eLABORATE Magazine featured Yugo’s article titled ‘Ultimate Zirconia Coping Design: Understanding Aesthetics and Function’ in Sep/Oct 2012 (published language: English).
Introduction:
It has been a decade since Zirconia became available to the dental market, and it is now one of the premier alternatives to Porcelain Fused to Metal (PFM) restorations.
There are always pros and cons in selecting any material, and dental professionals need to treat their chosen material properly to ensure the longevity of the restorations.
One of the common issues facing Zirconia is the potential fracture of the layering ceramic/porcelain (Fig 1 & 2). The general causes of the problem are:
- Firing program/conditions,
- Parafunctional stresses,
- Ceramic-Zirconia compatibility, and
- Insufficient support in the sub-structure.
Understanding of function/occlusion, visualisation of the outcome and an appropriate substructure design: taking these factors into account will significantly reduce the risk of failure.
According to a study by 3M ESPE1, the fracture/chipping rates of PFM restorations were 1.36%, whereas Lava Zirconia fractured or chipped at a rate of $1.20% over 44,133 restorations in total (in 2012).
Many people still believe that PFM is a better choice, but the studies show otherwise. Zirconia is no longer considered a new material, and there is no reason for us not to relegate PFM to a thing of the past.
In my opinion, we can prevent the fracture even further than the studies (close to 0%) by adding sufficient support within the restorations sub/superstructure.
The “$64,000 question” is where to apply the support and how much without losing the aesthetic benefits of Zirconia.
This article presents studies in understanding the aesthetics and function – the ultimate goal of Zirconia coping design.
Aesthetic Zone:
The ‘Aesthetic Zone’ can be defined as any dentoalveolar segment visible upon full smile from an objective standpoint2. Subjectively, it has been described as any dentoalveolar area of aesthetic importance to the patient.
Additionally, the ‘Aesthetic Window’ is what’s within the lips. When working within the aesthetic zone, we need to carefully determine the treatment plan, prep design and material selection to maximise the aesthetic outcome.
Fig 3 shows an aesthetically pleasing smile with a medium lip-line. Neither the upper nor lower teeth show the margin fully. There is just enough interdental papilla showing in the upper anterior region to emphasise the individual teeth within the aesthetic zone.
Fig 4 shows a gummy smile – the patient has a high smile-line, and only the upper teeth are visible. Thus, the aesthetics of the upper cervical third become an essential part of the success of the case if we were to treat the upper restorations.
Fig 5 shows a low lip-line with the lower dentition being more visible than the upper, so the aesthetic demands become higher in the visible region – the mandible. As can be seen, the aesthetics are more compromised on the maxilla for that reason.
Fig 6 literally shows all the teeth in the mouth! In fact, the patient’s main concern was the aesthetics of the molars due to the buccal corridor, which is not in harmony with the soft tissue.
As can be seen above, each patient has different expectations based on their aesthetic zone. It is essential for dental professionals – clinicians and technicians to communicate regarding the patient’s requirements.
Upon the assessment of the aesthetic zone, what happens in the non-aesthetic area? The clinical instructions, clinical philosophy and patient’s expectations are our priority. Still, in general, we can apply more support / thicker substructure in the non-visible areas of the non-aesthetic zone.
It is a trade-off: the more layering porcelain applied to achieve better aesthetics, the weaker the restorations become. Thus, the more monolithic Zirconia we can obtain for the support, the stronger they become, but the less aesthetically pleasing they become.
Understanding oral stress – Occlusion:
Understanding the distribution of stresses under occlusion is essential when designing proper support within the sub/superstructure of the Zirconia framework.
Figs 7-12 show examples of a typical maxilla on the left and a mandible on the right. Functional cusps (Figs 7 & 8) and marginal ridges (Figs 9 & 10) are the areas in which the most support is necessary because they bear significant stress under occlusal load.
On the upper, the functional cusps are in the palatal region within the non-aesthetic zone to create a robust substructure without compromising the aesthetic outcome.
On the other hand, lower functional (buccal) cusps can appear within the aesthetic zone. Thus the substructure needs to be designed more carefully. The goal is to apply enough support without negatively affecting aesthetics.
The marginal ridges are the most common area of failure of fracture of veneering ceramics/porcelain (c.f., Fig 1). This area, in particular, must be supported as much as possible while still leaving enough space for layering ceramics/porcelain.
When considering anterior guidance (Figs 11 & 12), Zirconia backing can be applied on the upper as it is outside the aesthetic zone (i.e., being invisible under normal circumstances).
It is impossible to see this area without a mirror behind the teeth, so there is no reason for us to compromise durability by applying aesthetic porcelain in this area.
No one would appreciate having a fracture, as seen in Fig 2, requiring a remake of their restorations!
On the lower anterior, coping design can vary depending on the occlusion, aesthetic zone, clinical philosophy and the patient’s desire for the outcome.
Zirconia backing at the lingual aspect is still preferred for strength as it’s not within the aesthetic zone.
If we can afford to have more substantial substructure without visible aesthetic compromise, that should be the choice we should make.
Coping Design:
Figs 13-40 show the tooth structure: natural dentition or ideal final shape (left); ideal Zirconia coping design against the final shape (2nd from the left); combined tooth structure with the final shape, Zirconia coping design and natural dentine structure (middle); natural dentine structure against the final shape (2nd from right); and the dentine structure on its own (right).
To a certain extent, we need to consider the natural dentine structure and carry them through to the coping design.
