Whereas metal-supported porcelain crowns—which used to be associated with an opaque, lifeless appearance and a grey line at the gum line—have now been replaced by zirconium (zirconia) restorations, which are regarded today as the pinnacle of aesthetics and durability. In this article, we will examine zirconium crowns – this 'high-tech' product of modern dentistry – in every detail, from their technical specifications and application processes to their advantages and care tips.
Zirconium dioxide (zirconia) is the oxidised form of the naturally occurring metal zirconium, yet it behaves like ceramic (porcelain) within the mouth. The zirconia used in dentistry is a multi-phase material known as 'partially stabilised zirconia', capable of withstanding high masticatory forces.
In the early 2000s, zirconia was an opaque, chalk-white substance used solely as a framework material due to its limited aesthetic properties. However, today, the material's translucency has been enhanced to such an extent that monolithic (single-piece) blocks capable of perfectly replicating natural tooth enamel are now being produced.
There are several key reasons why zirconium has become popular in dentistry, including its versatility in crowns, bridges, and implant abutments, demonstrating its broad clinical utility and benefits for various restorative needs.
- Superior Aesthetics: Thanks to its ability to transmit and reflect light, zirconium perfectly mimics the colour and translucency of natural teeth.
- Incredible Durability: As the strongest ceramic in dentistry, its 'transformation toughening' mechanism offers reassurance of long-lasting restorations for both professionals and patients.
- Biocompatibility (Tissue-Friendly): Free of metal, resistant to corrosion, and compatible with gum tissue, it provides a safe, worry-free option for those with metal sensitivities, reassuring both professionals and patients.
- Digital Precision: Thanks to CAD/CAM (computer-aided design and manufacturing) technology, it fits the tooth with millimetre-level accuracy.
No two cases are the same, and modern zirconia blocks are available in a range of types to suit different needs:
- UTML (Ultra Translucent): Offers the highest light transmission (51 per cent). It is particularly ideal for anterior crowns, veneers (laminates) and inlays/onlays.
- STML (Super Translucent): Offers a balance between strength and aesthetics (49% light transmission). Can be used for up to three-unit bridges in the posterior region.
- ML and HT (High Translucent): Possess very high flexural strength (1125 MPa and above). They are the preferred choice for long bridges and complex implant-supported restorations.
- Multilayer Structure: Modern blocks are manufactured with a four-layer structure (enamel, transition and body layers) so that, like the natural structure of the tooth, they are more opaque at the cervical region (gum line) and more translucent towards the tip.
Zirconium dental crowns are 'high-tech' restorations that meet both aesthetic and functional expectations, and are widely used in modern dentistry. The most suitable candidates for these crowns are as follows:
Patients with high aesthetic expectations
As zirconium can perfectly mimic the colour and translucency of natural teeth, it is ideal for patients seeking an aesthetic smile. The following candidates are particularly suitable:
- Patients with Severe Discolouration: This is one of the most effective solutions for patients with third- and fourth-degree tetracycline staining—particularly caused by medication use during childhood—where whitening (bleaching) methods have failed to produce results.
- Those Seeking a Natural Appearance: It is suitable for patients who wish to avoid the black lines that metal-supported porcelain can create at the gum line and who want the porcelain to look more vibrant without the metallic sheen.
For Those Seeking Functional Durability and Strength
Zirconium's high mechanical strength and fracture resistance make it suitable not only for front teeth but also for the posterior region:
- Areas Subject to High Masticatory Forces: In situations requiring a high load-bearing capacity, such as the molars in the posterior region, the use of 'monolithic' (single-piece) zirconia is preferred as it eliminates the risk of porcelain chipping.
- Cases Requiring Bridges: It can be used across a wide range of applications, from single-tooth crowns to multi-unit bridges, and even full-arch restorations.
Candidates with Metal Allergies or Seeking Biocompatibility
One of the most important properties of zirconium is that it has a metal-free structure. Therefore:
- Those Seeking Biocompatible Restorations: It is one of the best options for patients seeking a material that provides excellent compatibility with body tissues, is corrosion-resistant and has low thermal conductivity.
