As we all know, the smoother we make our preparations - especially in regards to indirect restorations - the better. 

Smooth preparations not only facilitate easier and better fabrication, they also result in increased long-term predictability of our restorations. While any restoration benefits from a smooth preparation with the increased use of all porcelain restorations and CAD/CAM, the need for smooth restorations has never been more critical. 

Two of the simplest and most cost-effective ways we can improve the smoothness of our preparations is to take a look at the burs or cutting instruments we are using and the hand pieces that we use to dive them. The three instruments I have found most easy to use, particularly for margin refinement, are fine grit flat-ended modified cylinder diamonds, a fine grit round-ended cylinder diamond bur or a pointed friction grip on which the point on the stone has been removed (essentially converting it to a modified cylinder stone).

Keep in mind there is a hidden danger when using any one of these instruments that will be disastrous if not appropriately managed. The danger is carrying the bur too far laterally onto the tooth, which will result in the “up-curve” of the bur cutting a sharp lip at the edge of your margin, which will make it very difficult, if not impossible, to fabricate an excellent fitting restoration.

To avoid this, if you are using a flat-ended modified cylinder, make sure to use the flat apical end of the instrument, which will mean that a small portion of the bur will be hanging off the tooth and towards the soft tissue. If using a round-ended cylinder bur, you must only use slightly less than half the diameter of the bur on your margin.

Equally, if not even more critical, is the handpiece used to drive your instrument of choice. While our high-speed handpieces are great for efficient initial preparation, they are far from the best at allowing us to achieve the smoothest final preparation. For that, we really need much slower hand pieces that provide a better tactile feel. The most ideal handpiece, or at least my favorite, is a 1:1 attachment for my electric motor.

Another option with less of an initial investment (especially if you are not yet using electric motors) is a friction grip attachment for your slow-speed air motor. When using either of the burs listed above for your final margin refinement, it’s best to move the bur in one constant direction around the tooth rather than in a back and forth motion, as the back and forth motion will inadvertently leave small steps in your margin preparation. Lastly, place a finger from your opposite hand to help balance and steady your movement around the tooth.

John R. Carson, DDS, PC, Spear Visiting Faculty. www.johnrcarsondds.com 


Comments

Commenter's Profile Image Dave
August 1st, 2013
Great techniques John. Thanks for sharing. If you use an electric 1:5 friction grip for your main prep, then turn down the speed for finishing with fine diamonds and stones, do you still need a 1:1 attachent? What advantage do you get by buying a 1:1 head? I have always wondered???
Commenter's Profile Image John R. Carson
August 1st, 2013
Great question Dave, I had wondered the same thing in the past as well. The answer is while you can slow down a 1:5 and get some of the same benefits the electric motors and their attachments are designed to run at full speed and when we slow them down it puts more strain on them while this strain is not typically a big issue for short periods of time, it is best to keep these periods short to maximize the life of the motor and attachments. The other advantages are that 1:1's tend to be more durable and since there is a limit to how much you can slow down an attachment with a 1:1 you can achieve slower speeds if needed.
Commenter's Profile Image Alex Quintner
August 2nd, 2013
One danger not mentioned is the potential for heat damage when using a fine grit bur to refine your prep. Due to the low cutting efficiency I have found that there is a higher likelyhood to press harder using these burs to great a smooth one pass cut which even with copious irrigation can heat up the tooth. My point is that this requires special attention so as no to overhear the pulp especially given that in most cases when we are refining our preps we are working over relatively thin tooth structure.
Commenter's Profile Image John Carson
August 2nd, 2013
Excellent point Alex! We have to make sure too much heat is generated. Although not stated when refining preps I recommend a light touch, even for the one direction final pass. In addition I also use irrigation with my 1:1 electric. Thanks for bringing this up as I should have stated this originally.
Commenter's Profile Image Kirk
August 2nd, 2013
What about using hand instruments to get rid of the dreaded "J-Margin?" We tend to forget about all those chisels, hatchets, gingival margin trimmers, etc., that we had to buy in dental school, and, lately, I have begun to pick them up again, with, in my opinion, excellent results over what I can do with rotaries alone.
Commenter's Profile Image John R. Carson
August 3rd, 2013
Excellent point Kirk, they can be used for very nice results as well!! Thanks for brining up this up! Loving all the great comments!
Commenter's Profile Image Mike Hagley
January 13th, 2014
A # 6 round bur rotated slowly in a slow speed handpiece with a light touch (rotating from axial wall toward margin) also works well to smooth shoulders for CAD/CAM restorations. make sure only "inside" half of bur is contacting prep to avoid 'J' ". Credit to CERECDoctors.com