Influence of preparation design and ceramic thicknesses on fracture resistance and failure modes of premolar partial coverage restorations.
BACKGROUND
Preparation designs and ceramic thicknesses are key factors for the long-term success of minimally invasive premolar partial coverage restorations. However, only limited information is presently available on this topic.


OBJECTIVE
The purpose of this in vitro study was to evaluate the fracture resistance and failure modes of ceramic premolar partial coverage restorations with different preparation designs and ceramic thicknesses.


METHODS
Caries-free human premolars (n=144) were divided into 9 groups. Palatal onlay preparation comprised reduction of the palatal cusp by 2 mm (Palatal Onlay Standard), 1 mm (Palatal-Onlay-Thin), or 0.5 mm (Palatal Onlay Ultrathin). Complete-coverage onlay preparation additionally included the buccal cusp (Occlusal Onlay Standard; Occlusal Onlay Thin; Occlusal Onlay Ultrathin). Labial surface preparations with chamfer reductions of 0.8 mm (Complete-Veneer-Standard), 0.6 mm (Complete-Veneer-Thin), and 0.4 mm (Complete Veneer Ultrathin) were implemented for complete veneer restorations. Restorations were fabricated from a pressable lithium disilicate ceramic (IPS-e.max-Press) and cemented adhesively (Syntac-Classic/Variolink-II). All specimens were subjected to cyclic mechanical loading (F=49 N, 1.2 million cycles) and simultaneous thermocycling (5°C to 55°C) in a mouth-motion simulator. After fatigue, restorations were exposed to single-load-to-failure. Two-way ANOVA was used to identify statistical differences. Pair-wise differences were calculated and P-values were adjusted by the Tukey-Kramer method (α=.05).


RESULTS
All specimens survived fatigue. Mean (SD) load to failure values (N) were as follows: 837 (320/Palatal-Onlay-Standard), 1055 (369/Palatal-Onlay-Thin), 1192 (342/Palatal-Onlay-Ultrathin), 963 (405/Occlusal-Onlay-Standard), 1108 (340/Occlusal-Onlay-Thin), 997 (331/Occlusal-Onlay-Ultrathin), 1361 (333/Complete-Veneer-Standard), 1087 (251/Complete-Veneer-Thin), 883 (311/Complete-Veneer-Ultrathin). Palatal-onlay restorations revealed a significantly higher fracture resistance with ultrathin thicknesses than with standard thicknesses (P=.015). Onlay restorations were not affected by thickness variations. Fracture loads of standard complete veneers were significantly higher than thin (P=.03) and ultrathin (P<.001) restorations.


CONCLUSIONS
In this in vitro study, the reduction of preparation depth to 1.00 and 0.5 mm did not impair fracture resistance of pressable lithium-disilicate ceramic onlay restorations but resulted in lower failure loads in complete veneer restorations on premolars.
Associate Professor, Department of Prosthodontics, School of Dentistry, Albert-Ludwigs-University Freiburg, Germany. Electronic address: petra.guess@uniklinik-freiburg.de.
Petra C
PC
Guess
Stefan
S
Schultheis
Martin
M
Wolkewitz
Yu
Y
Zhang
Joerg R
JR
Strub
D000134:Acid Etching, Dental / Q000379:methods; D001641:Bicuspid; D001732:Bite Force; D002484:Cementation / Q000379:methods; D002516:Ceramics / Q000737:chemistry; D003776:Dental Porcelain / Q000737:chemistry*; D017267:Dental Prosthesis Design; D019232:Dental Restoration Failure; D003799:Dental Stress Analysis / Q000295:instrumentation; D003801:Dental Veneers; D006801:Humans; D007284:Inlays; D008422:Materials Testing; D010756:Phosphoric Acids / Q000737:chemistry; D019279:Resin Cements / Q000737:chemistry; D013499:Surface Properties; D013696:Temperature; D013997:Time Factors; D019231:Tooth Preparation, Prosthodontic / Q000379:methods
C508050:IPS e.max Press; D010756:Phosphoric Acids; D019279:Resin Cements; C078484:Syntac dentine adhesive; C493120:lithia disilicate; D003776:Dental Porcelain; C103502:Variolink; C023506:Glass ceramics; C030242:phosphoric acid