In a two-year study, Austrian researchers have investigated the behaviour of magnesium-alloy implants in mammalian femurs

Biodegradable implants in bone tissue have been attracting increasing interest in the last few years, thanks to the considerable advantage they have over iron and zinc-based ones. They function like a permanent implant initially, but degrade in the physiological environment until they disappear completely. This eliminates the need for follow-up surgery to remove the implant after the tissue has healed. But, balancing the degradation with the bone’s natural healing timescale can be challenging, and there’s a particular lack of data on how the body processes the rare-earth elements now used in implants.

To investigate these mechanisms, Austrian researchers looked at in-vivo degradation of two magnesium alloys, implanted into the femurs of male rats. Their paper, published in the latest issue of Acta Biomaterialia [DOI: 10.1016/j.actbio.2016.06.025] looked specifically at the long term distribution of released ions, coupled with the bone response, over a period of 24 months. Magnesium has similar mechanical properties to bone, and its degradation timescale can be ‘tuned’ by alloying.

Alloy ZX50 was found to degrade fully after four months, whereas in some locations on the bone, pins made from WZ21 (which contains yttrium) remained in place for up to 24 months. In both cases, the bone recovered to their original condition. Magnesium concentrations in the surrounding bone were found to peak within a month of implantation of WZ21 pins, though never exceeding tolerance limits. For both alloys, magnesium levels had returned to normal after 24 months. High variations in yttrium distribution were observed throughout the bone during WZ21 pin degradation; in some regions, reaching a thousand times higher than the basal level. But again, yttrium concentration had dropped to almost zero by the time the pins had fully degraded.

Hydrogen gas bubbles occurred in both alloys, with extensive gas formation observed in alloy ZX50. This led to areas of cell displacement, and while these areas continued to shrink after the pin had degraded, their role in the consolidation stage of bone regeneration makes them important in clinical applications. In addition, while the yttrium was seen to ‘disappear’, further study will be needed to verify its safe excretion. The authors recommend that organ pathology studies should be undertaken to demonstrate the absence of systemic toxicity of yttrium and other rare-earths.


F. Amerstorfer, S.F. Fischerauer, L. Fischer, J. Eichler, J. Draxler, A. Zitek, M. Meischel, E. Martinelli, T. Kraus, S. Hann, S.E. Stanzl-Tschegg, P.J. Uggowitzer, J.F. Löffler, A.M. Weinberg, T. Prohaska, “Long-term in vivo degradation behavior and near-implant distribution of resorbed elements for magnesium alloys WZ21 and ZX50”, Acta Biomaterialia (2016) Article in Press. DOI: 10.1016/j.actbio.2016.06.025