New Paradigm for Crown Preparation: Great White Ultra Carbide Instruments

by George Freedman DS, FAACD, FACD

The standards of dental care have evolved rapidly over the past 50 years.1 Today’s best practice modalities require both tooth conservation and clinical efficiency.2 These concepts are not always mutually compatible. The efficient, and preferably rapid, removal of existing tooth structures and restorative materials must be accomplished with minimal heat generation during the preparation phase.3 As clinical efficiency is increased with faster and more aggressive cutting tools (FIG 1), it is clinically imperative that tooth preparation avoid the excessive heat generation that could possibly damage the remaining tooth structures and endanger the health of the pulp.4,5

In most clinical situations, water and air coolants are utilized in conjunction with high-speed bur preparation to reduce the risk of thermal damage to the tooth.6 The clinical efficiency of tooth preparation is largely dependant on the shape and design of the cutting bur, and the number of steps that comprise the overall treatment. The more often that the dentist must change burs during tooth cutting, the more time- consuming the process, and the less efficient the technique.

Practitioners use both visual and tactile clues to determine tissues to be removed. Darker dentin is assumed to be affected by caries; it should be removed (unless, of course, it is re-hardened secondary dentin). Lightly colored dentin and enamel are presumed to be healthy tissues. For the dentist to observe color differences during preparation, the bur’s rotation should remove debris as quickly and effectively as possible (FIG 2).

The earliest dental burs were manufactured from a variety of metals that were harder than natural tooth structure. With time, steel became the preferred bur metal. Developments in particle-to-metal adhesion technology resulted in the first diamond burs. These burs were preferable to steel for high-speed tooth preparation. The subsequent introduction of carbide cutting instruments was a leap forward for dentistry; carbide offered more effective tooth preparation with less surface striation than diamonds. More recently, cross-cuts and innovative attack angles were introduced to the carbide cutting shank to make preparation better, faster, and easier (FIG 3).

In the past, dentists have tended to favor diamond burs for extra-coronal tooth preparation while carbide burs have been used largely for intra coronal cutting.7 The relative popularity of carbide and diamond burs varies considerably in various parts of the globe, largely due to local availability, cost, and education.8 One factor that is often not considered by the clinician is that as diamond burs are used, their cutting efficiency tends to decrease dramatically. Their cutting diamonds tend to wear down and debris accumulates in the bur cavities (FIG 4), reducing efficiency.9 In order to compensate, dentists tend to press harder on the tooth with the bur in order to maintain the earlier cutting efficiency. Inadvertently, this actually decreases the efficiency of the procedure and increases the potential for heat generation.

Diamond burs tend to grind tooth structures while carbide burs cut these same materials. This leads to the conclusion that crown and bridge preparation, where rapid and effective gross tooth reduction is required and desirable, is best accomplished with carbide instruments.

Recent research has indicated that when a crown or onlay restoration is to be bonded to the tooth surface, carbide bur preparation can improve the bond to the dentin.10 A more effectively bonded crown increases the longevity of the restoration by decreasing leakage, and thereby the possible adhesive failure of the restoration. Carbide burs typically generate a smoother surface and a partially visible smear layer.11 This smear layer may be more easily dissolved and incorporated by self-etching primers, thus providing a stronger hybrid layer. This results in higher bond strengths.12 Cross-cut carbide burs improve the retention of crowns cemented with zinc phosphate by approximately 50%. Thus, the use of finishing burs on axial walls is discouraged.13

Current concepts of conservative dentistry dictate that a minimum of healthy tooth structure be removed during the preparation prior to the restorative process. Natural enamel and dentin are very likely the best dental materials in existence. Tooth structure conservation is thus inherently a desirable dental objective. Consequently, minimally invasive procedures that allow a greater part of the healthy tooth structure to be preserved are preferable (FIG 5).14 The patient also benefits greatly from minimally invasive dentistry. There is typically less discomfort during treatment, and a greater likelihood that the repaired tooth will last a lifetime.

The dental profession tends to take burs for granted. They are frequently used for patient treatment every day, and their effectiveness and efficiency can have dramatic impact on the practice. It is interesting to note that if the practitioner uses burs that are just 10% more efficient, the savings in operative time can easily increase practice billing significantly without any corresponding increase in overhead. Thus, the entire revenue increase goes directly to the bottom line. Generally burs are one of the least expensive components of the dental armamentarium, at least relatively. A small difference in bur cost can often make a major clinical impact. The most important parameter to consider is to select the best bur for the job, keeping in mind that the small added expense of opting for a premium instrument can pay off handsomely. Some burs are designed for single use. They can be sterilized and re-used, but often exhibit a significantly decreased cutting efficiency. Other burs are designed to be sterilized and re-used.

