Rotary instruments - endurance and power
What high-speed instruments are currently available for the field of restoration and prosthetics? What are the current technological trends and what new developments are being worked on in terms of motor, chuck system, light, cooling or maintenance? The following information is intended to provide up-to-date advice on the crucial issue of what to consider when buying your instruments.
The basics: there are two systems used for driving the bur; an air system and an electrical system.
In the air system, a distinction is made between turbines and air motors. With turbines, the bur is driven directly by a rotor. This rotor has an impeller which is acted upon by compressed air. Turbines can achieve an idle mode speed of as much as 330,000 to 400,000 min-1.. The working speed is about half of the idle mode speed, i.e. 150,000 to 250,000 min-1. depending on the contact pressure. The maximum power of 10-22 watts is also achieved in this speed range. The air motor drives the bur indirectly via a transmission instrument - the “contra-angle or straight handpiece”. The air motor achieves a speed of approximately 25,000 min-1. Contra-angle handpieces are supplied with various different transmission and transition ratios. An air motor with a contra-angle handpiece with a transition of 2:1 will achieve a bur speed of approximately 12,500 min-1.
Electric motors can achieve an idle mode speed of up to 40,000 min-1 which corresponds to a bur speed of 200,000 min-1 with 1:5 contra-angle handpieces. The maximum power is greater than
60 watts and a torque of approximately 3 Ncm is achieved.
Electrically driven contra-angle handpieces are therefore not slowed down or stopped during tooth preparation if the bur comes into contact with different tooth structures or prosthetic materials. They cut at almost constant speed irrespective of the load. The bur is centred significantly better by contra-angle handpieces than by turbines. The bur vibrates considerably less in a contra-angle handpiece than with turbines.
Improved centring means greater precision, less time expenditure and less heating of the tooth substance during cavity preparation. The trend for electric motors began in Europe mainly due to the difficulties and costs involved in retrospectively laying air lines in existing buildings. It transpired that electric drives are not only easier to install but are also more efficient to operate.
Several decades later, electric motors are pre-eminent in Europe and Asia and are also becoming increasingly popular in North America, as the innovative developments in design, material, ergonomics, torque and light make dental practice better, quicker and easier.
The wide range of straight and contra-angle handpieces satisfies both general and clinical requirements. Most manufacturers offer a selection of both, some instruments have been designed to cover most dental applications, while others have been customized for specific clinical applications.
Turbine vs. contra-angle handpiece
The advantages of a turbine are its simple and robust construction, low purchase price and the significantly lower weight. Over the years, however, a significant problem has emerged in the shape of the damage to hearing caused by the high-frequency noise emissions from turbines; electric motors, on the other hand, are generally quieter and easier on the ears than turbines. A further advantage of using electric motors can be identified if we consider the amount of tooth substance removed per time unit: they are superior to turbines for grinding with water-cooling .
Let there be light! But where and what kind?
Instruments with light have become mainstream over the last two decades. This light is normally generated by halogen lamps and conducted to the treatment site via glass rods. Improved illumination of the treatment site using light instruments is desirable, or even a necessity, in all fields. In the meantime, instruments with light have become standard in modern practice. The use of LEDs (light emitting diode) is a new development in the field of light instruments. Turbines with LED lighting have been available since 2007. The use of these robust and shock-resistant LED chips results in a longer lifespan and improved illumination of the treatment site than with comparable halogen lamps.
Selection criteria: Head technology
The smaller the head, the better the access to and visibility of the treatment site. When making a decision to purchase, the working height (head + bur) should be considered in addition to the diameter and the height of the head. The smallest turbines reach a working height of about 21 mm (with a bur length of 19 mm). The heads of these miniature turbines have a diameter of about 10 mm and a height of about 12 mm. These minimal dimensions nevertheless achieve high-power performance.
In order to reach these performance levels, some manufacturers even use two impellers in the turbine. When slowing down, the turbine’s rotor sucks in air from the immediate surroundings. There is a risk that contaminated air will be sucked in. Modern turbines have what is known as a hygienic head. In the hygienic head system, for example, bypass channels prevent the intake of external air.
A turbine’s idle mode speed (around 400,000 -1 min) is generally an indicator of the cutting rate. The advantage of electric motors clearly lies in the fact that speed and torque can be very easily controlled. Brushless electric motors offer the possibility of controlling speed in a range from about 300 to 40,000 min-1. These motors provide stable torque over the entire speed range. More and more dentists are starting to move towards using electric motors. If lifespan, hygiene, wear and sterilizability are taken into account, then brushless electric motors are preferable to motors with brushes.
