ORE Part 2 · Dental Manikin (DM) guide
Class II (DO) Cavity Preparation
A Class II (Class 2) disto-occlusal (DO) cavity in a premolar: an occlusal outline with a retentive lock and a distal box that must clear its contacts. The challenge is keeping the width to gauge without undermining the buccal cusp, removing unsupported enamel at the box and protecting the adjacent tooth.
Our video demonstrates this on an upper premolar. In the exam, the tooth and exercise are set on the day.
What examiners look for
- The occlusal outline completed first, at a consistent depth with a smooth, flat floor
- Straight or convergent walls — never divergent
- Width to gauge throughout, including the lock
- A proximal box with straight walls and no ledge where it meets the occlusal floor
- Buccal, palatal and gingival contacts opened, with no unsupported enamel
- The buccal cusp preserved and the adjacent tooth undamaged
Common mistakes
- Divergent walls, which won't retain the filling
- An unsupported shelf of enamel left at the box
- A ledge between the occlusal floor and the box
- Over-widening instead of small cuts and frequent checks
- Breaking the marginal ridge without a band and wedge in place
Candidate questions
In what order should I cut a Class II DO cavity?
Complete the occlusal outline at full depth first, then break the marginal ridge and form the box. Clear the contacts and refine the width to the condenser last.
Are straight walls acceptable?
Straight or convergent walls are acceptable; divergent walls aren't. Keeping the bur upright naturally tends towards convergence.
How do I protect the adjacent tooth?
Place a matrix band with a wedge before you cut near the contact.