Adjusting the occlusion

When adjusting the occlusion on an zirconia teeth caps, there is no need for concern about opaque or metal show-through because the colour of the restoration is homogeneous throughout the restoration. Shape and contour adjustments are easily accomplished using a diamond or small green stone and any conventional extraoral ceramic polishing kit. The restorations are very strong, and occlusal adjustments can be made before the unit is cemented, allowing for final polishing of the restoration in the laboratory, rather than in the patient’s mouth.

Resistant to fracture

Because the restoration is so resistant to fracture, conservative preparation is encouraged; because the restoration is homogeneous in nature, there is no aesthetic ceramic component to fracture during occlusal loading. Research has found MZ crowns are less abrasive to opposing natural dentition than most other full-coverage restorations. The tissue response to MZ is very favourable. Resistance to bacterial colonization helps keep the soft tissue surrounding MZ restorations much healthier-looking at re-care visits.

Require less time to fabricate

Perhaps the greatest advantage of MZ crowns is the fact that they require less time to fabricate. As the impression is scanned for fabrication of the crown or a digitial impression is taken sometimes – it removes all the intermediately steps as we do in conventional PFM crowns.

Wear of opposing tooth structure

Wear means “loss of material from a surface”. Wear of tooth material is related to several factors, such as mechanical contact, surface roughness, grain size, fracture toughness, occlusal load, temperature, chemical reactions, environment and lubrication. Surface conditions is one of the most crucial factors, therefore, different kinds of surface treatments should be applied on the restorative materials in order to prevent damage of natural antagonist teeth.

There are two common surface treatment techniques for monolithic zirconia, such as polishing (manual/machine) or glazing (glass coating/firing) to improve the aesthetic appearance of the restoration and to obtain smooth surface texture. Diamond points, rubber wheels and abrasive pastes are used in polishing procedures. Glazing is performed by firing a thin coating of glass on the surface or by firing the restoration up to temperature required for glazing.

The wear ability of monolithic zirconia — can be clearly observed that polished zirconia had the lowest wear on the antagonists compared to glazed zirconia. This result was attributed to the fact that glazed zirconia loses the thin glaze after a short period of clinical function, with the result of appearance of the rough and more abrasive surface of zirconia. It was also stated that glazed layer is easily removed by chairside occlusal adjustments.

Surface roughness

Preparing a smooth surface for ceramic restorations is considered as an important step because increased surface roughness associated with improper surface treatment can increase wear rate of the opposing natural teeth and can compromise the clinical performance of the restorations.

Polished surfaces of monolithic zirconia were smoother than glazed surfaces. It was stated that polished zirconia showed a lower surface roughness than glazed and ground zirconia. These differences may be due to the different polishing (machine or manual) and glazing (glass coating, firing) techniques, or different study protocols. It was known that machine polishing results in a significantly higher surface gloss of ceramics than manual polishing with tools for intraoral polishing.

Fracture strength

Fracture of zirconia showed the highest strength when luted with adhesive resin or glass-ionomer cements, compared to lithium disilicate and feldspathic ceramics. Similar results were obtained with monolithic zirconia crown with a thickness of 1 mm was found equal to metal-ceramic crowns. It was also reported that strength of monolithic zirconia was higher than veneered zirconia, lithium disilicate and metal-ceramics.

Optical properties 

The creation of acceptable aesthetic result with monolithic zirconia restorations is challenging because they are mono-layered restorations. Application of colouring liquids, surface characterization, glazing and polishing of zirconia are the procedures to look like natural teeth. Significantly improved colour adaptation to adjacent teeth is accomplished with colouring of the monolithic zirconia structures, followed by individual colour characterizations achieved by surface painting.

The colouring liquids with different colour intensities are applied with a paintbrush prior to sintering.

The translucency of the monolithic zirconia restoration is also essential for optimized aesthetic outcome. However, an increase in crystalline content and framework thickness in order to achieve high strength would generally result in lower translucency. Zirconia has higher contrast ratio compared to glass ceramics, and can be clinically applied with a minimum thickness of 0.4 mm.