Recent research at the University of Rochester, Eastman Dental Center, jointly undertaken by the Prosthodontic and the Mechanical Engineering Departments, examined the efficiency of various dental burs with respect to cutting rate and load needed to complete standardized preparations in Macor samples. Both air driven and electric handpieces were tested. The cutting rate represents the speed at which the bur (reflecting its material composition and design) cuts through a standardized material. The faster the speed, the more efficient the preparation. The load measures the operator pressure needed to cut effectively. A higher required load will cause greater operator fatigue at the end of a long working day.

In the air-driven high speed handpiece, the SSW Great White Ultra had a significantly greater cutting rate than the other burs tested (FIG 6). In addition, the Great White Ultra bur required the least load, or operator pressure, for effective preparation (FIG 7).

Similar results were observed for electric high speed handpieces. The SSW Great White Ultra had a cutting rate significantly greater than the other burs tested (FIG 8) and required the least load, or operator pressure, for effective preparation (FIG 9).

In practical terms, the Great White Ultra burs cut between 11-35% faster than the other burs tested. This can save the practitioner between 1-3 minutes on a 10 minute preparation procedure. The decreased load also translates into greater operator comfort.

Dental bur design has developed varying flute angle and cutting characteristics that are specific to the intended task. Operative, cavity and crown preparation carbide burs have flutes (dentates) that are designed deep and wide, creating a more aggressive cutting of enamel with increased speed and efficiency (FIG 10). Operative burs are either straight-bladed or cross-cut. Straight-bladed burs cut more smoothly but are slower, particularly with harder substrates. Cross-cut burs tend to cut faster, but may create more vibration. Finishing burs have more flutes, closer to
gether and shallower, than operative instruments (FIG 11). This design allows for fine finishing and polishing of dental materials or tooth surfaces.

The Great White Ultra Bur (SS White Burs, Lakewood NJ) represents a new category of crown preparation burs; it is more sharply dentated than earlier cross-cut burs. The design creates a bur that cuts faster, with less vibration in both tooth structures and other dental materials (FIG 12). The bur cuts faster and smoother because it does not “grab” or “catch” the substrate, and thus does not stall in harder materials. The novel design creates less stress on the remaining tooth structure and less frictional heat that may irritate the pulp and damage the supporting periodontal structures. The aggressive cutting angle (FIG 13) of the Great White Ultra allows the operator to use less pressure on the tooth during preparation (resulting in decreased tooth heating and dentist fatigue). (FIG 14).

The goals of conservative tooth preparation include:15

1. Re-contouring the remaining tooth and restored structures to a specified shape and size to accommodate a crown

2. Providing a depth guide on all surfaces including the occlusal to allow the crown to have sufficient bulk and strength to withstand occlusal and other intraoral forces

3. Completing the preparation process with a single pass by one bur on the buccal, lingual, mesial and distal

4. Creating the intended marginal finish, whether shoulder or chamfer, at the same time as accomplishing the gross preparation of the other surfaces

5. Developing a surface that is suitable for bonding the indirect restoration

6. Remaining conservative of tooth structure

7. Preparing the tooth quickly and efficiently for both patient and dentist comfort.

For most dentists, the cutting speed tops the list of features that are important in selecting dental burs. Carbide manufacturers have produced a variety of designs and shapes that are intended to reduce the time that it takes a practitioner to prepare the tooth for a crown.

The Great White Ultra bur cuts quickly and smoothly through enamel. It negotiates amalgam and other restorative materials with minimal clogging and no drag or stalling in these harder materials. The bulk reduction in the crown preparation phase can be accomplished with a single instrument (FIG 15). The highly dentated body of the Great White Ultra cuts efficiently and quickly, and combined with the smooth tip, help to provide two reduction actions in one single pass with a single bur (FIG 16). The rounded, non cross-cut tip provides smooth, precise, and controlled margins with the same cutting motions as the gross reduction preparation.

There are two preferred marginal anatomies for crown preparation, the chamfer and the shoulder. Accordingly, two margin-specific clinical series of burs have been crafted. The Great White Ultra 856 Series develops a rounded axial-gingival margin providing a chamfer finish for the preparation (FIG 17). The Great White Ultra 847 Series creates a 90 axial-gingival wall and provides a shoulder margin for crown restoration (FIG 18).

