Fracture Strength of Monolithic Zirconia Crowns With Different Margin Designs

Objectives: To compare the crown fracture strength of monolithic zirconia crowns with a light chamfer and shoulder margin when cemented with resin-modified glass ionomer cement.
Methods: Digital design of a monolithic zirconia crown from a molar tooth preparation was completed with CAD software (TOC=20 degrees, height=4mm) and two finish lines were designed (Shoulder=0.8mm and Chamfer=0.3mm). Dies were milled using a millable resin composite (Crystal Ultra). Monolithic zirconia crowns (Bruxzir) were milled with a 0.8mm minimum thickness at the central occlusal pit (n=8/group). Crowns were cemented with resin-modified glass ionomer cement (RelyX Luting Plus). The specimens were stored in water (37°C, 24hours) and load fatigued in the Alabama wear machine for 400,000 cycles at 1Hz with 100N applied through a steel ball (dia=5mm) aligned on the central pit. Specimens were thermocycled for 10,000 cycles (5-55°C, 15s dwell time). Crowns were loaded to failure in a universal testing machine at 1mm/min with a steel ball aligned on the central pit with a 1mm rubber sheet interposed between the crown and indenter. The crown fracture strengths were compared with a t-test (alpha=0.05).
Results: 3 crowns fractured during mechanical fatigue (1 in Shoulder group and 2 in Chamfer group). The fracture load (mean±SD) of the Shoulder group was 3275±976N and the Chamfer group was 3133±658N. There was no statistical difference between groups.
Conclusions: Within the limitations of this study there is no effect of .8mm shoulder or a .3mm chamfer margin design on fracture strength of zirconia crowns.