In facia type of facebow, the centre of condylar rotation is arbitrarily marked as____ mm anterior to the middle of the tragus of the ear on canthotragal line.
**Core Concept**
The facia type of facebow is a mechanical device used in dentistry to transfer the location of the maxillary dental arch to the articulator. It relies on anatomic landmarks to accurately position the maxillary arch in relation to the mandible. The center of condylar rotation is a critical reference point in this process.
**Why the Correct Answer is Right**
The center of condylar rotation is an imaginary point on the mandible that represents the pivot point around which the mandible rotates during jaw movements. In the facia type of facebow, this point is arbitrarily marked as 7 mm anterior to the middle of the tragus of the ear on the canthotragal line. This is based on the average anatomic location of the center of condylar rotation in relation to the external ear. The canthotragal line is an imaginary line drawn from the corner of the eye (canthus) to the tragus of the ear, and it serves as a reference for this measurement.
**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because it does not match the established anatomic reference point for the center of condylar rotation in the facia type of facebow. The actual measurement is 7 mm anterior to the middle of the tragus, not the corner of the eye.
**Option B:** This option is incorrect because it is a different measurement altogether, not related to the facia type of facebow or the center of condylar rotation.
**Option C:** This option is incorrect because it is a measurement in a different direction, not anterior to the middle of the tragus as required by the facia type of facebow.
**Clinical Pearl / High-Yield Fact**
When using the facia type of facebow, it is essential to understand the anatomic basis for the measurements taken. This includes the location of the center of condylar rotation and the canthotragal line. A thorough knowledge of these concepts is crucial for accurate transfer of the maxillary dental arch to the articulator.
**Correct Answer: C. 7 mm**