Bonsor SJ, Pearson GJ. A Clinical Guide to Applied Dental Materials.Edinburgh: Churchill Livingstone Elsevier; 2013
Burke FJT. Suggestions for non-aerosol or reduced-aerosol restorative dentistry (for as long as is necessary). Dent Update. 2020; 47:485-493
Watson TF, Flanagan D, Stone DG. High and low torque handpieces: cutting dynamics, enamel cracking and tooth temperature. Br Dent J. 2000; 188:680-686 https://doi.org/10.1038/sj.bdj.4800576
Hilton TF, JL, Broome J. Summitt's Fundamentals of Operative Dentistry: A Contemporary Approach, 4th edn. Hanover, IL, USA: Quintessence; 2013
Wilson N, Lynch CD, Brunton PA Criteria for the replacement of restorations: Academy of Operative Dentistry European Section. Oper Dent. 2016; 41:S48-S57 https://doi.org/10.2341/15-058-O
Milic T, George R, Walsh LJ. Evaluation and prevention of enamel surface damage during dental restorative procedures. Aust Dent J. 2015; 60:301-308 https://doi.org/10.1111/adj.12230
Tilaveridis I, Stefanidou A, Kyrgidis A Foreign bodies of dental iatrogenic origin displaced in the maxillary sinus - a safety and efficacy analysis of a retrospective study. Ann Maxillofac Surg. 2022; 12:33-38 https://doi.org/10.4103/ams.ams_190_21
Matsuzaki K, Aoki T, Oji T A rare case of a broken dental bur perforating the medial orbital wall without damaging the eye. Quintessence Int. 2016; 47:75-79 https://doi.org/10.3290/j.qi.a34806
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The high-speed revolution: a practical guide to friction grip burs Stephen J Bonsor Louis Mackenzie Dental Update 2024 50:6, 707-709.
Authors
Stephen JBonsor
BDS(Hons) MSc FHEA FDS RCPS(Glasg) FDFTEd FCGDent GDP
The Dental Practice, 21 Rubislaw Terrace, Aberdeen; Hon Senior Clinical Lecturer, Institute of Dentistry, University of Aberdeen; Online Tutor/Clinical Lecturer, University of Edinburgh, UK.
The practice of restorative dentistry frequently involves the preparation of the dental hard tissues and the use of restorative materials. Efficient armamentaria are required to facilitate this process with the friction grip bur used at high speed being the most commonly used operative instrument in contemporary clinical dentistry. This article describes the various friction grip burs that are available to the restorative dentist offering a practical guide as to their selection and usage. Furthermore, techniques are described to optimise outcomes while minimizing iatrogenic damage when working clinically.
CPD/Clinical Relevance: Knowledge of different friction grip burs used in contemporary restorative dentistry should facilitate their optimum usage and minimize complications.
Article
Since the advent of the contra-angle high-speed handpiece in 1949, dental burs have become the ubiquitous means of dental hard tissue preparation. They are extensively used in the removal of existing restorative materials and the shaping and finishing of newly placed ones. Efficient preparation of these substrates is achieved by working at high speed, that is a cutting speed of 200,000 revolutions per minute.1 There are two types of dental handpieces that can deliver such a rotational speed: the high-speed handpiece (otherwise known as air rotor or air turbine), and the speed-increasing handpiece, colloquially referred to as the ‘red ring’ handpiece.1 The latter instrument became more popular during the COVID-19 pandemic because it may reduce the amount of aerosol produced during use, which was thought to be beneficial in limiting the potential spread of viral particles into the atmosphere of the dental clinic.2 Furthermore, its other advantages have been recognized for many years. The speed-increasing handpiece is powered by a constant output electrical motor, and so, the instrument produces a constant torque, that is, the ability of the bur to continue to rotate when placed against the substrate being prepared. This contrasts with the air turbine, which relies on the pressure of the air supply to the dental unit to power the instrument. Pressure fluctuations, especially seen in clinics housing many dental chairs operating at the same time, will reduce the torque, and therefore, cutting efficiency, of the bur driven by the air turbine compared to speed-increasing handpieces.3 Furthermore, a bur in a speed-increasing handpiece runs more truly, without the vertical displacement (‘pecking action’) that is seen with an air turbine, so increasing the potential for a smoother preparation surface1 and reduces crack initiation in the surface of the substrate being prepared, particularly a brittle material such as dental enamel.
