![]() Bob Winter created these multi-radius burs. The multi-radius creates a 3-dimensional contour at the end of the bur. The ends of the diamonds are designed in such a way so as to reduce the chance of creating a “J-hook”. One solution to the “J-hook” problem is to utilize a diamond that has a multi-radius end rather than a round end. ![]() An open margin will result from this “J-hook” design. Ceramic cannot be fired to a thin edge less than 0.3mm. The “fin” of tooth structure remaining makes it impossible for fabrication of a closed margin. Overpreparation occurred because the diamond was placed beyond the bur radius. The “J-hook” is created by overpreparing the tooth with a round-ended diamond or carbide. One of the most common errors in preparing shoulder margins is the creation of a “J- hook“ finish line. No matter the choice of diamond, the goal is to create a smooth shoulder that can be accurately impressed, conventionally or digitally. If used properly, the end shape will create a shoulder contour and margin depth in a very precise manner. Various diameters and grits provide aid in the speed and smoothness of the tooth preparation and the margin. The shape of each design is developed in such a way to create the proper contours necessary for the intended preparation. Many different diamonds are available on the market. Various high-speed diamond rotary instruments are commonly used for crown preparations. The diamond shape and contour is important for successful margin preparation. Creating these different margins requires different tools to achieve success. The various margin types are necessary for adequate restoration strength and material support. A deep chamfer is required for a metal-ceramic restoration. And a rounded shoulder margin is needed for an all-ceramic crown. A knife-edge margin may be selected for a gold or metal crown. Different designs are necessary depending upon the type of restoration to be fabricated. By applying this concept, the tooth can be prepared for the crown of choice that will satisfy all necessary requirements for strength and color.Ĭrown margins are the critical factor in restoration fit. The diagnostic wax-up acts as the guide in this outcome-based process. ![]() Final shade, material of choice and proposed crown contours are all determined before we prepare the tooth. This concept reflects the idea that we begin the tooth preparation process with a picture or a vision of the outcome. In the Restorative Design Workshop, we speak of outcome-based design. There are many factors that go into designing a crown that will achieve the desired outcome. Obtaining good preparation design and contouring is a skill that requires practice and consistency. Whether the final crown restoration is waxed and pressed or digitally fabricated and milled, the preparation smoothness and the accuracy of the margins will be the determining factor for healthy tissues, good fit and a pleasing result. Precision, accuracy, and sharpness are key features to fabrication, maintainability and predictability over the long term. Margin design is fundamental to the fit of the restoration. But the most critical factor for restoration success and longevity is the margin. Proper contouring, smooth surfaces, rounded and softened corners are important attributes that provide for successful outcomes. Tooth preparation is the key to achieving these goals. Be biologically acceptable to the tissues Create restorations that are esthetically pleasingģ. Maintain structural integrity of the toothĤ. There are four primary goals of tooth preparation:ġ. A frequent question is about tooth preparation. “Hey Bonk, how do you get good crown margins?”Ī great question! And very important, too. Many dentists ask me questions about techniques and procedures in my practice.
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