Notes on Operative Dentistry and Endodontic Viva Voce

O perative Dentistry and Endodontic Viva Voce :

Mercury-free alloys:

These are Gallium-indium alloys. They are made up of Ag-Sn particles in Gallium-indium liquid.

Gain substitutes for the Hg in dental amalgams. Gallium melts at 28°C and can be used to produce liquid alloys at room temperature.

Disadvantages of Mercury-free alloys

i. Surface roughness

ii. Marginal discoloration

iii. Tooth fracture due to expansion

iv. Difficult manipulation.

If the adjacent proximal boxes are of different sizes, prepare the larger first so that the preparation for the smaller can be kept more conservative, and conversely restore the smaller first and the larger, last.

Bevels

Single beveled instruments. It has bevel on only one side. E.g. Enamel hatchet, GMT

Beveled instruments

It has beveling on both sides.

To determine whether the instrument has right or left bevel, the primary cutting edge is held down and pointing away, and if the bevel appears on right side then it is the right instrument of the pair.

Mesial bevel can be determined by observing the inside of the blade curvature. If the primary bevel is visible, it is the mesial bevel. If primary bevel is not visible, it is a distal bevel.

Sharpness of the instrument can be tested by resting the cutting edge on a hard plastic surface. If the cutting edge digs over the surface, the instrument is sharp. If it slides, then the instrument is dull.

In other method, observe the primary cutting edge under the light. If the light reflects, it indicates a blunt edge. Sharp edges don’t reflect the light.

Number of blades:

1. Number of blades is always even.

2. Excavating burs have 6-10 blades.

3. Finishing and polishing burs have 12-40 blades.

4. Greater the number of blades, smoother will be the cutting at slow speed.

Steel burs are mainly used for finishing procedures.

Cross cuts are needed on fissure burs to obtain adequate cutting efficiency, even at low speeds. They are not used at high speeds as they tend to produce an unduly rough surface.

Vibrations above 1300 rpm are imperceptible to the patients, so sensations are lost at high speed range.

Temperature during cavity preparation should not exceed 130°F or 60°C. When using high speed, low pressure and water coolant should be used to prevent overheating.

Pressure is directly proportional to heat generated. Whenever rpm is increased, pressure has to be reduced. It should not exceed 4 ounces when using high speeds, and not more than 12 ounces, when using low speeds.

Noise level more than 75db causes hearing damage.

Glass bonds best to enamel than dentin and cementum.