When we talk about materials in surgical instruments, we must take into account that there are some characteristics that all surgical materials need to fit. As we can see in the article ‘Which metals are commonly used for surgical instruments?’, when we talk exclusively about classical surgical instruments, it is clear that they have to accomplish certain requisites regarding safety, durability and mechanical properties.
But surgical techniques and the medical industry are changing so fast that the old criteria are becoming more flexible and there are some exceptions in specific situations.
Steels used in surgical tools
The most commonly used material for surgical tools is, without any doubt, stainless steel (inox). To be considered surgically suitable, it has to contain at least 13% chromium . This is because of its relatively low price, good corrosion resistance and mechanical properties.
One of the most used grades of stainless steel is grade 316L (the low carbon variant of 316). It is alloyed with molybdenum, giving the tool a better corrosion resistance, which is important in the sterilization process. This is a classical method, exposing the tool to pressurized, saturated steam, typically at 134 °C for three minutes or 121 °C for 15 minutes in an autoclave). This stainless steel is non-hardenable.
We need to take into account the application of the tool. For example, for cutting instruments, like tissue scissors, we would choose high-carbon steels (HCS) alloyed with chromium: types 440 and 420. These don’t have the corrosion resistance of 316L but are hardenable. HCS are extremely strong yet more brittle. They offer better improvement in mechanical properties from heat treatment and longer service life than medium-carbon steels.
Nowadays, there is a broad spectrum of proprietary stainless steels with subtle differences in composition that provide special features for a specific application. Some examples are the Sandvik 4C27A medical strip used for surgical needles, the CHRONIFER M-15 X (431 (X)) used for pliers, pincers or scissors, the Dumostar which is more elastic, being perfect for tweezers and forceps or the Dura 4110 used for cutting surgical tools.
Combining materials for advanced surgical tools
When we need to add characteristics to an instrument, there is the possibility to change only the part that needs it. One example could be for improving grip (like for the Kelly forceps). These are usually serrated with tungsten carbide inserts.
There are some further cases in which the industry has successfully adapted to the necessities of the surgical field. One great example is the scalpel. Until 1915 these were built only in one piece, but the sterilisation method of that time dulled the blades and there was the necessity of having a very sharp instrument. Hence, Morgan Parker invented the two-piece scalpel that we know today, in which the reusable handles are made of 316L stainless steel and the replaceable blades are made usually of 440 high-carbon steel (carbon content up to 0.6-1.2% by weight)  or subtypes of martensitic stainless chromium steel .
Expanding the range of materials available for surgical tools
Nowadays, new steriliszation methods are widely spread (like plasma sterilization), so it is not required that the tool tolerates high temperatures anymore . This fact has led to the penetration of polymers and plastics in the surgical tool industry, leading to the implementation of a wide spectrum of new concepts for surgical tools and new techniques to fabricate them. One great example is the use of 3D printed personalized surgical guides to lead the surgery .
Materials requirements for biomedical implants and prosthesis
In this section, we again see stainless steel type 316L, because its price is a common choice for biomedical implants. We have to note that for medical implants, however, it is necessary that the final product follows sanitary standards to avoid bacterial growth and toxicity derived from the implant material.
This is covered by the ASTM F138 standard for surgical implants . So, generally required is what we call stainless steel 316LVM (UNS S31673), which is nowadays only achieved by vacuum melt. We also need to have in mind that 316L is an alloy with nickel, therefore it can cause allergic or immune reactions in certain patients .
Because of the necessity for greater durability and mechanical stress resistance required for prostheses and some implants, a new actor comes into play: the titanium alloys. Ti-6AL-4V grade 5 has its greater strength, and Ti-6AL-4V ELI grade 23 has lower O2 content, an improved ductility and fracture toughness with some reduction in strength. The advantages of using titanium over stainless steel are greater biocompatibility, greater fatigue strength, greater corrosion resistance, lower weight and lower Young’s modulus.
The latter is beneficial in avoiding a misbalanced stress distribution in the peri-implant area, caused by the high differences in YM between bone and prosthesis. The grade 23 is already validated for detailed surgical procedures. Another alloy, Ti-6Al-7Nb, is also attractive as it is more bio-compatible due to the absence of vanadium .
For medical implants, the high-cycle fatigue strength (avoiding cracking thus will be the determinant factor for fixation) is more important than fracture toughness (which is only important when a crack has already occurred, therefore the toughness should not be a selection-limiting factor) .
We have to notice again that, aside from the application, the environment in which the implant is going to be used also determines the material required.
For example, a heart valve prosthesis is in contact with circulating blood, hence it is very important to avoid the blood clotting on its surface. For this purpose, a good example of a coating would be the On-X Pyrolytic Carbon.  It is important to avoid the displacement of such valves, so if they are secured to the surrounding tissue with a polyethylene terephthalate mesh, this allows for the body’s tissue to grow while incorporating the valve .
Some attempts have been made with ceramic materials. One great example being prostheses with yttrium tetragonal zirconia polycrystals (3Y-TZP). After a series of failures in 2001, their use in orthopaedic surgery has been reduced by 90% . These materials, nevertheless, are widely used in dentistry for the fabrication of dental crowns and fixed partial dentures .
There are also new techniques of production bursting in the implant/prosthesis production, again we see the example of the 3D printing used to create personalized implants/prosthesis for our patients even using metals like titanium  or bio ceramic-polymer composites to produce Hydroxyapatite scaffolds that will be colonized by bone cells .
As the medical industry evolves, new materials are emerging that provide new features that make them more suitable for certain specific applications. These include better resistance to corrosion, greater hardness, greater tenacity, etc. Likewise, these enable new therapeutic options such as 3D printing of custom implants.
Normally, these materials are combined with more traditional materials that are still widely used for their proven applicability, their low price and their pre-existing presence in the industry.
“By writing articles for Matmatch, I am able to give a part of me to society by sharing knowledge and inspiring new ideas”.
Hugo Herrero Antón de Vez
Doctor of Medicine (MD)
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