Cause of fracture of occlusal rest:
- a- Shallow preparation in marginal ridge.***
- b- Extension of rest to central fossa.
- c- Improper centric relation.
The correct answer is a- Shallow preparation in marginal ridge.
A shallow preparation in the marginal ridge is the most common cause of occlusal rest fracture. The marginal ridge is the raised portion of enamel that forms the border between the occlusal surface and the marginal surface of a tooth. When preparing an occlusal rest, it is important to reduce the marginal ridge sufficiently to create a rest that is thick enough to withstand occlusal forces. If the marginal ridge is not reduced enough, the rest will be too thin and weak, and it will be more likely to fracture.
- Option b, "Extension of rest to central fossa,":
is not a common cause of occlusal rest fracture. The central fossa is the depression in the center of the occlusal surface of a tooth. Extending the occlusal rest to the central fossa can sometimes weaken the rest, but it is not as common a cause of fracture as a shallow preparation in the marginal ridge.
- Option c, "Improper centric relation,":
is not a direct cause of occlusal rest fracture. However, improper centric relation can lead to occlusal interference, which can put excessive stress on the occlusal rest and make it more likely to fracture.
Therefore, the most important factor in preventing occlusal rest fracture is to ensure that the marginal ridge is reduced sufficiently during preparation.
