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Careful! Your new light may not cure your
bonding agent. Here’s why ....
(In the beginning there was camphorquinone
and halogen lights, and all was good)
by Nelson Gendusa,
DDS Director of Research
Until the early 90’s virtually all light-cure resins used camphorquinone
(CQ) as the initiator. When the CQ molecule is hit by photons of
certain wavelengths (primarily the blue wavelengths around 460nm),
it generates the free radicals that trigger the curing reaction.
Until the early 90’s virtually all curing-lights used a halogen
bulb to create these photons. Halogen bulbs generate a relatively
broad spectrum that runs from 370nm (deep violet) to 515nm (bluish-green.)
So ... until the ‘90’s curing a restoration was like shooting
at a small target with a shotgun. You were certain to score a hit,
but you wasted a lot of buckshot in the process.
Fig 1: Think of your curing light as a gun and the initiator in
the resin as the target. The halogen light is a "shotgun."
It hits the CQ "target" but scatters considerable buckshot
around it.
Then about ten years ago two things occurred that individually were
brilliant ... but together, created some problems.
Remember, at that time virtually all light-cure composites were
using CQ as an initiator. So engineers decided to design a curing
light that zeroed in on that small 460nm peak. By shooting bullets
(or even lasers) instead of buckshot, these new lights would be
less wasteful. They’d cure faster and maybe even produce less
heat. The results of this engineering were the plasma arc light,
the curing laser and the new diode light.
Fig 2: Touch&Bond's unique initiator puts the target at the
other end of the halogen range. As you can see, the same halogen
shotgun blast hits both the T&B and CQ target.
Fig 3: But lasers, LEDs and most PAC lights aren't
shotguns ... They're "rifles" (or maybe in the case of
lasers, "rayguns.") They blast the CQ target dead center.
And completely miss T&B.
About the same time, the polymer chemists started experimenting
with new initiators. There are some limitations to CQ. For example,
CQ can restrict the film thickness of a bonding agent. And it can
affect the all-important air inhibition layer necessary to bond
to the overlying composite. Furthermore, CQ is yellowish, so when
it’s used in composite, it can prevent those "whiter-than-white"
bleached shades some patients now demand. So the chemists started
using initiators other than camphorquinone. They figured that since
everybody was using a halogen light with a broad spectral range,
they could employ initiators that reacted to a different part of
that range. Keeping the shotgun metaphor, fig 2 shows what the chemists
were “aiming” for.
But you can see the problem. Just about the time some light manufacturers
started shooting bullets, the resin manufacturers moved the target!
And if your light doesn’t generate the proper wavelengths,
it doesn’t matter how bright it is or how long you shine it
on the restoration. It may look like it’s cured due to the
heat of some lights, but it’s not well polymerized.
The moral:
If you use a non-halogen light, you should check whether it’s
compatible with any new material you try. This requires two phone
calls: First, call the material supplier and ask what it’s
“initiation peak” and “curing range” are. Then
call your light manufacturer to find out if it hits that target.
To save some of you the calls:
Epic-TMPT composite - Any curing light
Epic-AP composite - Any curing light
TotalBond adhesive cement - Halogen or Apollo with 430 tip
Touch&Bond bonding agent - Halogen only
Brush&Bond bonding agent - Any curing light
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