## 4 Discussion
From this viewpoint, the glass-ceramics proposed in this work also have some limitations, but they exhibit interesting advantages compared to the majority of dental glass-ceramics commonly proposed in the literature [24]. Materials used for dental implantology should be able to quickly bond to host alveolar bone. TT-CEL2, as demonstrated by in vitro bioactivity tests (Figures 4 and 5), shows a highly bioactive behaviour: in fact, after immersion for just one week in SBF, a compact layer of HA formed on its surface. As reported by many authors, the presence of a HA layer, mimicking the bone mineral, \begin{table}
\begin{tabular}{c c c} \hline
**Property** & **Dentine** & **Enamel** \\ \hline Bending strength (MPa) & 30–120 & 60–200 \\ Elastic modulus (GPa) & 18–26 & 70–100 \\ Hardness (GPa) & 0.7–0.8 & 3.0–5.5 \\ Fracture toughness (MPa\(\cdot\)m\({}^{1/2}\)) & 2.4–2.5 & 1.0–1.5 \\ \hline \end{tabular}
\end{table} 
Table 5: Mechanical properties of a natural tooth (data from [5; 54; 55; 56; 57; 58]). promotes cell adhesion and migration, thus allowing the formation of a stable interface between the implant surface and the host bone [18; 20; 24]. It was also demonstrated that osteoblasts preferably attach and spread on HA nano-sized crystals [61]. By comparing the mechanical properties of TT-CEL2 with the data reported in Table 5, it is possible to observe that the bending strength and fracture toughness of TT-CEL2 are comparable to those of dentine. The toughness of traditional glass-ceramics is usually below 1 MPa\(\cdot\)m\({}^{1/2}\), and hence the fracture toughness of most bioceramics proposed in the literature is still far from being appropriate to produce artificial tooth roots. Moreover, it should be considered that the periodontal ligament contributes roughly 1 MPa\(\cdot\)m\({}^{1/2}\) to the fracture toughness in the sound tooth, as it acts to sustain part of the applied load [59]. Thanks to its excellent bioactivity, strength, and toughness, TT-CEL2 may be suggested for a tooth root replacement. However, the elastic modulus and hardness of TT-CEL2 are higher than those of dentine; in particular, the hardness is one order of magnitude higher. With regard to the materials used for restorative dentistry, they should have proper aesthetics; in addition, bioactivity on the surface of dental restorations must not occur [62]. TT-SCNA qualitatively exhibits an excellent whiteness, which reproduces the appearance of tooth enamel. Moreover, as assessed by soaking in SBF, TT-SCNA has a bioinert-like behaviour. By comparing the data reported in Tables 4 and 5, it is possible to observe that the bending strength and Young's modulus of the TT-SCNA approach those of tooth enamel; in addition, the toughness of TT-SCNA is three times higher than that of natural enamel. Therefore, TT-SCNA shows a great potential as material for restorative dentistry; for example, it could be used to fill enamel cavities or to produce partial/total dental crowns. Future assessment of the translucent characteristics of TT-SCNA will allow drawing more definite conclusions on the aesthetic suitability of this material for dental restoration [63]. As regards TT-FaGC, the situation is more complex. This material exhibits bioactive properties, even if not so good as TT-CEL2, and thus it might be suggested for dental implantology. However, the fracture toughness of TT-FaGC is lower than that of natural dentine. Potential suitability of TT-FaGC for enamel restoration may be suggested, too, considering that its bending strength is comparable to that of enamel and its Young's modulus is not much lower than that of enamel. In addition, the toughness of TT-FaGC is twice as high as that of enamel. Finally, the presence of FA crystals could impart an anti-cariotic effect to the material inducing enamel remineralisation, as reported by some authors [64].