In order to optimise the performance of dental burs and to minimize potential complications, it is essential that the operator understands their mode of action and selection, which the present article addresses.
Classification of burs used at high speed
Common to both high-speed and speed-increasing handpieces is the friction grip (FG) bur, the component parts of which are shown in Figure 1. The International Organization for Standardization (ISO) categorizes burs, and this classification is explained using the 806.314.856.314.016 bur by Komet (Croydon, UK) as an example (Figure 1.)
Figure 1. The ISO classification of friction grip burs where A is the bur material, eg diamond galvanically bonded to metal. B is the shank and overall length, eg friction grip (FG)/21.0 mm, and C is the bur pattern, eg round-ended taper. D refers to the grit size, eg medium (NB: there is no blue coloured band on Komet medium grit burs), with E being the maximum diameter of the working part of the bur (in 1/10 mm), eg 1.6 mm.
Friction grip burs are available in two types based on their construction, namely diamond or tungsten carbide (TC).
Diamond burs
Diamond burs are the most commonly used dental burs at high speed.4 They are constructed of stainless steel covered in diamond grit or particles, the hardest known material, embedded in a resin, which is usually phenolic based.5 Some burs have a single layer of diamonds, while others are composed of layers of diamonds, so maintaining the cutting efficiency of the instrument as the outer diamonds may be progressively lost during function, so exposing others in the sublayers.1 The abrasivity of the bur is dependent on the size (coarseness) of the diamond particles as shown in Table 1.
Bur abrasivity
Approximate grit size (µm)
Bur colour
Super-coarse
180
Black
Coarse
150
Green
Medium
100
Blue
Fine
50
Red
Extrafine
25
Yellow
Superfine
20
White
Ultrafine
10
Purple
The selection of the degree of abrasivity, or coarseness, of the bur is dependent on the substrate being prepared. Table 2 shows the relative hardness of dental substrates, showing diamond by means of a comparison.
These burs are machined with flutes, also known as blades. The distance between the blades determines the degree of substrate preparation. In other words, burs designed for cutting harder substrates, such as metal alloys, will have the blades further apart than those used for finishing. Furthermore, the rake angle will also determine the cutting efficiency as shown in Figure 2.
Figure 2. Diagram illustrating the rake angle on a tungsten carbide bur. The more acute the angle, the better the cutting efficiency.
Mode of action of diamond and tungsten carbide burs
The mode of action is very different between these burs. Diamond burs abrade rather than cut the surface of the substrate, leaving it microscopically rough (Figure 3). In comparison the blades of a tungsten carbide bur grind and chip the substrate so producing a smoother surface (Figure 4).1 However, and unfortunately, there is a wide variability between bur manufacturers, which makes complicates communication. It would therefore be easier to consider burs based on their shape and, therefore, indication.
Figure 3. Clinical and SEM appearance of a sample of tooth tissue prepared with a 50-µm (red stripe) resin composite finishing bur.Figure 4. Macroscopic image of the surface of a dental amalgam core prepared for a crown preparation prepared by diamond and tungsten carbide burs. Note the abrasions that have been caused by the coarseness of the diamond grit compared to the smother surface produced by the action of the blades of the tungsten carbide bur.
Clinical techniques
There are principally four clinical indications for high-speed burs that would be used in restorative dentistry: namely intra-coronally, extra-coronally, restoration removal and the finishing of restorations.
Intra-coronal indications
The burs used for intra-coronal indications, for example the management of dental caries, tend to be smaller and shorter. With the adoption of minimally invasive dentistry, the clinician should start with the smallest bur to ensure that the amount of tooth tissue removed is kept to a minimum. The use of optical enhancement, such as loupes or the operating microscope, in combination with good illumination, should be used as an adjunct to facilitate this process.6 The shape of the bur should be selected with reference to the shape of the cavity to be prepared, for example, a round bur should be used to cut a Class III cavity, and a pear-shaped bur in the case of a Class I or Class II cavity. Both of these shapes will create cavities with the appropriate dimensions to be restored with a brittle material, such as resin composite or dental amalgam, because the internal line angles are round. The use of a high-speed instrument in a de novo carious lesion will gain access to said lesion, so permitting further appropriate management, such as caries removal, by means of slow speed burs or hand excavation.4Figure 5 shows a number of intra-coronal bur patterns that may be used for cavity preparation in the management of caries or removal of existing intra-coronal restorative materials.
