Spinning Down the Tooth: Advances in Crown Preparation
To keep pace with innovations in materials and technology, practitioners should be aware of new cutting instruments to more quickly and efficiently remove crowns and/or gain access through them for endodontic procedures.
Crown and bridge restorations remain the “bread and butter” of general dental practices and a large part of prosthetics practices. Yet, some practitioners are still using older methods and materials despite advances in technology that have changed how many clinicians prepare teeth and manage the materials from which fixed prosthetics are fabricated.
Advances in Handpieces
To date, air or electric have been the predominant methods of choice for handpieces, and both have positive and negative attributes. The main drawback for air-driven handpieces relates to force versus speed. Although an air-driven handpiece may have the bur spinning at 400,000 rpm, as soon as the bur contacts the tooth, the added resistance results in a drop in speed to 140,000 to 200,000 rpm, depending on the hardness of the material being cut.1-3 The practitioner’s instinctive reaction to resistance typically is to press the bur towards the tooth more, but that only increases resistance, further decreasing instrument speed. With electric handpieces, this is not an issue because the unit senses the increase in resistance (torque) and, in turn, increases the speed to keep the torque in a specified range. Therefore, although their speed is typically one-third of the speed of an air-driven handpiece, electric units cut more efficiently and faster because they deliver constant torque, which equates to cutting efficiency.4,5 Electric handpieces, however, are typically heavier and bulkier than air-driven models due to the mechanics needed in the handpiece to drive the shaft connected to the bur.
New handpiece technology combines air and electric to provide a hybrid unit (Midwest® Stylus™ ATC, DENTSPLY Professional, www.StylusATC.com). This unique instrument utilizes proprietary Speed-Sensing Intelligence (SSI), connecting the handpiece to the electronic control unit, which enables constant maintenance of bur speed based on the load applied to the bur. Speed is therefore maintained at 330,000 rpm, creating a more consistent transfer of power to the bur and thereby overcoming the cutting issues associated with air-driven handpieces. The Stylus ATC handpiece also has the lighter weight and more ergonomic dimensions of an air-driven unit and may be an alternative for practitioners who seek an instrument with greater power but are not comfortable handling the size and weight of electric handpieces.
More Efficient Crown Preparation
Many practitioners typically first consider using diamonds when preparing a tooth to accept a crown, reserving the use of carbides for removing old filling materials and preparing teeth for direct restorations. However, diamonds may not be the most efficient rotary instrument for crown preparation. Diamonds and carbides cut tooth structure differently. Diamonds grind away tooth structure, and the grit of the diamond—fine, medium, coarse, or super course—determines how quickly tooth structure is removed. In contrast, carbides “mill” away tooth structure using blades rather than the rough surface of the diamond to cut. Because a very hard substance (enamel) is overlaying a softer material (dentin), efficiency relates to how quickly the enamel can be removed from the tooth.
Crown and bridge carbides have been introduced based on knowledge gained in the machining industry. When machinists want to cut a material—whether it’s plastic, wood, or metal—they use rotary carbides with sharp toughened blades with cross-cuts (dentate cuts to the blade). Diamonds are reserved for smoothing already-cut surfaces. Milling (cutting) tooth structure is more efficient and faster than grinding. The more quickly and efficiently the preparation is performed, the less trauma is transmitted to the underlying pulp. These crown and bridge carbides are available in both chamfer and shoulder designs (for axial/proximal reduction and margin development) as well as a large football (for occlusal reduction). While these carbides can be used in both air-driven and electric handpieces, the author has found they cut more quickly and efficiently in electric handpieces. Unlike diamonds, which most practitioners use multiple times before discarding, these crown and bridge carbides should be restricted to single patient use, as their cutting efficiency decreases rapidly with each use. While diamonds can be autoclaved with no significant effect to their cutting ability, carbides lose the edge to the cutting blades during sterilization. A smooth non-dentated tip on these carbides provides a well-defined margin. Among these products are Razor Prep™ Carbide (Axis/Sybron Dental, http://axis.sybronendo.com) and Great White® Ultra (SS White, www.sswhiteburs.com).
For practitioners who nonetheless prefer using a diamond for crown preparation, a “guide pin” style diamond should be considered. These diamonds have a non-cutting tip that is narrower than the width of the diamond-covered portion of the instrument. The tip extends approximately 1 mm past the end of the diamond surface and assists in retracting the soft tissue to decrease gingival abrasion during preparation. As the instrument is used to prepare the axial and proximal walls, the guide pin limits the depth of cut, so over-preparation in areas that are difficult to visualize directly is avoided. Depending on the diamond used, depth of cut varies from 0.4 mm to 0.8 mm, allowing uniform tooth reduction to a depth ideally suited to the crown material that will be selected. Additionally, the tip will limit apical direction of preparation as it contacts the crestal bone, preventing the margin from being placed subcrestal. Among these products are NTI® Guide Pin Diamonds (Axis/Sybron Dental), Guide-Pin Diamonds (Brasseler USA®, www.brasselerusa.com), and Pin-Diamonds® (Komet USA, www.kometusa.com).
