Life expectancy of modular Ti6Al4V hip implants: influence of stress and environment.

Stress dependent electrochemical dissolution is identified as one of the key mechanisms governing surface degradation in fretting and crevice corrosion of biomedical implants. The present study focuses on delineating the roles of mechanical stress and chemical conditions on the life expectancy of modular hip implants. First, material removal on a stressed surface of Ti6Al4V subjected to single asperity contact is investigated experimentally to identify the influence of contact load, in-plane stress and chemical environment on mean wear rates. A range of known stress levels are applied to the specimen while its surface is mechanically stimulated in different non-reactive to oxidizing aqueous environments. Evolution of surface degradation is monitored, and its mechanism is elucidated. This phase allows estimation of Preston Constant which is later used in the analysis. Second phase of the work is semi-analytical and computational, where, based on the estimated Preston constant and other material and process parameters, the scratch propensity (consisting of magnitude of scratch depth and their frequency per unit area) due to micro-motion in modular hip implants is estimated. The third phase views these scratches as initial notches and utilizes a mixed-mode fatigue crack propagation model to estimate the critical crack length for onset of instability. The number of loading cycles needed to reach this critical crack length is then labeled as the expected life of the implant under given mechanical and chemical conditions. Implications of different material and process conditions to life expectancy of orthopedic implants are discussed. It is observed that transverse micro-motion, compared to longitudinal micro-motion, plays a far more critical role in determining the implant life. Patient body weight, as well as proximity of the joint fluid to its iso-electric point play key roles in determining wear rates and associated life expectancies of modular hip implants. Sustained aeration of joint fluid, as well as proper tolerancing of mating surfaces, along with a proper choice of material microstructure may be utilized to extend implant life.

[Damage to a hip endoprosthesis caused by high-frequency electrocautery]. / Schädigung eines Hüftendoprothesenschafts durch Einsatz eines Hochfrequenzmessers.

Contact between high-frequency cauterising instruments and metal endoprostheses can cause visible flashovers. The resulting local heating might partially transform the microstructure of the prostheses. In the present case a flashover to the in situ titanium hip endoprosthesis during the revision of a fractured ceramic head decreased the fatigue strength of the prosthesis and ultimately caused its failure. During revision surgery it is essential to prevent contact between in situ metal components and high-frequency cauterising instruments.

Aneurysm clips made of titanium: magnetic characteristics and artifacts in MR.

PURPOSE: To evaluate the magnetic characteristics, artifact formation, and implant safety of titanium aneurysm clips for use in MR imaging. METHODS: Aneurysm clips made of titanium alloy TiAl6V4 were tested in a magnetometer to determine their magnetic susceptibility and in a 1.5-T MR imager using both a geometric phantom and an animal model. A commercially available alpha-Phynox clip served as the reference standard. RESULTS: We found minimal magnetization and a significant reduction in image artifacts with the titanium clip as compared with the Phynox clip. CONCLUSION: The titanium clips improve image quality, biocompatibility, and patient safety in medical MR applications.

[Titanium aneurysm clips and their advantages in diagnostic imaging]. / Titananeurysmaclips und ihre Vorteile in der bildgebenden Diagnostik.

Aneurysms clips made of a titanium alloy (TiAl6V4) were used in clinical practice for the first time. The design of the clips is identical to the routinely used Yasargil series. In 30 patients, 38 symptomatic and asymptomatic aneurysms were fixed with 45 clips. Metallurgical advantages of the new alloy are better biocompatibility, less magnetic susceptibility, and lower X-ray density. The postoperative imaging results are superior to the conventionally used alloys with respect to artifact reduction in computed tomography, angiography, and magnetic resonance imaging. With a follow-up period of 7 months, a statement on biocompatibility cannot yet be given.