03 May Which luting agent for fixing indirect restorations?
Dr Pasquale Venuti
The tooth preparation for indirect restorations (inlays, onlays, crowns, veneers) cannot provide any macro-mechanical retention (undercuts) – as we may do for direct restorations (direct amalgam or composite), because we need an axis of insertion for seating them.
Therefore, for fixing an indirect restoration to the tooth structure, we need to use a luting agent. The luting agent may adhere to residual prepped dental structure and to the indirect restoration’s intaglio surface by means of
- micromechanical retention
- chemical retention
The main luting agents we use in dentistry are
- Glass Ionomer cements (including Resin Modified Glass Ionomer)
- Zinc Phosphate cements
- Resin Cements (with or without bonding agent)
Glass Ionomer cements adhere to the residual dental structure both chemically and micromechanically.
Zinc Phosphate cements and Resin Cements (with or without bonding agent) adhere to the residual dental structure mainly micro-mechanically. Today, thanks to the presence in some bonding agents of molecules like MDP, we may get some chemical adhesion of the resin cements both to the dental structure and to the intaglio surface of the indirect restorations.
Glass ionomer cements and Resin cements adhere to the intaglio surface of the restoration mainly micro mechanically.
Even if we refer to resin cementation as an adhesive procedure, we should admit it is an improper lexicon. We should use the word adhesive procedure for glass ionomer rather than for resin cements.
Glass ionomer, Zinc Phosphate or Resin Cements?
This depends on the form of resistance (geometry) of the residual dental structure receiving the indirect restoration.
Many books and papers refer to the form of retention and form of resistance about indirect restorations. I ponder the lexicon is inaccurate, because we cannot offer any form of retention with the indirect restoration, differently from what we may do with direct restoration. That’s why we should refer on the kinematic point of view just to the form of resistance.
If you have no form of resistance to dislodgment (insufficient geometry), as happens with veneers and onlays, we should use resin cements, preferably with bonding agents. Resin cements offer more strength in tension and in shear and bonding agent offers better micro mechanical retention both on tooth structure and on the intaglio surface of the restoration.
If you have a form of resistance, as happened with most of the crowns, we may use every luting agent. In that case, I strongly prefer Phosphate Zinc cement and Glass Ionomer cements.
Why preferring Zinc Phosphate and Glass Ionomer when the prepped residual dental structure has a form of resistance?
The question is “Why don’t we just bond everything?”. The answer is that bonding procedures expose to may issues:
- bonding takes time
- bonding requires multiple steps….more steps can introduce more chances of mistakes
- bonding needs a strict isolation
- bonding requires to prepare the surface of the tooth and the intaglio if restoration with air abrasion
- cleaning up the excess of resin cement and bonding agent is a tough and long task
- more expensive procedure
Using the luting agent as Zinc Phosphate or Glass Ionomer makes the luting procedure:
- one step
- does not require strict isolation
- no need to air abrasion
- easy removal of excess of cement
- they have a long track record of decades. They have shown brilliant results in vivo on long
The only material that makes things a little bit confusing is lithium disilicate (e.max). In Vitro studies have shown that monolithic e.max luted with resin cements and bonding agents will create a stronger restoration. However, several clinical studies (in vivo) have shown that the success rates of Emax luted with glass ionomer vs. resin cementation with bonding agent – in presence of adequate form fo resistance – is the same.
A metanalysis by Maroulakos and coworkers (2019) have analyzed the clinical performance of ceramic crowns on natural abutments when adhesive or conventional cementation was employed. The analysis reported that the survival rates for adhesively cemented lithium disilicate crowns ranged from 83.5% to 100%, whereas the survival rate reported for conventionally cemented lithium disilicate crowns was 98.5%. Survival rates for adhesively cemented zirconia crowns ranged from 83.3% to 100%, whereas those reported for conventionally cemented zirconia crowns ranged from 82.0% to 100%.
The authors conclude that, based on current evidence, adhesive and conventional cementation results in comparable clinical outcomes for both lithium disilicate and zirconia tooth-supported single crowns.
For resuming, my suggestion is to use zinc phosphate cement and glass ionomer cement for luting crowns on abutments with an optimal form of resistance and to reserve bonding procedures with resin cements to all clinical situations in which we have no form of resistance.
In the next article, we’ll consider how to evaluate the form of resistance of the prepped residual dental structure, in order to differentiate among crowns with optimal and not optimal geometry.