IntroductionDental burs are used to clean residual adhesive off teeth after orthodontic bracket removal. These burs incur wear degrada-tion in the process. Can they be re-used on another patient or should they be single use only? Here we employ non-destructive 3D imaging of used burs by micro-CT to address this question.

Orbitals: Pantone 293Bruker: Schwarz

Petra Koch1, Paul Zaslansky2

Charité - Universitätsmedizin Berlin, Germany Center for Dental and Craniofacial Sciences1Department of Orthodontics, Dentofacial Orthopedics and Pedodontics 2Department for Operative and Preventive Dentistry

As many teenagers and parents know, contemporary ortho-dontic treatment relies on aligning and moving teeth using adhesively attached orthodontic appliances (Fig. 1A). Dental composites and resins are used to bond orthodontic brackets (Fig. 1B) to the outer, typically labial tooth surfaces. Upon com-pleting treatment, months or years later, both brackets and dental materials need to be removed (Fig. 1C).

Figure 1

Figure 1: To align teeth (A) orthodontists use brackets (B) and wires that apply forces to change tooth position and angle. Treatment ends (C) with bracket removal, when the teeth and dental arches have reached aesthetic, harmonious well-matching relations leading to functional occlusion and pleasing smile.

Micro-CT as a Tool to Help Optimize Orthodontic Bracket Removal Procedures

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Removal of the adhesive remnants is an integral part of treat-ment. It is a clinical challenge for the treating orthodontist, who must completely remove the adhesive from the tooth, without causing damage to any of the teeth. High-speed water-cooled burs are commonly used, requiring patience and precision and full patient cooperation. A variety of grinding tools are available for this task, for example, tungsten carbide burs (Fig. 2A, B).

Questions arise regarding the optimal use of such tools, and how to use them best.

Material and MethodsFigure 3 shows typical steps involved in post-orthodontic adhesive removal, using the flame-shaped H48L tungsten carbide bur (Komet Dental, Gebr. Brasseler GmbH & Ko. KG, Lemgo, Germany). The flutes (Fig. 2) of the bur are tapered with an angle of 52° in the direction of rotation and a moder-ate quasi-orthogonal angle of 20° with respect to the shaft axis. This reduces chances of possible damage to tooth sur-faces, if/when the bur touches enamel. This specific bur has 12 flutes and a diameter of 0.14 mm and comes with differ-ent shafts designed to meet all hand pieces available on the market.

Typical Clinical use of the Bur Includes:

�� Removal of the brackets with pliers; detachment frequently occurs in the bracket-adhesive interface (Fig. 2B)�� Grinding away of the composite residues from the tooth surface by repeatedly moving the water-cooled rotating bur across the polymer on the outer surface. The clinician must apply gentle pressure, allowing sufficient time for the bur to remove the adhesive �� From time to time, compressed air is used to dry the surfaces and find remaining contaminated tooth surface patches�� Polishing of the tooth surface is used to remove residues. For this, slow-speed rubber cups are used�� Application of fluoride is often indicated to increase enamel resistance to erosion or bacterial contamination.

Figure 2

Figure 3

Figure 2: These example burs are designed to gently remove polymer and composite adhering to the outer tooth surface. Although informative, the optical images (A) provide only partial information about the 3D design of the tool with little information on how it operates. Tomographic imaging (B) helps to better understand its mode of action.

Figure 2: These example burs are designed to gently remove polymer and composite adhering to the outer tooth surface. Although informative, the optical images (A) provide only partial information about the 3D design of the tool with little information on how it operates. Tomographic imaging (B) helps to better understand its mode of action.

MicroCT 3D perspective

To study the wear of the bur, micro-CT was used to record the 3D shape and details of the geometry of flute edges, using the high-resolution settings of a desktop instrument. The bur was imaged with a Bruker SkyScan 1275 micro-CT (Bruker micro-CT, Kontich, Belgium) mounted on a thin sample holder allowing positioning close to the X-ray source. The high den-sity of elements in the bur necessitated using the higher source energy settings of 100 kV (yielding µA) and a CU filter helped remove lower energies reducing significant edge arte-facts. Best scans were obtained using 360° rotations, 6 µm effective pixel size. The image volumes were reconstructed using NRecon (V. 1.7.4.2, Bruker micro-CT, Kontich, Belgium).

Figure 4

Figure 5

Figure 4: Economic considerations in choosing bur use, characteristics and contribution to clinical workflow

Figure 5: Dental carbide wear in 2D sections along the bur axis (H48L tungsten carbide, Komet Dental, Gebr. Brasseler GmbH & Co. KG, Lemgo, Germany): Whereas obvious rounding of the edges is seen near the shaft (compare A with D, read/green arrows) extensive wear is observed in the central area(compare B and E, red/green arrows), with notable but less significant effects at regions ¾ up towards the tip (compare C with F, red/green arrows).

Whereas 3D images may be produced instantaneously using CTvox (V3.0, Bruker micro-CT, Kontich, Belgium), the reconstructed data in slices is readily available as TIF or PNG files to be analyzed in any other package. Examples of typical cross-sectional slices obtained using the open-source ImageJ packages (https://imagej.net, V 1.52n) and its derivative, Fiji. Stacks of images make it easy to select, compare and measure changes in shape at precisely the same height in the sample both before and after use. In this manner it becomes possible to examine bur abrasion related to usage. Direct comparison between scans of a bur before and after a bracket removal session reveal selective zones

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Results & Conclusion

Superimposition of the virtual data obtained before and after bur use revealed that wear occurs in the central region up to two-thirds of the height of the bur. Wear never exceeded 1 mm in any direction (Fig. 6). The flutes are worn off sig-nificantly after usage and would thus require more force and longer treatment time to remove remaining composite on the tooth surface, with possible danger of tooth overheating.

These observations from 3D micro-CT imaging lead to a cur-rent recommendation to use a bur for just one debonding ses-sion in a single patient. However, comparisons with different usage protocols and bonding materials are still required to assess how general this single-use recommendation should be.

with subtle but important regions of wear (Fig. 5). Rounding of flute edges is seen to be symmetric around the bur axis but inhomogeneous and non-uniform at different cross-sections along the bur length. Although the properties (strength, wear resistance) of the materials used for bracket adhesion are known, little is known about the relationship between wear patterns during composite removal and the technique used by the practitioner. Future investigations will make it possible to quantify such measurements to reveal dimensions and loca-tions in space that can serve as input for numerical modeling.

The 3D nature of the reconstructions lends itself to more extensive 3D analysis. By conversion of the data into binary (black and white) slices and conversion into industry-standard STL-files (export function of the inbuilt Fiji volume viewer) a more detailed analysis of wear is possible. It is possible to visualize and quantify the area of wear and carbide substance loss is by programs that are able to align and compare the 3D data. One such example is the 3D analysis software Geomagic Control (3D Systems Inc., Rock Hill, S.C., U.S.A.) showing (Fig. 6) differences in the 3D deviation (mismatch, in 3D) between the original unused, and worn bur. It can be seen that greater wear is observed in the central region of the flutes and at the bur tip. These are likely to relate to contribu-tions of the clinical technique used, the adhesive properties and the geometry and hardness of the bur.

Figure 6

Figure 6: Superimposition and comparison of the bur volumes before and after use reveal important regionally varying differences in the amount of remaining substance (Tungsten carbide in this case). The green traces indicate a mismatch (averaging at about 130 µm in the flutes) with a clear trend for loss, as opposed to yielding or deformation of the flutes.