for extended periods.
Residues from previously used instrument wraps are often overlooked as a cause for damage to the instruments. Responsible staff should check to see that reusable wraps are properly rinsed so that residues that are high in alkaline or acid do not remain in the wrap. A simple test can determine if the wraps are acid or alkaline in nature. Hard water areas are prone to cause damage to instruments by forming deposits on the instruments after using detergents that are not hard water tolerant.

Manufacture of surgical instruments

Most instruments have three stages of manufacture – forging, fitting polishing.
Forging
The stainless steel arrives at the factory in the form of bars where it is first cut to the general size of the instrument and formed into a rough forging which resembles the end shape.
Fitting
A fitter will then straighten out this rough forging and remove excess metal by grinding. He will shape the joints at this stage and put the serrations on the jaws using a special milling cutter. The pieces are then assembled to form the instrument. The instrument is then set correctly before they then go through two very important steps which will affect the quality for the finished instrument – hardening and tempering.
The metal is hardened by heating it to about 1600F and then cooling it rapidly in water. This treatment sets up strains in the metal which are then relieved by tempering, a process whereby the steel is reheated to a lower temperature. This results in a decrease of hardness to the desired level. Allowance has to be made for the different thickness of an instrument – for example not all of the shaft of a needle holder is hardened to the same level because of the different thickness of the shaft, and a different cooling rate is required which makes the shaft stronger where it needs to be. To avoid fractures at these points impact blasting on the surface relieves stresses and cuts down the chance of fractures without reducing the hardness. Impact blasting also results in an antiglare effect on the surface that some prefer.
The instrument is etched with any required identification marks, usually using laser-engraving techniques.
Polishing
Polishing is a process used to achieve a smooth surface on the instrument. It is extremely important to polish an instrument because the passivation process leaves microscopic pits where the carbon molecules were removed. Polishing also builds a layer of chromium oxide on the surface of the instrument. Through regular handling and sterilization the layer of chromium oxide will build up and protect the instrument from corrosion. In some circumstances, that is why you will notice older instruments less corrosive than new ones. The newer instruments have not had the time to build up the chromium oxide layer. To polish the instrument, blasting with very fine beads, using no more than 60 PSI pressure dry air will not abrade the passive oxide layer but simply indent it. Using this process results in a nice luster by scattering the light.

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