A considerable proportion of contemporary endodontic practice involves cutting access cavities through large composite or amalgam restorations, and various types of crown. The conventional air turbine handpiece (AT) is thus often tested to its limit. Slow progress and stalling lengthen what is often a difficult and technical procedure. The purpose of this article is to consider the features of an appropriately geared electrically driven handpiece as an alternative.
The Borden Airotor of 1957 produced speeds of up to 300,000rpm, but in common with successive developments lacked torque and was thus easily stalled when called upon to cut resistant materials such as crown alloys. The speed of an AT typically drops up to 40 per cent or more when contact occurs between the bur and material to be cut, as air pressure is insufficient to maintain the rotational speed of the turbine.
Speed increasing electric handpieces (EH) (red ring) are commonly available with a 5:1 increasing gear ratio, giving bur speeds of up to 200,000rpm, with micromotors running at between 1000 to 40,000rpm.
There are limited studies investigating the cutting power and efficiency of EH units. Cutting efficiency may be defined as the maximum capacity for removal of tooth structure, for a given force, over a determined period of time. Cutting power is nowadays quoted in Watts (W). Most modern air turbines generate less than 20W peak, with the mean figure for an EH being just under 40W. Ercoli et al compared AT and EH units in cutting simulated enamel and found the EH to have a greater cutting efficiency, especially as the preparation progressed, and when used with a carbide bur.
Particularly relevantly to endodontic access preparation are the findings of Choi et al, they concluded that there is a significant interaction in cutting efficiency between the type of material cut and the handpiece used. High noble metal alloy, amalgam and machinable glass ceramic were all more easily cut with the EH. However, Choi et al also observed that as harder material was cut, the force exerted by the operator will increase, and it is likely that these conditions will demonstrate an even clearer advantage in favour of an EH unit.
Christensen observed in 2002 that EH units exhibited the following well acknowledged advantages:
- High levels of torque with very little stalling
- Quiet in use, with a reduced potential for hearing damage and a less disturbing noise for patients
- Higher precision cutting due to superior bur concentricity
- Less vibration and smoother in use
- Disadvantages include:
- Greater cost
- Greater weight than an AT. Weight and handling characteristics of EH units vary greatly, with the latest units being lighter and better balanced.
- Greater torque and cutting power requires some habituation to avoid overheating teeth (and restorations) in some situations
It is also worth noting that where teeth are beyond endodontic salvation and require sectioning prior to conservative removal, electric units carry no risk of surgical emphysema.
There is a wide range of these handpieces on the market. It is worth evaluating several. My personal view is that an LED powered illuminated unit is to be strongly recommended, and that as in general, you get what you pay for.
References available on request.