The Great White Ultra bur kits organize a variety of shapes and sizes that are typically used in routine crown preparation. The bonus is that once the correct bur is selected, the entire preparation can often be completed without changing to another instrument. Bulk reduction and a smooth margin are created with the same reduction instrument.

Clinical Case 1

The preparation of the bicuspid crown is very rapid and straightforward. A single pass of the Great White Ultra bur reduces the bulk of the tooth at the height of curvature and finishes the chamfer margin simultaneously (FIG 19). The interproximal preparation must be accomplished without marring the surface of the adjacent tooth. One of the thinner GWU burs may be used (FIG 20). The buccal surface is not smoothed out with a disc or diamond; the striations created by the bur increase the surface area available for adhesion (FIG 21). The occlusal reduction is completed to provide 1.5-2.0mm clearance for the crown (FIG 22). The completed preparation, ready for impressions, is viewed from the occlusal (FIG 23). The entire circumferential preparation was completed with a single Great White Ultra bur in a single pass.

Clinical Case 2

The molar crown preparation is begun on the buccal surface (FIG 24) and continued circumferentially. The bulk and marginal preparations are completed at the same time. The completed preparation, ready for impressions, is viewed from the occlusal (FIG 25). The stone model is verified against the intra-oral preparation, and the crown is tried on extra-orally (FIG 26). If the fit on the model is correct, then the crown is tried intra-orally, and cemented on to the prepared abutment (FIG 27). A circumferential preparation that has even depth throughout and adequate space for the restoration, as well as a well-defined margin (whether chamfer or shoulder), results in a well-fitting and long-lasting crown.

Clinical Case 3

Some practitioners prefer to use depth grooves to guide crown preparation. The Great White Ultra bur is well suited to this task. The depth grooves are placed quickly and evenly to the desired preparation depth (FIG 28, images 1-4), at the same time that the location of the margin is determined. The depth grooves are joined, maintaining the selected depth of the preparation and the location of the restorative margin (FIG 29, images 1-2). The occlusal surface is reduced to an ideal depth and shape (FIG 29, image 3) and the preparation, completed within a matter of minutes, is viewed from the occlusal (FIG 29, image 4).

It is reasonable to expect that Great White Ultra burs can be used for multiple tooth preparations, and that they can be cleansed effectively between patients. There are two important steps to follow for the proper sterilization of multiple-use tungsten carbide burs:

1. Burs should be cycled through an automated washer such as the Hydrim (SciCan, Toronto, Canada), that provides rapid and effective washing, rinsing, and drying with a single push of a button. (The instruments may be cleaned manually, but they should be pre-soaked to loosen debris and handled with extreme care to avoid skin punctures. Avoid cold sterilizing solutions that contain oxidizing agents which can weaken carbide burs. Ultrasonic systems can be used, as well. The re-use of solutions in these systems is less than ideal, however. Separate the burs from each other in a bur block during ultra sonic emersion to prevent damage to the cutting surfaces. Brush any remaining debris away with a stainless steel wire brush. Rinse and dry the burs.)

2. It is only at this point that sterilization can be initiated. The importance of this step cannot be overstated. Only the effective sterilization of burs eliminates the threat of cross contamination to patients and staff. Steam autoclaves will effectively sterilize carbide burs but some units may allow surface corrosion to develop. Metal bur blocks may promote galvanic corrosion, and should be avoided. Both dry heat sterilizers and chemi-claves can be used without corroding or dulling carbide burs.

Conclusion

Great White Ultra burs are an innovative solution for the crown and bridge tooth preparation process. The differential reduction provided by the varied cross-cutting of the bur’s active surface allows intraoral multi-tasking. Great White Ultras simplify the clinical procedure by reducing the circumferential bulk of the tooth and preparing the final margin at the same time. Rapid cutting, less structural stress, and a more adhesive surface are additional advantages. OH

Dr. George Freedman is a founder and past president of the American Academy of Cosmetic Dentistry, a co-founder of the Canadian Academy for Esthetic Dentistry and a Diplomate of the American Board of Aesthetic Dentistry. He is Visi
ting Professor, Universita di Firenze, Florence, Italy. Dr. Freedman sits on the Oral Health Editorial Board (Dental Materials and Technology) is a Team Member of REALITY and lectures internationally on dental esthetics and dental technology. A graduate of McGill University in Montreal, Dr. Freedman maintains a private practice limited to Esthetic Dentistry in Markham, Canada.

Oral Health welcomes this original article.

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