Metals Used for Surgical Instruments

Out of all the materials available on the earth, only a few satisfy the design and regularity requirements for use in surgical instruments. In this article, we will evaluate the important properties needed to be considered for surgical instruments.

Mechanical Properties

We know that different metals have very different properties. Finding the best properties to suit the needs of each type of surgical equipment is vital. Some of the issue to consider include:-

Ductility and malleability

The metal must be relatively malleable so it can be shaped without causing flaws. However, it must not be too malleable as it will need to hold its shape once manufactured. Depending on the instrument/application, the metal used may also need to be fairly ductile. Many surgical instruments are long and thin, e.g. scalpels, forceps, scissors, etc. They should not break off during the operation. The lowest modulus required for surgical instruments is 100GPa.

Ability to withstand high temperatures

Surgical instruments are subjected to elevated temperatures during the sterilisation process. The instruments are sterilised by scolding hot steam at a minimum of 121 degree Celsius temperature for at least 30 minutes. There should be minimum dimensional change in the instrument after repeated sterilisations.

Wear and tear

Surgeons require instruments that perform correctly every time they are used. Wear and tear affects surgical instruments as after a certain usage they will not perform as required. For example scissors may become blunt and not cut tissue easily.

Resistance to corrosion

Surgical instruments are increasingly exposed to bodily fluids, tap water and cleansers. Bodily fluids such as blood or pus generally contain chloride ions, which can corrode the instruments. They are also washed under the tap water to remove bodily fluids from them. The water may contain high concentration of minerals such as chlorine, sodium and magnesium. These can cause severe stains and corrosion if left to sit on the instruments. Hospitals generally use distilled water to partly eliminate the problem.

Various cleansers are used to clean the instruments from bacteria and infections. They could be either an alkaline solution or an acidic solutions with varying pH. This acidic/alkaline solutions may, over time, corrode the surgical instruments.

Biocompatibility

In simple terms, biocompatibility is a material’s compatibility with living tissue. Biocompatible materials do not produce a toxic or immunological response when exposed to the body or bodily fluids. Elements such as nickel, chromium, and cobalt are likely to cause allergic and tissue hypersensitivity problems.

Magnetic Properties

Surgical instruments are sometimes subjected to a magnetic field in the operating room such as a from an MRI, which generates a magnetic field of ~1.5 Tesla. This magnetic field can affect surgical instruments in numerous ways including:-

• Unwanted movement caused by the magnetic field interactions (i.e. the missile effect)
• Heating of instruments because of radio frequency (RF) power deposition
• Artifacts associated with the use of the instrument

Non-magnetic instruments such as stainless steel, titanium alloys etc are recommended in such environments.

Glare

By anodizing the surgical instruments, manufacturers change the surface properties of the metal. This makes the surgical instruments non-reflective. When the surgeon/lab operators are working under operating lights or a microscope, an anti-glare surface is important.

Ergonomics

Lightweight surgical instrument are simply easier to handle, especially during long surgical procedures, dissections or when performing repetitive tasks. This means that titanium surgical tools are more suitable than their stainless steel counterparts.

Metals Used for Surgical Instruments

There are a range of different metals used for surgical instruments because of their individual characteristics. These include:-

Stainless Steel

Stainless steel is the most commonly used metal for surgical instruments. It is a highly corrosion and rust resistant alloy as well as being strong and durable. Stainless steel is also non-magnetic and has an excellent strength to weight ratio, making the instruments easy to use during surgeries.

The choice of steel is determined according to the desired:-

• Flexibility
• Hardness
• Tensile strength
• Malleability

These requirements are based on the operation to be performed using the surgical instrument. Some types of steel can be hardened, others types can’t, depending primarily on the carbon content of the steel. The alloy is composed of varying amounts of iron ore and chromium. The large quantities of chromium gives the steel its “stainless” properties. The chromium forms a thin layer on the surface, known as a “passive layer,” which protects against corrosion.

The most commonly used steel composition in surgical instruments is as follows:-

Carbon	0.17-0.25%
Silicon	1.0%
Manganese	1.0%
Phosphorous	0.045%
Sulphur	0.043%
Chromium	12.0-14.0%

It is not difficult to see why stainless steel is so popular but there are also other metals used in the production of surgical instruments.

Tungsten Carbide

Tungsten carbide (TC), an alloy of tungsten and carbon, is used in the manufacture of various instruments. These include:-

• Needle holders
• Scissors
• Pin cutters
• Pliers
• Wire tighteners

Since the TC is harder than the steel, it offers exceptional durability. Usually the TC is soldered or welded to the jaws/working ends of instruments. TC inserts that are soldered can be separated from the instrument and replaced.

Aluminum

Certain instrument parts and cases are manufactured from aluminum, which is lightweight. Aluminum is treated with an electrochemical process called anodization which forms an oxide layer on the surface of the aluminum. The oxide layer offers great corrosion resistance. Certain cleaners, disinfectant solutions and abrasive brushes can damage the protective layer.

Titanium

Titanium is becoming more widely used, particularly in the manufacture of implantation devices. These are used to repair fractures, e.g. plates and screws as the metal has extremely high biocompatibility. It is also used for microsurgical instruments, where its light weight is an important factor in avoiding surgeon fatigue. Titanium also has anti-glare properties.