Features of fracture of prosthetic tooth-endocrown constructions by means of acoustic emission analysis.

OBJECTIVE: The study aims at comparing the fracture resistance of different restorative materials used in dental endocrown restorations and respective endocrown restorations under a quasi-static compressive load using acoustic emission (AE) method.

METHODS: Five restorative materials were used in this study. The restorative materials were manufactured into discs 13mm in diameter and 5mm thick, which were then divided into 5 groups and included into Type 1: Group B: zirconium dioxide (Prettau zirconia); Group C: ceramics (IPS e.max Press); Group D: metal ceramics (GC Initial MC+Nicrallium N2 BCS); Group E: composite resin (Nano Q); Group F: luting cement (RelyX™ U200). Twenty-five extracted human molars were divided into 5 groups and included into Type 2: Group A: control, no restoration; Group BE: restored by zirconium dioxide endocrowns; Group CE: restored by ceramic endocrowns; Group DE: restored by metal ceramic endocrowns; Group EE: restored by composite resin endocrowns. An increasing load was applied to the center of the samples with a hard steel ball until a fracture occurred. The loading rate was 0.12mm/min. An AE system was used to monitor the fracture of the samples. The load corresponding to the first AE event and the final fracture load were used to evaluate the fracture resistance of the restored teeth. The data were analyzed using ANOVA and Tukey's post hot test (α=0.05).

RESULTS: A lower threshold of 220μV was selected to exclude spurious background signals. For the initial fracture load of Type 1 samples, Group F (0.029kN)<Group E (0.039kN)<Group D (0.056kN)<Group C (0.253kN)<Group B (intact). The same trend was found for the final fracture load, i.e., Group F (1.289kN)<Group E (1.735kN)<Group D (3.362kN)<Group C (6.449kN)<Group B (intact). For the initial and final fracture load, statistically significant differences (p<0.05) were found between group C and the others groups. For the initial fracture load of Type 2 samples, Group EE (0.069kN)<Group DE (0.072kN)<Group CE (0.148kN)<Group BE (2.511kN). For the final fracture load, Group EE (1.533kN)<Group CE (2.726kN)<Group BE (3.082 kN)<Group DE (3.320kN). The initial fracture load of the ceramic samples is somewhat higher than that for the endocrown restorations with the endocrowns made of this material (0.253 and 0.148kN, respectively). At the same time, for the metal ceramic and composite resin samples, the initial fracture loads are somewhat lower than in case of compression of the endocrown restorations with the endocrowns made of these materials (0.056 and 0.072kN; 0.039 and 0.069kN, respectively). The final fracture load of all the samples of the dental materials exceeds the strength of the respective endocrown restorations. The final fracture loads of the endocrown restorations with zirconium dioxide and ceramic endocrowns (3.082 and 2.726kN, respectively) are significantly lower than the final fracture load of the respective endocrown materials (intact and 6.449kN, respectively).

SIGNIFICANCE: Dental restorations should be made of high-strength materials. Zirconia displayed the highest fracture strength, while composite resin had the lowest fracture strength out of the materials used for the endocrowns. For teeth restored with endocrowns, the use of metal ceramics as endocrown material may lower the risk of failure during clinical use.