gms | German Medical Science

Objectives: Osteoporosis is characterized by an imbalance of bone resorption and formation leading to the weakening of bone mass and density that increases the fracture risk. In elderly, conventional permanent implants, including titanium and stainless steel, can induce stress-shielding leading to bone loss and increased risk of peri-implant fractures. To overcome disadvantages associated with permanent implants in fragile bones, bioresorbable magnesium (Mg) implants would constitute a promising alternative for elderly patients, especially under osteoporotic conditions. Moreover, since osteoporosis is linked to a low-grade inflammatory state comparable to magnesium deficiency and oral Mg supplementation has been shown to improve bone mass density, we hypothesized that Mg-based implants support bone formation due to the release of Mg ions. The aim of the study was to investigate, whether a lean magnesium-zinc-calcium alloy supports bone formation in osteoporotic and old Sprague Dawley rats.

Methods: Twenty female Sprague Dawley rats were divided into three groups: 15-months old ovariectomy-induced osteoporotic, 15-months old healthy and 6 weeks old juvenile healthy animals. Mg-based (Mg<0.5% wtZn<0.5% wtCa; ZX00) pins (l=8mm, d=1.6mm) were transcortically implanted into the proximal metaphysis of the tibiae. Osteoporosis progression and implant degradation were visualized with in vivo low-medium resolution micro-computed tomography (µCT) immediately after as well as 2, 6, 12, 15, 18 and 24 weeks after implantation. We calculated implant volume/surface, gas evolution and degradation rate.

Results and Conclusion: In vivo µCT imaging demonstrated continuous osteoporosis progression over three months after ovariectomy. After calculation, implant volume showed a relative decrease of 39.95% in osteoporotic, 26.15% in old healthy and 11.99% in juvenile healthy rats 24 weeks after implantation. Moreover, hydrogen gas evolution was significantly increased in osteoporotic rats even after 2 weeks when compared to old and juvenile healthy rats. Computation of the degradation rate revealed an enhanced degradation rate in osteoporotic compared to old and juvenile healthy rats, respectively. In this study we wanted to investigate whether the Mg-based alloy ZX00 supports bone formation and osseointegration in osteoporotic rats due to the release of Mg ions. Interestingly, implant volume loss, hydrogen gas evolution and degradation rate per day were tremendously increased compared to old and juvenile healthy rats. Due to the enhanced gas evolution, bone in-growth and osseointegration could not be observed via µCT imaging. We suggest that the low-grade inflammatory state associated with osteoporosis reduces the pH which subsequently enhances implant degradation compared to the old healthy rats. Further studies are needed to investigate the underlying process of the enhanced degradation rate and histological investigation is warranted to characterise the surrounding of the implant and the bone-implant interface.