A better way to fix bones

Michael J. Walter
Medical Alloy Specialist
Carpenter Technology Corp.
Reading Pa.

Recent tests on a new, essentially nickel-free stainless alloy, BioDur 108 (ASTM F2229-02), have shown it is significantly stronger, in both annealed and cold-worked conditions, than any of the nickel stainless alloys currently serving in biomedical implants and medical devices.

The test results show that the alloy can be an alternative to the three austenitic stainless alloys — BioDur Type-316LS alloy (ASTM F138), BioDur 22Cr-13Ni-5Mn alloy (ASTM F1314), and BioDur 734 alloy (ASTM F1586) — the most commonly used stainless alloys for bone plates and screws, as well as fracture, trauma, and spinal-fixation components.

BioDur 108 alloy has demonstrated corrosion resistance similar to that of BioDur 734 and BioDur 22Cr-13Ni-5Mn alloys. It handles corrosion significantly better than the widely used alloy BioDur 316LS. And in biocompatibility tests, it also scores favorably and thus qualifies as a candidate for biomaterial applications.

PUT TO THE TEST
For testing, conventionally cast electroslag remelted ingots of BioDur 108 alloy were converted into billets for subsequent processing into various-sized bars and wires. Samples then saw various anneal and cold-work conditions and were tested in accordance with ASTM standards against the three commonly used alloys in equivalent metallurgical conditions.

Technicians measured mechanical properties of the machined specimens including yield and tensile strength, impact toughness, and fatigue resistance. They checked corrosion resistance by calculating and measuring critical crevice-corrosion temperature (per ASTM G48, Method D) and by completing PREN and pitting tests per ASTM G48, Method A.

BioDur 108 maintains its austenitic structure because of its high (1%) nitrogen content. High nitrogen also contributes to the alloy's high strength, ductility, and corrosion resistance.

BioDur 108 alloy yields at approximately 606 MPa (88 kpsi) in the annealed condition. In contrast, yield strength for BioDur 316LS is approximately 241 MPa (35 kpsi), typical of austenitic stainless steels with low nitrogen. The nitrogen-strengthened BioDur 734 and 22Cr-13Ni-5Mn alloys — with more nitrogen than BioDur Type 316LS alloy but less than BioDur 108 alloy — typically yield at about 448 MPa (65 kpsi) when annealed. Additionally, BioDur 108 alloy's high nitrogen content enhances the effect of cold work, thus boosting alloy strength.

Cold reduction also affects alloy yield strength. Tests of BioDur 108 show that its yield strength is approximately 20% higher than 22Cr-13Ni-5Mn and is more than double that of BioDur 316LS at common 69% cold reductions.

BioDur 108 alloy is tough just like most austenitic alloys. Standard 10 10-mm Charpy V-notch specimens of annealed alloy exceed the capacity of common testing machines at room temperature. So it is best to use subsize 10 5-mm Charpy V-notch impact specimens when testing for impact energy.

High-nitrogen austenitic alloys like BioDur 108 tend to have "ductile-to-brittle" transitions resembling those of ferritic alloys. BioDur 108 alloy's ductile-tobrittle transition is suppressed below 0°C. This transition takes place at about 20°C. Therefore, BioDur 108 alloy is a candidate for applications above 20°C.

The nitrogen in BioDur 108 alloy helps boost its strength. High nitrogen also significantly improves its fatigue resistance. Technicians did rotating-beam fatigue tests on annealed alloy specimens having an ASTM #5 grain size and an ultimate tensile strength of 930 MPa (135 kpsi). They observed a fatigue limit of about 380 MPa (55 kpsi), or roughly 41% of the ultimate strength. This fatigue limit is essentially that of 22Cr-13Ni-5Mn alloy and higher than that found in BioDur 316LS.

CORROSION RESISTANCE
Austenitic alloys' ability to resist corrosion relates strongly to their levels of chromium, molybdenum, and nitrogen. BioDur 108 alloy has significant amounts of these elements and thus resists pitting and crevice corrosion — critical properties for materials used in medical devices.

Technicians compared BioDur 108 alloy with 22Cr-13Ni-5Mn and BioDur 316LS alloys for several qualities: calculated Pitting Resistance Equivalent Number (PREN), measured weight loss in ferric-chloride pitting tests (ASTM G48 Method A), and in tests of calculated and measured critical crevice corrosion temperature (CCT) per ASTM G48 Method D. BioDur 108 alloy had a higher calculated PREN, the least amount of weight loss in pitting tests, and the highest calculated and measured CCT.

Technicians have confirmed the relative corrosion resistance of the alloys by testing for corrosion fatigue limits in distilled water solution and a standard Ringer's solution at 37°C (98°F). In addition, BioDur 108 alloy passed the ASTM A262 Practice A requirements for resistance to intergranular corrosion.

BIOCOMPATIBILITY
BioDur 108 alloy also underwent biocompatibility tests to further evaluate its potential use as a biomaterial. Tests compared it to the most commonly used nickel-bearing austenitic stainless steels. Extensive biocompatibility tests conducted by Toxikon Corp., Bedford, Ma., showed BioDur 108 alloy met all test standard requirements. Toxikon's findings include:

Cytotoxicity: The alloy was noncytotoxic and meets Elution test, ISO 10993 requirements.

Irritation: Test samples did not exhibit any signs of erythema, edema, or necrosis. Toxikon concluded the alloy was a negligible irritant.

Acute systemic toxicity: There were no signs of toxicity. Test samples met the requirements of ISO 10993-11, Systemic Injection Test.

Pyrogenicity: Test samples met ISO 10993-11 for the absence of pyrogens.

Mutagenicity: Alloy samples were nonmutagenic based on the test methods employed.

Implantation with histopathology: There were no signs of toxicity after 14 and 28-day implantation tests. Hemocompatibility: Toxicon concluded alloy samples were nonhemolytic.

Carpenter Technology Corp. can provide more details about test procedures and standards met.