A new, essentially nickel-free, biocompatible stainless steel alloy for implantable orthopedic devices meets the needs for high levels of strength, toughness, and resistance to corrosion and fatigue. BioDur 108 alloy, developed by Carpenter Technology Corp., Reading, Pa., (cartech.com) is for use in applications that include bone plates, bone screws, fracture, spinal fixation components, and hip and knee parts. It is an alternative to the three BioDur alloys — Type 316LS, 22Cr-13Ni-5Mn, and 734.

In tests meeting ASTM standards, BioDur 108 exhibited significantly higher strength in annealed and cold-worked conditions than any common nickel-containing stainless alloys used in medical applications. BioDur 108 demonstrated corrosion resistance equal to alloys 734 and 22Cr-13Ni-5Mn, and greater than BioDur 316LS. Biocompatibility tests for the new material also proved favorable for use in biomaterial applications.

The new alloy's 1% nitrogen content maintains its austenitic structure and contributes to high levels of strength, ductility, and corrosion resistance. Its typical yield strength is about 88 ksi in annealed conditions, while for BioDur 316LS it's about 35 ksi. Nitrogen-strengthened BioDur 734 and 22Cr-13Ni-5Mn alloys, with more nitrogen than Type 316LS but less than 108, exhibit about a yield strength of about 65 ksi.

BioDur 108 has a yield strength about 20% higher than 22Cr-13Ni-5Mn and more than double BioDur 316LS at a cold reduction of 69%. In annealed conditions, room-temperature impact energy levels for standard 10 × 10 mm Charpy V-notch specimens exceed the capacity of common testing machines.

The ductile-to-brittle transition response in BioDur 108 is suppressed to below 0° C. This transition takes place at about 220° C, making the material useful above this temperature.

High nitrogen levels also let the material develop high resistance to fatigue. Rotating-beam fatigue tests conducted on specimens from annealed bar stock having an ASTM #5 grain size and an ultimate tensile strength of 135 ksi showed fatigue limit of about 55 ksi, or 41% of the ultimate strength, equal to 22Cr-13Ni-5Mn and higher than BioDur 316LS.

Corrosion resistance in austenitic alloys is related to chromium, molybdenum, and nitrogen levels. Because these levels are high in BioDur 108 alloy, it offers high resistance to pitting and crevice corrosion.

Compared to 22Cr-13Ni-5Mn and 316LS alloys, BioDur 108 had a higher calculated pitting resistance equivalent number, the lowest weight loss in ferric chloride pitting tests, and the highest calculated and measured critical crevice-corrosion temperature.

Testing for corrosion-fatigue limits in a distilled-water solution and standard Ringer's solution at 37°C confirmed the relative corrosion resistance of the alloys. In addition, 108 passed ASTM A262 Practice A requirements for resistance to intergranular corrosion.

BioDur 108 met all requirements of biocompatibility test standards. It is non-cytotoxic and met requirements of the Elution test, ISO 10993. Samples did not exhibit signs of erythema, edema, or necrosis. Testers observed no signs of acute systemic toxicity. Test samples met requirements of ISO 10993-11, Systemic Injection Test, and ISO 10993-11 for the absence of pyrogens.

In addition, samples were concluded to be non-mutagenic and non-toxic because no signs of toxicity were exhibited after being implanted with histopathology in 14 and 28-day test periods. Finally, the alloy samples were concluded to be nonhemolytic.