In a Class II preparation, the isthmus is the portion of the cavity that connects which two parts?

• A. Occlusal Portion and Proximal Portion
• B. Proximal and Cervical Portions
• C. Lingual and Buccal Walls
• D. Occlusal and Buccal Walls

Answer: (A)

In a Class II preparation, the cavity is built with an occlusal portion that handles the chewing surface and a proximal portion that extends into the adjacent surface. The isthmus is the narrow connection between these two parts, acting as the bridge that links the occlusal portion to the proximal portion so the restoration can extend smoothly from the biting surface into the proximal area. This connection must be wide enough to allow proper filling and condensation of material while preserving the tooth’s walls, and it is specifically the link between occlusal and proximal portions rather than a connection between walls or to the cervical area.