Off with the Old
As newer materials for crown fabrication have been introduced, the challenge has become determining how to best remove these crowns when they need replacement or how to best gain access through them to initiate endodontic treatment. Current materials used for fixed prosthetics range from all-metal, which can be non-precious (base metal), and those containing gold and other precious alloys, as well as porcelain-fused-to-metal and the newer all-ceramic materials such as zirconia. Each of these materials presents different challenges when removal is desired. When removing fixed prosthetics, carbides are generally used for cutting metal, while diamonds are reserved for cutting ceramics. The key is to select the most efficient rotary instrument for cutting a given material.
While all carbides can cut metal, some are not well designed for that task, so when using non-metal–cutting carbides, the practitioner can expect bur breakage and rapid dulling of the blades when in contact with metal. With this in mind, some manufacturers have developed metal-cutting carbides specifically designed to section the harder non-precious metals, which are more difficult to cut than precious metals. These specialty carbides are dentated instruments that can aggressively cut the hardest metals. They should be used with a light stroking motion and irrigation; after being used, they should be disposed of and not reused for another patient. When sectioning or accessing through non-precious metals, several carbides may be required to complete the task because they can dull quickly on harder metals. Among these products are Carbide Crown Cutters (Horico® Dental, www.horico.de), Great White® Gold Series (SS White), Razor Carbides (Axis/Sybron Dental), H4MC crown cutter (Komet USA), and SabreCut™ (Brasseler USA).
Removing ceramic crowns or accessing through them for endodontic procedures can be quite a clinical challenge, especially with the increased use of zirconia as either an underlying coping or as a monolithic material. Unlike metal, ceramics cannot be cut using carbides, because as the carbide contacts the ceramic surface its cutting blades are immediately obliterated due to the abrasiveness of the ceramic. Thus, diamonds are needed to cut or access through ceramics, essentially abrading away the ceramic material.
Zirconia has been the greatest challenge in clinical use compared to other ceramic materials due to its much harder consistency. Standard diamonds typically demonstrate diamond particle removal from the instrument’s shaft, shortening the life of the diamond. This is especially the case if irrigation is not used during the cutting procedure, as the instrument heats up on contact with the ceramic and the particles fall off. Manufacturers have, thus, developed diamonds specifically for cutting zirconia as well as other harder ceramics that are being fabricated. Interestingly, use of coarse diamonds is not needed, as medium and fine grit diamonds have been found to be as efficient as coarse ones.6 One clinical issue when cutting ceramics intraorally is microcracking. This is not a concern if the restoration is to be removed, but can be a clinical issue when creating access for endodontic treatment. To minimize this problem, it is recommended that while using irrigation, a round diamond be used in a circular motion, outlining the access shape desired. Once penetration through the ceramic has been achieved, the practitioner can switch to a longer diamond to widen the access to the desired shape. Utilization of a safe-ended diamond (no particles on the apical tip) adds a measure of safety, because it will prevent deeper penetration of the instrument while the access is widened. Among these products are Lion diamonds (Horico), ZR-Diamonds™ (Komet USA), DuraBraze™ (Brasseler USA), Great White® Z (SS White), and ZIR-CUT™ (Axis/Sybron Dental).
Tooth preparation materials and technologies continue to advance. Practitioners should be taking advantage of these improvements to provide better, more efficient treatment and learn how to better manage care.
1. Matsui K. Change of load and revolution number when types of cutting instruments attached to air turbine handpiece are used (author’s transl) [in Japanese]. Shikwa Gakuho. 1982;82(4):517-539.
2. Monagahn DM, Wilson NH, Darvell BW. The performance of air-turbine handpieces in general dental practice. Oper Dent. 2005;30(1):16-25.
3. Electric Handpieces. The Dental Advisor. 2005;22(2);508-510. https://www.cda-adc.ca/jcda/vol-72/issue-6/508.pdf. Accessed March 25, 2014.
4. Choi C, Driscoll CF, Romberg E. Comparison of cutting efficiencies between electric and air-turbine dental handpieces. J Prosthet Dent. 2010;103(2):101-107.
5. Eikenberg SL. Comparison of the cutting efficiencies of electric motor and air turbine dental handpieces. Gen Dent. 2001;49(2):199-204.
6. Christensen GJ. Endo access through ceramics: Are cracks a problem? Clinicians Report. 2012;5(10):1,3-4.
About the Author
Gregori M. Kurtzman, DDS, MAGD
Private Practice, Silver Spring, Maryland;
Former Assistant Clinical Professor, University of Maryland School of Dentistry, Department of Endodontics, Prosthodontics, and Operative Dentistry, Baltimore, Maryland