Introduction:
The adhesion of composite resins is obtained by etching the enamel with phosphoric acid. It is the result of capillary penetration of surface irregularities. The rate of capillary penetration depends on the wetting and viscosity of the sealant.

Aim of the study:
To present the quality of bond of dental fillings to hard dental tissues.

Material and methods:
Commercial dental composites were used in the study: a liquid consistency the so-called flow type (Flow Art by Arkona) and hybrid composite with microfiller (Gradia Direct by GC) and twenty experimental composites. The flow-type composite contains mineral fillers, which constitute about 60% of its composition. It is cross-linked with visible light from the blue range (430-500 nm). The commercial composites have been modified with synthetic hydroxyapatite at different percentages: 2%, 5% and 8% by weight. Two types of hydroxyapatite powder were used in the study: commercial <200 nm and synthesized <25µm. Molars and premolars were used in the study.

Results:
The adhesion of composite material to the enamel and composite material is a mechanical process. For this purpose, etching with phosphoric acid is used, which prepares the enamel surface. This process allows the adhesive resins to penetrate deep into the enamel, resulting in a permanent bond.

Conclusions:
When analyzing the photographic material, no gap was found between the dental filling and the tooth. Noticeable is the layer that has been identified as hybrid, which is described in the literature. At higher magnifications, the so-called microspheres can be observed that indicate bonding of composite material to hard dental tissues.