Elsevier

Dental Materials

Volume 24, Issue 5, May 2008, Pages 633-638
Dental Materials

Strength influencing variables on CAD/CAM zirconia frameworks

https://doi.org/10.1016/j.dental.2007.06.030Get rights and content

Abstract

Purpose

Many studies in the dental literature look at the effect of different surface treatment methods on the flexure strength of zirconia where polished zirconia has been used as control. However, zirconia is subjected to different types of surface damage as a result of the CAD/CAM milling procedure and also to damage produced by other laboratory procedures in use daily.

Aim of the work

The aim of this work was to evaluate the effect of different surface treatment methods and in particular the effect of the CAD/CAM milling procedure on the flexure strength of zirconia frameworks.

Materials and methods

At least 20 zirconia bars (17 mm × 2 mm × 1 mm) for each group were prepared by either cutting and polishing zirconia milling blocks or by using a CAD/CAM device (Cercon) which left behind characteristic surface features related to the milling process. The fully sintered bars received either of the following surface treatments: air-borne particle abrasion (with 50 and 120 μm aluminum oxide particles, or both). Some bars received a heat treatment commonly used in baking veneer ceramics before or after particle abrasion. The surface roughness was measured for all bars, which were finally loaded in a three-point device. The fractured bars were examined using scanning electron microscopy. Data were analyzed using one-way analysis of variance and survivability was estimated using Weibull analysis (α < 0.05).

Results

There were significant differences in the flexure strength (in MPa) between the tested groups subjected to different surface treatments which can be categorized into four strength levels: (1074–1166 MPa) for polished zirconia and the CAD/CAM bars that were particle abraded (50 μm Al2O3) whether with or without heat treatment (936 MPa) for the ground bars that were particle abraded (50 μm Al2O3), (708–794 MPa) for CAD/CAM bars and for the polished bars that were particle abraded (120 μm Al2O3), and (546 MPa) for the ground bars that were particle abraded (120 μm Al2O3) being the weakest. There was a strong correlation between flexure strength and the severity of surface damage as indicated by surface roughness (R2 = 0.912). Scanning electron microscopy revealed different types of surface and subsurface damage produced by the different surface treatments.

Conclusions

The surface damage produced by the CAD/CAM milling procedure significantly reduced the strength of zirconia which could be further weakened by different surface treatment methods resulting in unexpected failures at stresses much lower than the ideal strength of the material. It is advised to consider the effect of the CAD/CAM procedure on the characteristic strength when designing zirconia-based fixed partial dentures.

Introduction

The introduction of zirconia to the dental field opened up the design and application limits of all-ceramic restorations. The superior mechanical properties of zirconia combined with the state-of-the-art CAD/CAM fabrication procedure allowed for the production of large and complex restorations with high accuracy and success rate [1].

The strength of zirconia can be directly influenced by different surface treatment methods which exert different degrees and types of surface damage. These areas of surface flaws act as stress concentration sites and even though they are microscopic in nature, they act as potential sites for crack initiation and propagation [2]. Dental literature has focused on studying the effect of different surface treatments on the strength of zirconia-based materials and reduction in strength was generally associated with the degree of surface damage [3], [4], [5]. On the other hand, some studies reported an increase in strength after air-borne particle abrasion and related such finding to the creation of compressive fields as a result of the induced tetragonal–monoclinic transformation of the surface crystals [6], [7], [8], [9].

A point worth noting is that in some of the previous studies polished zirconia was used as a reference point, while grinding with different grits of silicon carbide paper and air-borne particle abrasion at a high pressure were commonly used as examples of different levels of surface damage. On the other hand, under daily circumstances, zirconia is subjected to a different type of surface damage as a result of the milling procedure, which leaves behind characteristic trace lines and different patterns of surface damage and flaws [5]. Additionally, the combined effect of the CAM milling procedure and common laboratory procedures such as air-borne particle abrasion and multiple firing cycles used in baking the ceramic veneer, could be different from that expected for polished or disc-ground zirconia [10]. The mechanical properties and the long-term stability of CAD/CAM zirconia will be a function of the exerted surface damage, the degree of transformation, and the loading environment in terms of peak stresses and number of cycles [11].

The aim of this study was to evaluate the damage induced by the CAD/CAM milling procedure, combined with different surface finishing procedures, on the mechanical properties of zirconia and to analyze the interaction between these variables.

Section snippets

Materials and methods

At least 20 zirconia bars (17 mm × 2 mm × 1 mm) for each group where prepared by either of the following methods: cutting zirconia milling blocks in a sawing machine (ISOMET Q2 1000, Buehler Ltd., Lake Bluff, IL) using a diamond coated disc saw (ground bars) or by using CAD/CAM technology (Cercon, Degudent GmbH, Hanau-Wolfgang, Germany) where individual bars were milled by the machine using wax replicas (CAM bars). The bars were sintered in the relevant manufacturer equipment (Cercon Heat uses a 6.5 h

Results

Statistical analysis revealed significant differences in the three-point flexure strength values between the groups tested as a result of the different surface treatment methods applied (F = 35.5, p < 0.000). According to strength values, the groups tested could be divided into the following categories: (1074–1166 MPa) for polished zirconia and CAM and polished bars which were particle abraded (50 μm Al2O3), (936 MPa) for ground bars that were particle abraded (50 μm Al2O3), (708–794 MPa) for CAM bars,

Discussion

As the main function of the underlying framework is to support the ceramic veneer and to carry the loading forces, different laboratory tests were used to evaluate the internal strength of zirconia frameworks. The design of fixed partial dentures can be considered as a simple beam and different flexure strength tests are frequently used for strength evaluation [4]. On the other hand, standard flexure strength tests do not take into account important factors such as the effect of design,

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