Innovative Composite Evaluation Method:  Cavity Adaptation Combined with Bond Strength

Objectives: Loss of interracial adaptation due to shrinkage stress or weak adhesion are current problems occasionally overlooked. This study aimed to evaluate the adaptation at the floor and microtensile bond strength (MTBS) in the same class-I cavity, and examine their correlation. Methods: Cylindrical cavities 3 mm in diameter and 1.7 mm in height were prepared in human molar teeth. The cavities were treated by a two-step self-etching adhesive (Clearfil SE Bond; Kuraray Medical, Japan) in one group, and an all-in-one adhesive (Xeno V; Dentsply, Germany) in the other group. They were then bulk-filled using a flowable composite(SDR; Dentsply). Each specimen was three-dimensionally scanned using a single-mode fiber-based swept-source optical coherence tomography (OCT; Santec, Japan) at 1310 nm center wavelength. Gap formation at the cavity base was quantified on cross-sections obtained from the 3D scans according to the backscatter signal intensity from the interface; a significant peak was indicating gap. The same cavities were then sectioned by a low-speed diamond saw to produce MTBS beams 0.7 mm in lateral dimensions, and subjected to loading at a 1 mm/min cross-head speed. Results: The majority of gaps were observed when the all-in-one bonding system was used. The two-step self-etch adhesive showed a significantly better adaptation, accompanied by a higher bond strength. When the data were pooled, Spearman's rank correlation showed a moderate correlation between length percentage of gap-free interface at the cavity floor and mean MTBS value of the beams obtained from the same specimen (rho = 0.67, P < 0.05). Conclusions: While different in nature, cavity floor adaptation and MTBS followed similar trends in both groups. The combination of OCT imaging tool with the bond strength test may offer further insight to the actual performance of materials under more clinically relevant situations, compared to testing of adhesion on flat dental surfaces.