How wide should the isthmus areas be in a class I preparation?

• A. 0.5-0.8 mm
• B. 1.4-1.6 mm
• C. 2.0-2.4 mm
• D. 1-1.2 mm

Answer: (D)

The width of the isthmus in a Class I preparation is the narrow channel that connects the buccal and lingual aspects between the occlusal walls. It should be just wide enough to provide a stable core of amalgam and adequate access for condensation, while preserving as much tooth structure as possible to maintain strength. About one to 1.2 millimeters hits that balance nicely. This width keeps enough tooth structure to resist occlusal forces and maintain cusp integrity, yet allows enough space for proper placement and condensation of amalgam against the walls. If the isthmus is made wider, it undermines cuspal strength and resistance form; if it’s too narrow, it impedes caries removal access and compromises the bulk and adaptation of the filling.