1.6 mm FG chuck system high-speed instruments
Press-button chuck systems are the current standard. With this system, no tool is needed to change the bur. It should be possible to change the bur with a very low actuating force. However, the actuating force must also not be too low in order to prevent unintentional actuation, for example by touching the patient’s cheek. It is essential that there is sufficient retention force to securely clamp the bur.
Swallowing or inhaling a bur would be life-threatening for the patient. This places high demands on manufacturers because very high centrifugal forces are created at these speeds. A simple chuck system which can be operated rapidly, but which provides sufficient retention force to securely clamp the bur, is advantageous.
Option of retrofitting electric motors
Around the world, many dental units are only equipped with compressed air and air-based instruments. The unit often does not have a power supply or control unit for an electric motor. These units can easily be fitted with a table-top control unit. These control units are connected to the existing supply hose of the air-based instrument. The speed can also be controlled using the existing foot-operated starter or via the table-top control units.
Spray system, cooling
There are two main reasons for spraying the treatment area with air and water. On the one hand, the tooth is cooled to prevent the pulp from overheating and, on the other hand, the removed material is taken away to ensure perfect visibility.
Studies by Sharon C. Siegel, M.S., D.D.S. and J. Anthony von Fraunhofer, M.S.C., Ph.D., F.A.D.M., F.R.S.C. also prove that there is a correlation between the number of spray channels and the cutting rate. Instruments with several spray channels have a significantly higher cutting rate than instruments with only one spray channel. Studies by H. H. Martin and H. A. Gleinser, Freiburg, provide information on the correlation between spray flow rate, the number of nozzles and the increase in temperature of the tooth substance during preparation.
Turbines and high-speed contra-angle handpieces with one-, two- and three-output spray systems were investigated. In summary, these studies came to the following conclusion: a 3-nozzle spray system using 50 ml spray water per minute results in the smallest temperature increases. With less water, e.g. 15 ml/min, the temperature increases greatly, even in the case of multi-nozzle systems. Instruments with 5 spray channels were one innovation introduced in this field in 2007.
Instruments with several spray channels offer greater efficiency, improved visibility, less risk of malfunction if one channel becomes blocked and improved safety for the patient. Having several channels ensures that even if the neighbouring tooth is in the way, the remaining channels will still provide sufficient cooling.
Improved visibility of the treatment site is always desirable. A head-worn dental light does not provide sufficient illumination due to the cramped conditions in the mouth, which are disrupted by a large number of dental instruments and hands.
Instruments with an integrated light source, which illuminate the treatment site directly, are needed. Instruments with halogen light which emerges from glass rods directly onto the head only a few millimetres from the bur have become standard in recent decades. The illumination is restricted to the vicinity of the bur. Instruments with LED (light emitting diode) light were manufactured for the first time in 2007. With a colour temperature of 5500 K and a light intensity of 25,000 lux, LEDs supply daylight-quality light directly to the treatment site.
Positioning the LED directly on the instrument head provides large-scale, diffuse illumination of the entire treatment area. Contra-angle handpieces with LED light, which function without a power supply from the dental unit were an innovation in 2009. The power for the LED is produced using a generator integrated in the instrument, and the generator being driven by the induction air. This generator technology has already been successfully used in oral surgery instruments since 2007.
Light instruments need to be sterilizable and thermo washer disinfectable so that they can be fully integrated into everyday practice and the hygiene process. The improved visibility enables more precise restoration and prosthetics work. This means less stress and improved treatment quality for the patient and dentist.
In order to meet the hygiene requirements, turbines, contra-angle handpieces and motors must be sterilized after every patient. The instruments must be able to be separated from the hose and mounted quickly and easily. Finally, dentists’ chairs should not be obstructed and employees should not be burdened by complicated and time-consuming installation tasks. Couplings also need to be sterilizable.
An effective maintenance system is crucial for an efficient maintenance process. Different designs of instruments from different manufacturers require specific maintenance procedures. It is important to choose instruments which require a simple maintenance process that can easily be carried out in everyday practice.
Manufacturers offer maintenance units tailored to the maintenance requirements of the particular instruments. These units are highly recommended, as regular maintenance has a huge effect on the lifespan of the instruments.
Every instrument is sterilized many times a day. Only high-quality instruments can cope with this many cycles without suffering impairment of functions or power. The sterilization procedures used must comply with the manufacturers’ guidelines so that the lifespan of the instruments is not unnecessarily shortened. The sterilization process may not exceed the maximum permitted temperatures. Vacuum steam sterilizers are generally considered to be gentle and reliable.
* published in APDN June 2009 by Michael Pointner (Graduate Engineer), Norbert Thuminger (Graduate Engineer)