Resistance to fracture of two all-ceramic crown materials following endodontic access.
BACKGROUND
There is currently no protocol for managing endodontic access openings for all-ceramic crowns. A direct restorative material is generally used to repair the access opening, rendering a repaired crown as the definitive restoration. This endodontic procedure, however, may weaken the restoration or initiate microcracks that may propagate, resulting in premature failure of the restoration.


OBJECTIVE
The purpose of this in vitro study was to evaluate how an endodontic access opening prepared through an all-ceramic crown altered the structural integrity of the ceramic, and the effect of a repair of this access on the load to failure of an all-ceramic crown.


METHODS
Twenty-four alumina (Procera) and 24 zirconia (Procera) crowns were fabricated and cemented (Rely X Luting Plus Cement) onto duplicate epoxy resin dies. Twelve crowns of each were accessed to simulate root canal treatment therapy. Surface defects of all accessed specimens were evaluated with an environmental scanning electron microscope. The specimens were repaired with a porcelain repair system (standard adhesive resin/composite resin protocol) and were loaded to failure in a universal testing machine. Observations made visually and microscopically noted veneer delamination from the core, core fracture, shear within the veneer porcelain, or a combination thereof. A Kruskal-Wallis test was used to determine if a significant difference (alpha=.05) in load to failure existed between the 4 groups, and a Mann-Whitney test with a Bonferroni correction (P<.0125) was used for multiple comparisons. A Weibull analysis was also used to estimate the Weibull modulus and characteristic failure for each group.


RESULTS
All specimens exhibited edge chipping around the access openings. Some displayed larger chips within the veneering porcelain, and 4 zirconia crowns showed radial crack formation. There was a significant difference in load to failure among all groups with the exception of the alumina intact and repaired specimens (P=.695). The alumina crowns generally showed fracture of the coping with the veneering porcelain still bonded to the core, whereas the zirconia copings tended not to fracture but experienced veneering porcelain delamination.


CONCLUSIONS
Endodontic access through all-ceramic crowns resulted in a significant loss of strength in the zirconia specimens but not in the alumina specimens.
Wood
Kelly Copps
KC
Department of General Dental Sciences, Marquette University School of Dentistry, Milwaukee, 53233, USA.
Berzins
David W
DW
Luo
Qing
Q
Thompson
Geoffrey A
GA
Toth
Jeffrey M
JM
Nagy
William W
WW
D000537:Aluminum Oxide; D003442:Crowns; D003776:Dental Porcelain; D017269:Dental Prosthesis Repair; D019232:Dental Restoration Failure; D003799:Dental Stress Analysis; D008422:Materials Testing; D003765:Models, Dental; D018915:Root Canal Preparation; D018709:Statistics, Nonparametric; D016019:Survival Analysis; D015040:Zirconium
D003776:Dental Porcelain; D015040:Zirconium; D000537:Aluminum Oxide; C028541:zirconium oxide