The translucency of Zirconia copings can mimic the colour of a natural dentine substructure, thus the knowledge (understanding and controlling of the opacity/translucency) of the aesthetic factors coupled with an understanding of the functional relationships within the aesthetic zone and occlusion will allow the design of copings for the ultimate in both durability and aesthetics.
Upper Incisor:
This tooth is in the aesthetic zone. The labial aspect is not relevant to the occlusion, so as long as the thickness of the layering ceramics/porcelain doesn’t exceed more than 2mm, no support is necessary.
Interproximal openness is essential for a better aesthetic outcome and control of the layering ceramics/porcelain.
The incisal height of the coping is related to the final colour/wear patterns of the tooth.
The more worn the dentition being working on, the higher the height of the substructure incisally.
As can be seen, the palatal aspect is fully covered (apart from the incisal third) with monolithic Zirconia for anterior guidance.
Polished Zirconia is superior for biocompatibility and wears against natural dentition, so it is essential to give a high polish/lustre finish where the substructure is visible in the final restorations.
The palatal aspect should be fully covered with monolithic Zirconia, apart from the interproximal and the incisal third, as explained above.
A uniform amount is reduced at the interproximal and labial surfaces so that veneering porcelain can maximise aesthetics, with full Zirconia backing on the palatal aspect (which is outside of the aesthetic zone).
Upper Caine:
The design of the labial view should be reasonably close to the shape of the internal dentine structure as long as the layering ceramic/porcelain is less than 2mm in thickness.
It is in the aesthetic zone with no centric relation to be considered.
The most significant differences, when compared to the internal dentine structure, are the cervical third and palatal aspect.
Sufficient room for layering ceramics/porcelain on the cervical third for better aesthetics, with Zirconia backing for support.
It is essential to consider the lateral excursion/canine guidance.
The contact points in the centric relation areas should be covered with Zirconia.
Interproximal openness is necessary and follows the natural dentine structure on the incisal third for a better aesthetic outcome.
The characteristics are similar to the upper central from this view.
Upper Premolar:
The internal dentine structure lacks adequate support on the marginal ridges, which must be reproduced within the Zirconia substructure/framework.
The incisal third of the coping can remain similar to the internal dentine structure.
Applying a Zirconia collar on the cervical in the palatal aspect increases the strength of the restoration. Extra support on the palatal cusp is necessary, height-wise, due to the occlusal force from the functional cusp.
The labial aspect is similar to the internal dentine structure while creating sufficient room on the cervical third for layering ceramics/porcelain for a better aesthetic outcome.
The height of the Zirconia collar at the cervical should be 0.5mm-1.0mm for presentation purposes (i.e., not as visible, yet maintaining a good amount of strength).
The height of the palatal cusp should be greater than the buccal cusp for functional reasons. The palatal aspect is not in the aesthetic zone, plus it has a functional/working cusp.
There are significant differences in the marginal ridges between the coping design and the internal dentine structure.
More considerable support in this region is a must for the longevity of the restorations.
The buccal aspect is in the aesthetic zone with a non-functional cusp, so no support is necessary as long as the thickness of the layering porcelain doesn’t exceed 2mm.
Upper Molar:
Similar to the upper premolar, except for the presence of two cusps.
Similar to the upper premolar, except for the presence of two cusps.
Similar to the upper premolar, except for the presence of two cusps.
Similar to the upper premolar, except for the presence of two cusps.
Lower Incisor:
Similar to the upper central.
Similar to the upper central.
Similar to the upper central.
Similar to the upper central.
Lower Premolar:
There is a lack of internal dentine structure support on the marginal ridges, and the height towards the tip of the cusp; this support needs to be created within the Zirconia coping.
The buccal aspect is in the aesthetic zone and has a functional cusp.
The aesthetics come from layering ceramics/porcelain on the buccal aspect. However, we need to increase the height of the substructure in comparison to the internal dentine structure for greater support.
Sufficient room is left buccally on the cervical third for the aesthetics created by layering ceramics/porcelain, and a Zirconia collar is retained on the lingual aspect.
The lingual cusp is a non-functional cusp, so no support is necessary as long as the thickness of the layering ceramics/porcelain doesn’t exceed 2mm.
We need to follow the shape of the internal dentine structure with adequate support on the marginal ridges.
Prepared similarly to the upper premolar, although the differences in support on the buccal cusps and lingual cusps are reversed in regards to its height.
Sufficient room is left on the occlusal table for achieving pleasing aesthetics, as this area is generally visible while speaking or smiling/laughing with an open mouth.
Lower Molar:
Similar to lower premolar, except for the presence of two to three cusps.
Similar to lower premolar, except for the presence of two to three cusps.
Similar to lower premolar, except for the presence of two to three cusps.
Similar to lower premolar, except for the presence of two to three cusps.
Postscript:
Due to the advancement of the material in Zirconia (i.e., increased level/degree of translucency of the Zirconia blocks), it is understandable to recognise the growing popularity of monolithic restorations.
However, the ceramic/porcelain layering technique remains highly sought-after to reproduce the natural aesthetics adequately.
I’ve constructed thousands of Zirconia restorations with the way presented above for the past decade, and the failure rate is honestly closed to zero (which my clients can confirm).
What’s important is for the dental professionals to be on the same page in the coping design and communicate the level and degree of strength and aesthetics required for each case.
References:
[1] http://solutions.3m.com.au/wps/portal/3M/en_AU/3M-ESPE-APAC/dentalprofessionals/
products/category/digital-materials/lava-zirconia/
