A-About Surgical Instruments
- Surgical instruments can be made from different types of alloys, such as stainless steel, titanium ...etc.
- The most common and economic alloy for manufacturing wide range of surgical instruments is the hardened stainless steel.
B-Metallurgical Background
- Stainless steel is a generic term commonly used to describe a group of Iron-based alloys which exhibit a phenomenal resistance to rusting or corrosion because of its chromium content.
- Stainless steel Types:
- 1. Ferritic:
- Ferritic alloys are obtained when Chromium is added to Iron to develop corrosion resistance without steel-type hardening. Ferritic alloys are mainly used for construction, and for use in automobiles.
- 2. Martensitic: (400 series)
- Martensitic alloys result when Chromium is added to steels with carefully chosen Carbon contents, which enables the material to be hardened by heating. They range from the least expensive and most widely used 410 grade, to 440-C grade which is used for knives and surgical tools e.g. forceps, scissors, rongeurs, curettes, trephines, knives and needle holders, due to its great hardness. Martensitic types offer the ideal combination of corrosion resistance, superior mechanical properties and resistance to abrasion and erosion.
- 3. Austenitic: (300 series)
- Nickel is the next most important alloying element, the most famous alloy being "18-8" containing 18% Chromium and 8% Nickel. Unlike the atomic structure of Ferritic or Martensitic Steels, Austenic Steel has a face-cantered cubic form made stable by the presence of Nickel. This gives the Austenitic Steels unique properties of workability, toughness, and corrosion resistance. Austenitic steel is used where cutting edges and strength are not required, such as retractors.
C-Standards for Surgical Steel
The surgical steel should be achieves bio-compatibility with human body .The PH and ionic conditions within the human body (body fluids)cause a relatively hostile corrosion environment for metals .The severe corrosive environment of the body is largely caused by the presence of chloride ions ,which have a highly corrosive effect particularly on stainless steel .So that ,the surgical steel must contain a percentage of Nickel ,to resist corrosion ,thus improving surgical steel ability to remain its integrity within the body's internal environment. Also the corrosion resistance and bio-compatibility of surgical steel depends greatly on its surface finish. This is due to the fact that; the finer the surfaces finish the higher the Chromium layer. Electro polishing has been shown to give the most superior finish and highest level of corrosion resistance, this is because it gives a superior surface finish and removes surface contaminants which can cause corrosion.