Dental restorations made of zirconium oxide (synonym: zirconia, Zirconia, ZrO2) are ceramic workpieces with outstanding material properties in terms of their biocompatibility, stability and aesthetics. The range of applications of the material extends from conservative restorations (tooth-preserving restorations) through crowns and bridges to implant prosthetics. The advantages of zirconium oxide as a tooth replacement material lie in its excellent biocompatibility (biocompatibility) and the esthetics that can be achieved with the tooth-colored ceramic. Only the opacity (lack of light transmission) of conventional zirconium oxide material has a detrimental effect, so that as a monobloc (monolithic, without subsequent ceramic veneering) it is generally only used for posterior teeth. The development of zirconium oxide, whose translucency (partial light transmission) is more similar to natural tooth enamel in its optical properties, has expanded the range of applications. However, patients with bruxism (“teeth grinding“) may find the hardness of the material, which is higher than that of natural tooth substance, disadvantageous, as pronounced grinding and/or pressing may damage the enamel of the antagonistic teeth (the teeth of the opposing jaw). In addition, the risk of chipping, the shearing off of veneering material from the base framework of a crown or bridge, is significantly increased. Zirconia materials in monolithic processing (manufactured in one piece without veneering material) released for patients with bruxism are intended to address this problem. Material properties
Zirconium dioxide (chemical characterization: ZrO2 + HfO2 + Y2O3 > 99 %) belongs to the so-called oxide ceramics and exhibits properties comparable to metals or even superior to them in some cases. As a ceramic, it is corrosion-free and is characterized by high flexural strength and fracture toughness. It has been used successfully in orthopedics for many years. In addition, the poor thermal conductivity of ceramic benefits the pulp (the tooth pulp), which is thus exposed to less thermal stimuli. With high hardness and flexural strength, zirconium oxide has a high modulus of elasticity (synonyms: modulus of elasticity, coefficient of elasticity, modulus of elongation, tensile modulus, Young’s modulus) – a measure of the resistance that a solid body offers to its elastic deformation. Concessions to esthetics and abrasion or attrition behavior (wear caused by chewing or grinding) inevitably affect the material properties. For example, translucent zirconia results in lower fracture toughness, which somewhat limits the range of indications compared to conventional zirconia.
| Material properties | Zirconia (Cara Zr) | Translucent zirconia (Cara Zr trans) |
| Modulus of elasticity [GPa] | 200 – 220 | 210 |
| Fracture toughness [MPa/m2] | 10,0 | 5 |
| Flexural strength [MPa] | 1.180 | 1.200 |
| Hardness | 1.200 – 1.300 HV10 | 1.300 HV10 |
Methacrylate-based acrylics can be used to attach zirconium oxide dentures to the prepared (milled) teeth. If the patient is hypersensitive to this material, another advantage of zirconium oxide is that – unlike a large proportion of other ceramic materials – it can also be cemented with conventional cements based on zinc phosphate or glass ionomer. The ceramic material is selected from three basic colors (Cara Zrtrans e.g. light, medium, intensive). In the case of a monolithically fabricated crown or bridge, a more precise color match can be made with stains. If only the crown or bridge framework is made of zirconium oxide, which is subsequently customized with a fired-on ceramic veneer, the most demanding esthetic results can be achieved, for example for the anterior region.
Indications (areas of application)
- Inlays (inlay filling)
- Onlays (form of dental prosthesis in which the occlusal surface is not completely covered).
- Veneers (veneers)
- Monolithic single crowns
- Frameworks for veneered single crowns
- Frameworks for veneered bridges – up to 7 units of translucent zirconia, up to 16 units of conventional zirconia.
- Monolithic bridges of the same span
- Free-end bridges – made of conventional zirconia with a pendant length of no more than one premolar width (the width of an anterior molar).
- Primary telescopes (inner telescopes) for telescopically anchored prostheses.
- Abutments – one-piece or two-piece abutments for implants.
- Brackets – fasteners commonly used in orthodontics for fixed appliances.
- Allergy / intolerance to metal alloys.
- Allergy/intolerance to fastening plastics
Contraindications
- Bruxism (grinding and pressing) – In this case, the use of zirconia should be carefully considered and, if necessary, the indication should be strict even for zirconia approved for bruxism in monobloc processing.
- Bridges with more than two adjacent pontics (pontics to replace missing teeth) – exception: bridge between the lower canines to replace four incisors.
- Free-end bridges with pendant dimensions of more than one premolar width (premolar = anterior molar).
- Loosened abutment teeth (teeth to wear a bridge).
- Hypersensitivity to zirconia
Before the procedures
- Planning of the dental prosthesis with consideration and education about alternative materials.
- Pre-prosthetic measures – e.g. conservative restorations (tooth preserved restorations), root canal treatments, periodontal therapy (treatment of periodontal diseases), etc.
The procedures
- Preparation – First, the teeth are prepared (ground) to receive the zirconia denture in the dental office. Until the denture is completed, the teeth are protected by a temporary (transitional) restoration.
- Impression – An impression of the dentition is required for production in the dental laboratory. This is obtained either conventionally by means of impression materials based on silicone or polyether, or digitally by optical scanning with an intraoral camera, which allows 3D images in the entire mouth.
- Transfer – The conventional impression is transferred in the dental laboratory into a detailed working model made of special plaster, which in turn is transferred by scanning into the program of a CAD-CAM unit (CAD: Computer Aided Design, computer-aided construction; CAM: Computer Aided Manufacturing, computer-aided milling technology). Alternatively, the data of the digital impression are transferred to the design program.
- CAD – The denture is now designed on the computer. Here, great care must be taken to the framework design. On the one hand, this involves demands on the material thicknesses in the crown wall area or at the connection points to pontics, which differ between 0.3 and 1 mm depending on the expected load and indication. On the other hand, the shaping of a framework to be veneered takes place in such a way that a maximum of 2 mm is calculated for the veneer thickness and consequently the zirconium oxide framework is already designed in the form of reduced teeth before the subsequent ceramic veneering.
- CAM – The established method for the production of the zirconia workpiece is the computer-assisted milling technique. For this, the previously planned design is worked out in a complex three-dimensional milling process from a factory-made, chalky-soft blank, the so-called green body. This is significantly less stable and about 25% more voluminous than the finished ceramic, but the milling process is therefore more time-saving and less material-intensive. Nevertheless, there is also the alternative of milling an already hard sintered (ready-fired) ceramic block.
- Sintering firing – The milled blank is subjected to a subsequent sintering firing (heating almost to melting temperature, usually under pressure), whereby the final material properties are achieved by a previously calculated volume shrinkage and pore reduction.
- Monobloc individualization – If the workpiece is a monolithic crown or bridge, color individualization can be carried out by applying paste-like stains and subsequent glaze firing. This provides a significant expansion of the color spectrum beyond the basic colors.
- Ceramic veneering – A crown or bridge framework, on the other hand, is elaborately veneered by multiple applications of ceramic veneering material in different color layers and sintering. This allows the most aesthetically pleasing results to be achieved, as subsequent veneers have enamel-like translucency.
- Insertion – The completed denture is inserted in the dental office.
After the procedures
- Timely control date
- Regular recalls (re-presentations) to the dentist to keep the dentures functional for a long time.
Possible complications
- Fracture (break)
- Chipping – shearing of the ceramic veneering material from the zirconia framework.
- Detachment of the denture from the tooth due to detachment of the luting cement.
