ORE Part 2 · Dental Manikin (DM) guide
Class II (MO) Cavity Preparation
A Class II (Class 2) mesio-occlusal (MO) cavity in a molar for an amalgam restoration, combining a retentive occlusal lock with a mesial proximal box. The challenge is placing the outline correctly between the mesial cusps and clearing every contact while keeping the width to the condenser.
Our video demonstrates this on an upper first molar (26). In the exam, the tooth and exercise are set on the day.
What examiners look for
- An outline that follows the fissure pattern, with the lock in the buccal fissure
- A lock and dovetail that give mechanical retention
- A smooth, flat floor and straight or convergent walls
- Mesiobuccal, mesiopalatal and gingival contacts all opened, with no unsupported enamel
- A condenser that moves freely through the whole cavity, including the lock
- The adjacent premolar protected and undamaged
Common mistakes
- Leaving an unsupported shelf of enamel at a contact
- A step or ledge on the buccal wall of the box
- A lock too narrow for the condenser, or widened everywhere instead of where it binds
- Tilting the bur so the walls diverge
- Cutting the box without a matrix band and wedge protecting the neighbouring tooth
Candidate questions
Why does an MO amalgam preparation need a dovetail?
Amalgam doesn't bond, so it relies on mechanical retention. The occlusal lock keeps the restoration interlocked in the cavity.
What if the condenser fits the box but not the lock?
Find exactly where it binds and cut only there, in small strokes with the bur upright, checking as you go.
How do I protect the adjacent tooth?
Place a matrix band and wedge before cutting near the contact. It protects the neighbouring tooth and improves your view.