Composition of Surgical Stainless.

By C.Milligan Date 07-12-2004 11:47

Greetings,

I am interested in learning more about the requirements for Austenitic stainless steel alloys that are classified as "Surgical-Grade." I have searched the forum and caught a brief thread from another member that directed me to this section of the FDA's site: http://www.fda.gov/ora/science_ref/bulletins/lib3994.html It simply states: "The minimum chromium level is 11.50 % or greater for all classes and types of sugical stainless steel with the exception of Precipitation Hardening steel of type XM-16 which can be as low as 11.00 % chromium. " I have a client that is requiring a repair/modification with "Surgical-grade stainless steel." Thet go on in their GMP policy code to describe acceptable materials for this area to be type 316, and 316L. The part about "Surgical-Grade" was only spoken word, and not policy. So the decision about what material to use is simple... Follow the customers written policy. In fact the client actually mentioned that the phrase "Surgical-grade" had sort of become a colloquialism at their facility and that it had become synonomous with type 316 stainless. They are not alone however (unfortunately)... Several contacts in the pharmaceutical and medical fields have tossed around the same "Metallurgical slang."
If I understand correctly type 304 has a typical percentage of between 18 and 20% chromium, and type 316 is 16-18%. That could allow type 304 to be considered "Surgical-grade" could it not? What is actually used to determine whether or not a particular alloy is considered "Surgical-grade?" (Especially austenitic alloys.)

Best regards and thanks!

Chris

By swnorris (****)

Date 07-12-2004 15:18

Hi Chris,

As you stated, Type 304 has a chromium content of 18-20% as opposed to a 16-18% content with Type 316. The nickel content in Type 304 (8 -10.5%) is lower than Type 316 (10-14%). The presence of chromium gives resistance to corrosion and tarnishing, and the presence of nickel increases the corrosion resistance. For what it's worth, the low end total percentages of chromium and nickel are equal (26%) for both types, but the high end total percentages for Type 304 are lower (30.5%) than Type 316 (32%). Then, consider the addition of about 2.5% molybdenum to Type 316, which gives it better overall corrosion resistant properties than 304, which contains no molybdenum. Therefore, Type 316 is usually the next step up if Type 304 does not provide enough corrosion resistance. There is a paragraph on the link below under "Austenitic" which seems to support your notion, as it mentions Type 304 surgical stainless in the last sentence:

http://chemistry.about.com/cs/metalsandalloys/a/aa071201a.htm

By Bonniweldor (**)

Date 07-12-2004 22:14

I intuit that "surgical grade" stainless is informal nomenclature. If the surgical implement intends to be rigid or hold a sharp edge, a 300 series material does not appear to be best choice, unless, perhaps, the material is severely work hardened before forming into the final implement shape. The 17-4 PH material would seem to be preferred for rigid sharp implements. If 304 is actually used for the surgical implement application, 316 appears to offer only little more.

By andy

Date 07-13-2004 10:17

The Handbook of stainless steel (publisher McGraw HIll editors Peckner and Bernstein 1977) has a chapter on stainless steels in medical devices. I'm sure you'll find similar advice in the ASM metals Handbooks also.

Stainless steels in the medical field fall into two brad categories: those used for implant, screws etc and stay in the body (biocompatability) and those that are used for instruments, scalpels and the like. For cutting edges, martensitic stainelss steels (400 series) are generally used.

In the HofSS, in the 1960's ASTM F4 sub committe was formed to standardize materials use din surgical implants and standards have been published. The recommeded alloy chemistry is not identical to AIDSI standards so grades 316 and 317 may fit the specification. There doesn't seem to be any mention of a 304 grade.

The chemical composition quoted is:

C 0.03 max, Mn 2.00 max, P 0.03 max, S 0.003 max, Si, 0.75 max, Cr 17.0-20.0, Ni 10.0 - 14.0, Mo 2.0 - 4.0, Fe balance. The designations in this chapter are 316LVM (I think the VM refers to vacuam remeklted.

If your client is in the medial field then clearly you need to provide a 'surgical grade' material that conforms to the corre6tc spec. If the client is not involved in the medical field then probably 316L may be suited but depending on the service environment even 316 may not be suitable. That as they say is another story...

Useful??

By C.Milligan Date 07-13-2004 11:37

Thanks everyone!

It seems as though the material composition is going to be different depending on the intended use. (That is logical enough...) I suspect that there is a subjective test that different materials must pass to be considered for use as an instrument or implant. It would make sense that the user would be more concerned more with alloy performance than composition. The composition data may be derived emperically from materials that simply met the performance criteria.
In the case of my customer, they admitted that "surgical-grade" was a misused moniker. Their requirements are for type 316, or 316L and simply require material test reports (MTR's), passivation and analysis. They are a pharmaceutical manufacturer and not a medical facility. In fact the above requirement is only for "product-contact" and "production areas". Many of the items we supply are simply glass bead finished T304.

Thanks again to all who responded!

C.Milligan