It isn’t unusual for clinicians to regularly see patients who have clinical failures or complications related to poor occlusion. An essential part of...
BUR SIZES AND SELECTION FOR OPTIMUM PREPARATIONS
A bur consists of three parts: the head, the neck, and the shank. The burs used in handpieces are usually fabricated from tungsten carbide, but they could also be made with diamond particle coatings.
Burs are one of the most frequently used mechanical devices in any dental office. Selecting the correct size and type of bur is essential for clinical success.
A bur consists of three parts: the head, the neck, and the shank. The burs used in handpieces are usually fabricated from tungsten carbide, but they could also be made with diamond particle coatings. The coating has varying degrees of roughness depending on the nature of the treatment. Ceramic and zirconia burs are increasingly utilized. Ceramic burs are suitable for caries removal and are used in restorative procedures.
A wide range of sizes and configurations is available, and the bur selected by the clinician will depend on individual preference, the type of procedure, and the bur’s effectiveness in a given clinical situation. Selecting the most appropriate bur is critical for safely and effectively removing dental hard tissues. Choosing the correct type of bur also increases ergonomics for the clinician and reduces discomfort for the patient.
Tungsten Carbide and Diamond Coated Burs
Tungsten carbide burs have blade-shaped heads to produce the desired cutting action. These cutting properties vary according to the blade’s degree of angling and positioning. Some of these properties are outlined below:
- A bur with a more obtuse angle produces a negative rake angle, increasing the longevity and strength of the bur.
- Burs with more acute angles produce positive rake angles, so cutting is quicker, but the bur will become blunt sooner.
- Crosscuts or additional cuts across the blades increase cutting efficacy. Crosscut burs are faster due to less debris build-up.
- Flute angles and cutting characteristics in burs are designed for each specific for the task.
- Cavity preparation burs have wider and deeper flutes that cut into enamel more efficiently and quickly.
- Operative burs are crosscut or have straight blades that cut more slowly but smoothly.
- Diamond coated burs create a smooth shape, and the level of smoothness achieved depends on whether you select a fine, medium, or coarse-coated bur.
Bur shapes depend on the clinician’s preference and the patient’s required treatment. Configurations include tapered fissures, inverted cones, round, and pear-shaped. Each of these are available in a variety of sizes and diameters. Flat fissure, crosscut, and plain tungsten carbide burs all have the same cutting force when used in a traditional air-driven handpiece, so their shape doesn’t appear to influence the cutting power of the bur.
More specialized restorative burs are used for specific tasks, including depth-cutting burs, which have horizontal ridges against the diamond fissures to guide the depth of the cut. End-cutting burs are used to shape the floor of mesial and distal tooth preparations for Class II cavities with smooth sides. These burs reduce the risk of affecting the surface of the adjacent tooth with the bur’s cutting surface. End-cutting burs can also be used to shape the pulpal floor of Class I and II cavities while avoiding the risk of a cutting surface meeting the prepared cavity walls.
More recently, tungsten carbide burs have been developed. These burs have a unique geometry, because they are more sharply dentated compared with a crosscut bur. Tungsten carbide burs cut more quickly and efficiently through enamel, amalgam, composite, and metals, all while producing less vibration. The design of these burs helps reduce stress on the tooth and surrounding periodontal tissues, because they require less pressure to initiate and complete a cut. Cutting is more accurate and generates less heat. Similarly, fine crosscut tungsten carbide burs are suitable for all types of restorative dentistry and can accomplish multiple tasks, saving time because there is no need to swap burs during a procedure.
Choosing the Correct Type of Bur for Specific Clinical Situations
Removing a Dental Crown
A diamond fissure bur is most effective when cutting through porcelain. After removing the porcelain, it’s easiest to remove the metal substructure using a tungsten carbide bur. The tooth structure underneath can be prepared with a tapered-fissure bur.
Removing an Amalgam
A tungsten carbide bur is the most efficient and fastest way to remove an existing amalgam. Pear-shaped burs and fine crosscut burs are both good burs to select.
Removing Dental Caries
Dental caries are removed using a round tungsten carbide bur at a slow speed. Good sizes to select are sizes two, four, six, or eight. Using a round tungsten carbide bur at a slower speed removes only a minimal amount of dental hard tissue while more effectively removing the softer areas of decay. Where cavities are deeper, it reduces the risk of pulpal exposure. Some clinicians may prefer to use a ceramic round bur. Once all signs of caries are removed, a finer and smoother diamond or carbide bur can complete the preparation before restoration.
Preparing for an All-Ceramic Restoration
All-ceramic restorations need a 90o exit angle but require a rounded internal angle. A modified-shoulder diamond bur provides a flat 90o exit angle and a rounded internal angle. When using any bur, it is possible to create a reverse margin inadvertently if the depth cut of the bur exceeds the diameter width at the tip. You often won’t notice this problem until a model is poured. One solution is to use an end-cutting bur or to choose a bur size that matches the desired depth of the preparation.
Finishing burs are made from tungsten carbide and have flutes that are closer together and shallower compared with operative burs. Tungsten carbide or diamond finishing burs remove excess composite and smooth the restoration before polishing. They create an optimal smoothness, reducing the potential for biofilm development on the restoration surface.
Maintaining Reusable Burs
After completing the patient’s treatment, it is essential to examine the condition of the burs closely and discard any that are worn or damaged. Using a damaged bur will affect the quality of the preparation and could cause trauma to the hard tissues. Additionally, a damaged bur will cut less efficiently and may cause overheating of the handpiece as it increases power to compensate. After pre-soaking them to loosen debris, any burs suitable for reuse should be cleaned and sterilized. Diamond burs may be cleaned using an enzymatic cleaning solution that prevents the diamond coating from dulling during sterilization.
Please be reminded that should you wish to discuss a case in more detail, our experienced technical team is here to assist you.