surgical instruments and stainless steel

Here are a few interesting points we’ve pulled together regarding surgical instruments and stainless steel…

Different chemical compositions are suitable for different instruments. BS 5194 : Part 1:1991 gives a more in-depth breakdown for the uses, but as a basic guide, instruments should be manufactured from the following (a few examples are shown below):

Hardenable (Martensitic) European Steels:

AISI 420S29 EN56B –  artery forceps, needle holders, dissecting forceps and clamps – things that need a bit of flex and give

AISI 420S37 EN56C – Bone cutters, punch forceps, some scissors – anything that needs to hold an edge

AISI 420S45 EN56D – For knives, chisels, gouges and osteotomes. Small Ear, Nose and throat knives, scissors, super cut scissors, screwdrivers – these items need to be very hard

As a general rule, the more a harder edge is required to be on the instrument for cutting, the more carbon the instrument will contain. The carbon allows the steel to be hardened, so it doesn’t bend during use and also can be sharpened so it retains its sharpness for cutting. However, during maintenance this is very relevant, as you must ensure you maintain this hardness.

Chromium is the part of the composition of the steel that gives it the shiny chrome presentation. This also helps with rust resistance, along with nickel and the more chrome, the more likely it will be to resist corrosion.

A crucial part of instrument finishing is the Passivation Process, this is a Nitric Acid surface treatment to remove foreign bodies and oxidise the surface of the material to increase corrosion resistance. Passivation should only be performed by trained technicians, who have suitable protective equipment and knowhow.

The ability of an instrument to re-process through sterile services, without any problems i.e. rusting and corrosion, is a very good sign of its quality, however there are many factors such as AWD used and chemicals used for cleaning. This is particularly important, as most instruments are made from Martensitic Steels, and therefore have more chance of corrosion if for example, they are left wet. As an analogy, think of the staining on your cutlery in the dishwasher if left wet.

A copper sulphate test may be used to check for improper material selection and we may do this, if passivation indicates poor materials. This is also very useful, when we are processing repairs and refurbishing instrument trays, as it aids identification of poor quality steels, which may contaminate whole trays with rust. The test can also be used to check passivation has been carried out correctly, as it identified any free iron, remaining on the surface of the stainless steel, which would be a corrosion risk.

Any surgical instruments you purchase in the UK, which are CE marked should be correctly passivated and corrosion resistant. You can carry out a simple test which is recommended in the British standards for Surgical Instrumentation (BS 5194 series), to detect any free iron and chromium depletion.

What you need:

1. Copper sulphate solution

2. Distilled water

3. Ethanol

The instrument is manually cleaned and immersed in ethanol, then dried. Instrument is dipped in the copper sulphate solution for 6 minutes at room temperature, removed, washed with distilled water and wiped. You can then check for copper deposits, which if significant will show up as pink marks, where potential areas for corrosion are detected.

Every pieced of stainless steel that is used to manufacture a surgical instrument, should have a mill or certificate of conformity (manufacturers version of this). As a buyer, this may tell you where the steel came from, to what grade it is, its exact composition but ultimately the stand it is manufactured to. Any company that is manufacturing and selling instruments should be able to present you with information such as standards instruments and materials are manufactured in accordance with.