Variability in the Chemical Content of Orthopedic Alloys

The chemical contents of the various metal alloys used in orthopedic implants is usually given, by ASTM standards for example, as an allowable range for the lesser alloying elements with possibly some other elements listed as smaller maximum and/or minimum allowable amounts. The percentage of the major element, according to the standards, is obtained by 100 percent, minus the sum of all of the other enumerated elements (i.e., direct measurement is not required).
 
For example, the ASTM F90 specification for “Wrought Cobalt-20Chromium-15Tungsten-10Nickel Alloy for Surgical Implant Applications” allows for chromium to be between 19.00-21.00 percent, tungsten 14.00-16.00 percent, and nickel 9.00-11.00 percent. Ranges are also given for carbon (0.05-0.15), manganese (1.00-2.00), and a maximum value for iron of 3.00 percent. Cobalt is listed in the composition table as “balance” with a footnote that says “Approximately equal to the difference between 100 percent and the sum percentage of the other specified elements. The percentage of the cobalt difference is not required to be reported.” Focusing on the major alloying elements, their minimum total is 42 percent and their maximum is 48 percent. This leaves the approximation of cobalt as 52-58 percent, without reference to possible presence of elements other than those listed.
 
Other ASTM standards state the balance calculation a bit differently. For example, in ASTM F2146 for “Wrought Titanium-3Aluminum-2.5Vanadium Alloy Seamless Tubing for Surgical Implant Applications” the language is “The percentage of titanium is determined by difference and need not be determined directly or certified.” The specification in F2146 is also silent about any other elements—cobalt for example—so the “determination” of the amount of titanium by difference would include any cobalt, or anything else, that happened to be present. I would assert that “determined” as used in F90 is more forceful and even less accurate than “approximately,” although I believe both to be incorrect. Is it really a determination or approximation, is it actually an assumption, or something else?
 
Recently, additional language has been added to some orthopedic material specifications (e.g., F1058 for “Wrought 40Cobalt-20Chromium-16Iron-15Nickel-7Molybdenum Alloy Wire, Strip, and Strip Bar for Surgical Implant Applications”) that limits the customer’s ability to use the specification to reject a lot based on reported chemical analyses. Section 6.2.2 states that “All commercial metals contain small amounts of elements other than those which are specified. It is neither practical nor necessary to specify limits for unspecified elements, whether residual elements or trace elements, that can be present. The producer is permitted to analyze for unspecified elements and is permitted to report such analyses. The presence of an unspecified element and the reporting of an analysis for that element shall not be a basis for rejection.” The final sentence here is of possible greatest concern, since it says that the buyer cannot reject material whose chemical analysis includes elements not specified in the standard, with the only limitation that the amounts be “small,” “residual,” or “trace”—without definitions for any of. This seems like seller’s language to me, since it removes the buyer’s ability to reject the material based on the ASTM specification. To retain this right, the buyer would have to add contractual language seeking to override this limitation. It is of interest that other standards still retain the right to reject based on the actual chemical analysis. Section 6.2.1 of F90 states that “Acceptance or rejection of a heat or lot of material may be made by the purchaser on the basis of this product analysis” without apparent limitation as to what elements in what amounts were called out in the analysis. This lack of limitation—and perhaps customer rejections—seems to have triggered the new language, which has appeared in new and revised standards. Of course, a material supplier could resolve this issue by not analyzing for or not reporting elements not listed in the specification.
 
Given the prescribed variability and possible presence of unlisted elements, the next question is whether other material properties in the specification would be compromised by deviations from expected elemental content. If so, the material would fail the specification based on property values and the variability would then be resolved. If not, material with unlisted elements in uncertain amounts would appear to meet the specification, leaving the question of whether these deviations matter. This question cannot be answered a priori, but it should be noted if a maximum carbon value of 0.15 percent matters in F90, similar small amounts of other things might also matter.
 
I suggest material specifications should use accurate language, although one might argue as long as we all know what we are doing it doesn’t matter what we call it. This, of course, assumes everyone does indeed know and understand. Approximated is better than determined, but assumed is more explicit and better expresses the uncertainty about what is actually there. Standards should not attempt to usurp contractual agreements between buyer and seller.