Retentive grooves of a proximal box in a class II cavity prepared for amalgam should be:
**Core Concept**
Proximal retentive grooves are a critical component in class II amalgam restorations. They enhance the retention of the restoration by creating an area of stress concentration at the dentino-enamel junction (DEJ) of the buccal and lingual walls. This stress concentration helps to resist the forces acting on the restoration, thereby preventing its displacement.
**Why the Correct Answer is Right**
The correct answer is C. Elongated and rounded in dentine at the buccoaxial and linguoaxial Line angles extending from the gingival wall to the axiopulpal line angle. This is because the retentive grooves should be placed in dentine, not at the DEJ. The grooves should be elongated and rounded to provide a mechanical advantage and to prevent crack propagation. The buccoaxial and linguoaxial line angles are the best locations for the grooves as they provide maximum retention and resistance to forces. The grooves should extend from the gingival wall to the axiopulpal line angle to ensure maximum retention.
**Why Each Wrong Option is Incorrect**
**Option A:** Sharp and elongated at the dentino enamel junction of the buccal and lingual walls is incorrect because the grooves should be placed in dentine, not at the DEJ. Sharp grooves can also lead to crack propagation and restoration failure.
**Option B:** Sharp and well defined extending from the gingival to the occlusal cavosurface angle along the buccal and lingual line angles is incorrect because the grooves should be elongated and rounded, not sharp. The occlusal cavosurface angle is not a suitable location for the grooves as it may compromise the occlusal surface.
**Option D:** Short and rounded at the dentino enamel junction of the buccal and lingual walls is incorrect because the grooves should be placed in dentine, not at the DEJ. Short grooves may not provide sufficient retention and can lead to restoration failure.
**Clinical Pearl / High-Yield Fact**
When preparing class II amalgam restorations, remember the "Rule of 3/4s": the retentive grooves should be 3/4 of the way from the gingival wall to the occlusal surface, 3/4 of the way around the dentine, and 3/4 of the way from the DEJ to the dentino-dentine junction.
**✓ Correct Answer: C. Elongated and rounded in dentine at the buccoaxial and linguoaxial Line angles extending from the gingival wall to the axiopulpal line angle**