Influence of Retainer Design on Shear Bond Strength in Fixed Partial Dentures: An In Vitro Comparative Study

Resin-bonded fixed dental prostheses (RBFDPs) are considered conservative alternatives to conventional fixed partial dentures for replacing missing teeth. However, concerns regarding retention and debonding remain, particularly in posterior regions. This study evaluated the influence of different retainer designs on the shear bond strength (SBS) of fixed partial dentures.

Fifteen monolithic zirconia fixed partial dentures were divided into three groups: conventional overlay-retained FPD (OVL), modified overlay-retained FPD (MOVL), and conventional full-coverage crown-retained FPD (FCC) (n = 5 each). Standardized preparations were performed on epoxy resin dies, and all restorations were cemented using dual-cure resin cement. Shear bond strength testing was carried out using a universal testing machine, followed by failure mode analysis under a stereomicroscope. Statistical analysis was performed using the Kruskal-Wallis and Chi-square tests.

Group C (FCC) showed the highest mean shear bond strength (1492.70 ± 87.25 N), followed by Group B (MOVL) (1201.60 ± 186.22 N), while Group A (OVL) demonstrated the lowest values (469.40 ± 190.57 N). A statistically significant difference was observed among the groups (p < 0.001). Adhesive failure predominated in Group A, whereas mixed failures were more common in Groups B and C. Failure mode distribution showed no statistically significant difference (p = 0.086).

The modified overlay-retained FPD demonstrated significantly higher shear bond strength than the conventional overlay design and showed performance comparable to the conventional full-coverage crown-retained prosthesis. Within the limitations of this in vitro study, the modified overlay design may serve as a conservative and clinically acceptable alternative for posterior fixed partial dentures.