- Implant Patients: It is frequently preferred for implant-supported prostheses, abutments and complex implant-supported structures.
Paediatric Patients (Paedodontics)
Zirconia crowns are also a suitable option for children in certain situations:
Early Childhood Caries: Pre-fabricated zirconia crowns are successfully used in the treatment of widespread caries in milk teeth (particularly in the anterior region) due to both their physical durability and aesthetic advantages.
Specific Candidate Groups Based on Technical Requirements
The candidate profile can be further refined depending on the zirconia series to be used:
- Anterior Aesthetics: The UTML (Ultra Translucent) series, which offers very high light transmission, is best suited for veneers, inlays/onlays and anterior crowns.
- Posterior Region and Bridges: The STML, ML or HT series, which offer higher flexural strength, are more suitable for patients requiring molar restorations and bridgework.
The zirconia crown process begins in the dentist's chair and continues in the laboratory using specialised equipment:
Preparation and Tooth Shaping
The tooth must be reduced in size to allow the permanent crown to be fitted. Typically, a 'chamfer' 0.9 mm deep is prepared at the gum line; 1.2 mm is removed from the sides and 1.8 mm from the occlusal surface.
Digital Impression and Design (CAD)
An impression of the prepared tooth is taken with a digital scanner and converted into a three-dimensional computer model. The anatomical shape of the crown is designed on this model.
Milling (CAM) and Sintering
Once the design is complete, the zirconia blocks are milled on computer-controlled milling machines. The milled teeth are initially approximately 25 per cent larger than their actual size, as they will shrink during firing (sintering). The teeth are fired in special high-temperature kilns at 1450°C–1550°C.
Colouring and Finishing
After sintering, colouring solutions or paints may be applied to give the tooth a natural appearance. The teeth are then glazed in vacuum kilns to achieve a smooth, glossy finish.
Tetracycline Stains and Zirconia
In cases of severe (tetracycline) staining caused by antibiotics taken during childhood, tooth whitening (bleaching) is generally unsuccessful. Zirconia crowns are the most effective way to mask such severe discolouration and provide the patient with a 'flawless smile'.
Zirconia in Paediatric Dentistry
Pre-fabricated zirconia crowns are successfully used in cases of widespread decay in primary teeth due to their durability and aesthetic qualities. Research indicates that zirconia crowns have a higher success rate in children compared to other composite-based methods (strip crowns).
Micro-Layering and the 'Mini-Max' Principle
In modern laboratories, a very thin layer of porcelain, just 0.2–0.6 mm thick, is applied to monolithic (single-piece) zirconia to enhance aesthetics. This method is known as 'micro-layering'; it achieves maximum aesthetics with minimum effort (the Mini-Max principle), whilst also minimising the risk of porcelain chipping.
Colour Stability and Ageing (LTD)
- Colour Change: Studies show that the colour of monolithic zirconia lightens slightly with ageing, whilst porcelain-veneered zirconia darkens slightly. However, these changes generally remain within clinically acceptable limits.
- Staining: Heavy smoking and excessive consumption of staining beverages such as tea, coffee, cola and Nescafé may cause staining on the zirconia surface.
Selection and Effect of Cement (Adhesive)
Due to the semi-translucent nature of zirconia, the colour of the cement applied underneath can affect the final appearance. Studies have shown that A2-shade cements generally produce a more aesthetic and natural result than transparent ones.
- Porcelain Chipping: When porcelain is layered onto a zirconia framework, the risk of chipping can reach up to 30 per cent. Monolithic designs should be preferred to minimise this risk.
- Staining: Hand oils, plaster dust or metal residues that come into contact with the surface prior to sintering during production may cause white or dark stains after firing.
- Cracks: Procedures such as removing the tooth from the sintering furnace before it has cooled can cause thermal shock and result in cracks.
To ensure that zirconium dental crowns (zirconia) are long-lasting and retain their aesthetic appearance, there are critical points to bear in mind regarding both daily care and lifestyle habits. The following points should be observed after the procedure:
Thorough Oral Hygiene and Plaque Control
Oral hygiene is the most important factor for the success of zirconia restorations.
