metal ion volume in heat treated versus as-cast MOM devices

[ErinSD]

I'm looking to validate some information I've received. I could not find the answer in the FAQ or past posts; please accept my apologies if this is a dumb question.

In metal-on-metal applications (in this case, a hip replacement prosthesis), are more metal ions generated when the two metal components are "heat treated" versus when the metal components are "as cast"? Are there any other factors to consider when attempting to reduce the metal ion volume in general?

Thanks for any information and/or pointers to other sources for this answer.

- Erin (a newbie) in San Diego

 

[kenvlach]

Erin,
It's not really clear to me what you are asking or what your concerns are. Are you perhaps concerned with bicompatibility, and thus, oxidized nickel on the surface which may be leached?

Strictly speaking, neither heat treating nor casting produces any metal ions, only metal atoms (except when combined with nonmetals such as carbon, oxygen and sulfur). However, heat treated surfaces as well as castings are typically covered with some amount of oxide depending upon whether a non-protective or protective atmosphere or a vacuum was used during processing. In any event, the surface would be cleaned and passivated or given a biocompatible coating before implantation. There are many applicable standards, e.g., ASTM F86, 'Standard Practice for Surface Preparation and Marking of Metallic Surgical Implants.'

http://www.astm.org/cgi-bin/SoftCart.exe/DATABASE.CART/REDLINE_PAGES/F86.htm?L+mystore+nswg5662

 

[ErinSD]

Thanks for replying. Sorry I was not more clear. My understanding is that when the two components of the metal-on-metal (MOM) hip replacement device (the hip socket side and the femoral head side) rub together when in use in the body (walking, etc), metal ions are generated. I've seen numerous studies indicating this, and specifically stating that metal ion counts are highest in the first two years that the implant is in use. These studies generally admit that we don't know if the resulting increase in metal ions in the body are a problem or not.

Just in case they are, I'm looking for a MOM device (which includes both the hip socekt and femoral head components) that specifically aims to reduce metal ions in the body. Some manufacturers claim that "as cast" metal parts, when rubbed together as in a hip socket use, produce far fewer metal ions in the body than when "heat treated" parts are used in the same way. Most of the components I'm looking at are chromium/cobalt alloys, if that makes a difference.

Thanks - Erin

 

[kenvlach]

OK, it's not something I'm familar with, but perhaps you're dealing with 3 concurrent phenomena:
1) metal-to-metal wear creating metal particles (e.g., a clump of Co & Cr atoms),
2) dissolution of metal particles by saline body fluid, dissolving them in ionic form (e.g., Cr⁺³ or Co⁺²),
3) possible leaching of more soluble metals from the surface, leaving the surface enriched in the less soluble alloying elements.

Commonly, wear creates roughness which accelerates the wear process. But of course, there are situation such as piston rings in engine cyclinders, in which wear is reduced after 'breaking in.'

The leaching step is similar to the passivation process used for stainless steels. Basically, soaking in nitric acid preferentially dissolves iron and nickel from the surface, leaving a more corrosion resistant, chromium-enriched surface.

As the heat treated vs. as-cast issue, it will depend upon the alloy, but heat treated may be better simply because it provides options for altering microstructure and hardness as desired. There are also coating & surface modification procedures such as nitriding which are possibly described to non-metallurgists as 'heat treatment.'

This article may be useful: 'UNIQUE PROPERTIES REQUIRED OF ALLOYS FOR THE MEDICAL AND DENTAL PRODUCTS INDUSTRY'

http://crswnew.cartech.com/wnew/TechArticles/TA00010.html

I suggest registering on Carpenter Technology's site & getting their medical alloy brochures:

http://www.cartech.com/products/wr_products_stainless_medaloy.html

E.g.,
"BioDur® Carpenter CCM® Alloy
[Alloy composition] Single Figures are Maximums [wt.%]
0.10 C, 1.00 Mn, 1.00 Si, 26.00/30.00 Cr, 5.00/7.00 Mo, 1.00 Ni, 0.25 N, 0.75 Fe, Bal. Co

A vacuum induction melted and electroslag remelted nonmagnetic cobalt-chromium-molybdenum alloy exhibiting high strength, corrosion resistance, and wear resistance. High nitrogen, low carbon, wrought version of ASTM F75 cast alloy. Meets the requirements of ASTM F799, ASTM F1537, ISO 5832-4 and ISO 5832-12. Has been used in the orthopedic implant industry."

Found a really interesting paper, lots of microscopy; seems to be a senior thesis (B.S.) by Douglas J. Wood at San Jose State Univ. describing "Co-Cr-Mo alloy ball press fit onto a Ti-6Al-4V alloy 'stem' (i.e., a 'modular' system). Modular hip prostheses have been found to produce wear debris via various wear processes."

http://www.engr.sjsu.edu/WofMatE/projects/srproject/index.htm#contents.html

 

[TVP]

Erin,

As kenvlach explained, debris particles that are generated due to metal-on-metal wear is the issue you need to understand. Converting metal atoms into ions occurs due to interaction with various bodily fluids. I am not an expert on Co-Cr alloys, so I don't want to comment on the cast vs. wrought philosophy with regards to wear resistance. What I will tell you is that wear is an incredibly difficult subject, with many different factors that interact including microstructure, surface roughness, etc. You may want to review a reference such as ASM's Handbook of Materials for Medical Devices:

http://www.azom.com/Sale.asp?SaleID=265

 

[Robertmet]

Yes it can be complex.Plates and pins should be not only the same alloy but from the same batch of metal to prevent galvanic corrosion which may also create painful electric currents..Passivation of SS cleans impurities from the surface and creates a thick oxide coating.Reaction to metal implants is an individual thing and about 10 % must be replaced with another material or removed because of reaction to the body.The metal and reaction products must be compatable with the body.I haven't read much recently but in the past the Vitallium [Co-Cr] was the best overall.Metals also because of their rigidness tend over time to push through the bone .

 

[EdStainless]

The generation of ions would have to be via a combined wear/corrosion mechanism. Wear itself will only generate solid metal debris.

The galvanic issue is more related to comfort than corrosion that could lead to failure, unless there are other metals in the persons body also.
Material properties, surface finish, cleaning, and many more will factor in. Does anyone use an artificial "break in" cycle on such implants? Cycling the parts under load in an electrolyte might be an easy way to reduce the breaking period issues. A similar thing is done to components that go into ultra high vacuum applications.

= = = = = = = = = = = = = = = = = = = =
Rust never sleeps
Neither should your protection

http://www.trent-tube.com/contact/Tech_Assist.cfm