The relationship between the working edge of the instrument and the tooth surface is:
**Core Concept**
The question is testing the understanding of the relationship between the working edge of an instrument and the tooth surface in dental anatomy. The working edge is the part of the instrument that comes into contact with the tooth surface, and it is crucial in determining the instrument's effectiveness and safety.
**Why the Correct Answer is Right**
The correct answer is related to the concept of the "angle of approach" and the "axis of rotation" of the instrument. The angle of approach is the angle at which the instrument approaches the tooth surface, while the axis of rotation is the imaginary line around which the instrument rotates. The working edge of the instrument should be in contact with the tooth surface at a 90-degree angle to the long axis of the tooth, and the axis of rotation should be parallel to the long axis of the tooth. This ensures that the instrument is effective in removing decay or shaping the tooth without causing damage.
**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because it does not take into account the importance of the angle of approach and the axis of rotation in determining the effectiveness and safety of the instrument.
**Option B:** This option is incorrect because it suggests that the working edge of the instrument should be at a 45-degree angle to the long axis of the tooth, which is not the optimal angle for effective and safe instrument use.
**Option C:** This option is incorrect because it implies that the working edge of the instrument should be at a 180-degree angle to the long axis of the tooth, which would be ineffective and potentially damaging.
**Clinical Pearl / High-Yield Fact**
A good rule of thumb in dental anatomy is to always approach the tooth surface at a 90-degree angle to the long axis of the tooth, and to use instruments with a working edge that is designed for the specific task at hand (e.g. excavator, drill, or scaler).
**Correct Answer:** C.