Chapter 19 Cutting instruments
Cutting instruments are, of course, not dental materials per se, but the effect they have when used with dental materials and on tooth tissues is of great clinical significance. This chapter discusses the equipment used to cut and polish both dental materials and tooth tissues. Other techniques such as sandblasting, air abrasion and laser cutting, which have found many indications in modern dentistry as they are much less invasive, are also described.
The removal of a variety of materials can be very demanding. It is remarkable that it is possible to remove tooth tissue (enamel and dentine) and all restorative materials (including ceramics, metals and plastics) using the same implement, the dental handpiece. In other walks of life, specific dedicated machines are used to cut different substrates.
Working at high speeds (in excess of 180 000 rpm)
• High-speed handpiece, also known as an air rotor or air turbine (Figure 19.1). As its name implies, it is driven by a supply of compressed air.
Both the air driven high-speed instrument and the speed-increasing handpieces use friction grip (FG) burs, the bur being the cutting component of the system (Figure 19.3). The bur is slid into the jaws of the chuck while these are open and then the jaws are released to grip the bur shank. Box 19.1 illustrates how a friction grip bur should be inserted correctly into a high-speed or speed-increasing handpiece.
A high-speed or speed-increasing handpiece should never be run without fully inserting the bur into the chuck. If the bur is only partly inserted into the chuck, it may be released during use and cause intraoral soft tissue injury or be lost into the patient’s oropharynx. In addition, any eccentricity in the running of the chuck will be exaggerated. This will damage the chuck and the bur. If the bur is too short to access the operating site, it should be changed for one of a greater cutting length (see fig 19.31, p. 335).
Strictly speaking, handpieces, particularly those running at high speeds, should not be run without a bur. However with a quality, well-balanced product, there will be no damage if this is done inadvertently.
The turbine is powered by compressed air, which passes up the central lumen of the instrument. The air pressure is usually 3 bar (43.5 psi) although this varies depending on the handpiece manufacturer’s advice. This column of air then strikes the blades of a windmill in the handpiece head, causing it to rotate. The chuck is at the centre of the windmill. As the windmill rotates, so does the bur. The internal components of a high-speed handpiece are shown in Figures 19.4 and 19.5.
Reproduced with kind permission of W & H.
Reproduced with kind permission of W & H.
Surrounding the chuck is the bearing housing, which is held centrally within the head of the handpiece by a plastic ring. This housing must be made to very precise tolerances to prevent the bur running eccentrically. Failure of the bur to run centrally will cause:
• The bur to judder. This leads to vibration that is then transmitted to the material being cut, causing cracking and crazing. This vibration may also be unpleasant for the patient. It can cause the bur to break as it may snatch against the cutting surface.
Inside the bearing housing are seven or eight ball bearings, which run freely inside a cage called a ball race. The ball bearings are surrounded by a phenolic resin that lubricates their movement in the same way as oil lubricates the moving parts of a car engine. The race holds the shank of the bur, allowing it to rotate smoothly along a central axis with minimal friction. These ball bearings are made of either stainless steel or a ceramic material. There has been a move in recent years towards ceramic ball bearings, primarily because ceramic, is a harder material than stainless steel and wears less. It is also lighter in weight.
As significant heat is generated during cutting due to friction, it is critical that effective cooling is provided over the whole cutting surface of the bur. Water is generally used to cool the bur. It is transported to the handpiece head via fine tubes within the body of the handpiece and exits via a number of small outlet holes, which are aligned to deliver the water onto the bur (Figure 19.6).
Reproduced with kind permission of W & H.
It is desirable to have at least four holes, preferably more. This is because during use the position of the handpiece may prevent one or more holes directing water to the bur. The water may be also deflected away from the bur by the surface being cut. This compromises the supply of water to the bur and hence the cooling efficiency (Figure 19.7).
