Dental drill

High-speed handpieces work at cutting speeds over 180,000 rpm. They are technically categorised into Air Turbine and Speed-increasing depending on their mechanisms. In a clinical setting, however, Air turbine handpieces are most often referred to as "High-speeds". Handpieces have a chuck or collet, for holding a cutter, called a burr or bur.

Power

Air turbine used in a dental handpiece

Correlation between rotational speed and torque

Correlation between rotational speed and turbine output power

The turbine is powered by compressed air at about 35 pounds per square inch (~2,4 bar),[1] which passes up the centre of the instrument and rotates a windmill in the head of the handpiece. The centre of the windmill (chuck) is surrounded by bearing housing, which holds a friction-grip burr firmly & centrally within the instrument. Inside the bearing housing are small, lubricated ball-bearings (stainless steel or ceramic) which allow the shank of the burr to rotate smoothly along a central axis with minimal friction.

Failure of the burr to run centrally causes a number of clinical defects:

• The burr will judder; this will cause excessive, damaging vibrations leading to cracking and crazing in the material being cut. It is also an unpleasant experience for the patient.
• Eccentric cutting - this will result in irregular removal of the surface, meaning more tissue than necessary is removed.
• Decreased control - due to irregular cutting, it is more difficult for the Dentist to control movements

Electric motors cannot turn as fast as air turbines. To power a high-speed handpiece, gears are needed to increase the speed of an electric motor, often by a ratio of 1:5.[2] For this reason, electric handpieces are also called speed-increasing handpieces, working at cutting speeds over 180,000 rpm.[3]

• Speed-increasing handpiece is driven by electrical motor, also known as micromotor.
• The power to the handpiece is provided by the micromotor.
• Within the handpiece is internal gearings which allow the friction grip burr to rotate at a constant speed and torque.
• Therefore the power is provided by micromotor and internal gearings.

Slow speed handpieces work at a much slower rate that high speed and speed-increasing handpieces, and are usually driven by rotary vane motors, instead of air turbines. They work at a speed between 600 and 25,000 rpm. The internal gearings are very similar to that of a speed-increasing handpiece. The main difference between the two is that slow speed has internal gearing and they use a latch grip burr as opposed to a friction grip burr.

Designed to work at slower speeds.

A collection of various burrs used in dentistry.

Dental rotary instruments - boreri

A dental burr is a type of burr used in a handpiece. The burrs are usually made of tungsten carbide or diamond. The three parts to a burr are the head, the neck, and the shank.[4]

The heads of some burrs (such as tungsten carbide burrs) contain the blades which remove material. These blades may be positioned at different angles in order to change the property of the burr. More obtuse angles will produce a negative rake angle which increases the strength and longevity of the burr. More acute angles will produce a positive rake angle which has a sharper blade, but which dulls more quickly. The heads of other commonly used burrs are covered in a fine grit which has a similar cutting function to blades (e.g. high speed diamond burrs).

There are various shapes of burrs that include round, inverted cone, straight fissure, tapered fissure, and pear-shaped burrs. Additional cuts across the blades of burrs were added to increase cutting efficiency, but their benefit has been minimized with the advent of high-speed handpieces.[4] These extra cuts are called crosscuts.

Due to the wide array of different burrs, numbering systems to categorise burrs are used and include a US numbering system and a numbering system used by the International Organisation for Standardisation (ISO).

The instrument needs to be disinfected after every usage to prevent infection during succeeding incisions. Due to the mechanical structure of the device, this must not be done with alcoholic disinfectant, as that would destroy the lubricants. Instead it has to be done in an autoclave after removing the drill, washing the instrument with water and lubricating it.[5][6] The United States Food and Drug Administration classes burrs as "single-use devices",[7] although they can be sterilised with proper procedures.

Foot-powered dental drill

The Indus Valley Civilization has yielded evidence of dentistry being practiced as far back as 7000 BC.[8] This earliest form of dentistry involved curing tooth related disorders with bow drills operated, perhaps, by skilled bead craftsmen.[9] The reconstruction of this ancient form of dentistry showed that the methods used were reliable and effective.[10] Cavities of 3.5 mm depth with concentric grooves indicate use of a drill tool. The age of the teeth has been estimated at 9000 years. In later times, mechanical hand drills were used. Like most hand drills, they were quite slow, with speeds of up to 15 rpm. In 1864, British dentist George Fellows Harrington invented a clockwork dental drill named Erado.[11] The device was much faster than earlier drills, but also very noisy. In 1868, American dentist George F. Green came up with a pneumatic dental drill powered with pedal-operated bellows. James B. Morrison devised a pedal-powered burr drill in 1871.

Chayes M33 with Buffalo drive belt.

The first electric dental drill was patented in 1875 by Green, a development that revolutionized dentistry. By 1914, electric dental drills could reach speeds of up to 3000 rpm. A second wave of rapid development occurred in the 1950s and 60s, including the development of the air turbine drill.

The modern incarnation of the dental drill is the air turbine (or air rotor) handpiece, developed by John Patrick Walsh (later knighted) and members of the staff of the Dominion Physical Laboratory (DPL) Wellington, New Zealand. The first official application for a provisional patent for the handpiece was granted in October 1949. This handpiece was driven by compressed air. The final model is held by the Commonwealth Inventions development Board in Canada. The New Zealand patent number is No/104611. The patent was granted in November to John Patrick Walsh who conceived the idea of the contra angle air-turbine handpiece after he had used a small commercial-type air grinder as a straight handpiece. Dr. John Borden developed it in America and it was first commercially manufactured and distributed by the DENTSPLY Company as the Borden Airotor in 1957. Borden Airotors soon were also manufactured by different other companies like KaVo Dental, which built their first one in 1959.[12]

Current iterations can operate at up to 800,000 rpm, however, most common is a 400,000 rpm "high speed" handpiece for precision work complemented with a "low speed" handpiece operating at a speed that is dictated by a micromotor which creates the momentum (max up to 40,000 rpm) for applications requiring higher torque than a high-speed handpiece can deliver.[13]

Starting in the 1990s, a number of alternatives to conventional rotary dental drills have been developed. These include laser ablation systems[14] and air abrasion devices (essentially miniature sand blasters) or dental treatments with ozone.

• MedTerms definition for Drill, dental
• "Dental drills - enemy of the people?" from the British Dental Association museum
• Australian Dental Journal:1 p59-62

Wikimedia Commons has media related to Dental drills and bits.