Evaluation of Retentive Force and Long-Term Performance of 4 Mol% Yttria Partially Stabilized Zirconia (4Y-PSZ) Double Crowns.

PURPOSE

This study aims to evaluate the effects of taper angle and the number of insertion-removal cycles on the retention force of 4 mol% yttria partially stabilized zirconia (4Y-PSZ) double crowns over time.

MATERIALS AND METHODS

Primary and secondary crowns were fabricated using 4Y-PSZ with taper angles of 2°, 4°, and 6° (n=15). Retention force during crown removal was measured after applying 50-N and 100-N loads. Insertion-removal cycles were performed at 0, 2500, 5000, 7500, and 10,000 cycles, and retention force was assessed at each stage. Contact conditions between the inner and outer crowns were evaluated via color mapping at 0 and 10,000 cycles. Two-way ANOVA was used for statistical analysis, followed by Tukey's test for multiple comparisons.

RESULTS

Retention force significantly decreased with smaller taper angles and higher cycle numbers (p<0.01). For a taper angle of 4°, the initial retention force of 10.36 N decreased significantly to 5.98 N after 10,000 cycles (p<0.01). For a taper angle of 6°, the initial retention force of 1.64 N dropped to 0.03 N after 10,000 cycles. Color mapping revealed continuous contact along the axial surface, with strong point contacts, particularly in the central axial region. Smaller taper angles exhibited stronger contact points, which decreased with repeated cycles, indicating changes in contact conditions.

CONCLUSIONS

Higher retentive forces were achieved with increased loading pressures and smaller taper angles, while repeated insertion-removal cycles led to a progressive reduction in retention. These findings provide foundational data supporting the clinical application of 4Y-PSZ double crowns.