Recently, Watts et al.  investigated the substitution of CaO by MgO on the bioactive glass structure. By increasing the MgO content, both the glass transition temperature and dilatometric softening point decrease, while the thermal expansion coefficient rises. This is due to the considerably poorer bond strength of Mg–O in comparison to Si–O, which weakened the overall glass network . The presence of magnesium as a trace LDC000067 in the melt-derived 46S6 glass composition produced effects such as improving the bioactive glass dissolution and increasing the silica gel layer thickness . The surface reactivity of glass in SiO2–P2O5–CaO–MgO was investigated and it was shown that MgO in the glass indirectly advances the early stage of mineralization by favoring the silicon atom with non-bridging oxygen speciation . Increased surface reactivity by increasing MgO/CaO ratio was observed, which can play an important role in glass bioactivity. On the other hand, some in vitro results indicate that MgO has a detrimental effect on apatite formation. The rate of glass degradation gradually decreases with the increase of MgO, and the formation of apatite layer on glass surface is retarded , ,  and . It was shown that the apatite layer formation rate slows down when MgO content is above 7 mol%.