The purpose of this study was to evaluate the effects of the incorporation of hydroxyapatite (HA) derived from cuttlefish bone on the chemical and mechanical properties of glass-ionomer cements (GIC). The null hypothesis was that the GICs modified with HA would not show significantly better performance in terms of mechanical and chemical properties than the materials without HA added to the powders of two commercially available GICs. Methods Two materials were used in this study: Fuji II LC and Fuji IX GP Extra. Cuttlefish bones (Sepia officinalis L.) from the Adriatic Sea were used for the hydrothermal synthesis of HA. Cuttlefish bone-derived HA was ground and sifted through a 180-µm-sized sieve leaving the HA powder in the form of hexagonal column crystal aggregates with a diameter of <180 µm. The fluoroaluminosilicate glass powder and HA powder were hand-mixed with a mortar and pestle for 20 min to obtain as uniform a distribution of HA as possible. The prepared powder was then mixed with the polyacid liquid by spatulation. The research was conducted through three types of testing. First, the compressive strength (CS), diametral tensile (DTS), and flexural strength (FS) were tested on a universal testing machine. Four groups (n=15) were prepared for each material. The first group was without the addition of HA, while 2, 5 and 10% by weight of HA were added to the three experimental groups. The samples were stored in distilled water for 7 days before testing. In the second part of the research, the properties of microhardness and surface roughness were tested. The samples for measuring microhardness and surface roughness were made in Teflon molds 5 mm in diameter and 2 mm deep, and stored in distilled water at 37 °C for 7 days before testing. Microhardness (4 groups for each material, n=80) was measured on a Vickers hardness tester at a load of 980 g for 15 seconds, and one sample from each group was separated for the SEM analysis: those samples were vacuum dried and coated with gold particles, and then examined for topographic and compositional changes (with a JSM-6400 Electron Microscope equipped with the NORAN System 6 X-Ray Microanalysis System, JEOL, Tokyo, Japan). Surface roughness was measured with a contact profilometer. The third part of the research was the measurement of fluoride releases from the GIC. The fluoride release was measured with an ion-selective electrode after 24 hours, 7 days, and 45 days of stirring, on 6 samples per group. The samples were made in individual Teflon molds with a diameter of 8 mm and a depth of 2 mm. ANOVA with Tukey's post-hoc test was used for statistical analysis. Results The first type of research: The control and experimental groups of Fuji II LC showed significantly higher values of CS, DTS and FS compared to the Fuji IX GP Extra groups. CS and FS were highest in the Fuji IX GP Extra control group. The difference in CS between the Fuji IX GP Extra control group and the Fuji IX GP Extra with 2 wt% HA was significant. The Fuji II LC group, modified with 10% by weight of HA, showed significantly higher FS compared to the Fuji II LC control group (p <0.05), but the difference was not significant compared to the Fuji II groups with 2 wt% and 5 wt% HA. The FS of the Fuji IX group modified with 5 wt% HA was significantly higher than the FS of the Fuji IX group without HA. However, there was no significant difference in DTS between the Fuji II group without HA and the Fuji II groups modified with 2 wt%, 5 wt%, and 10 wt% HA. There was also no significant difference in DTS between the Fuji IX group without HA and the Fuji IX groups modified with 2 wt%, 5 wt%, and 10 wt% HA. The second part of the research: The microhardness values obtained for the Fuji II LC modified with 10 wt% HA were significantly higher compared to the other two groups with modified powder. The lowest microhardness is observed for 2 wt% HA, but Tukey test showed that microhardness of the Fuji II with 10 wt% HA was significantly higher. In the Fuji IX GP Extra groups, it was observed that the increase of HA concentration decreased microhardness. However, only the difference between 10 wt% HA concentration and 2 wt% HA concentration as well as the difference between 10 wt% HA concentration and the group without added HA particles were significant. There was no evidence that microhardness for the group without HA particles and the groups with 2 wt% HA and 5 wt% HA differ. The comparison of materials with respect to SR showed a significant difference between (p<0.0001) Fuji II LC and Fuji IX GP Extra, with higher SR values for the HA-modified materials. The third part of the research: The level of released fluorides was statistically significantly higher in Fuji IX Extra GP than in Fuji II LC, regardless of the HA modification, and the groups modified with 2 and 5 wt% HA showed significantly increased fluoride release at all three time points. The sample with 10 wt% HA showed significantly higher amounts of released fluorides than the group without HA, but lower than the samples with 2 and 5 wt% HA. With respect to time, it can be noticed that the concentration of the released fluoride ions rate significantly decreased with time (p < 0.001). This decrease was significantly different for the two materials (p < 0.001). When observing the effects of adding HA to the materials, the fluoride release was different when compared to the group without HA added. The sample with 10 wt% HA showed significantly higher amounts of released fluoride than the group without HA, but lower than the samples with 2 and 5 wt% HA. Conclusions In this doctoral dissertation, all null hypotheses were rejected, and it was proved that the addition of micro-HA derived from the cuttlefish bone powder to the Fuji IX and Fuji II powders did not improve the FS, CS and DTS of the chemical curing of the Fuji IX groups; the addition of micro-HA obtained from the cuttlefish bone powder to Fuji II LC improved the mechanical properties of the Fuji II LC group, and at a content of 10 wt% HA, there was a significant improvement in FS. The porous HA incorporated in Fuji IX GP Extra had a negative effect on FS, CS and DTS; CS was significantly reduced in the Fuji IX GP Extra 2 wt% HA and Fuji IX GP Extra 10 wt% HA groups, and SC is significantly reduced in Fuji IX GP Extra 5 wt% HA, compared to the Fuji IX GP Extra without HA particles group. Otherwise, the addition of HA particles in Fuji II LC had a positive effect on CS, DTS and FS. The Fuji II LC groups modified with 10 wt% HA showed the most favorable results compared to FS. This study showed that the addition of micro-HA derived from the cuttlefish bone powder to Fuji IX GP Extra and Fuji II LC did not improve SR and reduced the MH values in all groups except in the Fuji II LC group with 10% by weight of HA.The samples of Fuji IX GP Extra modified with 10 wt% HA showed the most favorable results after 24 hours, and 2 and 5 wt% HA improved the concentration of released fluorides at all three time points.