Digital versus conventional veneering of zirconia and cobalt chromium crowns: Fracture load before and after thermomechanical aging.

STATEMENT OF PROBLEM: To complement the digital workflow for manufacturing fixed dental prostheses, both high-strength frameworks and esthetic veneers should be designed and fabricated digitally. However, how the fracture load of digitally veneered restorations compares with conventionally fabricated restorations is unclear.

PURPOSE: The purpose of this in vitro study was to examine the fracture load of digitally and conventionally veneered zirconia and cobalt chromium crowns initially and after thermomechanical aging.

MATERIAL AND

METHODS

Milled zirconia and cobalt chromium copings for a maxillary canine were fabricated (N=96). Digital veneers were milled and connected to the copings with a sintered ceramic slurry. The conventional veneers were fabricated by using a master mold, and the crowns were bonded to the cobalt chromium abutments. Half the specimens were subjected to 6000 thermal (5 °C to 55 °C, 60 seconds) and 1 200 000 mechanical (50 N, 1.5 Hz, 0.7 mm lateral movement) cycles opposed by steatite antagonists, and the fracture load was determined. Fracture types were categorized, and scanning electron microscopy performed. The data were analyzed with a 3-way global univariate analysis of variance, t test, the Pearson chi-squared test, and the Weibull modulus (α=.05).

RESULTS

Unlike the framework material (P=.316) and artificial aging (P=.064), the veneering protocol affected the fracture load (P=.007). Digital veneers (range: 2242 to 2929 N) led to lower values than conventional veneers (range: 2825 to 3166 N), which was significant for aged cobalt chromium copings (P=.024; 2242 versus 3107 N). Conventionally veneered crowns showed lower Weibull moduli after thermomechanical aging (range: 3.2 to 3.5) than initially (range: 7.8 to 11.4). The copings of all the zirconia specimens fractured, while chipping occurred with the cobalt chromium specimens.

CONCLUSIONS

The high fracture load values of the veneered crowns, even after simulated 5-year aging, indicated sufficient mechanical properties (nearly 4-fold the average occlusal force of 600 N) for the successful clinical application of digitally veneered zirconia and cobalt chromium copings.