Abstract
The different capacities of magnesium in the metallic form (Mg-metal) and magnesium oxide (MgO) to stimulate bone healing are possible clues in the search for products that may promote bone healing. Since both Mg-metal and MgO can be assumed to release comparable amounts of Mg2+ ions during their reactions in the tissue where they have been implanted, it is of some importance to follow this process and analyze the resulting mineral formation in the tissue at the implantation site. Implants of MgO were inserted into rat tibia, and the bone healing was compared with sham-operated controls. Samples were taken after 1 week of healing and analyzed by histology, environmental scanning electron microscopy equipped with an energy dispersive x-ray spectroscopy analyzer, and time-of-flight secondary ion mass spectrometry (ToF-SIMS). Callus bone was seen in sham-operated controls after 1 week of healing. Implantation of MgO impaired the callus bone formation by replacing bone with apparently mineralized areas, lacking osteocytes and were denoted, amorphous bodies. Elemental analysis showed increased levels of Ca (7.1%), P (3.7%), and Mg (0.2%) in the bone marrow of MgO-treated animals versus sham-operated controls Ca (2.4%), P (2.3%), and Mg (0.1%). The Ca content of the cortical bone was also significantly increased (Ca, 29% increase) in MgO-treated animals compared to sham-operated controls. The Ca content of the cortical bone of sham-operated animals was also significantly (p < 0.05) higher than the corresponding value of untreated animals, which means that the surgical trauma induces an altered composition of the bone mineral. The Ca/P ratio was 1.26-1.68, which is compatible with that of mineralized bone with different contents of organic materials. Analysis of bone sections using ToF-SIMS showed the presence of hydroxyapatite (HA) and MgCO3 in the bone marrow and in cortical bone. Analysis using x-ray photoelectron spectroscopy of Mg, MgO, and MgCO3 after incubation with cell culture medium (DMEM), in vitro, showed binding of CaPO4 at the Mg and MgO samples. The Ca/P ratio was 0.8, indicating a higher P content than that expected for HA. Exposure of human embryonic stem cells to Mg species preincubated in DMEM resulted in HA production by the cells. Thus, two sources of CaPO4 in the bone marrow of MgO-treated bone were defined, catalytic formation on Mg-species and synthesis from activated stem-cells. The presented data suggest that bone healing near Mg implants is congruent with the fracture healing of bone, boosted by high HA levels in the bone marrow. In this context, the different capacities of Mg-metal and MgO to catalyse the formation of HA can be important clues to their different bone promoting effects.
Highlights
Magnesium implants have, since long, been known to stimulate bone formation in vivo.1 The application of the metal as an implant material has been hampered by the formation of hydrogen gas during Mg corrosion.2 The gas a)Electronic mail: hakan.nygren@gu.se development in the tissue can be avoided by implanting Mg corrosion products, like magnesium oxide (MgO).3 Implantation of MgO has been reported to stimulate bone formation, but the effect was intermittent and of short duration.3 It meets with the difficulty in explaining the different effects of Mg-metal and MgO with differences in the levels of Mg-ions in the tissue
Mg corrosion products may function as substrates facilitating the formation of low crystalline, carbonated HA (CHA), where the high Mg ion concentration inhibits its transition to crystalline HA.14
The present study was performed to elucidate the effects of calcium phosphates, formed by catalysis, on the surface of Mg corrosion products, on human embryonic stem cells in vitro, and on bone tissue in vivo when implanted into rat tibia
Summary
Calcium phosphates, including hydroxyapatite (HA), precipitate on the surface of corroding Mg.. To gain a better understanding of the corrosion process and how it might facilitate mineralization in vivo, we applied energy dispersive x-ray spectroscopy (ESEM/EDX), ToF-SIMS, and XPS for chemical and elemental analyses of bone mineralization.. Mg corrosion products may function as substrates facilitating the formation of low crystalline, carbonated HA (CHA), where the high Mg ion concentration inhibits its transition to crystalline HA.. Several studies have been carried out on Mg-metal and its corrosion, but little data are available on the effect of MgO on mineral formation in vivo. The present study was performed to elucidate the effects of calcium phosphates, formed by catalysis, on the surface of Mg corrosion products, on human embryonic stem cells (hESCs) in vitro, and on bone tissue in vivo when implanted into rat tibia
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