STP684

    Crevice Corrosion of Implant Alloys—A Comparison of In-Vitro and In-Vivo Studies

    Published: Oct 1979


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    Abstract

    As part of a program to evaluate a family of high-strength, high-ductility stainless steels (TRIP steels) for use in orthopedic implants, the crevice corrosion resistance of one of these steels (containing 4 percent molybdenum) was compared with the resistances of two alloys in current clinical use (Type 316L stainless steel and cast Vitallium alloy). The three materials were subjected to two types of in-vitro test and two types of in-vivo tests. In one in-vitro test, two-plate crevice assemblies of each material were weighed before and after a nine-month period of immersion in a physiological saline solution; concurrent potential measurements were made on noncreviced specimens. A rapid in-vitro electrochemical technique, called the PPR test, was used to determine crevice corrosion (or pitting) propagation rates and the potential, Eprot, below which crevice corrosion (or pitting) cannot occur. By comparing Eprot values with the long-term corrosion potentials, it was possible to predict the relative susceptibilities of the alloys to crevice corrosion. In one in-vivo test, bone/metal crevices were produced by inserting small cylindrical coupons of each alloy into holes drilled into the femurs of six mongrel dogs. In the other in-vivo test, bone plates of each alloy were fixed with mating screws to each humerus and to each femur of four rhesus monkeys: this type of specimen provided metal/metal and metal/bone crevices and most closely simulated expected service conditions. The cylindrical coupons, the bone plates, and the tissue surrounding each specimen were examined after exposure times of up to two years. The results of the two in-vitro tests and of the in-vivo tests in rhesus monkeys were entirely consistent: each test indicated that the TRIP steel is fairly susceptible to crevice corrosion, that Type 316L staineiss steel has limited susceptibility, and that cast Vitallium alloy is essentially immune to this form of localized corrosion. However, all alloys were much more resistant to crevice corrosion when implanted into dog bone, and the TRIP steel performed as well as the Type 316L stainless steel, though not as well as the cast Vitallium alloy. Thus, although we have demonstrated that it is possible to obtain good agreement between in-vitro and some in-vivo tests, the inconsistencies in the in-vivo results underline the need for a reevaluation of the animals and the in-vivo test techniques commonly used for assessing the suitability of candidate implant materials.

    Keywords:

    implant materials, in-vitro, analysis, in-vivo, analysis, austenitic stainless steels, cobalt alloys, cobalt-chromium alloys, dogs, monkeys, concentration cell corrosion (crevice corrosion), orthopedics, implantation, electrochemical techniques


    Author Information:

    Syrett, BC
    Senior metallurgist and biologist, Physical and Life Sciences Group, SRI International, Menlo Park, Calif

    Davis, EE
    Senior metallurgist and biologist, Physical and Life Sciences Group, SRI International, Menlo Park, Calif


    Paper ID: STP35947S

    Committee/Subcommittee: F04.19

    DOI: 10.1520/STP35947S


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