by Gary Alex, DMD
There is a trend in dentistry toward simplification. This is readily
apparent with the emergence of so-called universal bonding agents
and cements that manufacturers claim can be used for virtually all
adhesive and cementation scenarios. While universal adhesives and
cements have the potential to reduce product inventory and simplify
placement protocol, a universal material may not, in fact, be the best choice in all
clinical situations.
When it comes to cementation there are many factors to consider before deciding
on the use of a specific class of cement and placement protocol, including the nature of
the tooth and restoration substrate being cemented or bonded, inherent retentiveness
of the preparation, access and control of the working area, ease of use and cleanup, and
aesthetics. While other cementation options exist, and excluding porcelain veneers,
the majority of dentists today use one of three distinct classes of materials for the fi nal
cementation of their restorations.
- Resin modifi ed glass ionomers (e.g. FujiCem-2 GC, 3M ESPE RelyX Luting Plus)
- Dual-cure self-adhesive resin cements that are placed without the use of a separate
dentin bonding agent (e.g. 3M ESPE RelyX Unicem 2, Kerr Maxcem,
BISCO Bis-Cem and GCG-Cem)
- Dual-cure resin cements that are used in conjunction with a separately placed
dentin bonding agent (DB A) (e.g. 3M ESPE RelyX Ultimate Adhesive Resin
Cement, BISCO Dual-Link and Ivoclar Multilink)
Resin Modified Glass Ionomers (RMGIs)
I consider RMGIs to still be one of the best, and one of the most underrated,
cementation options. In fact, unless retention is an issue, RelyX Luting Plus
(3M ESPE) is my default cement for all full coverage zirconia, PFM and cast
gold restorations. In all cases, the intaglio surface of the restorations should
be sandblasted prior to cement placement. In addition, I like to use a zirconia
primer such as Z-Prime (BISCO) on the intaglio surface when cementing zirconia
restorations.
RMGIs have many positive attributes, including good physical properties,
low solubility, chemical bond to tooth structure, low film thickness, significant
fluoride release, anti-microbial activity, proven long-term track record and low
incidence of post-operative sensitivity. One of the biggest clinical advantages
of RMGIs is that they are very easy to mix, place and clean. In fact, cement
cleanup is generally much easier with RMGIs when compared to resin cements.
This fact alone makes RMGI an attractive cementation option. RelyX Luting
Plus also has a tack-cure option that allows almost immediate cement clean-up
in the margin areas.
On the downside, RMGI cements are not as retentive as the two classes of
resin cements that have a distinct advantage over RMGI cements when it comes
to bonding restorations on or in minimally retentive preparations. This is because
resin cements have the potential to bond more durably to both the tooth structure
and the restorative material. Resin cements are also generally the best choice when
placing porcelain restorations that can be etched with hydrofluoric acid as resin
cements bond extremely well to etched porcelain and optimize overall assembly
strength. In addition, resin cements are often a better choice for restorations placed
in the aesthetic zone, because RMGI cements tend to be opaque and can show
through many restorations (such as translucent lithium disilicate) which results in
unacceptable aesthetics.
Self-adhesive Resin Cements
Dual-cure self-adhesive resin cements were developed as an alternative to traditional
resin cements. When initially developed, the use of resin cements first
required conditioning of the tooth tissues (usually with phosphoric acid) followed
by the application of a DBA. Self-adhesive resin cements eliminated this step by
essentially having the cements themselves condition and prime the tooth substrate.
You could say the etching and priming steps are essentially built into the chemistry
of self-adhesive resin cements. Elimination of the etching and priming steps
saves considerable time, and this class of cements is currently very popular among
clinicians. In fact, one product in this class, 3M ESPE RelyX Unicem 2, has been
the best selling cement in the USA for several years. Cements in this category are
generally stronger, have better physical and esthetic properties, are more versatile
and more retentive than RMGI's.
On the downside, these cements have much less fluoride release and cleanup
can be more difficult compared to RMGIs. I strongly advise those using self-adhesive
resin cements to allow these materials to self-cure on their own and then
follow-up with light-curing to ensure maximum polymerization. There may be
several good reasons for allowing the cement to self-cure before light curing. In
principle, the chemical interactions that need to occur for a self-adhesive resin
cement to actually interact with tooth tissues take some degree of time and
should take place while the material is in a fluid state. In this way, molecular
movement and interaction with tooth tissues is optimized. If the cement is tackcured
or light polymerized immediately after seating the restoration, the cured
cement may limit the degree of molecular interaction because it's harder for
things like molecules to move around in the set cement. In addition, there is a
window of opportunity during the self-cure mode when the resin cement gels
up and is relatively easy to remove. Conceptually, there may also be advantages
in terms of stress development by allowing the resin cement to cure more slowly
during the self-cure mode, providing more time for both the tooth and restoration
to adapt as stress develops.
Resin Cements used with a DBA
ith the emergence of self-adhesive resin cements, dentists might ask why even
use a resin cement with a DB A anymore, as it takes more time and additional steps.
The primary answer is bond strength. If the goal is to optimize bond-strength,
then a DBA followed by a traditional resin cement is still the way to go. I routinely
use products such as Scotchbond Universal Adhesive (3M ESPE) along with RelyX
Ultimate resin cement or All-Bond Universal and Dual Link (BISCO) in situations
where I want maximum retention (such as minimally retentive preparations
and wings for resin bonded bridges). A significant problem with this class of resin
cement is that cleanup of the fully set cement can be extremely challenging. As with
self-adhesive resin cements, I recommend allowing the cement to self-cure prior to
light curing and cleaning before fully set.
Conclusion
I think the idea of a universal cement is laudable, but not necessarily practical.
At this point, I think clinicians have to tailor their choice of what cement to
use based on the specific clinical situation, the material they are placing, the best
current available research, and their own clinical experiences. This requires that
clinicians be knowledgeable about a range of cement categories in order to select
the most appropriate cement for the job.
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