Rotary Instrumentation
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Direct answer
At 250,000 to 400,000 rpm the air-turbine handpiece cuts enamel efficiently with light pressure and a water spray, while slow-speed contra-angles below roughly 30,000 rpm finish caries removal and polishing where torque and control matter more than speed. Bur selection follows shape and material: carbide fissure and round burs cut tooth structure at high speed, diamond points grind enamel and ceramic, and stones and rubber impregnated abrasives finish and polish restorations. The pair of rules that protect the pulp are constant water cooling and intermittent, light-pressure cutting, because uncooled high-speed cutting raises intrapulpal temperature enough to kill the pulp even when the cavity itself stays shallow.
What you must remember
- Speed bands: slow speed up to about 30,000 rpm for caries excavation, finishing and polishing; high-speed air rotor roughly 250,000–400,000 rpm for enamel and dentine removal with copious water spray.
- High speed demands light pressure, a water spray (air and water together) and intermittent cutting; heat, not depth, is the main danger.
- Bur shank types: handpiece (HP) straight shank, latch-type (RA) for slow contra-angle, friction-grip (FG) for the air rotor.
- Bur shapes by G.V. Black's numbering tradition: round (No. 2, 4), inverted cone (No. 33 1/2, 35), straight fissure (No. 57), tapered fissure (No. 245), pear (No. 330) — the 245 remains the workhorse occlusal bur.
- Materials: steel burs cut slowly and dull fast at high speed; tungsten carbide burs hold edges at high speed; diamond abrasives grind enamel, ceramic and resin best.
- Finishing chain: fine diamonds and 12-fluted carbide finishing burs, then aluminium oxide discs (Sof-Lex type) and rubber points and cups with polishing paste.
- Stones: green (carborundum, silicon carbide) for gross reduction; white (aluminium oxide) for smoother finishing of resins and enamel.
- Alternative cutting: air abrasion with 27–50 µm alumina particles, Er:YAG laser ablation and ultrasonic tips — all no-pressure options valued in minimally invasive work.
Choosing the bur for each stage of a Class I preparation
Begin with a 245 tapered fissure carbide bur in the air rotor under a full water spray: enter the fissure at the selected point and drop the full 1.5 mm in one pass, because the bur's own length is the depth gauge and repeated shallow passes produce irregular floors. Carry the outline through the involved fissures with the same bur, finishing walls flat and margins clean. Now change philosophy, not just speed: carious dentine is removed at slow speed with a round carbide bur (size 4 or 6) or a spoon excavator, because at slow speed you feel the transition from soft, leathery infected dentine to hard affected dentine — tactile feedback the air rotor erases. If the fissure stains deeply but is not cavitated, a small round bur or a fine diamond opens it just enough for a sealant or preventive resin restoration rather than a full box. For finishing after restoration, the sequence reverses in fineness: 12-fluted carbide or fine diamond to contour, medium-to-superfine aluminium oxide discs wet-polished to gloss. Every step uses the lightest pressure that cuts, and the spray never stops while the bur touches tooth.
Where students slip
The recurring exam question is "why does the air rotor need water spray?" and the expected answer runs through frictional heat: uncooled cutting at high speed raises pulpal temperature, and classic primate experiments by Zach and Cohen showed that an intrapulpal rise of just 5.5 degrees Celsius already kills a proportion of pulps — hence cooling is not optional. The second slip is bur misuse: using a steel bur at high speed (it dulls within seconds and then burns rather than cuts), using a diamond on carious dentine (it plugs and smears instead of flaking), or reaching for a green stone on a composite surface (gross scratches that no disc sequence afterwards fully removes). In vivas, candidates also mix up latch-type and friction-grip shanks — RA burs sit in the slow contra-angle with a latch catch, FG burs friction-fit into the turbine chuck.
Frequently asked questions
What pressure and cooling does high-speed cutting require?
Light, intermittent pressure with a continuous air–water spray; heavy pressure stalls the turbine and frictional heat endangers the pulp.
Which bur is the standard for an occlusal Class I outline?
The 245 tapered fissure carbide bur in a high-speed handpiece, its length serving as the depth guide for the 1.5 mm pulpal floor.
Why are steel burs restricted to slow speed?
Their edges blunt rapidly at turbine speeds, after which they generate heat and burnish rather than cut dentine.
What do green and white stones contain?
Green stones are silicon carbide (carborundum) for aggressive reduction; white stones are aluminium oxide for finer finishing.
What is air abrasion used for?
Cutting small, conservative cavities and roughening surfaces with a stream of 27–50 µm alumina particles — vibration-free and often anaesthesia-sparing, though dusty and needing isolation.