I expect to make slight occlusal adjustments when I insert any indirect restoration. The “high” spots are generally on cusp tips, marginal ridges or on the convexity of inclines. It is rare to adjust contacts that affect the details of occlusal anatomy, such as the central groove.
If adjustment needs to be made to the groove, either the technician did not adequately establish the proper depth to the groove or the clinician did not reduce the tooth enough to create the anatomy without making the restoration too thin. If occlusal adjustments affect the occlusal anatomy, re-establishing the grooves or anatomy is very difficult on e.max restorations and nearly impossible with monolithic zirconia.
Grinding with a diamond bur to create a groove or detail in the anatomy is one of the most difficult things to do, even in the laboratory. The pointed diamonds used are very inefficient, so the temptation is to grind at high speed, use course diamonds and apply more pressure on the bur. RESIST THIS TEMPTATION! It is well documented in clinical literature that surface and subsurface damage from grinding on ceramic post-insertion can cause fractures or chipping of the ceramic.1, 2, 3
One major concern with attempting to re-establish detail in the anatomy after cementation is the fact that the thickness of the ceramic is unknown. For example, Ivoclar recommends a minimum thickness of 1.5 mm for e.max in functional areas. If the e.max is thinner than 1.5 mm in the central fossa, it is at a higher risk of fracturing.
If you decide to attempt to re-establish grooves in a restoration, it is important that you use only a flame-shaped fine diamond bur at a maximum speed of 15,000 rpms. This will generate minimal heat, vibration or trauma to the ceramic. The diamonds will quickly wear off the shaft during the adjustment, so make sure that you have an adequate supply on hand before beginning the procedure. Polish with rubber points (Brasseler W16 MLD and W16 FLD for e.max, W16 MZR and W16 FZR for zirconia) and Brasseler’s diamond polishing paste using a bristle brush.
The best and safest option may be to leave a flatter occlusal morphology, rather than trying to re-establish anatomy.
(Click the link for more articles by Dr. Bob Winter.)
Bob Winter, D.D.S., Spear Faculty and Contributing Author
- Chang CW, Waddell JN, Lyons KM, Swain MV. Cracking of porcelain surfaces arising from abrasive grinding with a dental air turbine. J Prosthod. 2011; 20(8):613-620
- Song XF, Yin L, Han YG, Wang H. In vitro rapid intraoral adjustment of porcelain prostheses using a high-speed dental handpiece. Acta Biomater. 2008; 4(2):414-424
- Song XF, Yin L. Subsurface damage induced in dental resurfacing of a feldspar porcelain with coarse diamond burs. J Biomech. 2009:42(3):355-360