From Volume 40, Issue 3, April 2013 | Pages 194-200
Contemporary restorative procedures demand precise detail in tooth preparation to achieve optimal results. Inadequate tooth preparation is a frequent cause of failure. This review considers the electric high-speed, high-torque handpiece and how it may assist clinicians in achieving greater accuracy in tooth preparation.
Since the foot-powered dental drill was invented by James Beall Morrison in 1871, operative dentistry has relied heavily on rotary instrumentation. The air rotor was first introduced in 1957 and, for many clinicians, remains the most popular technique for high-speed tooth preparation1,2 (Figure 1).
In recent years, the profession has moved away from providing restorations that require conventional tooth preparations and, instead, moved towards the use of minimally invasive restorative techniques. The success of these techniques demands extremely accurate and precise tooth preparations.3–7 Ibbetson8 has suggested that a precise 0.5 mm axial groove preparation is important for the provision of predictable adhesive bridgework. Magne and Belser9 have demonstrated that porcelain restorations can be successfully used to restore damaged teeth in the presence of a strong porcelain to enamel bond. It should be appreciated, however, that enamel can be extremely thin at the amelocemental junction (acj) and unintentional overpreparation in this area is a frequent problem, leaving little or no enamel on which to bond.
Several clinicians believe that the optimal selection and use of rotary instruments are key to achieving more precise preparations.10–12 This has generated interest in the electric or speed-increasing handpiece, on the grounds that it offers high-speed cutting with greater tactile feedback compared with the air-turbine.
This paper may be useful to practitioners who feel that they may benefit from greater control with high-speed instrumentation and will consider the use of the electric handpiece in clinical practice, drawing comparisons with the air-turbine under the following major headings:
High-speed tooth preparation may be accomplished using a conventional (high-speed, low-torque air-turbine) or an electric (high-speed, high-torque) handpiece.
The high-speed conventional handpiece continues to be used as the main means of carrying out tooth preparation in dental practice.2 Rotary speeds of 300–500,000 rpm are generated by compressed air which is driven into the handpiece, propelling a small rotor or turbine mounted on bearings in the head of the handpiece.2,13
The electric handpiece is powered by an electric micro-motor capable of producing and maintaining constant ultra-high rotational speeds (200,000 rpm) while still retaining a high torque (Figure 2).
These high-torque values provide the clinician with greater tactile feedback for more controlled tooth preparation.10,14–16 While air driven rotors may produce greater average rotational speeds (340,000 rpm) than their electric counterparts, it should be realized that they do so at the expense of torque and hence tactile feedback.
When comparing the air-turbine with the electric handpiece, several factors merit consideration and these are summarized in Table 1.
| Ideal Features of a Dental Handpiece | Air-turbine | Electric Handpiece |
|---|---|---|
| Lightweight | Average weight 45 g | Average weight 65 g |
| Head size | Standard and mini sizes available from most manufacturers |
Standard head sizes widely available. Mini sizes are available from a small number of |
| Illumination | Fibre optic and LED light sources available | Fibre optic and LED light sources available |
| Smooth running and vibration free | Smooth cutting can be achieved but is accompanied by some vibration and noise | Smooth cutting can be achieved and high torque is effective at minimizing vibration and noise |
| Minimal stalling during cutting | Due to low torque, can stall when used to cut through ceramics and cast alloys | Higher torque prevents stalling even when cutting through ceramics and dental alloys |
| Accurate speed control | With an air-turbine handpiece, rotational speed is dependant on air pressure and is rapidly reduced by any increase in resistance | A constant speed can be maintained in electric handpieces, and they are equipped with a feedback system to prevent the bur from slowing as the operator applies load (when cutting a tooth, for example). This concept is similar to the cruise control system in a car. When cruise control is set and the car encounters a hill, the feedback system helps to maintain the set speed |
| Adequate cooling | Multiple spray ports | Often have additional spray ports compared to the air-turbine. Evidence suggests a better coolant effect is observed in electric handpieces |
| Facilitate easy bur changing | Push-button friction grip | Push-button friction grip |
| Cost | Moderately expensive | Handpiece is approximately twice the cost of an air-turbine and requires the additional purchase of an electric micromotor |
| Easy to maintain | Familiar maintenance process. May require more frequent repairs | Requires rigid adherence to manufacturer instructions to prevent problems |
As previously mentioned, there has been great interest in the electric handpiece as a means of producing accurate tooth preparations that improve the outcome of treatment rendered.
A study by Kenyon et al10 compared Class I cavity preparations cut by 86 dental students using both the electric and the air-turbine handpiece. The quality of the preparations was assessed by an uninvolved departmental staff member. The cavities prepared with the electric handpiece scored higher for preparation form and refinement than those prepared using the air-turbine, however, owing in part to the small sample size, these differences were not statistically significant.
There are many further case reports and expert opinions in the literature claiming that tooth preparations produced using the electric handpiece are more accurate than those made with the air-turbine. It should be appreciated, however, that long-term independent clinical studies are needed to validate these claims.
The cutting efficiency of dental rotary instruments is defined as their capacity to remove dental tissue against effort required, over a given interval.17 Eikenberg18compared the cutting efficiency of two electric handpieces to an air-turbine, using a glass ceramic block with similar thermal and mechanical properties to enamel (Macor, Corning Inc, New York). Both electric handpieces demonstrated greater efficiency in cutting the glass ceramic compared to the air-turbine.
A recent study15 compared the cutting efficiencies of electric and air-turbine handpieces using seven materials commonly encountered in operative dentistry, including Macor, an enamel substitute; amalgam, zirconium oxide, high noble metal alloy, noble metal alloy, base metal alloy and aluminium oxide. Each material was categorized according to its Vickers hardness values.
