A magnesium-based implant for its application in young, old and osteoporotic bone

Introduction The advancing prevalence of post-menopausal osteoporosis is associated with increasing age of the population. Osteoporosis is characterized by weakening of the bone mass and density thereby increasing the risk of bone fractures. In 2010, 3.5 million incident fragility fractures were recorded in the EU which also increases the economic burden associated with high healthcare 60 costs. Conventional and rigid titanium (Ti) and stainless steel implants are currently used for stabilization of bone fractures. In children, permanent rigid implants must be removed otherwise impeding longitudinal bone growth in many cases. In elderly, permanent implants can induce stress-shielding leading to bone loss and increased risk of peri-implant fractures. Especially osteoporotic patients with weak trabecular and cortical bone might have an increased risk for refractures due to the rigidity of Ti implants. Therefore, biodegradable magnesium (Mg) implants would constitute a promising alternative for both, children and elderly patients. Previous studies have already shown that the lean Mg-0.45 %wt Zn- 0.45 %wt Ca (ZX00) implant material can be successfully implanted into femoral bone of juvenile rats and tibial bone of juvenile sheep, thereby supporting bone formation with adequate gas evolution. Here we transcortically implanted ZX00 into the metaphyseal tibia of juvenile, old and osteoporotic rats and compared the degradation and osseointegration of ZX00 via in vivo micro-computed tomography (µCT). The primary outcomes measured included implant volume and surface as well as gas evolution over a time period of 24 weeks.

Methods Six-weeks old female Sprague Dawley® (SD) rats underwent bilateral, bicortical and metaphyseal implantation of ZX00 pins into tibiae. One-year old osteoporotic animals underwent ovariectomy to induce osteoporosis and after three months, ZX00 implantation was performed according to juvenile rats. In vivo µCT scans were performed 4, 8 and 12 weeks after ovariectomy in old animals to observe osteoporosis progression and 2, 6, 12 and 24 weeks after surgical intervention at a resolution of 56 µm per voxel, respectively.

Results In vivo µCT imaging demonstrated continuous osteoporosis progression over three months after ovariectomy. After ZX00 implantation, homogenous degradation of ZX00 over the entire study period of 24 weeks was observed in juvenile rats. Bone formation, osseointegration and bone in-growth were not influenced by gas evolution in juvenile rats. However, in osteoporotic animals ZX00 degradation was enhanced and induced more gas evolution when compared to juvenile rats.

Discussion In this study we compared ZX00 degradation in tibiae of juvenile, old and osteoporotic rats. ZX00 markedly differed between the groups. Therefore, we suggest that differences in bone metabolism as well as changes in pH might influence the degradation behaviour, which will need further elucidation.

Keywords magnesium implant, osteoporosis, bone formation

Korrespondenzadresse Nicole Gabriele Sommer, Medizinische Universität Graz, Univ.-Klinik für Orthopädie und Traumatologie, Auenbruggerplatz 5, 8036 Graz, Österreich, Germany

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