Fractured Bridge Abutment AKA Why I Now Use Keyways- By Thomas W. Mitchell, DDS, FAGD

Introduction:

I did this bridge 19(20)21 in 2000 BL (Before Dr. Lane Ochi). As Dr. Ochi pointed out when one abutment is much smaller than the other the force concentrations will weaken the abutment and it'll fail one way or another. Here #21 fractured horizontally at mid prep. Now all my bridges get a keyway at the distal of the smaller abutment.

Bridge 19(20)21 done in 2000. Now it's loose on abutment 21.

I cut the bridge at the 19-20 connector and removed the 20-21 segment, finding the occlusal 1/2 of #21 abutment still in the crown.

There was no significant decay on the abutment.

Being so old school I place 4 Minikin self-threading pins. I'm trying to protect the buildup from horizontal fracture.

Surpass Universal Adhesive and WO Titan flowable composite (by Apex Dental Materials, Inc.) co-cured around the pins.

Anchor buildup resin cement. I cut a box form in the distal of the prep to make room to place the keyway closer to the long axis so we get a better emergence profile.

This is 19 after removing the old buildup material.

Here's the case back from the lab. It's critical that the keyway be oriented so the most mesial abutment crown seats first and posterior section seats down into that abutment to hold it on.

It doesn't matter if the female key is connected to the mesial abutment or, as Dr. Lane Ochi designs, the male is attached to the mesial abutment so the keyway can be hidden.

Here's the female part of the keyway on the distal of 21.

Even though I did prep a big box form into 21 distal, the keyway still extends distally farther than I'd like. Here that's due to draw issues. Another advantage of inverting the keyway (to hide it) is then the male won't extend distally as far, reducing the leverage on the abutment. I prefer my orientation because the keyway is visible so there's no question of why that connector looks funny.

The mesial abutment is seated first and the distal section slides into place.

Before we add cement we always fill the female key with vaseline. Can you guess why and how I learned to do that?

Then normal cementation procedures and we're done.

Conclusion:

I now use keyways on all the bridges I do, placing them at the distal of the smallest abutment tooth to reduce the forces on that tooth and help prevent failure from abutment crown loosening or abutment prep fracturing. Keyways come as inexpensive plastic patterns and all labs have easy access to them. I doubt it's possible to incorporate them into zirconia bridges yet. Hope this helps.

Case by:

Thomas W. Mitchell, DDS, FAGD

Email:

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