- Plaque Buildup: Plaque buildup and gum problems resulting from inadequate oral care can negatively affect the colour and tissue compatibility of the restoration.
- Supportive Care: Your dentist may recommend chemical plaque control agents, such as mouthwashes containing chlorhexidine, to help maintain gum health.
Diet and Habits (Preventing Staining)
Although zirconia crowns are resistant to staining, excessive consumption of certain substances can compromise surface aesthetics:
- Coloured Beverages: Excessive consumption of coloured beverages such as tea, coffee, cola and Nescafé can lead to surface staining beyond clinically acceptable limits.
- Smoking: Heavy smoking is one of the key factors causing colour changes on the zirconia surface over time.
Awareness of Thermal and Environmental Factors
Zirconia crowns are materials affected by the dynamic and challenging conditions within the mouth:
- Hydrothermal Ageing (LTD): Zirconia is subjected to a process known as 'low-temperature degradation' (ageing) in the moist environment of the mouth and under constantly fluctuating temperatures. This process can lead to the formation of micro-cracks on the surface and the roughening of the material, which may pave the way for long-term colour changes.
- pH and Temperature Fluctuations: Constant pH fluctuations and thermal stresses within the mouth (very hot or very cold food and drink) can affect the material's optical and chemical stability.
Professional Monitoring and Care
- Regular Check-ups: Regular visits to the dentist (usually every six months) are necessary to check the marginal fit of restorations, gum health and surface smoothness.
- Mechanical Protection: If you have a habit such as teeth grinding (bruxism), the use of protective night guards to preserve the structural integrity of the crowns and protect the opposing teeth should be assessed by your dentist.
What is a zirconium dental crown?
It is a type of dental crown made from zirconium dioxide ceramic, which is metal-free and offers high durability and aesthetic appeal.
Does it hurt to have a zirconium crown fitted?
As the tooth preparation is carried out under local anaesthetic, you will not feel any pain.
Do zirconium crowns look natural?
Yes, their light transmission and colour closely resemble those of natural tooth enamel.
Do zirconium crowns discolour over time?
No, unlike metal-supported porcelain crowns, they do not form a black or grey line at the gum line.
Do zirconium crowns cause allergies?
Zirconium is a biocompatible material and, as it does not contain metal, it does not cause reactions in people with metal allergies.
How long do zirconium teeth last?
With proper care and oral hygiene, they can generally be used without any problems for 10 years or more.
Do cigarettes, tea or coffee stain zirconium?
Excessive consumption and poor oral hygiene can lead to clinically noticeable staining on the surface.
Are temporary crowns fitted?
Yes, temporary crowns are fitted to protect the tooth and maintain aesthetics whilst the permanent zirconium crowns are being prepared.
Can zirconia crowns be used in children?
Yes, they are used as an aesthetic and durable solution, particularly for severe decay in milk teeth.
What is monolithic zirconia?
It is a highly durable structure in which the entire crown is milled from a single block of zirconia, with no additional porcelain layer on top.
What is porcelain chipping?
It refers to the cracking or detachment of the aesthetic porcelain layer on the substructure; this risk does not exist with monolithic zirconia.
Do zirconia teeth cause sensitivity to hot and cold?
As they have low thermal conductivity, they are similar to natural tooth structure and generally do not cause sensitivity.
Can zirconia crowns be fitted on teeth with tetracycline staining?
Yes, it is one of the best options for masking this type of severe discolouration.
How much of the tooth needs to be prepared for zirconia crowns?
Generally, a preparation of 0.9 mm at the gum line and between 1.2 and 1.8 mm on the surfaces is sufficient.
Do zirconia crowns smell?
Properly fitted and cleaned zirconia crowns do not give off any odour of their own.
Is the colour of the crown adhesive important?
Yes, the shade of the adhesive (particularly A2 shades) affects the final colour and aesthetics of the crown.
Why are zirconia crowns more expensive?
Due to the cost of the material and the advanced technologies used in production, such as CAD/CAM, they are more expensive than traditional methods.
