Characterizing the performance of dental air-turbine handpieces

B.W. Darvell; J.E. Dyson    The University of Hong Kong, Hong Kong

1.1 Outline

After briefly outlining the general importance of air-turbine handpieces in dentistry (Section 1.2), a historical account of their development puts their present status into context (Section 1.3). However, in order to understand performance in general, it is necessary to recognize the large number of factors involved, and their complex interactions (Section 1.4). In essence, it is not yet possible to characterize the cutting performance of these devices, dependent as they are on the behaviour of cutter and substrate, amongst other things. Accordingly, it is as yet only feasible to document the physical aspects of the behaviour of the turbine itself (Section 1.5), but this leads to a number of figures of merit that may be used for product comparisons in an objective fashion that are tied to the physics of these machines. Even so, because of their internal complexity, primarily in terms of gas flow, it is necessary to resort to the ‘black-box’ approach and document input–output relationships, subsuming much unresolvable detail in some fitted parameters. Selection and application by the end-user nevertheless depends on a number of further issues of great importance, and these are discussed under the general heading of hazards (Section 1.6). The chapter closes with some general remarks on selection, usage, and areas where further study is essential.

1.2 General importance: applications, benefit

The dental air-turbine handpiece rapidly gained widespread acceptance by the dental profession after its introduction in the late 1950s, and it continues to be used as the main means of carrying out cutting work in clinical dental practice, whether of tooth tissue or restorative materials. In comparison with alternatives at the time, the reasons given for its usefulness included the following.

• Power: power-to-weight ratio very favourable, negligible transmission loss;

• Size: size and weight allow better control for long periods without tiring as well as good intraoral access;

• Speed: reduction of unpleasant vibration, finer control of cutting process;

• Effort: lower forces could be used yet with higher removal rates.

These considerations still appear to be pertinent.

1.3 Historical outline: development, features

A turbine is a motor in which a shaft is steadily rotated by the action of a current of fluid upon the blades of a wheel. Turbines powered by various fluids have evolved along several paths, and it is not possible to identify a single source for the development of dental systems.

The first air-powered dental engine design was patented in 18681 although in fact this was not a turbine but effectively a lobe pump operated in reverse. It was intended to be operated by mouth, foot bellows, or a compressed air vessel. The first true turbine dental handpiece, with a 13-bladed rotor, was patented in 1874,2 with similar suggestions for operation as the lobe pump. It received little attention from the profession. A water-powered device in 18773 also made provision for a fine stream of water to be directed as coolant onto the cutting instrument. A more elaborate device with a transmission clutch, a rotatable handpiece sheath, and revised mechanism for the attachment of cutting instruments followed in 1879,4 although details of the turbine rotor were not given and the drive fluid was not specified.

These machines were all somewhat bulky with their weight borne by the dentist’s hand. However, a water-powered engine, produced by S. S. White in 18815 avoided this problem by the motor being mounted on a floor stand. A flexible shaft transmitted the drive in a fashion similar to that of many foot-treadle engines of the time. Evidently, the problems were greater than the advantages. Improvements made to foot-treadle and electric dental engines in the late 1800s led to fluid-driven de vices falling by the wayside and, by the 1920s, the cord-arm drive had been adopted as the de facto standard means of transmission from an electric motor to the handpiece.6

In the 1870s, the maximum speeds were around 700 rpm (12 /s) and 1000 rpm (17 /s) for foot-driven and electrical devices, respectively.7 Speed, recognized to be beneficial, progressively increased, but success depended in part on suitable rotary cutting instruments being available, as well as improved means of cooling the cutting site. An electric engine of 1911 reputedly achieved up to 10 000 rpm (167 /s)6,8−10 separating discs and grinding tools worked more smoothly with less patient discomfort. However, the engine was unsuccessful because of overheating and seizure of the hand-piece bearings.6,9 Effective means of achieving such speeds did not become available until the 1940s.

Studies of vibration perception provided evidence in favour of increased speeds.11−13 The upper frequency threshold of vibration perception was found to be ~ 650 Hz, with maximum unpleasantness in the range 100200 Hz. Using burs, stones, and diamond instruments at 3000–4000 rpm (50–67 /s), the vibrations produced were ~ 110–150 Hz. An air-turbine handpiece was then developed (see below) specifically to produce vibrations above the limit of perception by virtue of its high rotation rate. In the end, it was concluded that, in procedures that resulted in the same range of temperature rise, high-speed devices could remove enamel some three times as fast and at 1/30th of the operating load, as well as with better control and less effort.14 Indeed, with proper cutting-site cooling, high-speed rotation was not only possible but practical, safe, and effective,15,16 with advantages for both patient and operator.17 In fact, it was said that ‘few pieces of equipment in dentistry have caused more changes and improved dental service to a greater degree than ultra-high-speed handpieces (i.e. those rotating at 1000–5000 /s)’,18 allowing improved patient response, shortened operating time, reduced vibration perception, and less patient and operator fatigue. For these reasons this development was described as ‘one of the most significant contributions to dental health service’.19

Nevertheless, high-rotation-rate cutting only became possible when instruments became available that could withstand such speeds. Until at least 1870, steel burs were the only cutting instruments available and these were individually shaped and finished by hand. The mass production of carbon steel burs began by the 1870s.8 Corundum (Al2O3) separating discs and stones were introduced in 18726,8,9 and provided the first satisfactory means of cutting enamel, although subsequently supplanted by carborundum (SiC).

Diamond grit cutting instruments were first advertised in 187820 but, being on a soft copper core, could not be used at high speed until the development in 1932 of galvanized bonding to harder alloy.6,8,9 Tungsten carbide burs followed in 1948 and proved to be extremely successful in high-speed applications.8 There has been no significant development since then.

The problem of heat generation remained, although recognized as much as 2000 years ago21 for surgical trephines. A cooling system was fitted to one handpiece in commercial production by 1874:22 water was applied from a rubber bulb though a hose and nozzle, but an integral system soon after was to apply a stream of water onto the cutting instrument.3 Many patented designs followed,23−38 some of which allowed compressed air to be applied to the cavity for debris removal. One even heated the air and water to minimize patient discomfort, and they could be released simultaneously. Alongside this, more effective aspiration was required.39,40 Hollow burs, through which air is passed to supplement the cooling provided by air and water jets, were devised in 197419,41 but failed to gain widespread use, despite a similar principle being used in surgical instruments.

The start of the modern turbine era may be 1941, when a patent claimed 25 000 rpm (417 /s) for a design using compressed air at 45 psi (310 kPa).42 The turbine rotor was unusual: a cylinder with a circular arrangement of holes through which air jets from two nozzles were directed. In addition, ball bearings were to be used (as opposed to the sleeve bearings of earlier handpieces), and the inner ball race of the bearing at the chuck was arranged to cause the jaws to open and close by a sliding action, thus facilitating the rapid change of instruments.

The first demonstrations of Norlén’s device in London, UK were in May 1958,43 although it had been patented in 1952.44 Turbine rotation was transmitted to the instrument via a mechanism in the body of the handpiece, which was interchangeable by a slip-joint connection. Multiple nozzles directed air onto inclined, slightly shovel-shaped turbine blades. Speed control was by means of adjusting the opening of vent holes. Said to reach a rotor speed of 120 000 rpm (2000 /s), a...