AISI (American Iron and Steel Institute) has identified the various formulations of steel
and catalogued using a number system. Various materials
used in surgical instrumentation have AISI numbers, like AISI 304, AISI 316L,
AISI 410/410X, AISI 420A, AISI 420B, AISI 440C.
ASTM (American Society of Testing and Materials) establish standards and specifications
for numerous materials, both metallic and non-metallic.
Some materials have an AISI number and some ASTM number.
Series 300 Stainless Steels are Austenitic or non-hard enable or non-magnetic.
The carbon content is as low as 0.08% that means these metals cannot be heat
hardened and will not hold a strong cutting edge. However, it may be 'work hardened'
through machining or forming. The presence of almost 18 % chromium and 8 % nickel
makes these metals highly corrosion resistant and guarantee a brilliant shine.
The term 18-8 stainless is used to describe this series.
Series 400 Stainless Steel alloys are magnetic type and heat hard enable. These have
relatively high carbon and low chromium percentages, along with traces of manganese (Mn),
silicon (Si), sulphur (S), phosphorous (P), nickel (Ni) Molybdenum (Mo) etc endowing the
metal with specific properties. Their use is a necessity whenever strength and temper are
required. The degree of hardness depends largely on the carbon level. This series is used
in most O.R quality instruments, cutting and non-cutting, because it has a high tensile
strength, can hold edge for a long time and is corrosion resistant. However, as there is
little or no nickel in this series it is susceptible to corrosion if not looked after properly.
BS 5194: Part 1 DIN 58298 ISO 7153-1
This is a relatively modern British innovation that has become in universal use for steel
instruments. It reaches the factories in the form of bars and sheets. Looking at these
lengths of rough and dark gray material, one wonders how it will be fashioned into
beautifully finished instruments. Such a transformation reveals craftsmanship at its best.
STAINLESS STEEL TYPE AISI 304:
This material is easy to machine and form. Instruments or components that do not
need to be hardened, such as speculums, retractor blades, hollow handles,
hospital hollow wares, nuts, screws and pins etc, are usually made from this grade.
STAINLESS STEEL TYPE AISI 316L:
It is a very high purity alloy. 'L' stands for low carbon. It is also called SMO. It is a
preferred material for making most orthopedic implants, such as plates and screws,
nails, staples, prostheses and aneurysm clips etc. Certain instruments that have
prolonged contact with
implants also utilize this material to avoid transfer of dissimilar metal particles
which could cause galvanic corrosion reaction on the implants.It is however important
to know that metal implants and prostheses can sooner or later fail since no metal
has yet been developed that will equal the revitalizing durability of living bone.
STAINLESS STEEL TYPE AISI 410:
This is a most commonly used grade for the manufacture of non-cutting instruments.
Typical examples are hemostats, assorted forceps, and retractors and so on. It is easy
to work with and has Rockwell Hardness in the range of C-40 to C-45. It is a misconception
that this grade is inferior in any way as it clearly complies with the British, German and
International Standards for application in surgical instruments.
STAINLESS STEEL TYPE AISI 420 & 440:
These are useful grades for cutting instruments such as scissors, knives, chisels, ronguers,
bone cutters, bone drills and taps etc.
AISI 420 can get Rockwell Hardness in the range of C-48 to C-50 but if hardness around C-55
is required then grade AISI 440 with more carbon is appropriate.
Type ASTM F67 - Commercially Pure grades 1, 2 and 4. It has the same strength as steel but
weighs only 60% of the steel. It is highly corrosion resistant but very difficult to work with.
Some microsurgery instruments, mandibular implants and weight bearing hip prostheses
are made from it. Its biocompatibility is far more superior to any other currently employed
implant material. It can be finished in a distinct blue color but repeated autoclaving may give
it a reddish tinge.
GRADE GC20 This is the hardest man made material, sometime also referred to as diamond.
It has a Rockwell of C-86. Cemented carbide is often called hard metal. Its inserts are sintered
in the jaws of high wearing instruments such as needle holders, scissors and wire cutters.
Traditionally these instruments are partly gold-plated for recognition.