This study showed no significant difference in cutting efficiency for materials with high hardness values, such as base metal alloy, noble metal alloy, aluminium oxide and zirconium. However, the electric handpiece had greater cutting efficiency for materials with lower Vickers hardness values (120–250 kg/mm2) such as amalgam, high noble metal alloy and Macor. The Vicker's hardness values for enamel and dentine are close to this range and have been reported19 as 268 kg/mm2 and 62kg/mm2, respectively. Hence, the electric handpiece may be more efficient at cutting dental hard tissues than the air-turbine.
Christensen20 has suggested that the constant torque and lack of ‘stalling’ of the electric handpiece are other advantages that make it more efficient at cutting than the air-turbine. This is supported by Pinero,21 who reported on the superiority of the electric handpiece over the air-turbine for the removal of defective metal ceramic crowns.
The electric handpiece may improve the comfort of treatment procedures for both the patient and the dentist.
The lightweight, slender design of the air-turbine is considered to be one of the factors that has contributed to its enduring popularity1,2 (Figure 3), while some clinicians believe that the heavier weight of the electric handpiece is a disadvantage.20,21
Several researchers have reported on the low noise levels emitted by the electric handpiece during operation.10,17,20,22 This may be less irritating for the patient compared to the high pitched noise of the air-turbine.
There are conflicting reports in the literature regarding the effect of vibrations from dental handpieces. It has been suggested that such vibrations may be responsible for symptoms of hand-arm vibration in dentists and may cause enamel cracking in teeth.17,23 Several authors have reported that electric handpieces offer lower levels of vibration compared with the air-turbine and therefore may be more comfortable to operate. However, a recent study by Poole et al22 found no distinction in vibration characteristics between the air-turbine and the electric handpiece.
A study by Watson et al17 examined the use of the air-turbine and electric handpiece as potential causes of iatrogenic enamel cracking. Their results showed no statistical difference in the amount of enamel cracking between the air-turbine and electric handpiece.
Air water spray cooling prevents a temperature rise in the dental pulp during high-speed rotary preparation. As reported by Zach and Cohen,24 a temperature rise of 5.5 °C led to loss of vitality in 15% of teeth.
Ercoli and co-workers25 compared the water-cooling effect of both the air-turbine and electric handpiece. Although the overall water-flow rate for both handpieces was similar (40 ml/min), it was noted that water was delivered from more spray ports in the electric handpiece. It was concluded that this produced a greater cooling effect compared with the spray from the air-turbine (Figure 4).
A further advantage of the additional spray ports has been demonstrated in a study by Siegel and von Fraunhofer.26 They observed a significant increase in rotary cutting efficiency by increasing the number of spray ports while maintaining the same volume of coolant.
Cost is a major factor when considering the purchase of new equipment. Electric handpieces must be coupled to a high-speed electric motor in order to produce high rotational speeds and retain torque. Contemporary dental chairs may come fitted with electric motors or they may be retro-fitted to an existing chair. In either case, the purchase of an electric motor is a significant investment, especially if equipping multiple surgeries. In contrast, most dental practices will already be equipped with a source of compressed air required to power an air rotor.
An assessment of several dental suppliers catalogues reveals that the cost of an electric high-speed handpiece normally outstrips that of an air-turbine by around 30%; although others state the cost of the electric handpiece to be 2–3 times more expensive than the air-turbine.15
According to some clinicians, electric handpieces are easier to maintain and may require less frequent repairs than the air-turbine.20 This is in contrast to reports in the dental literature that describe severe overheating in improperly maintained electric handpieces, leading to severe burning of dental operators.27 One further disadvantage is that, unlike air-turbines, electric handpieces do not begin to sound or feel differently when the gears are worn, leaving the operator unaware of potential problems.
Several authors report that maintaining an aseptic field is easier with the electric handpiece than with the air-turbine because a lower level of blood and saliva aerosol seems to be produced.25,28,29 This is supported by the published data of Hall29 who, in assessing the cross infection of domiciliary dental care, isolated a greater number of bacterial colonies from surfaces contaminated with aerosol from the air-turbine against those sprayed by the electric handpiece.
An important engineering improvement in electric handpieces is the development of brushless motors. A conventional brush motor has carbon brushes that transmit the electricity on to the rotor to make it turn. Carbon brushes wear down over time and need to be replaced, as well as producing carbon dust that can clog the motor. A brushless motor, which employs a magnetic field to turn the rotor, may help overcome these problems.27
Research is also being directed towards the development of a lighter weight electric handpiece which, it may reasonably be assumed, will improve acceptance.10,15,20
The electric handpiece is coupled to an electric motor which provides variable power and higher torque than the air-turbine. It provides a constant rotational speed which is helpful for lengthy or extensive preparations for fixed prosthodontics.
The higher torque of the electric handpiece, maintained while cutting, produces a marked change in tactile feedback and this may enable greater control over tooth preparations. However, this change in ‘feel’ means that a learning curve should be expected before one can use these instruments effectively. In addition, these units are heavier than conventional air rotors, and may lead to operator fatigue due to weight and balance of the motor component. For many dentists, a further drawback is the cost of purchasing additional handpieces, together with an electric micromotor.
Further research is needed to substantiate case report claims in the literature suggesting that tooth preparations produced with the electric handpiece are more accurate than those produced with the air-turbine.
The choice of handpiece for restorative dentistry is partly dependent upon the nature of the treatment planned and the experience of the dentist. Most restorative dentistry could be carried out with either the air rotor or the electric handpiece. However, specific considerations for the use of the electric handpiece are as follows:
