Research and Education
Long-term retention of zirconia crowns cemented with current automixed cements

https://doi.org/10.1016/j.prosdent.2020.04.014Get rights and content

Abstract

Statement of problem

Automixing and dispensing cements is a straightforward approach with consistent dosing. Previous studies have demonstrated clinically significant differences in crown retention between power-liquid and paste-paste forms of the same cement, as the composition between the 2 differs. A self-adhesive modified-resin (SAMR) and a resin-modified glass ionomer (RMGI) cement, originally offered as a powder-liquid, are now in common use as paste-paste automixed cements. With the increased use of zirconia restorations, the long-term retention of zirconia crowns for these 2 automixed cements should be evaluated.

Purpose

The purpose of this in vitro study was to determine whether zirconia crowns cemented with 2 automixed cements provided clinically acceptable retention after 6 months of aging with monthly thermocycling.

Material and methods

Extracted molars were mounted in resin and prepared with a flat occlusal surface, 20-degree taper, approximately 4-mm axial length, and with the axio-occlusal line angle slightly rounded. Prepared teeth were equally distributed into 3 cementation groups (n=12) to achieve nearly equal mean preparation surface areas for each group. Zirconia crowns (IPS ZirCAD LT) were fabricated with an added occlusal bar to facilitate removal of the cemented crowns. Cement space was set at 45 μm axially and 55 μm occlusally. After sintering and before delivery, the intaglio surfaces were airborne-particle abraded with 50-μm alumina at 275-kPa pressure for 3 seconds and then steam cleaned. Cements were the original powder-liquid RelyX Luting (RMGI; RXL) as the control, paste-paste, automixed systems RelyX Luting Plus Automix (RMGI; RXLA), and RelyX Unicem 2 Automix (SAMR; RXUA). Crowns were cemented under 196 N force, placed in an oven at 37 °C and 100% humidity during setting and then thermocycled (5 °C-55 °C) for 5000 cycles monthly for 6 months. The crowns were removed axially with a universal testing machine at 0.5 mm/min. Removal forces were recorded and dislodgement stress calculated by using the surface area of each preparation. One-way ANOVA was used for dislodgement stress and force. Chi-square test was used for cement location after testing (α=.05).

Results

RXLA demonstrated considerably lower crown retention (1.3 MPa) and differed significantly (P<.001) from RXUA (3.1 MPa) and RXL (3.1 MPa). Modes of failure showed most of the cement remaining only in the crown intaglio for RXLA for all specimens, whereas half of the crowns for RXL and RXUA demonstrated cement adhesion to both dentin and the intaglio surface, indicating cohesive failure of the cement at separation. As the Levene test was significant, the Games-Howell test was used for mean differences. The χ2 analysis was significant.

Conclusions

After long-term aging with monthly thermocycling, high-strength zirconia crowns were strongly retained by 2 (RXL, RXUA) of the 3 cements. Crown retention for RelyX Luting Plus Automix was less than half in comparison and with cement found only on the intaglio surface after separation.

Section snippets

Material and methods

The methods were similar to those in a previous study.9 Extracted, unrestored human molars were cleaned, disinfected in 0.5% sodium hypochlorite,16 and stored in fresh tap water until cementation. The teeth were mounted in stainless steel rings and secured with autopolymerizing acrylic resin. The occlusal surface was sectioned perpendicular to the long axis by using a thin diamond saw (Isomet; Buehler). With a high-speed handpiece fixed to a modified surveyor, molars were prepared by rotating a

Results

The results of ANOVA in testing for differences in mean stress among the 3 cements were significant (P<.001). The Levene test for equal variances was significant (P<.001), thus the Games-Howell procedure was used for mean comparisons. After pairwise testing of means, the removal stress for RelyX Luting (RXL) and RelyX Unicem 2 Automix (RXUA) were found to be similar in magnitude (3.14 and 3.09 MPa) and not shown to differ, and both were significantly greater than the mean removal stress for

Discussion

In a previous study, a paste-paste, hand-mixed RelyX Luting Plus cement demonstrated poor crown retention compared with the original powder-liquid, hand-mixed version of the RMGI cement.15 The rationale was stated to be that the composition of powder-liquid and paste-paste versions of the cements were different out of necessity to formulate 2 paste consistencies. The present null hypothesis of no difference for zirconia crown retention among the 3 cements was rejected because the automixed RMGI

Conclusions

Based on the findings of this in vitro study, the following conclusions were drawn:

  • 1.

    After 6 months of aging with monthly thermocycling, high crown removal stresses were recorded for RelyX Luting and RelyX Unicem 2 Automix (3.14 and 3.09 MPa) without a statistical difference.

  • 2.

    Both exhibited cement residue on dentin and the crown intaglio for half of the specimens.

  • 3.

    For the remaining crowns, RelyX Luting had cement principally on the prepared dentin, and RelyX Unicem 2 Automix crowns had cement

Acknowledgments

The authors thank Nakanishi Dental Laboratory for crown production; B&B Dental Ceramic Arts for scanning duplicate working dies; 3M ESPE for supplying luting agents; Bill Kuykendall, Lab Engineer UW Mechanical Engineering, for guidance at testing facility; SEM assistance from Hanson Fong, UW Materials Science and Engineering; Dr Amanda Patterson for assistance with tooth preparation, impressions and formation of working dies; and Dipl. Ing Stefan Rues, Dental Materials Section Head at the

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Supported by the Dean and Margaret Spencer Clinical Research Fund, University of Washington School of Dentistry.

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