Initially, porcelain fused to metal (PFMs) restorations were the clinician’s standard choice of material, because they offered reliable treatment outcomes for the patient.
However, the opaque porcelain used to mask the metal substructure, particularly in the cervical third, frequently causes a light gray appearance at the cervical margin. The effect can worsen after a few years if gingival recession occurs. Now, the demand for increased aesthetics and more cosmetically pleasing restorations has risen exponentially, corresponding with a significant fall in demand for PFMs. PFMs have been replaced with more clinically and aesthetically acceptable all-ceramic restorations, the most popular of which is zirconia.
Modern zirconia is entirely different from the initial products available, which often had a bone-white substructure that was difficult to completely mask. Even a slight adjustment to a restoration could expose the substructure. Now, the very latest pre-shaded zirconia is available in several different materials that are suitable for specific applications in the mouth, significantly improving its scope of use.
When restoring a tooth, the clinician must choose the correct material. Significant factors that influence selection include aesthetics and the overall strength of the prostheses. Popular all-ceramics choices include alumina and lithium disilicate materials. However, the characteristics of zirconia are appealing on multiple levels and suitable for a broader range of clinical applications compared with other all-ceramics.
Zirconia has been used as a biomaterial since the 1970s in implants and is currently the material of choice for complete hip replacements. The comparison with dental applications is interesting, because hip replacements are also load-bearing and in close proximity to osseous and vascular tissue. Both applications are exposed to significant wear.
The Superior Characteristics of Zirconia Restorations
Zirconia offers optimum properties for use in dentistry, including:
Superior toughness and fatigue resistance
Excellent biocompatibility and wear properties
Good material compatibility for patients concerned about the possibility of metal allergies or sensitivities
Similarity to titanium in that it doesn’t inhibit the osteoblasts (bone forming cells), which are essential for osseointegration
Dental zirconia is usually modified with yttria, an oxide that stabilizes the crystal structure transformation and improves the physical properties of zirconia so that it can be made in the tetragonal crystal structure at room temperature. The result is crack-resistant transformation-toughened zirconia.
When zirconia is subjected to stress, it goes through an instantaneous phase transformation to a monoclinic crystal structure that is approximately 4% larger in volume than its tetragonal form. This transformation clamps a microscopic crack shut, actively resisting crack growth. It’s a significant property, especially when fabricating a restoration that will be subjected to substantial chewing forces.
High translucency zirconia contains a higher percentage of yttria to increase aesthetics, resulting in a 10 to 15% cubic-phase zirconia, which is why the flexural strength is lower than traditional zirconia but still considerably higher than other restorative materials. Some specific formulations incorporate other elements, such as a fluorescence, which is added at the powder stage before blocks or discs are created.
Suitable for Multiple Clinical Situations
Zirconia offers advantages for many different clinical situations. For example:
Zirconia is ideal for single anterior and posterior crowns and for full-mouth rehabilitation.
The very latest zirconia materials produce highly aesthetic results anywhere in the mouth and are even suitable for patients with bruxism.
Because of their strength, anterior zirconia copings can be manufactured as thin as 0.3 mm, giving them a more natural, tooth-like appearance.
The reduced strength of the more translucent forms of zirconia remains at least as good as a PFM restoration but with improved aesthetics.
Monolithic zirconia is particularly suitable for posterior restorations and where bruxism is a concern and is naturally tooth-colored. Additionally, zirconia manufactured for patients with bruxism has proven to be far more gentle and wear-friendly to opposing tooth structure than initially anticipated.
Zirconia high translucent (Zirconia HT) is one of the newest products on the market. It is a versatile material suitable for anterior and posterior crowns and strong enough for three-unit bridges. While the flexural strength of zirconia HT is lower compared with solid or layered zirconia, it remains stronger than PFMs and is three times stronger than lithium disilicate. Laboratory tests have shown that the flexural strength and fracture toughness of zirconia is significantly higher compared with alumina or other aesthetic ceramics.
Zirconia restorations are fabricated using computer-aided design and computer-aided manufacturing (CADCAM).
The following are advantages of CADCAM manufacturing:
The technique produces precision-fitting restorations that require less chairside time for seating and bonding the prosthesis.
Reduced chairside time helps the dental office to operate more smoothly and efficiently.
Patients always appreciate it when their appointments are on time and require minimal or no chairside adjustments due to a well-fitting restoration.
Here are some additional reasons for choosing zirconia products for your practice:
Patients are becoming more knowledgeable about the availability of all-ceramic restorations, and it’s not uncommon for them to request zirconia by name.
The latest generation of zirconia is providing optimal outcomes for aesthetically crucial situations.
Clinicians who are more familiar with this product will often choose to work with zirconia exclusively, because they know it is reliable and worth the investment.
Zirconia restorations do not require any significant changes to your preparation or cementation techniques.
Initial concerns when adopting any new material are understandable, but years of research into the strength and reliability of zirconia restorations now exist, particularly where multiple unit prostheses are required. Lithium disilicate restorations, such as IPS e.max, are incredibly aesthetic and, up until recently, would have been the material of choice for anterior crowns. However, high translucent zirconia combines the desirable strength of zirconia with excellent aesthetics. The translucency achieved by some of the newer and more translucent zirconia materials is statistically equivalent to lithium disilicate.
Zirconia’s incredible strength has never been doubted, however earlier versions often required significant work, such as layering multiple porcelains to improve aesthetics. Now, the introduction of multilayered and pre-shaded discs makes it possible to produce restorations with remarkable flexural strength, aesthetics, and surface hardness. The latest materials exhibit excellent color consistency and reliable quality. Knowing the limitations of this material and when to select it is essential, but, when utilized correctly, zirconia products are well worth the investment.
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