What is surgical steel? The role of stainless in healthcare

What is the difference between surgical steel and stainless steel?

Stainless steel is a low-carbon steel, which contains a minimum of 10.5% chromium. It’s the chromium that reacts with oxygen to form a stable oxide bond on the surface to prevent rust. The more chromium, the more resistant to oxidation. Stainless steel also contains some nickel, which lessens corrosion resistance, but this can be offset with the addition of molybdenum.

This bond is a film and if damaged, will self-heal. This is critical for applications in healthcare, as damage in the form of fissures present a breeding ground for bioload bacteria. Stainless steels are also easy to clean and nonporous, which helps maintain a hygienic environment. Chemically inert, stainless steel can also be safely sterilised without suffering corrosion or degradation.

The application of stainless steel is widespread in healthcare. Stainless steel in medical devices are also common, but not all stainless steels are suitable for the medical industry.

For information such as what is stainless steel and what are the different stainless steel grades, check out our Understanding stainless-steel grades guide.

What is surgical steel?

There is no formal definition of what constitutes medical-grade stainless steels. So then what is it? Is surgical steel the same as stainless steel? No.

The difference between surgical steel and stainless steel - also known as medical-grade stainless steel - comes down to the level of corrosion resistance. Surgical steel has the higher resistance. These steels contain a minimum of 13% chromium. Surgical steel is cured and contains at least 0.2% carbon. Nickel reduces brittleness.

Stainless steel surgical instruments should be curable and ductile. Then there are implant stainless steels, which should not be hardenable.

Implant-grade stainless steel

Surgical stainless steels are used for non-permanent implant devices. The nickel content can be a concern. Although generally considered hypoallergenic for most people, the issue is corrosion and wear, which releases nickel ions or fretting debris into the body. Note, stainless steels are vulnerable to crevice corrosion. This refers to oxidation between parts, such as plates and screws, which are common in implant devices.

Nickel, like chromium, are carcinogens. According to the science of toxicology, a material’s impact on our bodies depends on several points:

• The form of the material
• The volumes that we’re exposed to
• How we’re exposed to the material

The risk from nickel derives from exposure in its raw form, but once alloyed with steel, the risk is minimal. Likewise, the amount of chromium encountered by patients is also considered non-hazardous.

Still, nickel-free surgical grade stainless steel, such as BioDur® 108, is available to offset any potential risks. If the implant device you’re designing is permanent, look at plastics, which offer many excellent choices. You can learn about them in What are medical-grade plastics?

Common medical stainless steels

Let’s look at the most common surgical grade steels and stainless steels used in healthcare. SAE is the Society of Automotive Engineers who developed the stainless-steel specifications, or grades, that we talk about.

304 stainless steel

Stainless steel 304 is considered a medical stainless steel. This grade is similar to grade 316. The difference between 304 and 316 stainless steel is that 316 contains molybdenum, which enhances corrosion resistance, while 304 does not. Still, 304 stainless steel corrosion resistance is high, which is why 304 stainless steel tubing is common in healthcare.

316 and 316L surgical steel

316 stainless steel is used to manufacture heat exchangers, jet engine parts, exhaust manifolds and so many more applications. It’s also used in the medical field, offering excellent machinability and good resistance to pitting.

Then there’s 316L surgical stainless steel. The difference between 316 and 316L stainless steel is in the carbon content. To qualify as 316L surgical grade stainless steel, the carbon content can’t exceed 0.03% – hence the “L” in the stainless-steel code, which stands for low carbon. This makes it even more corrosion resistant than 316.

316L is a biocompatible stainless steel when produced to ASTM F138 / F139 standards. The metallurgical composition of grade 316L stainless steel means it’s a low-allergy material. It’s also non-magnetic, so it won’t interfere with sensitive equipment.

316L has outstanding properties for manufacturing. Much of this has to do with the amount of nickel involved – between 10% and 15% – which is why it has excellent formability, weldability and ductility.

420 stainless steel

This grade is not an implantable material. It offers good ductility, however. In its annealed state and when polished, hardened or surface grounded, it has outstanding corrosion resistance, with a minimum chromium content of 12%. Grade 420 is magnetic in both the annealed and hardened conditions, although it is never used in the annealed condition. Under hardened conditions, 420 resists fresh water, alkalis, air, foods and mild acids.

Surgical steel grade 420 also has good strength and impact resistance. It’s also good for machining, thanks to its sulphur content of up to 0.03%.

440 stainless steel

Medical grade steel 440 is sometimes called a “razor-blade steel.” It exhibits excellent resistance to mild acids, alkalis, foods, fresh water and air. A smooth polished surface also helps grade 440 resist corrosion in tempered, passivated and hardened conditions.

Grade 440 stainless steel can be easily machined in its annealed state. Machining after hardening is extremely difficult, or impossible in some cases. There are two grade 440s used in the medical industry: 440B and 440C. The difference between the two lies in their carbon content. Both have a high amount of carbon, with 440C the greater amount, which is why this stainless steel is difficult to machine.

630 stainless steel

Another material not meant for implant devices. It has excellent wear resistance, which makes it ideal for surgical instruments. Grade 630 is also called 17-4 due to its surgical steel composition, which is approximately 17% chromium and 4% nickel.

630 grade stainless steel can be heat treated to high levels of strength and hardness. It also has good toughness in the base metal and welds, minimising warpage and scaling. Its corrosion resistance is similar to that of 304, while its low carbon content means it has excellent mechanical properties.

Medical metals: common applications for stainless steels

Grade	304	316	316L	420	440B	440C	630 (17- 4)
Orthopaedic implant devices	X		X
Dental implant devices			X
Surgical and dental instruments		X	X	X	X	X	X
Pharmaceutical equipment: sample & material handling		X
Stainless steel tubing	X
Bone fixation	X
Containers for hazardous materials	X
Wires	X
Curettes	X
Plates and screws	X
Prostheses	X
Syringes and needles	X
Catheters	X
Sensor probes	X
Hospital furniture	X	X

 

The details: Medical-grade steels

The chart below shows you the chemical composition of stainless steels typically found in the medical industry. The designations are SAE and EN, the standard for European stainless-steel grades, although SAE is also used in Europe.

Designation				Chemical Composition (Major elements only) % content is a maximum unless a range is indicated
SAE	EN			C, ≤		Mn, ≤		P, ≤		S, ≤			Si, ≤	Cr			Ni		Mo		N, ≤	Other Elements ≤, UOS
304	1.4301			0.08		2.00		0.045		0.03			1.00	18.0-20.0			8.0-11.0		–		–	–
316		1.4401		0.08		2.00	0.045			0.030	1.00			16.0-18.0		10.0-14.0			2.00-3.00		–	–
316L	1.4404			0.03		2.00		0.045		0.030			1.00	16.0-18.0			10.0-14.0		2.00-3.00		–	–
420	1.4021			0.15, ≥		1.00		0.04		0.03			1.00	12.0-14.0			–		–		–	–
440B	1.4112			0.75-0.95		1.00		0.04		0.03			1.00	16.0-18.0			–		≤0.75		–	–
440C	1.4125			0.95-1.20		1.00		0.04		0.03			1.00	16.0-18.0			–		≤0.75		–	–
630 (17-4)		1.4542	0.07		1		0.04		0.03			1			15.5-17.5	3.0-5.0		–		–		3.0-5.0 Cu; 0.15-0.45 Nb

 

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