Many modern handpieces now have a light in close proximity to the bur. This is directed at the cutting surface so that the area being worked on is illuminated directly rather by reflection via a mouth mirror, increasing the dentist’s clarity of vision. The original lights were small halogen bulbs, with the light being transmitted to the handpiece head using a fibreoptic rod. Halogen bulbs deteriorate with time in use and have relatively a short life. They are also expensive to replace. Furthermore, the glass fibreoptic rod was prone to breakage if the handpiece was dropped, and deteriorated with time due to repeated decontamination cycles in the autoclave. Many manufacturers have now moved onto using low-power light emitting diodes (LEDs). The resulting light is whiter, more intense and the working life is much longer. LEDs are also relatively stable in wavelength over their lifetime and low-power LEDs produce little or no heat. The rapidly developing LED technology may well see considerable advances in product design and usage featuring these components in future.
A badly balanced handpiece will compromise the accuracy of the dentist’s work and increase operator fatigue. Other factors such as the type of the tubing housing, the services (air, water and electrical cabling) to the coupling and how it is arranged on the dental unit will also contribute to the balance of the handpiece. When the handpiece is held in the working position the balance should be neutral or slightly toward the handpiece head.
It is largely the clinician’s preference whether a heavy or lighter handpiece is selected. A heavier handpiece will lead to operator fatigue more quickly than a lighter one. The materials used to manufacture the instrument influence the weight of the instrument. Materials such as peek (a fibre-reinforced composite material) are being used by some manufacturers to construct the internal components of the handpiece so reducing the handpiece’s weight. Brass is commonly used to construct handpieces but will make the instrument heavier. Stainless steel is also used for handpiece manufacture and lies between these two materials with respect to weight.
Handpieces are either knurled or smooth in their external design and this is very much personal preference. Many practitioners prefer the knurled finish as this provides more stability and control of the handpiece. This is especially so when wearing gloves as the dentist’s grip on the handpiece is enhanced. It is important that the knurling is not too deep or close together to avoid compromising cleaning and sterilizing of the instrument (Figure 19.8).
Size of head
In the past, the larger the head of the handpiece, the greater power could be generated. This needed to be balanced against the decrease in access to the site to be worked on as the dentist’s vision was perhaps compromised. Smaller heads overcome these disadvantages but as a consequence, torque of the instrument was reduced in the early designs. Technology has now evolved such that it is no longer necessary to compromise the head size for torque.
The importance of torque
One of the critical properties for any dental handpiece is torque (along with concentricity and noise). This is the ability of the bur to continue to rotate and therefore cut when pressure (approximately 70 g) is applied to the substrate. The free running speed of a turbine is in the order of 300 000–400 000 revolutions per minute (rpm). As the bur is applied to the tooth the bur slows to a cutting speed of between 180 000 and 200 000 rpm. The optimum cutting speed is approximately one half of the free running speed.
To allow the bur to continue to rotate, the power must be maintained. The relationship of power, torque and speed are illustrated in Figure 19.9. Power is difficult to maintain with an air supply as the air pressure is not supplied at a constant level and may fluctuate depending on the draw down of air from the compressor. This will have an effect on the speed of the bur and hence the torque. The air pressure should be set at 2–3 bar (29–43.5 psi) and should be confirmed using an air gauge. There may be significant fluctuations in air pressure due to the various demands on the compressor due to demand from other clinics in a multi-clinic practice.
Fig. 19.9 The parabola shows the performance characteristic, i.e. the relationship of speed of rotation of the bur to power. As cutting speed increases so does the power up to a point past which the power starts to decrease. The maximum power is equivalent to half the free running speed and this is therefore the ideal cutting speed. The torque decreases linearly as rotational speed increases, i.e. the bur requires more power to continue to rotate it at the same speed as the pressure applied to it increases. This is due to the increasing drag on the bur.
Indications for using an air rotor handpiece
The speed-increasing handpiece (Figures 19.2 and 19.10) is driven by an electrical motor, also called a micromotor. The handpiece is placed onto the coupling of the micromotor on the dental unit (Figure 19.11).
Reproduced with kind permission of W & H.
The importance of maintaining sufficient torque during cutting has been discussed previously. The free running speed of a 1:5 speed-increasing handpiece is the same as its cutting speed at approximately 200 000 rpm, i.e. the motor speed of 40 000 rpm multiplied by 5 is a 200 000 rpm bur speed. This is maintained by the electrical motor, which delivers a consistent amount of power so the torque will be maintained when the bur contacts the tooth on preparation. This means that the tissue removal achieved using an electric motor-driven handpiece may be more consistent and usually has a higher torque.