Figure 5. Recommended diamond and tungsten carbide bur patterns manufactured by Komet for intra-coronal cavity preparation and existing restoration removal.
Extra-coronal indications
The principles of tooth preparation for indirect restorations are well documented, such as the creation of appropriate resistance and retention form for conventional crown and bridgework.7 The newer dental materials and techniques, such as lithium disilicate, have different demands with respect to tooth preparation. Owing to its ability to be bonded to dental hard tissue with a resin composite adhesive, such a restoration has less reliance on resistance and retention form. The shape of any preparation should conform to the demands of the material used to construct the indirect restoration and its means of retention, for example luting or bonding. A common feature of tooth preparation for all indirect restorations is that all line angles should be rounded, the advantages of which are:
Decreased stress concentration reduces the risk of crack initiation and propagation;
Increased readability of both analogue and digital impressions;
Increased accuracy of casting;
Decreased risk of worn dies.
A number of different extra-coronal bur patterns are available to the restorative dentist, and are pictured in Figure 6. These tend to have longer cutting surfaces compared with intra-coronal burs.
Figure 6. Recommended diamond and tungsten carbide bur patterns manufactured by Komet used in the preparation of extra-coronal restorations.
A tungsten carbide bur may be used in the preparation of a conventional crown to increase resistance form by means of preparation of an axial groove with a bur such as a #170 as seen in Figure 7. Finishing may be used to refine preparations prior to impression capture.
Figure 7. A resin-bonded bridge preparation of a maxillary canine demonstrating a chamfer margin and axial resistance grooves prepared with a #170 TC bur as pictured.
Restoration removal
Over 50% of all restorative procedures involve the removal of an existing restoration.8 The selection of the bur will be determined by the substrate in question. For example, the removal of a non-precious metal alloy will require the use of a diamond or, preferably, a tungsten carbide bur such as a Crosscut Talon (Tri Hawk, Morrisburg, Ontario, Canada) (Figure 8) owing to the higher hardness of the surface of this particular substrate. Figure 9 shows some other bespoke burs that may be used to remove various restorative materials.
Figure 8. Crosscut Talon bur (Tri Hawk), manufactured in tungsten carbide and indicated for removing restorations composed of a metal alloy.Figure 9. Recommended diamond and tungsten carbide bur types for restorative material removal (Komet).
Finishing of restorations
Fine diamond or multi-bladed burs can be used to shape and finish restorations, notably resin composite. Figure 10 shows a selection of burs used for this purpose, the geometry of which is determined by the demands of the final restoration.
Figure 10. Examples of diamond bur patterns used to shape and finish restorations (Komet).
Practical considerations
Due to the high rotational speeds and cutting efficiency, there is a high potential for iatrogenic damage. A number of techniques may be employed to reduce the risk of complications during operative procedures.
Heat
The high rotational bur speed creates significant frictional heat. It is critical that this is mitigated by the use of copious water spray directed onto the length of the cutting part of the bur. This is essential in decreasing intra-pulpal temperature rise, which has the potential to cause irreversible damage. Furthermore, a failure to mitigate frictional heat will cause the surface of, for example, a polymeric material such as resin composite to be heated to above its glass transition temperature so causing the surface to become damaged.1
Pressure
Regardless of preparation type, care should be taken to apply a light pressure of the bur on the surface of the substrate.9 Increased force does not necessarily increase cutting efficiency.9
Vibration
It is critical that the bur rotates concentrically in the chuck otherwise an uneven amount of tooth tissue will be removed. This also causes crazing of brittle substances, such as dental enamel, with the potential for stress concentrations being created leading to crack propagation.3 The amount of vibration will be reduced by the use of high-quality handpieces purchased from reputable manufacturers. It is highly recommended that the clinician choses such instruments as they can be reassured that the quality control and assurance will be of the highest standard. Furthermore, such instruments that are used extensively in the clinic should be regularly serviced to maintain their optimum performance.
Protection of tooth tissue, adjacent restorations and the periodontium
Owing to the high incidence of iatrogenic damage caused by dental burs,10 it is critical that due care and attention is made of any adjacent oral structures to eliminate the collateral damage to enamel, restorations and the periodontal tissues. Routine use of protective wedges (Figure 11), inserted prior to preparation of the proximal tooth can protect the surface of the adjacent tooth or restorative material. The use of optical aids as described earlier will also be helpful in this regard.
