Effect of Digital Flow Veneering Techniques on Bond Strength between Zirconia Cores and Veneering Ceramics

Review Article

J Dent & Oral Disord. 2019; 5(3): 1119.

Effect of Digital Flow Veneering Techniques on Bond Strength between Zirconia Cores and Veneering Ceramics

Hallmann Lubica1* and Gerngrob Mark-Daniel2

¹Department of Prosthodontics, Propaedeutic and Dental Materials, School of Dentistry, Kiel University, Germany

²Institute of Material Science, Faculty of Engineering, Kiel University, Germany

*Corresponding author: Hallmann Lubica, Department of Prosthodontics, Propaedeutics and Dental Materials, School of Dentistry, Kiel University, Germany; Email: [email protected]

Received: November 21, 2019; Accepted: December 16, 2019; Published: December 23, 2019


Objectives: The aim of this study is to evaluate the effect of digital flow veneering technique on bond strength between zirconia framework and veneer.

Materials and Methods: An electronic MEDLINE search complemented by manual searching was conducted to identify the effect of digital flow veneering technique on the stability of the dental restorations.

Results: The long-term stability of a dental restoration depends on many factors: the veneering technique, the geometry of the framework design, the thermal mismatch (CTE) between the framework and the veneer, the veneering materials, the pigments, the veneer/core thickness ratio etc. The CAD/CAM engineering technique seems to improve the stability of restorations due to the simplicity and therefore high reproducibility of the fabrication process.

Conclusion: CAD/CAM veneering technique seems to be a promise method to improve the stability of dental restorations.

Keywords: Digital dentistry; Veneering ceramics; Zirconia core; Mechanical properties.

Clinical Significance: The digital veneering techniques avoids the human error and offers higher precision and reduced cost.


The development of high strength ceramics in dental materials has opened up the new opportunity to avoid the long-term and high costs of conventional dental laboratory technology required for the preparation of dental restorative and prosthetic device such as inlays, onlays, crowns, fixed partial dentures, and removable dentures. The conventional methods such as lost-wax precision casting, dough modeling and curing of acrylic resins, and layered powder sintering of veneers are well established, but the results depend on the time and the experience of the dental technician [1]. The difficulties encountered in producing dental devices using the high strength ceramics as well as the difficulties with conventional methods has initiated the development of new methods to overcome these problems. One of these solutions was the introduction of Computer Aided Design and Computer Aided Machined (CAD/ CAM). Duret and colleagues pioneered with such a dental CAD/ CAM system in 1971 [1-6]. The development and establishment of CAD/CAM technology in dentistry has changed the production of restorative materials. Most of them are recently produced using industrially modern processes, ensuring quality standards which are only difficulty achieved under practical laboratory conditions. In contrast to the traditional additive technique, their processing uses the substantive route by machining the blocks and blanks into their final shape. This procedure allows the application of high strength ceramics as yttria-stabilized tetragonal zirconia polycrystal (Y-TZP), alumina and their composites [7-21]. Recently, glass-ceramics such as lithium silicate, zirconia reinforced lithium silicate, feldspar, polymer containing materials, ceramic-network materials and recently, lithium-disilicate-strengthened lithium aluminosilicate are developed for CAD/CAM technology [22-29]. Dental Zirconia is opaque and must be covered with a veneering ceramic to achieve an optimum esthetic appearance in color and translucency. Different techniques, such as layering, pressing, CAD-on and rapid layer can be applied to veneer the core materials [10]. In the layering technique, the veneering ceramics are mixed with a modeling liquid, and the mixture is brush-applied on the zirconia core. Its layer thickness is larger than the final dimensions to compensate the shrinkage of veneers. Multiple applications and four firing processes are required for the final veneer. The pressing technique is based on the injection of porcelain onto a zirconia framework and has the advantage of eliminating the porosity within the veneer thereby improving the mechanical properties of the dental device. The CAD-on technique is based on coating the zirconia framework with a CAD/CAM lithium disilicate glass-ceramic layer using a fused glass solder. On the other hand, the rapid layer technique uses the cementation of CAD/CAM milled veneer onto the zirconia framework with a dual-cure resinbased luting agents [10].

The exact identical replica of natural teeth is one of the great challenges for dental technicians. In addition to knowledge of the shape and surface of natural teeth, they need to also master the handling of ceramics perfectly. The layered technique requires a highly skilled dental technician. In the pressing technique, a final contour anatomical waxing is prepared on the core. After elimination of the wax in a furnace, ceramics are heat-pressed to the core. This method has some advantages on the layering technique such as the reduction of processing time, increasing accuracy, offering a higher stability, and eliminating the shrinkage [11-30].

The initial enthusiasm attitude towards dental zirconia was decreased by the increase of restoration failures compared to metalceramic restorations. For zirconia cores, delamination (chipping) of the veneering ceramics from the core is a major failure mode [31- 43]. The chipping of veneers depends on many factors, such as the excessive tensile stress due to the thermal mismatch (CTE) between the veneer and zirconia framework, the geometry of the framework design, core/veneer thickness ratio, the interfacial bonding strength, the veneer itself, the pigments, and also the veneering technique. In the layering technique, the number of firings, the heating and cooling rate, the firing time and temperature, can affect the quality of the final restoration.

A large mismatch in the CTE between veneer and framework can lead to delamination, micro-cracking, or chipping in the veneer. Therefore, manufactures have developed veneering materials with a slightly lower of even identical CTE to that of the framework [43] Chipping of veneers is currently the subject of intensive investigations [8,9,32-36,44-51]. For this reason, recently, new veneering materials and / or new methods are introduced to use CAD/CAM technology in final restoration to increase the efficiency of laboratory processing and the quality of restorations.

Since the factors, which are responsible for the veneer chipping, are still under debate, avoiding the use of veneers is another possibility to prevent chipping. At the beginning, the veneers were used to improve the aesthetic properties of zirconia because of its poor translucency. The advances in dental material sciences improve the aesthetics of zirconia. For example, full-contour zirconia restorations employing internal and external stain techniques can be used, but these are limited to the posterior regions with little aesthetic demands [51]. So, for the anterior regions the use of veneers is unavoidable.

The aim of this study was to investigate the influence of the veneering techniques on the bonding strength between zirconia core and veneers, since there are hardly any studies about the stability in long term of zirconia restoration using the CAD/CAM veneering technique.


Shear bond strength (SBS)

Hafez et al. [52] have studied the shear bond strength between zirconia and veneers. They have applied two techniques: manual layering and press-on. According to these authors, irrespective of the surface technique, press–on veneering showed statistically a significantly higher mean micro-shear bond strength value (21.5 ± 4 MPa) compared to layering veneering with mean values of 16.8 ± 5.9 MPa (Figure 1).