Retention of 3Y- Zirconia Crowns Cemented to Conventional PMMA, Milled PMMA, and CR-PEEK Denture Bases After Thermal Cycling

Objective: The aim of this in vitro study was to assess and compare retention of 3Y-zirconia crowns cemented to various denture bases including conventionally processed PMMA, milled PMMA, and CR-PEEK.

Methods: Denture base cuboids of conventional PMMA, milled PMMA, and CR-PEEK were designed and fabricated with a tooth preparation to receive a zirconia crown with wings, 14 of each specimen. Forty-two molar 3Y-monolithic zirconia crowns were prepared with standardized dimensions (1 mm shoulder margin, 2 mm axial wall height, 20-degree taper) and minimum thickness of 1 mm. Surface area of the intaglio of each crown was assessed in square millimeters (mm²). Each denture base segment was sandblasted, treated with visio.link primer, and crowns were primed with MDP and cemented with dual-cure resin cement. After 24 hours, samples were thermal cycled between 5 and 55ºC for 5,000 cycles. Samples underwent a crown pull test using a universal testing machine. Crown retention force was recorded in Newtons (N) and crown retention strength (MPa) was calculated. Normality was assessed with D’Agostino-Pearson test. Data was analyzed using one-way ANOVA with Tukey post hoc tests. Significance set at α=0.05.

Results: The one-way ANOVA revealed significant differences between groups (F= 3.4540, p = 0.04186). The Tukey post-hoc revealed no statistically significant differences between groups. The milled PMMA and conventional PMMA (p=0.0661) as well as milled PMMA and CR-PEEK (0.0730) approached statistical significance. There was no difference in crown retention strength between conventional PMMA and CR-PEEK (p= 0.9988). The crown retentive strengths were highest for conventional PMMA (5.5390 MPa), with CR-PEEK similar (5.5066 MPa), and lowest for milled PMMA (3.9001 MPa). The most common Mode of Failure was adhesive failure, with conventional PMMA failing adhesively to the denture base (75%) while CR-PEEK and milled PMMA failed to the crown (71% and 62%, respectively). Conventional PMMA was the only group to have fracture of the tooth preparation. 

Conclusions: Bonding zirconia crowns to milled PMMA, conventional PMMA, and CR-PEEK denture bases results in acceptable crown retention strengths, with no significant difference between denture base materials.