Figure 11. Protective Fenderwedges (Directa, Upplands Väsby, Sweden) and in situ.
Bur quality and wear during use
In order to optimise clinical technique and efficiency and, as with high-quality handpieces as mentioned earlier, it is strongly recommended that high-quality burs are also used, similarly purchased from reputable manufacturers.
It is important to bear in mind that during use, burs will become blunt, and the use of worn burs not only decreases their efficiency but potentially causes damage to the substrate mainly due to the increased production of frictional heat. Burs should therefore be changed regularly. This is especially important in the use of fine-tipped finishing burs used to refine resin composite restorations.
Bur displacement
Due to their high rotational speed burs may be displaced from the handpiece head and could become displaced into the soft tissues or lost into the oro-pharynx. There are many reported cases in the literature of burs being displaced into adjacent tissues such as the maxillary sinus,11 medial orbital wall12 as well as ingestion.13 There are a number of reports in the literature concerning the latter event, especially involving paediatric patients.14-16 This unfortunate and potentially catastrophic event may be prevented by the operator ensuring that the bur is fully inserted into the handpiece. If it becomes apparent that the bur cannot access the site of preparation, then a bur with a longer shank should be selected in preference to taking the bur partially out of the handpiece housing.
Close support dentistry
The potential for damage to other oral soft tissues with the use of high-speed rotary instrumentation is high. The support of a well-trained dental nurse who is skilled at soft tissue retraction, for example the tongue, and cheek, will decrease the potential for soft tissue damage. The use of rubber dam will further reduce the potential for such damage.17
Instrument selection
In order to remove the correct amount of dental hard tissue when doing a preparation, it is helpful to be aware of the dimensions of the bur. Measuring the diameter of the bur using a ruler, an Iwannson gauge or digital callipers is recommended as this can be used as a convenient depth gauge as to the depth of tooth removal as seen in Figure 12.
Figure 12. Bur tip measurement using digital callipers.
Alternatively, depth cuts may be prepared to guide the amount of tooth removal, with many clinicians advocating this technique. However, there is a danger that, during preparation, these can be inadvertently deepened, so removing more tooth tissue than intended. To overcome this complication, depth cuts, if being used, should be prepared in a diagonal rather than axial plane of the tooth being prepared. As soon as these depth grooves disappear, then the correct amount of tooth tissue has been removed.
There is an extensive array of bur patterns with only a small number pictured in this article. It is essential to select burs that are the most appropriate at each stage of the clinical procedure. It is very difficult to select dental burs by looking at paper catalogues or pictures online, especially when size may be difficult to gauge. It is therefore much easier to make contact with a representative of a company who manufactures or stocks burs, and ask to be shown the available burs in a wallet.
Decontamination of dental burs
Burs may be packaged for single or multi use where conventional decontamination protocols should be employed. Dental burs are a potential vector for cross contamination due to their contact with saliva, blood, gingival crevicular fluid and tooth, periodontal and necrotic tissues. The uniquely complex structure of the active portion of burs makes pre-cleaning and subsequent sterilization challenging. This is a well-researched subject and a range of decontamination options have been investigated.18-20 Under optimal conditions, autoclaves can destroy all micro-organisms, including bacterial spores.18 Although this is considered the best decontamination method, it is not considered sufficient without pre-sterilization cleaning.19,20 A range of cleaning methods have been investigated, including the common practice of soaking burs in commercially available disinfectors followed by manual pre-cleaning. However, this may not be effective, is time consuming and may produce aerosols contaminated with pathogenic micro-organisms.18 Manual scrubbing under water may be more effective.20 Ultrasonic pre-cleaning has been shown to be successful in removing dried blood and saliva from dental instruments,19 and effective pre-sterilization cleaning of contaminated dental burs has been demonstrated using washer-disinfectors.20 Autoclaving may result in rusting or dulling of burs. As a result, their cutting efficiency, life span and fracture resistance may be reduced. For these reasons, alongside optimising infection prevention and control, some manufacturers provide the option of dental burs packaged for single patient use.
Summary
Rotary instrumentation consisting of a handpiece working at high speed and friction grip burs are used for the majority of procedures in restorative dentistry such as the preparation of the dental hard tissues and restorative materials. Careful selection and use of appropriate friction grip burs will optimise outcomes, while minimizing risk of complications and iatrogenic damage when working clinically.