Mode of cutting
A bur in a speed-increasing handpiece runs more smoothly compared with a bur in a turbine The bur in a turbine also moves axially (in and out) during use, resulting in a pecking motion being transmitted to the material being prepared. This pecking motion causes a rippling effect on the material, leading to the formation of microcracks (Figure 19.12).
Fig. 19.12 A photomicrograph showing the effect of pecking on the periphery of a cavity. Note the rough edge where chipping has occurred. The outline is irregular as result of the pecking and there is crazing of the surface adjacent to the margin.
To prevent microcracks, many operative dentists are now moving onto using speed-increasing handpieces in preference to turbines. This is particularly so when doing work that requires a smoother running bur and precision such as refining tooth preparations, tooth hemisections and polishing. These handpieces also produce less noise and vibration.
Comparison of high-speed and speed-increasing dental handpieces
|Type of bur used||Friction grip||Friction grip|
|Power source||Compressed air||Electric micromotor|
|Motion of bur||Rotation and pecking||Rotation only|
|Balance||Usually neutral||Motor end heavy|
Fig. 19.13 (A) A contra-angle handpiece and (B) a straight handpiece (W&H). A piece of green tape has been placed by the user to differentiate which clinic the handpiece belongs to so that handpieces are not misappropriated after the decontamination procedure.
The internal workings of slow-speed handpieces are essentially the same as previously described for the speed-increasing handpiece as shown in Figure 19.11. The main differences between them are:
Indications for slow-speed handpieces
Contra-angle handpieces are generally used for operative procedures such as the removal of dental caries and for polishing enamel and restorative materials intraorally. Straight handpieces are used in oral surgical procedures or for the extraoral adjustment and polishing of acrylic and metals. The speed of the handpiece will depend on the task. Table 19.2 lists the cutting speeds for common dental procedures and materials.
Most dental handpieces (except air turbines) have coloured rings on the body of the handpiece. These rings denote the handpiece’s internal gearings. This feature is illustrated in many of the photographs in this chapter. The colour codes are explained in Table 19.3.
|Red||Increase usually 1:5 but may also be other ratios|
|Green (or double green)||Reduction may be 2:1, 4:1, or 20:1 but not related to a specific ratio|
• Reduce the substantial heat generated by friction. This heat can lead to deleterious effects on the dental pulp (Figure 19.15). Water cooling is more efficient at dissipating the heat than air.
• Excessive heat also has detrimental effects on the substrate, possibly causing it to melt. This melted material can clog the cutting surface of the instrument so resulting in reduced efficiency of the cutting or polishing process. More heat will be generated with larger amounts of the surface area of abrasive in contact with the substrate surface.
Fig. 19.15 (A) Histological section showing the damage in the dental pulp after a cavity has been prepared with no water coolant. The boxed area shows a large number of inflammatory and round cells and the loss of continuity of the odontoblast layer. (B) Normal pulpal tissue is shown for comparison. The boxed area shows the odontoblast layer is undisturbed and columnar in shape with normal tissue beneath.
Indications for use of speed-decreasing handpieces
• Endodontic canal preparation. Root canals should be prepared using a slowly rotating file. Many endodontic handpieces also have a design feature to control the torque, with the aim of preventing endodontic file separation during use.
• Prophylaxis. As the speed is slower, the heat produced and transmitted to the tooth during polishing is reduced. The slower speed also reduces the amount of the prophy paste (see p. 345) being sprayed everywhere! This phenomenon can be further reduced by using a reciprocating prophylaxis handpiece (Figure 19.17).
Particles of prophylaxis paste can ingress into the handpiece and damage the internal workings. It is recommended therefore to use a dedicated handpiece for prophylaxis with a sealed or disposable head. This will preserve the condition and extend the lifespan of other handpieces used for precision procedures.
Traditional rotary instruments are not always successful for certain procedures such as the removal of overhangs and ledges particularly in interproximal regions. When a rotating bur is used to polish a surface, there is a tendency for it to create a groove. With each subsequent pass of the bur, it naturally falls back into this groove, so deepening it. For this reason, removal of interp/>