Assessing the long-term in vivo degradation behavior of magnesium alloys - a high resolution synchrotron radiation micro computed tomography study

Abstract

Biodegradable magnesium (Mg) implants are emerging as a potential game changer in implant technology in situations where the implant temporarily supports the bone thereby avoiding secondary surgery for implant removal. However, the consequences of the alteration in the degradation rate to bone healing and the localization of degradation and alloying products in the long term remain unknown. In this study, we present the long-term osseointegration of three different biodegradable Mg alloys, Mg-10Gd, Mg-4Y-3RE and Mg-2Ag, which were implanted into rabbit femur for 6 and 9 months. In addition, we have investigated the effect of blood pre-incubation on the in vivo performance of the aforementioned alloys. Using high-resolution synchrotron radiation based micro computed tomography, the bone implant contact (BIC), bone volume fraction (BV/TV) and implant morphology were studied. The elemental traces have been characterized using micro X-ray fluorescence. Qualitative histological evaluation of the surrounding bone was also performed. Matured bone formed around all three implant types and Ca as well as P which represent parts of the degradation layer were in intimate contact with the bone. Blood pre-incubation prior to implantation significantly improved BIC in Mg-2Ag screws at 9 months. Despite different implant degradation morphologies pointing toward different degradation dynamics, Mg-10Gd, Mg-4Y-3RE and Mg-2Ag induced a similar long-term bone response based on our quantified parameters. Importantly, RE elements Gd and Y used in the alloys remained at the implantation site implying that they might be released later on or might persist in the implantation site forever. As the bone formation was not disturbed by their presence, it might be concluded that Gd and Y are non-deleterious. Consequently, we have shown that short and mid-term in vivo evaluations do not fully represent indicators for long-term osseointegration of Mg-based implants.