Department of Biomedical Materials Science

Material Durability

Corrosion Fatigue

During the early 1990’s it became obvious to us that while many of the metals and alloys, traditionally used for biomedical applications, had been evaluated for their corrosion fatigue properties, their behavior had not been examined using standardized procedures in any way resembling the in vivo environment. Many of these alloys had been evaluated using solutions and temperatures, not even remotely representative of the in vivo conditions. As a result of an extensive review of the literature, and a fundamental interest in this area, we began corrosion fatigue research on stainless steels, titanium and titanium alloys as well as Co-Cr alloys used for biomedical applications. Our purpose was to determine the effects of a single solution (Ringers) as compared to de-ionized distilled water on the fatigue performance of these alloys at 37ºC. Because of our twenty year history of failure analysis of retrieved implants, the recognition of fatigue as the primary mechanism of implant fracture, and some of the confusion in the early literature, we have also attempted to correlate the fracture mechanisms with fracture surface morphology and fundamentals of metals structure. To date, we have published a number of papers in this area on titanium alloys and stainless steels and have worked with several industrial partners during the development of new materials and processing.

Failure Analysis

In addition to the failure analysis performed during laboratory evaluation of new alloys for stress corrosion cracking and corrosion fatigue, a large portion of our work is related to failure analysis of retrieved implants. Approximately 20 years ago, we began by setting up our own implant retrieval program to evaluate all implants removed from patients regardless of condition. Since that time we have continued to evaluate failed implants from a variety of sources outside the University Medical Center. Analysis is usually composed of compositional analysis, light microscopic examination including optical metallography where applicable, hardness evaluation, and scanning electron microscopy. Where available, this information is correlated with clinical record data. Results of these studies have allowed us to correlate information such as fracture surface morphology of retrievals with our basic research on the stress corrosion cracking and corrosion fatigue of the different implant materials. Because of the information that we have gleaned over the years, research in our own laboratories as well as the results of research by others, has become more easy to interpret and has led us to a greater understanding of the effects of materials processing on properties.

Stress Corrosion Cracking

Over the last several decades there has been a great deal of conjecture and some studies reported in the literature concerning the possibility for in vivo stress corrosion cracking of alloys used for biomedical applications. Many of the reported incidences identifying stress corrosion cracking as the mechanism of failure of retrieved implants occurred before the 1970’s. Most of these reports implicated 316L stainless steel while only a few ascribed this failure mechanism to certain grades of CP titanium. Most of the fundamental research studies were performed in saline solution at 37ºC and some were also performed using more aggressive media at elevated temperature. Because of a lack of definitive correlation between these early reports, and because the composition of implant quality 316L stainless steel has been redefined, we embarked upon a program to assess the stress corrosion cracking behavior of these materials under the same conditions. We have begun by evaluating both the entire process to failure of the three most commonly used stainless steels, CP titanium, two a/ß titanium alloys, and a ß titanium alloy. All of this research to date is in the literature or shortly will be. We are continuing with this specific work to include all of the alloys used for biomedical applications, as well as expanding this area of research to include studies on the crack propagation phase.