Zirconia , advantages in dentistry
Dental crowns are, simply explained, prosthodontics solution for missing tooth or largely damaged tooth. After tooth preparation of tooth and taking dental impressions, production of dental crown continues in a dental laboratory. There are many different ways for production of dental crown depending on the type of crown. When talking about different types of crowns it is actually related to the type of material used for crowns. For long period of time, metal-ceramic crowns were used as the best option for this type of reconstruction. These crowns consist of a core that is made of metal alloy (even golden alloy in many cases) covered with ceramic. With the progression of dental material technology, new solutions appeared. Today, the golden standard for production of dental crowns is zirconia. Zirconia is crystalline dioxide of zirconia, very strong material with great mechanical and aesthetic characteristics. It can be used as a core of future prosthodontics work such as crowns and bridges on existing, patients teeth or on implants, but also as a material for a full zirconia crown made of zirconia known as monolithic zirconia crowns. Differences between metal-ceramic crowns and zirconia crowns are numerous where many advantages goes for zirconia crowns.
Esthetics of zirconia
Primary, the greatest advantage of zirconia over metal-ceramic crowns is related to aesthetics in the way of superior translucency of zirconia crowns. This is usually the only advantage that patients will notice when deciding over better prosthodontics solution for themselves. When patients decide to have zirconia crown, it is also important to know the differences between different zirconia crowns. This is mainly related to monolithic or full zirconia crowns and ceramics fused to zirconia crowns. Monolithic zirconia crowns are made out of zirconia block and usually can’t be modified for better esthetics while ceramic fused to zirconia crowns have many options for esthetical improvements with ceramic layering, shading and later polishing. Basically, the esthetic problem with monolithic zirconia crowns is related to lack of translucency which can be modified with improved zirconia like CubeX (DentalDirekt). When we speak about better esthetic properties with zirconia crowns, beside better translucency that is the greatest advantage, next great advantage of zirconia crowns is that the metal color in the gingival area is absent. This was often a problem with metal-ceramic crown since the metal of these crowns appears as a shadow in the area of marginal gingiva, especially with thin gingival biotype. Another problem with metal-ceramic crowns in this area appears during the preparation and cementation. When your dentist prepares a tooth for metal-ceramic crown they make micro-lesions in marginal gingiva that can interact with metal in the crowns and lead to the appearance of “metal tattoo”. With zirconia crowns, all these effects can be avoided since there is no metal involved with zirconia crowns, and the natural and healthy look of marginal gingiva is achieved.
Strength and biocompatibility of zirconia crowns
Patients often insist on esthetics, but physical properties of the material are equally important. Zirconia has great strength which made it a great alternative to the metal core, but speaking of strength is not that simple because other physical properties of the material are taken into consideration, such as elasticity of the material, its resistance to pressure, torsion and other. Since the chewing forces are different in every part of the jaw, different materials may be suggested in the different clinical situations according to the material’s physical and esthetical properties.
Another significant characteristic is biocompatibility of dental materials. Biocompatibility can be explained as a biochemical interaction between material and oral tissues, including dental tissues so as soft oral tissues such as gingiva and oral mucosa. These interactions may manifest as irritation, infections, tissue proliferation as a consequence of chronical irritation or different kind of allergic reactions. With zirconia, these interactions are very rare and it is considered as a material of high biocompatibility. It has already been explained that “metal tattoo” can often appear as a result of mechanical trauma during the preparation of the tooth for a metal-ceramic crown. This can’t be count as a spontaneous interaction between the gingiva and metal alloy, but gingival inflammation known as gingivitis can easily be the consequence of hypersensitivity or allergic reaction to some of the elements from the metal alloy. It is also important to make a difference between allergic reaction and type of crown preparation. Preparation for zirconia crown differs from preparation for metal-ceramic crowns in the way of the position of crown margin. For metal-ceramic crowns deeper preparation is needed. This type of preparation is called subgingival margin placement, while for zirconia crowns equigingival preparation is possible. It means that the crown margin is in the level of marginal gingiva and that way the tissue around the crown stays preserved from trauma during preparation and after crown cementation. There is no “metal shadow” with zirconia and lighter pressure on surrounding tissue with consequent acute or chronic inflammation that not only is esthetically unacceptable but also leads to functional and biological damage of periodontal tissue with potential later complication. Presence of metal alloy, in the light of biomechanical properties, often leads to another problem called “galvanic effect”. This phenomenon may appear, in some cases, when more than one metal alloy is present in the oral cavity. Alloys can cause the development of electrical current in the mouth which is followed by sensations such as pricking or burning. It is clear that with zirconia crowns this phenomenon is not possible since there is no metal alloy present.
Implant and abutments made of zirconia
With a progression of dental material’s technologies, implant placement and its own development progress rapidly. Zirconia found its place even in this area of dentistry with the appearance of zirconia abutments as the greatest esthetical solution. Abutments are suprastructures of dental implants, that carries the crown. Implants and abutments are usually made of titanium which can give the same aesthetic effect like metal-ceramic crown known as “metal shadow”. Besides that, zirconia is more suitable material for soft tissue around the crown with fewer chances for any allergic reactions since it has better biocompatibility than titanium. This advantage of zirconium abutment has great importance in implantology. Even today, the problem of soft tissue around implants with the accent on the interdental papilla persists and its preservation is of huge importance for esthetics after crown cementation. It is understandable why is zirconia abutment again more preferable then titanium, especially in the esthetic zone- frontal part of the jaws.
The problem with zirconia abutments appears when it is placed directly on implant made of titanium. Since zirconium is harder than titanium, the implant may be damaged during the time and the future of the entire implant questionable. Luckily, there are solutions for this problem on the market. Mesoabutments are developed to solve this problem. Here we talk about custom-made abutments that have the basic part made of titanium, so the direct connection between abutment and implant is achieved through titanium to titanium interphase. Over this titanium base, custom-made in CAD-CAM system zirconia part is fabricated, so the future crown is connected to abutment through the zirconia to zirconia interphase. The alternative solution to the previously mentioned problem with soft tissue around titanium can be solved by implantation of full-zirconia implants and one of the leading companies on the market that produces these implants is Z-system from Switzerland.
Full ceramic made in Germany
Since there are specific technical requirements in laboratory procedure for zirconia crowns, but also with the entire in-office procedure zirconia crowns are more expensive than conventional metal-ceramic crowns. Besides that, different sorts of zirconia determine the final price of the crown. Zirconia used for crowns in our dental office are produced by DentalDirekt, Germany, and depending on patient’s needs there are few options when we are choosing the right type of zirconia. DDcubeX2 has the highest rate in the translucency of 49 %, and with flexural strength over750MPa. It is usually used for reduced and full contour bridges (up to 3 units), preferably for anterior region. On the other hand, DDcubeONE has an even better flexural strength (1000MPa), but it lacks in translucency compared to DDcubeX2, which makes it ideal for veneers, inlays and overlays, so as for full contoured and reduced bridge consisted of 3 units or more, so as for hybrid abutment crowns. DDBioZX2 and DDBioZ can both be used for hybrid abutment and bars thanks to their tremendous flexural strength with proportionally low translucency and consequent lack of esthetics.
Considering all explained properties of zirconia, it is clear that complexity of such prosthodontics restorations is a highly demanding process as for the patients, so as for dentist. However, when the patient has clear idea of their final dental look and with the right help of their dentist, the best solution will be easily made.