Fracture characteristics and translucency of multilayer monolithic zirconia crowns of various thicknesses

Chien-Ming Kang; Wen-Chieh Hsu; May-Show Chen; Hsuan-Yu Wu; Yuichi Mine; Tzu-Yu Peng

doi:10.1016/j.jdent.2024.105023

Fracture characteristics and translucency of multilayer monolithic zirconia crowns of various thicknesses

J Dent. 2024 Apr 24:145:105023. doi: 10.1016/j.jdent.2024.105023. Online ahead of print.

Authors

Chien-Ming Kang¹, Wen-Chieh Hsu², May-Show Chen³, Hsuan-Yu Wu⁴, Yuichi Mine⁵, Tzu-Yu Peng⁶

Affiliations

¹ Huayi Dental Laboratory, Taipei, Taiwan; School of Dentistry, College of Oral Medicine, Taipei Medical University, Taipei, Taiwan.
² School of Dentistry, College of Oral Medicine, Chung Shan Medical University, Taichung, Taiwan; Department of Oral Hygiene, Hsin Sheng Junior College of Medical Care and Management, Taoyuan, Taiwan; Research Center of Precision Biomedical Implants, College of Oral Medicine, Taipei Medical University, Taipei, Taiwan.
³ School of Dentistry, College of Oral Medicine, Taipei Medical University, Taipei, Taiwan; Division of Prosthodontics, Department of Dentistry, Taipei Medical University Hospital, Taipei, Taiwan.
⁴ School of Dental Technology, College of Oral Medicine, Taipei Medical University, Taipei, Taiwan.
⁵ Department of Medical Systems Engineering, Graduate School of Biomedical and Health Sciences, Hiroshima University, Hiroshima, Japan; Project Research Center for Integrating Digital Dentistry, Hiroshima University, Hiroshima, Japan.
⁶ School of Dentistry, College of Oral Medicine, Taipei Medical University, Taipei, Taiwan. Electronic address: typeng@tmu.edu.tw.

PMID: 38670331
DOI: 10.1016/j.jdent.2024.105023

Abstract

Objectives: Multilayer monolithic zirconia (M-Zr) crowns can be engineered to achieve gradational translucency and color intensity. However, this modification may compromise the mechanical strength, raising concerns regarding the ability of M-Zr crowns to withstand occlusal stresses. The effects of M-Zr crown thickness on translucency and ability to endure occlusal forces were investigated at different tooth positions (incisors, premolars, and molars). The objective was to determine the minimal thickness of M-Zr crowns used in tooth preparation to meet aesthetic and functional demands.

Methods: M-Zr samples (Vita A1) with four thicknesses (0.5, 1.0, 1.5, and 2.0 mm) were prepared and subjected to translucency testing using a digital colorimeter by 3-third and 9-square division methods. Crown-shaped M-Zr samples with three thicknesses (1.0, 1.5, and 2.0 mm) and three tooth positions (incisor, premolar, and molar) were digitally designed, and 2.0 mm metal abutments were fabricated. The samples were bonded to the abutments; their fracture characteristics were evaluated using a universal testing machine, and their fracture surfaces examined using an optical microscope. Statistical analyses included the Shapiro-Wilk test, Pearson correlation, and one-way and two-way ANOVA with a post hoc Tukey HSD test (α = 0.05).

Results: Color analysis results revealed a significant negative correlation between thickness and translucency (r < -0.96, P < 0.01), with the highest values in the incisal region. Cross-sectional profiles confirmed the uniform thickness and morphology of the digitally designed M-Zr crowns. The results of fracture strength analysis showed position-dependent variability, a strong positive correlation with thickness (r > 0.96, P < 0.01), and fracture strengths consistently exceeding 1200 N across all tooth positions. Fracture patterns indicated that thinner crowns at the incisors and molars were more prone to cracking, whereas those at the premolars demonstrated significantly higher strength (4872.51 N, P < 0.05), only with crack or even no fracture occurring at 2.0 mm.

Conclusions: Thickness significantly influenced both the translucency and fracture strength of M-Zr, with the tooth position playing an additional role, albeit to a lesser extent. Although thinner crowns exhibited lower strength at each tooth position, even at a thickness of 1.0 mm, fracture strength exceeding 1200 N was maintained, surpassing the typical occlusal forces. Thus, it can be asserted that M-Zr crowns with a minimum thickness of 1.0 mm can meet both aesthetic and functional requirements.

Keywords: CAD/CAM; Dental crown; Digital colorimeter; Fracture strength; Multilayer monolithic zirconia.