Hey Bonk! I had this frustrating situation the other day: the cement was hardening when I was cementing a final ceramic crown. Why did that happen? It was expressed from a previously used tube with a fresh tip.

Cementing final restorations can be a very stressful process, especially if the restoration is an anterior crown or veneer. Placement and alignment must be perfect. There is no room for error, as the esthetic zone shows all! Every detail must be considered and precision of the steps must be accurate. It's a bad day when things go poorly. 

The luting cement manufacturers have improved the materials and systems to help make our jobs easier and less stressful. Pre-filled cartridges with disposable mixing tips have resulted in greater accuracy of material and more predictable outcomes. The old days of measuring and mixing powders and drops on glass slabs have been replaced by dual tubes containing the equivalent of base and catalyst to be mixed thoroughly together.

But caution must be taken with these new components to ensure proper mixing. The dual-tube cements with mixing tips are not foolproof. It is easy to assume that because the plungers in these tubes are designed to express equal amounts of luting cement. But this may not be the case.

There is always the possibility that reverse suction/material contraction may affect the amount of expressed material. If this occurs, it is possible (and probable) to have unequal distribution of material through the mixing tip. This unequal mix could result in delayed setting time, improperly mixed cement, or worse, a quicker set that does not allow adequate working time for proper restoration seating.  All of these scenarios lead to restoration failure and possible remake.

There are two steps for a simple solution. The first step, to avoid one of these errors, is to express material from the tubes prior to placing a mixing tip. The plunger should be depressed until both catalyst and base components express from the ends of the tubes in equal amounts. A mixing tip can then be placed over the tube ends.

The second step is to depress the plunger to express a small amount of material from the mixing tips onto a mixing slab. This small amount should be discarded, as it is possible that this first bit of material expressed through the tip is not adequately or fully mixed. The subsequent mixed materials should be distributed properly and evenly into the restoration to be cemented. This two-step technique should be applied to all materials manufactured and distributed in these dual-tube elements.

This small, but important step in the luting process helps to provide a less stressful and more predictable end result for cementing restorations.

Jeff Bonk, D.D.S., P.C., Spear Faculty and Contributing Author - http://jeffreybonkdds.com

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