Fatigue resistance of CAD/CAM resin composite molar crowns

Abstract

Objective: To demonstrate the fatigue behavior of CAD/CAM resin composite molar crowns using a mouth-motion step-stress fatigue test. Monolithic leucite-reinforced glass-ceramic crowns were used as a reference.

Methods: Fully anatomically shaped monolithic resin composite molar crowns (Lava Ultimate, n=24) and leucite reinforced glass-ceramic crowns (IPS Empress CAD, n=24) were fabricated using CAD/CAM systems. Crowns were cemented on aged dentin-like resin composite tooth replicas (Filtek Z100) with resin-based cements (RelyX Ultimate for Lava Ultimate or Multilink Automix for IPS Empress). Three step-stress profiles (aggressive, moderate and mild) were employed for the accelerated sliding-contact mouth-motion fatigue test. Twenty one crowns from each group were randomly distributed among these three profiles (1:2:4). Failure was designated as chip-off or bulk fracture. Optical and electron microscopes were used to examine the occlusal surface and subsurface damages, as well as the material microstructures.

Results: The resin composite crowns showed only minor occlusal damage during mouth-motion step-stress fatigue loading up to 1700N. Cross-sectional views revealed contact-induced cone cracks in all specimens, and flexural radial cracks in 2 crowns. Both cone and radial cracks were relatively small compared to the crown thickness. Extending these cracks to the threshold for catastrophic failure would require much higher indentation loads or more loading cycles. In contrast, all of the glass-ceramic crowns fractured, starting at loads of approximately 450N.

Significance: Monolithic CAD/CAM resin composite crowns endure, with only superficial damage, fatigue loads 3-4 times higher than those causing catastrophic failure in glass-ceramic CAD crowns.

Keywords: CAD/CAM crowns; Fatigue; Fracture; Glass-ceramic; Resin composite; Weibull analysis.

Conflict of interest statement

Conflict of Interest

All authors declare no conflict of interest.

Copyright © 2015 Academy of Dental Materials. Published by Elsevier Ltd. All rights reserved.

Figures

Figure 1

Microstructure of the resin composite Lava Ultimate (left) and the glass-ceramic IPS Empress CAD (right, courtesy of Ivoclar). Both materials were polished down to 1 µm prior to imaging. In addition, IPS Empress CAD surface was acid etched with 40% hydrofluoric vapor (20 s) to allow a better examination of the crystalline content.

Figure 2

Single cycle load-to-failure fracture modes. (a,c) Resin composite crown (3271 ± 143 N); (b,d) Glass-ceramic crown (1465 ± 222 N).

Figure 3

(a) Occlusal view of a resin composite crown from the mild profile (indentation load of 1200 N at 170K loading cycles). (b) Details of the fatigue scar in a resin composite crown from the aggressive profile (indentation load of 1700 N at 90K cycles). Note in (b): minor spalling measuring 1255 µm and 1796 µm in the bucco-lingual and mesio-distal directions, respectively.

Figure 4

Damage modes in resin composite crowns. (a) Sliding contact induced partial cone cracks. (b) Occlusal contact induced median cracks. (c) Impact induced inner cone cracks. (d) Flexure induced cementation surface radial cracks. (e) Minor spalling (Depth ~300 µm).

Figure 5

Flexure induced radial crack in a resin composite crown. Note: a large defect in the abutment compromises the support for the cement.

Figure 6

Crack interactions with nanoparticle clusters in the resin composite.

Figure 7

IPS Empress CAD failure probability (Weibull distribution) versus load with 90% two-sided confidence bounds (dashed lines).

Figure 8

(a) Occlusal view and (b) buccal view of a typical fracture pattern of the glass-ceramic materials (indentation load of 475 N at 220,000 loading cycles). (c) Occlusal view and (d) section view of a glass-ceramic crown (indentation load of 550 N at 250,000 loading cycles) exhibiting extensive crack formation.