Laboratory simulation of Y-TZP all-ceramic crown clinical failures

Abstract

Clinically, zirconia-supported all-ceramic restorations are failing by veneer-chipping without exposing the zirconia interface. We hypothesized that mouth motion step-stress-accelerated fatigue testing of standardized dental crowns would permit this previously unrecognized failure mode to be investigated. Using CAD software, we imported the average dimensions of a mandibular first molar crown and modeled tooth preparation. The CAD-based tooth preparation was rapid-prototyped as a die for fabrication of zirconia core porcelain-veneered crowns. Crowns were bonded to aged composite reproductions of the preparation and aged 14 days in water. Crowns were single-cycle-loaded to failure or mouth-motion step-stress- fatigue-tested. Finite element analysis indicated high stress levels below the load and at margins, in agreement with only single-cycle fracture origins. As hypothesized, the mouth motion sliding contact fatigue resulted in veneer chipping, reproducing clinical findings allowing for investigations into the underlying causes of such failures.

Figure 1.

Finite element analysis results. (A) CAD-designed four-cusp right first molar preparation and (B) cement, zirconia core, and veneer solid models. (C) Assembled preparation, cement, zirconia core, and veneer solid models. (D,E) Resulting maximum principal stress obtained on the veneer solid under a 1200-N vertical load applied on the mesial-distal cusp of the veneer solid; and (F,G) resulting maximum principal stress obtained on the zirconia core solid under a 1200-N vertical load applied on the mesial-distal cusp of the veneer solid. Note the high maximum principal stress values observed in the bottom part of the zirconia core layer (F) at the region immediately below the load, and at the cervical region.

Figure 2.

Crowns made from the anatomically correct crown model. (A) Plastic prototype of the prepared tooth, (B) plastic prototype inserted into a dental arch and articulator, (C) all-ceramic crown with 0.5-mm zirconia core (D) crown cemented onto plastic prototype, embedded in acrylic resin in preparation for load testing.

Figure 3.

Fractographic evaluation of failed specimens. (A) Single load-to-fracture: hackles (black arrows) originating from crack initiated on bottom part of the core in a Lava-Espe crown (radial crack). (B) Sliding-contact mouth-motion step-stress fatigue crown: SEM and polarized light microscopy (insert) aspect of indenter damage on lingual cusp (circle), after sliding through lingual slope of buccal cusp and causing a chip-off (pointers). Note that the indenter extended damage to interproximal and lingual cusps by developing cone cracks (black arrows). (C) Most common failure mode was chipping of veneering porcelain not reaching the core/ceramic interface, as depicted in this occlusal view of the buccal cusp. Tooth (t), cement (c), Zr core (Zr), veneering porcelain (p). White arrow points to the cervical margin.

Figure 4.

Weibull distribution graph for the Lava all-ceramic crowns depicts unreliability vs. number of cycles (90% two-sided confidence bounds) with a load of 200 N. Weibull modulus of 1.7 indicates that fatigue was not a strong accelerator of failure.