Fatigue Resistance of Monolithic Ceramic Crowns: In Vitro Comparison with Teeth.

Purpose: To compare the tooth fatigue resistance to lithium disilicate, UTML (ultra-translucent-multi-layered zirconia), and Y-TZP (yttrium-stabilized tetragonal zirconia) monolithic crowns, tested using a new, simplified fatigue testing machine (Ball-mill) and digital wear analysis. Materials and Methods: Twenty extracted human molars were used. In addition, 60 monolithic crowns were produced and subdivided into 3 groups (n = 20) according to material (lithium disilicate 1.5 mm, IPS e-max CAD LT, IvoclarVivadent; UTML 1.5 mm, Katana STML, Kuraray Noritake; Y-TZP 1.0 mm, Katana, Kuraray Noritake). Teeth were subjected to three 10-min cycles of Ball-mill and to one 30-min cycle. For monolithic crowns, twelve 60-min cycles were performed (since in a preliminary pilot study no significant volumetric losses were observed before 60 min). At T0 and after each cycle, an analysis was carried out using a stereomicroscope (Wild M3C, Heerbrugg) to evaluate the wear morphology, an intraoral scanner (CS 3500, Carestream Dental), and software (Rhinoceros, McNeel) to calculate the volumetric loss of the samples caused by the wear. Results: The lithium disilicate and Y-TZP crowns showed a 100% survival rate and high wear resistance; UTML had 95% survival. Lithium disilicate and UTML showed many parting-cleavage fractures (>90%). The wear generated on teeth at T60 was statistically greater (p < 0.05) than the one of crowns, Y-TZP in particular (p < 0.0001); all couples of compared materials differed significantly (p = 0.0008), except for lithium disilicate and UTML crowns. From comparison with the anthropometric classification of Miles, it was estimated that 60 min of Ball-mill testing correspond approximately to an average clinical wear time of 18 years. Conclusion: The Y-TZP crowns showed the best results in terms of wear resistance. The lithium disilicate presented excellent resistance, but with uniform 1.5-mm thickness over the entire crown (marginal and occlusal area). The Ball-mill testing machine provided good results in reproducing the clinical conditions: its in vitro outcomes demonstrated the same mechanical characteristics of the 3 ceramics as shown by other fatigue testing machines. The investigation methodology was effective when using digital technology to compare materials and calculate the volumetric dental loss over time.