Material Characterization: How Low Is Low?

Metal alloys, such as those used in orthopedic and other implants, are characterized in a number of different ways, generally starting with their chemical composition. Compositions vary in order to obtain a variety of characteristics such as micro-structure, surface finish, flexibility, strength, deformability, corrosion resistance, and biocompatibility of the base material. Such properties can be altered by manufacturing processes, and biocompatibility, in particular, is better associated with finished devices and their application. It is generally the case that not all properties can be optimized at the same time so that, as in much of life, there are tradeoffs to be made. A simple example of such a tradeoff in the home is the difference between a carbon steel knife and a stainless steel knife. A carbon steel knife can be made much sharper than a stainless steel knife, but it requires much greater maintenance. A stainless steel knife is in general less sharp, but doesn’t corrode—even if left in the sink for an extended period. Thus, the popularity of stainless steel knives in which, curiously, the primary cutting function of a knife is sacrificed for low maintenance.
 
Many metal alloys for medical applications are covered by ASTM standards, which address chemical composition and other properties. The chemical composition is typically given as a range for the required alloying elements and a short list of selected other elements for which maximums are given. The dominate element is then stated as being “determined” or “approximated” by subtraction(i.e., it does not have to be actually measured). The presence of non-required elements is related to the practical difficulties and/or cost of achieving higher levels of purity. This is similar to the allowable number of insect parts in a jar of peanut butter, which I regret to tell you is not zero.
 
A “Classical” Stainless Steel
A classical implant, stainless steel—first standardized within ASTM in 1976—is F139 – Standard Specification for Wrought 18Chromium-14Nickel-2.5Molybdenum Stainless Steel Sheet and Strip for Surgical Implants. The chemical composition of this alloy is specified as requiring:
 

• Chromium: 17-19 percent
• Nickel: 13-15 percent
• Molybdenum: 2.25-3 percent

 
Also specified is the maximum allowable amount for 7 other elements—carbon, manganese, phosphorus, sulfur, silicon, nitrogen, and copper. With the exception of manganese, the other allowable amounts are all under 1 percent. The percentage of iron is obtained by difference and “is not required to be determined.” Note that there are many other elements that might be present about which the specification is silent. These would be, in effect, included in the “iron” content.
 
A Low-Nickel Alloy
Nickel, as an alloying element, has been implicated in allergic reactions in some people. Thus, it became desirable to develop low-nickel stainless steels such as F2229 – Wrought, Nitrogen Strengthened 23Manganese-21Chromium-1Molybdenum Low-Nickel Stainless Steel Alloy Bar and Wire for Surgical Implants, first standardized in 2002. The required elements for this material are:
 

• Chromium: 19-23 percent
• Manganese: 21-24 percent
• Molybdenum: 0.5-1.5 percent
• Nitrogen: 0.85-1.1 percent

 
Nickel is given as 0.05 max, reflecting that this is a low-nickel alloy, as compared to the 13-15 percent given previously in F139. The F2229 material can be found described by some suppliers as “nickel free” and/or “essentially nickel free.” Clearly it is not required to be nickel free, and “essentially” is less than definitive. Note also that manganese is required here as is nitrogen, both of which had maximum values in F139. Such is the art of creating alloys, or perhaps it is the art of creating patentable alloys rather than important functional differences. The low amounts of some of the required elements molybdenum and nitrogen, which can be under 1 percent, speaks to the potential importance of even such small quantities in alloy performance, yet unnamed elements are not accounted for in any amount, small or not. Besides nickel, other elements with maximum values in F 2229 are carbon, phosphorous, sulfur, silicon, and copper. Again, there are many additional elements that might possibly be present that are not mentioned at all. None-the-less, iron is stated as “approximately equal to 100 percent minus the other given elements.”
 
A New ‘Nickel-Free’ Alloy
Recently standardized is F3273 – Wrought Molybdenum-47.5 Rhenium Alloy for Surgical Implants. The required alloy element rhenium is given as 46-49 percent. Note that the two previously discussed alloys were silent about rhenium, which might fairly be described as one of the lesser known elements. Fourteen elements have stated maximum values. Not included in these 14 elements is nickel, chromium, or cobalt, about which the standard says nothing. The amount of molybdenum is stated to be “approximately equal to the amount obtained by difference.” Thus, it is not measured, and the amount obtained by difference might contain any number of other elements in unspecified amounts.
 
The ASTM Press Release for this material states the alloy “does not contain elements such as nickel, chromium, or cobalt.” However, this statement is not supported by the explicit requirements of the standard. In fact, the standard states that analysis for elements not listed as either required or limited is not required to verify compliance.
 
In summary, F139 requires nominally 14 percent nickel, F229 permits up to 0.05 percent nickel, and F3272 says nothing about nickel while being described outside of the standard as not containing nickel.
 
Specifications, Not Words
From a strictly technical perspective, the chemical composition of an alloy is defined by the specification and not by the words that might be used to describe it. Whether this distinction is actualized by device manufacturers, material suppliers, regulators, customers, and ultimately patients, is not clear. But at least, in my opinion, a material should not be called nickel free (or chromium free or cobalt free) if nothing about the specification precludes the presence of these elements. In addition, a disturbing trend in ASTM metal alloy standards is the inclusion of a statement such as “The producer is permitted to analyze for unspecified elements, and is permitted to report such analysis. The presence of an unspecified element and the reporting of an analysis for that element shall not be a reason for rejection.” In other words, elements about which the standard is silent can in fact be present, and the customer has to accept it no matter what elements those may be and in whatever amount they happen to be present. This invites a battle of conflicting disclaimer language in purchase agreements in which each party tries to set requirements that say, “My requirements count and yours do not.” This brings us back to the insects in peanut butter analogy. If a peanut butter manufacturer gave you its analysis of inspect parts and you didn’t like the number provided, under the above logic you would be required to buy it anyway.