There are also many non-ferrous materials that are used in instruments for their specific
properties. They may be easier to machine or form and more suitable for procedures or budget.
It is 92.5% pure silver (Ag). Current uses include trachea tubes, probes for eyes and hemostatic
clips. Silver will turn black with oxide. This does not interfere with the function of the instrument.
A bit of rub with or without silver polish or elbow grease will bring the original shine back.
These instruments should be handled carefully as they are expensive as well as delicate.
An alloy of silver and nickel that is less expensive and is used as an alternative to Sterling
Silver. It is also more durable and does not turn black after autoclaving.
BRASS AND COPPER:
Free cutting brass type 360 and nickel-copper Class A or B are easily formed, molded and
welded therefore useful for making instruments that are otherwise hard to make from steel
or require malleable features. Certain instruments made traditionally in these soft metals
are proctoscopes, malleable uterine sounds and retractor blades, catheters and cannulas.
In order to enhance surface wear these are usually plated.
There are many hard and soft grades. With its excellent heat dissipation capacity along
with durability and lightweight it is employed for making the container system for
sterilization and storage. We offer a durable yet economical system. Aluminum
requires a protective layer by hard anodizing that can also be in different colors for
identification. Some instrument handles, mallet heads and splints are also made
Rubber, Plastic, Silicon and Tufnol are increasingly used in a wide variety of application,
both reusable and disposable. Some of the useful characteristics are toughness,
flexibility, resilient, resistant to chemicals, insulating, no moisture absorption, heat
deformation, high impact strength, easy fabrication and low cost etc. In fact the
possibilities are endless. Plastic items are gradually enabling a new wave of
convenience and cost reduction in the operating room, a trend that will undoubtedly
continue to grow with the passage of time. The use of latex rubber, however, has been
discouraged because it contains certain protein that is said to cause skin allergy to some
patients and hospital staff. No significant hazards are expected from silicon products under
normal conditions of handling and use.
Carbon steel and non-ferrous metals instruments are protected from staining or discoloring
by plating of various types. Until the advent of stainless steel most surgical instruments
were plated. However many instruments are still more economical or effective to make
plated. Mixing of plated instruments with stainless steel instruments in the reprocessing
cycle should be avoided as it may result in transference of chrome to stainless that will
look like stains. All plated instruments should be routinely checked to ensure there
are no cracks or worn areas in the surface. In recent years the use of ultrasonic cleaning
has hastened the demise of plated instruments as it tends to strip the plating from the
Instruments or components made of copper or brass can be plated with silver by
electrodeposited coating. To enable the final silver deposit to adhere it will follow a primary
coating of silver strike solution, nickel or copper plating. The silver plating shall not be less
than 0.0010 inch in thickness. It is especially suitable on malleable instruments because
the layer of silver plating is so thin it does not crack or chip when instrument is bent.
Silver plating may turn black with silver oxide and may be given a good rub to get
the shine back.
Nickel adheres very well to carbon steel and is not removed easily. The surface is
made free from all flaws or defects which will be detrimental to the final finish.
Cutting edges are ground to remove all plating. There are not many instruments made
with carbon steel these days, except perhaps for veterinary use or some general-purpose
scissors. Moisture will act rather quickly to first stain and then corrode these instruments.
Carbon steel, copper and brass are all base metals that can be chrome plated.
Finish on all edges and surfaces shall be uniform and free from burrs, sharp edges
(except where required), crevices, grind marks, rough areas, cracks and overlaps.
Chrome plating often has a layer of copper or nickel underneath and therefore protects
instruments longer from corrosion and discoloration.
BLACK CHROME PLATING:
Black finish makes work easier under microscope or extra lighting because of lack of glare.
Until recently, the black or ebonized finish was achieved by dipping in a sodium salt solution
and then baking in oven. These salts, however, have now been reclassified as very toxic and
therefore unsafe to use. There are also some very strong disinfectants also now available t
hat can strip this type of black finish completely. We have therefore abandoned this process
and may only be able to offer instruments ebonized in Europe if necessary.