Indirect Restorative Tooth Preparation: Extreme Efficiency and Accuracy

Many options are available when preparing teeth for indirect restorative care.1-4 The ultimate goal of full-coverage tooth preparation should be the complete removal of enamel, existing restorative materials, and caries. This preparation must be accomplished in a fast, clean, and easy manner. One school of thought is to accomplish this using diamond burs. While this method is safe and unlikely to cause enamel fracturing, it is slow to cut enamel and even slower at cutting metal-based restorations such as amalgam, or sectioning existing cast metal copings and crowns.5-8 These problems occur because diamond surfaces fill with debris and clog, wear smooth, and can cause burnishing and overheating of the tooth surface. In response to this problem, carbide burs have been used to accomplish all of these tasks easily.

The techniques that employ carbide burs for these tasks have been studied in depth and have shown a favorable outcome versus diamond burs. In a study conducted at Ohio State University by Ayad, et al,9 105 standardized tooth preparations for complete crowns were performed using a modified milling machine on extracted human teeth with diamonds, tungsten carbide burs, and tungsten carbide finishing burs of similar shape. The prepared dentin was analyzed with a surface profilometer and a scanning electron microscope (SEM). Teeth completed with the tungsten carbide burs appeared to result in a smoother surface (1.2 µm).

Today, the use of carbide burs is a highly accepted method for fast, clean bulk reduction of enamel and removal of existing restorations and caries.10-13 However, there has been one problem with the “traditional” full-coverage preparation using 557 or similarly shaped carbide burs. Following the stripping of bulk enamel, a large, shoulder-type step would remain in the preparation, requiring significant finishing with a diamond to achieve an acceptable beveled margin, be it chamfer or other margin designs of choice. This article presents 2 case reports in which this problem is solved.

MATERIALS

Figure 1. The Great White Ultra crown and bridge preparation kit. Figure 2. The Great White 856 Series bur is designed to create a rounded axial-gingival line angle for full- and partial-coverage restorations.

In both case reports, the Great White Ultra crown and bridge preparation kit (SS White, Figure 1) is used, which accomplishes bulk reduction while simultaneously creating an ideal chamfer or shoulder margin (Figure 2).  These burs cut fast with minimal clogging or vibration. Geometric specifications have been designed into the Great White Ultra to produce a sharp, clean cutting tip that will produce an impression-ready margin finish line. In addition, these burs are manufactured with a process known as concentric engineering.

In a study conducted at the Department of Conservative Dentistry, United Medical School of Guy’s Hospital, Guy’s Dental School in London, Watson and Cook aimed to determine the degree of eccentricity between different tungsten carbide bur manufacturing techniques and to study the effect of bur inaccuracy on dental enamel.13 High-speed cutting interactions with dental enamel between carbide burs were studied by means of a video-rate confocal microscope. Videotape showing the interactions of high-speed rotary cutting instruments (at 120,000 rpm) was taken under simulated normal wet-cutting environments, and the consequent damage to the tooth tissue was observed as it occurred. Concentrically engineered bur types produced a superior quality cut surface at the entry, exit, and advancing front aspects of a cavity as well as less subsurface cracking. This study has shown that substrate damage is reduced with high-concentricity carbide burs.

Included in the kit are the following: the 856 series of chamfer preparation burs designed to create a rounded axial-gingival line angle for metal-to-ceramic restorations; the 847 series shoulder preparation burs designed to develop a 90° axial-gingival line angle suitable for anterior all-ceramic and PFM restorations; and the 379 football-shaped bur designed for occlusal and lingual reduction.

CASE REPORT NO. 1

Figure 3. A lower right first molar prior to preparation for a full-coverage restoration.
Figure 4. The cavity space was filled with a light-cured provisional material and light-cured. An impression was made in order to fabricate a provisional restoration following tooth preparation and final impression.
Figure 5. The entire occlusal table is reduced, including the amalgam restoration.

The patient presented for a consultation regarding his fractured tooth. The lower right first molar, tooth No. 30 (Figure 3), presented with the mesio-facial-occlusal portion of the coronal structure missing due to caries. In addition, the presence of a failing amalgam restoration contributed to the ongoing decay of the carious lesion, necessitating full-coverage treatment. Once the patient was locally anesthetized, the tooth was mocked up with Systemp (Ivoclar Vivadent,  Figure 4) and light-cured, followed by a triple-tray impression using a medium-body polyvinyl siloxane (Flexi Velvet, J Morita USA). The tooth was then prepared using the 856-016 Great White Ultra bur that has a tip diameter of 1.20 mm and a cutting head length of 8 mm. An initial 1.5 mm depth cut was made along the occlusal, and the entire occlusal table was reduced (Figure 5).

Figure 6. The bur easily removes all remaining amalgam, enamel, and caries while simultaneously creating a margin. Figure 7. Following the core buildup, the margin is refined.
Figure 8. The occlusion is properly reduced. Figure 9. The final crown preparation, ready for impressioning.
Figure 10. The laboratory-fabricated porcelain-to-metal crown immediately following cementation.

A depth cut was made on the facial of the molar using this same bur, and this action was continued circumferentially, removing amalgam, enamel, and caries while simultaneously creating a margin (Figure 6). A core buildup was then fabricated using Photocore (Kuraray North America), the margin was refined (Figure 7), and the occlusion was reduced using the Great White 379-023 (Figure 8). This football-  or egg-shaped bur has a cutting head length of 4.2 mm. The final, full-coverage preparation can be seen with subgingival facial margins, preserving the gingiva and creating crisp subgingival margins (Figure 9). Upon the patient’s return for case delivery, the porcelain-fused-to-metal crown (Aesthetic Porcelain Studios Dental Laboratory) was luted to the tooth preparation using a resin-reinforced glass ionomer cement (Fuji II, GC America, Figure 10).

