The primary goal of monolithic or layered zirconia metal free crowns is to restore the function, form and esthetics without affecting any of the remaining natural teeth. With its greater strength and a tooth like color, it can be best used in replacing anterior and posterior teeth.
A monolithic (full contour) anatomical crown is made in a single block and has good strength. It is devoid of chipping and does not affect the opposing natural teeth, which makes it suitable for any patient. These monolithic crowns are therefore best used in replacing posterior teeth, which are primarily subjected to chewing forces.
The anterior teeth however need better translucency to best mimic the natural teeth. Metal free crowns in esthetic zones are hence layered with ceramic materials, for better staining and shading to get maximum simulation of natural teeth.
Why posterior crowns should not be layered?
(Importance of monolithic crowns for posterior teeth)
The posterior teeth are the primary teeth of chewing and are provided with cusps and grooves (elevations and depressions) on the upper surface to aid in crushing the food particles and therefore better mastication. When replacing these teeth, if the crowns are layered, they are eventually prone to chipping due to heavy chewing forces, leaving a flat the tooth surface.
This will compromise the chewing capacity, reduce the height of the crown, may cause TMJ pain when multiple teeth are involved resulting in patient discomfort and dissatisfaction. Monolithic Zirconia metal free crowns are devoid of any layering and inherently possess all the cups and grooves, which will not chip off. It not only maintains the height of the back teeth, but also provides better chewing efficacy. Monolithic Zirconia, with all these advantages, proves to be the ideal choice for replacing posterior teeth.
This picture is demonstrating why monolithic metal free zirconia crown is best for posterior teeth. As we can see, layered zirconia metal free crowns have become flat due to chipping of the layers over time as a result of masticatory forces. Whereas the monolithic zirconia crown with self-incorporated grooves and fissures makes it immune to chip, maintaining the anatomy and helping in better chewing, without any compromise.
The choice of the crown depends on the tooth that is being replaced. If it is the posterior teeth, where majority of the chewing forces are there, one can consider getting a PFM or a monolithic metal free zirconia crown. But as the chewing forces are greater, the porcelain may cause an unintended damage to the opposing natural tooth. Apart from which, the porcelain may eventually be chipped off leaving a flat surface with the metal part exposed. Hence a monolithic zirconia crown, which provides excellent chewing efficiency, no damage to the opposing natural tooth, having good strength and better esthetics would be the ideal choice.
Zirconia Layered with ceramic are exclusively indicated in replacing anterior teeth where they best mirror adjacent natural teeth.
| Best crown for anterior tooth or implant | Layered zirconia crown |  |
| Best crown for posterior tooth or implant | Monolithic zirconia |  |
| Best bridge for full arch implant | Nexxzr bridge |  |
Few cases like a single missing tooth, with inadequate bone to favour implant placement might need a dental bridge i.e., replacing the missing tooth by taking support from adjacent teeth. There are few types of dental bridges which can be used to replace a missing tooth. Certain considerations are however to be made to place a successful bridge.
Missing tooth is replaced by taking only one healthy tooth as a support. This type of bridge flexes more, affecting the natural tooth to a greater extent.
Missing tooth is replaced using metal framework as a support on the adjoining healthy teeth.
A long span of missing teeth are replaced using few implants. Cantilever bridges and Maryland bridges are seldom used as they does not serve the purpose of functionality of replacing teeth. They only help in providing an aesthetic advantage of eliminating the space of missing teeth.
One or more missing teeth are replaced by gaining support from the adjacent teeth which are otherwise healthy. The healthy natural teeth are reduced on all sides to place the bridge over them.
- The healthy natural teeth are to be filed down to take support for missing tooth
- If more teeth are missing, the bridges tend to flex, exerting unequal forces on the supporting teeth.
- The health and longevity of the otherwise healthy teeth are compromised.
- Chances of food accumulation and micro leakage is more when proper marginal fit is not give, leading to caries progression.
- The supporting teeth might need root canal treatment or the periodontal health might be compromised if not properly maintained.
The tooth reduction of supporting teeth might compromise the general health and longevity of the healthy natural teeth.
These bridges tend to flex under constant chewing forces as the missing tooth does not have a proper support of its own. Conditions where more teeth are missing, requiring a long span bridge further flex, exerting unequal forces on the adjacent teeth.
Also, there is a greater chance of food lodgement and caries progression along the margins of the teeth.
All these factors determine the success rate of bridges. Certain modifications in the preparation of teeth, treatment plan and material used to fabricate them will eliminate most of these problems giving a much better prognosis of dental bridges.
Although Porcelain fused to metal (PFM) bridges are widely being used, the newer material Zirconia has many advantages over the former. Few of them are being listed below:
Zirconia material has a greater flexural strength and fracture toughness when compared to the PFM. They easily bear the constant chewing forces and show a very little tendency to fracture.
The CAD CAM designed Zirconia metal free crowns offer a precise marginal fit which will give a perfect crown to tooth attachment and does not allow microleakage.
Zirconia being a metal free material is highly biocompatible and does not have any tissue reactions. Hence the missing tooth is made in such a way that there is minimal or no gap between the underlying gum and the tooth. This reduces food lodgement and consequent ill effect on the natural teeth.
Inherent high strength of Zirconia material makes it fracture resistant at a minimum thickness of 0.5mm. Hence not much tooth structure of the otherwise healthy tooth is reduced. This also will not compromise much of the health of natural teeth.
Zirconia, have high flexural strength of around 1200MPA and a great fracture resistance. Hence it is indicated in posterior bridges where occlusal forces are more.
Having known all the advantages of Metal free Zirconia over Porcelain fused to metal crowns, it is also imperative to know which crown to choose over implants. There are two types of dental implant systems being used.
Conventional Multi-Unit Implant – where the abutment is separately attached to the implant body with the help of screws.
Single-Piece Basal Implant – The entire implant comprises of a single component with self incorporated abutment.
This implant design determines how the permanent crowns/bridges are fixed onto them. When multi unit implants are used, to place a PFM/Zirconia bridge, a metal sleeve is made in the metal core, which helps in fixing the bridge to implants with the help of a screw.
Apart from all the advantages of zirconia over PFM, which were described, when crowns/bridges are placed over implants, certain additional factors are considered to allow better retention and their prognosis.
The bridge seems to be esthetically pleasing, but the crown along the margins of the metal sleeve is compromised and is more prone to fracture under forces.
Also, the multiple layers of metal, ceramic, screw and the cement require a lot more space, which in turn affects the size of the abutment. The abutments are greatly reduced affecting the retention of these bridges.
Using a single piece implant, eliminates this metal sleeve, as the abutment is self-incorporated, and is not fixed with mini screws. It also greatly reduces the additional 
space required for screws and ceramic layers.
High Strength of monolithic Zirconia (metal free) even at a minimum thickness (as low as 0.5mm) makes it possible to maintain the height of the abutment. Adequate height of these abutments increases the surface area for cement to properly adhere the bridge to implants, ameliorating the long-term retention of bridge.
NexxZr bridge is a complete zirconia bridge, which is a combination of monolithic metal free Zirconia on the posterior teeth and layered metal free Zirconia on the anterior teeth.
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