What’s Your Veterinary Instrument Manufactured From?
You may wonder why veterinary instruments are mainly manufactured from stainless steel, however, that ‘stainless steel’ can be prone to rust and corrode if it is not cared for correctly.
There are different chemical compositions used for each type of instruments. They can be categorised into different groups and the British Standards for the manufacture of surgical instruments breaks these down(BS 5194 : Part 1:1991).
As a basic guide. here are a few and their descriptions for veterinary instruments:
Hardenable (Martensitic) Stainless Steel:
AISI 420S29 EN56B – artery forceps, needle holders, dissecting forceps and clamps – things that need a bit of flex and give. So common vet instruments would be things such as Olsen Hegar Needle holder, Gillies Scissor Needle Holder, Spencer wells, Rochester Pean, Dunhill artery forceps and Spencer Wells Artery forceps.
AISI 420S37 EN56C – Bone cutters, punch forceps, some scissors – anything that needs to hold an edge. The most common examples here are those popular blunt edge curved scissors and Mayo and Metzenbaum Scissors. These stainless steel has slightly more carbon in than the steel used for artery forceps etc, so it can be hardened to allow it to hold a cutting 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. The common examples here may be obvious in small animal dental, such as coupland chisels and coupland gouges,
As a general rule, the harder an instrument is required to be for performance i.e. 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 with correct repair and refurbishment processes.
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 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 a veterinary instrument to re-process by washing and autoclaving, without any problems i.e. rusting and corrosion, is a very good sign of its quality, however there are many factors such as the washing process 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; water stains and mineral deposits left on your instruments can kick start corrosion.
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.
Every piece 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. As a manufacturer of orthopaedic implants and veterinary instruments we go one step further and also have third party testing carried out on our stainless steels, to ensure they are to the correct specification.