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