As I teach first year dental students how to prep a tooth for a full gold crown, get an impression, pour and mount models, wax-up, cast and polish, they are almost always amazed at all the required steps and how any part of the process can compromise their semester s major project. Perhaps their greatest realization is how much their preps affect the final result. Problems such as discontinuous or double margins, knife edge prep margins, and inadequate occlusal reduction haunt them throughout the project. Very few of the final results have been what I would consider adequate for clinical acceptance. But the procedure is valuable simply for them to have the realization of the requirements for success. Cerec dentistry is not really different. Each step will have an effect on the final result and the basic training we receive after purchasing our units focuses on prep, powder, and picture. Although state laws may vary and most have not addressed the Cerec procedures specifically, those three steps are the basic responsibility for the hands and skills of the doctor. Initial frustration with the Cerec experience is not uncommon because the requirements for success are different than for most lab processed crowns. Like the first year dental students, when the realization of those different requirements become reality in the mind of a doctor, a paradigm change occurs. Cerec success becomes reality and the future of dentistry comes to the present. In the First Quarter issue 2011 of Cerecdoctors.com magazine, Sameer Puri wrote a great article, Cerec Clinical Preparation Guidelines. He addressed occlusal and axial reduction, separation from the adjacent teeth, and the ferrule effect. Following up on that basic information, I propose to review some reasoning for preparation parameters and some clinical preparations and discuss why they work or what problems they may have. Back in the days of PFM crowns, (for me) all crown preps looked about the same, just different size for molars or bicuspids. With Cerec crowns, very few preps look the same because I try to let the tooth direct the preparations but keeping milling requirements and end result esthetics in mind. My first consideration is to look at the requirements that a milling machine will have to mill a successful restoration. As the tooth preparation is designed, an operator should think inside the box. That is, think of where the milling bur has to go (inside the tooth and approaching from the apex) and what it has to achieve. If the preparation is perfectly designed for a PFM restoration, the milled restoration would have some difficulty. For those doing Lava crowns, realize that the framework is also milled and will have the same problems a Cerec crown would have with the same clinical preparation. The width of the MCXL step bur is 1.4 mm wide and has a blunt end. As the bur approaches the inside of the cusp tip, it has to either stop short of the cusp tip (fig 1) which would leave unmilled porcelain and the crown will not seat completely, or remove all porcelain interfering with the
cusp (fig 2) and thin out the restoration, risking perforation or weakening of the structure and future fracture. Therefore, in order to avoid overmilling, all remaining cusps must be left more than 1.4 mm wide and have a flat surface for the bur to be able to duplicate the surface internally (fig 3). The next paradigm shifting realization occurs with the concept of line angles. Since the days of G.V. Black, dentists have been coerced into preps that have sharp line angles and straight lines for ease of restoration fabrication and retentive properties, whether for partial veneer gold restorations or amalgams. In those cases, if a prep had a gingival seat that was rounded, failure on the prep grade was ensured. With a good milled restoration, the preparation demands smoother transitions, roller coaster slopes and curves and no sharp line angles. The basis for that requirement is that all margins are milled in endo mode. This means that if the milling bur encounters a tight spot, rather than overmilling to get all the porcelain out of the way, it will stop and leave the excess and the crown will not seat as shown previously in figure 1. Internally, when the step bur mode is in effect, overmilling occurs. If that mode were applied to the margin regions and overmilling occurred, there would be an open gap and the crown would not fit. With the internal areas you have the choice to command the bur to mill endo mode or step mode, but near the margins, the software ALWAYS mills in endo mode. Figure 4 illustrates an onlay with sharp corners (which draw) but the crown doesn t seat because of the inability of the bur to mill out those corners(see Sam s pic in his level 2 presentation). Now lets look at a few preparations.
In this prep, a Sombrero or J margin creates a real challenge for the step bur. It would be impossible for it to mill out the porcelain to fit into this trench. Plus the fact that the tooth spike could easily fracture off, leaving an open margin. In the next prep, the knife edge margin preparation would leave the axial wall of the crown so thin it would not likely survive the milling process, or the designer would have to make the margins and the axial wall so thick, it would make the resulting crown unacceptable. Even without considering milling requirements, this crown design is doomed to failure due to lack of axial reduction. Other issues with this case include very rough margins in areas where you can see them, and poor imaging on the facial surface In this preparation, the deep distal decay destroyed a lot of structure, but with the preparation design, the operator was able to maintain a very significant amount of enamel for great bonding possibilities in the buccal and mesiolingual cusps. Note the smooth transitions from the proximal box forms up and over the remaining cusps. In the central fossa area, a wide isthmus exists, but the remaining buccal and lingual cusps are wide enough for the step bur to fit inside and mill the crown accurately. One would expect some overmilling from some of the roughness in the pulpal floor, but not likely of great significance.
In these preparations, note lack of line angles and the ramping indicated by the green arrows. Across the occlusal, extra reduction was placed to allow for more clearance and a keyway for the crown to seat accurately. This preparation provides very significant conservation of the lingual cusp and buccal and proximal walls providing resistance to rotation and accurate seating. Note again the lack of line angles in transition from proximal to cusp. Notice also the lingual cusp is fairly flat, but not a horizontal flat top. In a case where the lingual of an upper cusp is reduce only the two mm with the reduction bur, if the slope of the cusp is followed from the central pit through the cusp, it will naturally follow this sloped contour. Then
when the design for the crown is done, the internally sloping contour of the crown will maintain adequate thickness for the margin to survive. Remember the direction of the enamel rods (perpendicular to the surface) and the prep should follow that same direction. Consider this preparation: In this case, the horizontal flat top is not perpendicular to the cavo surface which results in a fragile thin margin which will likely not survive the milling process, leaving a chipped, open margin. However, in the following case, the flat top is lower than the cusp curvature and the margin would easily survive because in that position, the prep angle follows the direction of the enamel rods.
However, the buccal cusp is rather thin above the pulpal floor and could cause overmilling in that area. Here is where buildup might thicken up the buccal cusp so that overmilling won t occur. In the next example, the operator chose to drop a lingual step to provide some mechanical resistance form, provide a buccal and lingual ferrule effect and maintain buccal and lingual enamel. The central groove was prepped lower to allow for the occlusal anatomy and adequate thickness of the restoration and smooth transition from the proximal areas prevent the binding at sharp corner. Inlay and inlay/onlay combinations create another range of issues to consider and are technically more difficult to achieve. However, the principles of smooth margins and transitions and preps with adequate reduction remain important factors to success. Leaving narrow cusps less than 1.5 mm wide create
problems for the milling unit to be successful. That limit may be approaching as illustrated with the red arrow in the following diagram. The yellow arrow indicates a better, more easily milled cusp: In conclusion, I would suggest that the operator have in mind the result he wants before he starts the preparation. First step, get the required occlusal reduction, then remove old restorations and decay. Then blending box forms into the remaining structure with the consideration for enamel conservation and esthetics, particularly on the facial surface. In consideration of using a crown buildup, it should not be used just for lengthening walls for the conventional preparation, but for bulking up thin cusps or areas the milling bur can t fit into from the apical approach. Smooth the prep, especially the margins and move forward to the powder and picture steps toward a sucessful restoration. Steven F. Nielsen, DDS