CASE REPORT NO. 2

Figure 11. The lower left first molar with an open distal contact and subgingival carious lesion.

The patient presented with an open contact between the lower left first and second molars (Figure 11), with a defective amalgam restoration in the lower left first molar and a carious lesion at the tooth’s distal margin that was detected by explorer examination. Radiographic examination confirmed that there was a significant radiolucent area associated with the disto-marginal aspect of the amalgam. It was decided that in order to preservethe maximum amount of tooth structure while maintaining high restorative strength, a ceramic polymer onlay preparation would be created.

Figure 12. The facial amalgam is easily removed using a Fissurotomy bur. Figure 13. The preparation is restored using a flowable composite resin.
Figure 14. Amalgam removal is initiated, and the onlay prepared simultaneously. Figure 15. Continued removal of the remaining occlusal core and completion of the truing of the axial walls.
Figure 16. This same bur is used to remove the most distal portion of the remaining defective amalgam and decay while continuing to create an inlay box preparation. Figure 17. The occlusion is reduced.
Figure 18. The nearly completed onlay preparation prior to removal of any remaining unsupported enamel. Figure 19. The laboratory-fabricated ceramic polymer onlay immediately following resin bonding.

The facial amalgam was removed using a Fissurotomy bur (SS White, Figure 12) and was replaced with a flowable composite (Figure 13). This was followed by the removal of the existing amalgam restoration using the Great White 845-016 onlay preparation bur (Figure 14). This bur has a cutting diameter of 1.10 mm and length of 4 mm, allowing for complete, rapid removal of the amalgam restoration while simultaneously creating divergent axial walls (Figure 15). This was followed by the removal of the distal portion of the existing amalgam restoration along with any associated caries using the Great White 847-018 flat-end taper bur (Figure 16). With a cutting tip diameter of 1.50 mm and a cutting length of 8 mm, this bur allows easy access into narrow, long areas such as the distal box preparation areas of lower molars. Because the isthmus of missing tooth structure in the buccolingual was greater than one third of the entire bucco-lingual width of the tooth, the cusps required coverage. Rapid cuspal reduction was accomplished using the Great White 379-023 occlusal reduction bur (Figures 17 and 18).

At the delivery appointment several weeks later, the onlay was luted to place using a dual-cure resin cement (UltraBond Plus, DenMat, Figure 19).


References

1. Bass EV, Kafalias MC. Systematized procedure of crown preparation. J Prosthet Dent. 1989;62:400-405.

2. Zena RB, Khan Z, von Fraunhofer JA. Shoulder preparations for collarless metal ceramic crowns: hand-planning as opposed to rotary instrumentation. J Prosthet Dent. 1989;62:273-277.

3. Laufer BZ, Pilo R, Cardash HS. Surface roughness of tooth shoulder preparations created by rotary instrumentation, hand planing, and ultrasonic oscillation. J Prosthet Dent. 1996;75:4-8.

4. Dalvit DL, Parker MH, Cameron SM, et al. Evaluation of margin angles of collarless metal ceramic restorations. Gen Dent. 2004;52:148-150.

5. Schuchard A, Watkins EC. Cutting effectiveness of tungsten carbide burs and diamond points at ultra-high rotational speeds. J Prosthet Dent. 1967;18:58-65.

6. Elias K, Amis AA, Setchell DJ. The magnitude of cutting forces at high speed. J Prosthet Dent. 2003;89:286-291.

7. Matson E, Kikuchi HK. Appraisal of the cutting efficiency of rotary instruments of tungsten carbide at high speed [in Portuguese]. Rev Assoc Paul Cir Dent. 1981;35:150-155.

8. Atkinson AS. The significance of blade geometry in the cutting efficiency of tungsten carbide dental burs at ultrahigh speeds. Br Dent J. 1983;155:187-193.

9. Ayad MF, Rosenstiel SF, Hassan MM. Surface roughness of dentin after tooth preparation with different rotary instrumentation. J Prosthet Dent. 1996;75:122-128.

10. Price RB, Sutow EJ. Micrographic and profilometric evaluation of the finish produced by diamond and tungsten carbide finishing burs on enamel and dentin. J Prosthet Dent. 1988;60:311-316.

11. Eames WB, Nale JL. A comparison of cutting efficiency of air-driven fissure burs. J Am Dent Assoc. 1973;86:412-415.

12. Galindo DF, Ercoli C, Funkenbusch PD, et al. Tooth preparation: a study on the effect of different variables and a comparison between conventional and channeled diamond burs. J Prosthodont. 2004;13:3-16.

13. Watson TF, Cook RJ. The influence of bur blade concentricity on high-speed tooth-cutting interactions: a video-rate confocal microscopic study. J Dent Res. 1995;74:1749-1755.



Dr. Shuman maintains a full-time private practice outside Baltimore, Md, emphasizing reconstructive and aesthetic dentistry. He is a fellow of the Academy of General Dentistry, a fellow of the Pierre Fauchard Academy, and a member of the American Dental Association. Since 1989, Dr. Shuman has published more than 50 dental research and clinical articles that have appeared in numerous dental journals. He presents seminars and hands-on courses and has produced several educational videos including “The Joy of Clinical Dentistry” and “Do Your Dentures Suck?” as well the book Creating the Denture Practice of Your Dreams, which includes an instructional CD-Rom. He can be reached at (877) 4-SHUMAN or by visiting ianshuman.com.

Disclosure: Dr. Shuman received research support from SS White in preparation for this article.

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