By Mary Yakas, The DENTAL ADVISOR
One of the questions we receive routinely from dentists is whether or not to choose an air-driven or electric handpiece. For routine restorative dentistry, a traditional air turbine handpiece can easily handle tooth preparation, finishing and polishing. It allows maximum access and operator comfort with a familiar feel. In certain clinical situations, the power and control of an electric handpiece will offer great advantages. Crown and bridge preparation puts a heavy load on the bur, and efficiency will be increased by using an electric handpiece, especially in cases involving multiple teeth. The same can be said for removing existing porcelain and metal restorations. With the increasing use of high-power magnification and optical scanning of preparations in CAD/CAM technology, dentists are becoming increasingly aware of the quality of their tooth preparations. Since minor imperfections in margin detail become obvious when viewed on a high-resolution monitor, dentists are looking for every advantage to perfect their work, including choosing an electric handpiece.
The power of a handpiece is a function of torque and turbine speed. High-speed air turbine handpieces commonly operate between 27-42 psi, with operating speeds of 300,000-500,000 rpm. These handpieces are used with a “brush” or “feather” stroke to avoid constant, heavy load on the bur. When subjected to a heavy load, speed and torque decrease, resulting in decreased efficiency or stalling. Turbine design and bearing configuration are major factors in the performance and durability of high-speed handpieces. Traditionally, steel components have been used. The use of ceramic ball bearings is increasing, because they are harder, lighter and more wear resistant than steel. Ceramic balls reduce wear on the bearing and they produce fewer wear particles, which in turn increases turbine life. They can also reduce the noise generated by the air passing through the bearing. Manufacturers specify the maximum air pressure tolerances for each handpiece model. Exceeding air pressure recommendations will cause the turbine to work beyond its capabilities and thus fail prematurely. Manufacturers are continually refining the features of high-speed handpieces to make them more efficient and user-friendly.
The maximum speed of a handpiece is not indicative of the power or torque output. Handpieces with larger heads and larger turbines may operate at lower speeds than handpieces with smaller heads, but provide greater torque and power for cutting. Increasing torque and decreasing turbine speed can reduce noise levels. For increased cutting efficiency, handpieces with full-size heads are recommended. When access is diminished, one of the small-head handpieces may be a better choice clinically.
A 360-degree swivel attachment between the handpiece and the hose will reduce the drag and stress on the operator. A quick disconnect is a feature for ease of cleaning and lubrication. Different types of chucks are available, including push-button, latch and those requiring a tool. For convenient one-touch bur changing, the handpiece should have a push-button chuck.
The handpiece should have adequate air and water spray ports for cooling and rapid removal of debris produced by cutting. The mist of air and water spray should be directed at the bur and reach its entire cutting surface. Multiple ports typically produce a cleaner field.
Illumination from the handpiece is designed to eliminate shadows and increase depth perception in the oral cavity. The light source may be located in the handpiece, the coupler or a remote source. Removing the bulb from the handpiece is desirable in terms of increasing the longevity of the light after repeated sterilization cycles. When the bulb is located in the coupler, an efficient way of transmitting the light to the head of the handpiece is through a solid glass or cellular optic rod.
Electric handpiece control units
Most electric handpieces can be retrofitted to existing units in treatment areas. After connection to air and water lines, the electric motor control box is mounted to the delivery unit or placed on the bracket tray. Wired or wireless control display units are available for placement in a convenient location for the operator. The handpiece can be operated by the existing foot control. The motor speed is easily adjusted to provide precise control of the rotation of the bur for a specific procedure. Most current systems offer a digital display of the motor speed, some with touchscreen control.
Among attachments for electric motors, most of the head sizes are the same. Most procedures can be done with two attachment heads. Typical high-speed ranges are 1,000 to 200,000 rpm, and lowspeed ranges are 100 to 40,000 rpm. Ultra low-speed handpiece attachments allow endodontic procedures, some with torque control and auto reverse.
Many electric motors utilize small carbon brushes in the gear systems. Over time, the carbon brushes rub against the rotors leaving carbon dust that needs to be cleaned and brushes that require replacement. Many of the new motors are brushless, reducing the amount of maintenance needed. While most attachments have two gears, some provide three gears to better distribute the load. This feature reduces the wear on the bearings, resulting in less failure over time. While the maximum speed of an electric handpiece (200,000 rpm) is approximately half that of its air-driven counterpart, the ability to maintain a constant speed under load is what gives the electric handpiece the power advantage. The constant torque of an electric handpiece eliminates the stalling or reduced speeds experienced when using an air-driven handpiece to cut through crowns or other dense materials.
Constant torque and stabilization of the bur allow a concentric motion as speed is maintained.
The gears of the electric handpiece lock in the bur in to prevent wobble of the bur that is often experienced when using an air-driven handpiece. The sweeping motion used with an air-driven handpiece can result in irregularities at the margin of a preparation; the motion of the electric handpiece can produce a more smooth, even margin.
Maintenance and lubrication
Proper protocol requires handpieces to be sterilized between patients; therefore, they will be subjected to hundreds of sterilization cycles (autoclave or chemiclave) during their lifetime.
Performance is compromised with repeated sterilizations, meaning there is a reduction in speed, power, and fiberoptic output. It is critical to follow the manufacturers’ instructions for lubrication and routine care. Some manufacturers offer handpieces with factory-lubricated, sealed bearings that do not require regular lubrication.