Purpose: To evaluate the effect of the occlusal auxiliary retentive feature (OARF) on retentive force of crowns to commercially pure titanium (cpTi) dies, with and without crown preparations. Methods: Sixty cpTi dies were produced by wire-electric discharge machining, half of them with a truncated cone-shaped crown preparation (1 mm in height and 20° wall taper), and the other half without crown preparation. The cpTi dies were then assigned two equal groups: 1. Without OARF and 2. With OARF (3 mm in diameter by 2 mm in depth). A casting of an Au-Pd alloy (Castwell M.C. 12% gold, GC) specimen for each die was fabricated and seated onto a cpTi die with the following luting cements: zinc phosphate cement (ZPC; Elite cement 100, GC), resin-modified glass ionomer cement (RMGIC; Vitremer, 3M-ESPE), and resin-based luting cement (RBLC; Super-Bond C&B, Sun Medical). The specimens were then immediately immersed in 37° C deionized water for 24 hours prior to tensile loading at 1 mm/min in an universal testing machine until fractured. Statistics: Two-way ANOVA and Fisher's PLSD. Results: Two-way ANOVA for each luting cement revealed that there was a significant difference between the OARF in groups ZPC and RBLC (p=0.001 and p=0.039), whereas no significant difference was found in group RMGIC (p=0.072). With the presence of the axial wall, tensile force was significantly increased in group RBLC (p=0.017). Surface area (mm²) of each die without and with crown preparations were (without/with OARF) 28.3/38.9 and 64.9/74.9,respectively. Conclusion: The crown retention is enhanced by an increase of the surface area when luting with resin-based luting cement. However, when luting with zinc phosphate or resin-modified glass ionomer cement, OARF preparation has only a small effect on improving the crown retention; in particular, the abutment has a short axial wall and large wall taper.

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