ORE Part 2 · Dental Manikin (DM) guide
Class II (MOD) Cavity Preparation
A mesial-occlusal-distal cavity for amalgam — two proximal steps joined across the occlusal surface. The challenge is placing each step dead-centre in its contact, taking each to the depth that just opens the contact and no further, and joining everything at an even occlusal depth that the condenser passes through freely.
Our video demonstrates this on an upper premolar. In the exam, the tooth and exercise are set on the day.
What examiners look for
- Each proximal step started dead in the middle of its contact, parallel to the long axis of the tooth
- Steps extended buccally and palatally to their line angles, with the contacts opened by 0.5 mm on both sides
- Depth taken only as far as needed to open the gingival contact — the distal step runs deeper than the mesial
- An even occlusal outline at 1.8–2 mm, following the fissure with a V toward the buccal cusp rather than a straight gouge
- A cavity wide enough for the condenser to pass freely along its whole length
- Unsupported enamel shells removed cleanly, with straight step walls and no damage to the adjacent teeth
Common mistakes
- Chasing depth past the point where the contact opens — the premolar curves inward below the contact, so going deeper suddenly opens it far more than 0.5 mm
- Cutting the enamel shell with mesial-distal strokes instead of thinning it with buccal-palatal strokes until it chips away
- Through-and-through strokes across the occlusal surface, flattening the buccal cusp's V
- Driving the steps mesio-distally — step work is always buccal-palatal
- Wedging against the working tooth instead of the non-working tooth, losing both protection and visibility
- Spinning the bur at full speed for the last touch of a contact — let it slow down, or use the thin lower third of a flame bur
What the official DM Guidance states
- Each box clears the contact point by about 0.5 mm gingivally and bucco-palatally
- Boxes wide enough mesio-distally to be in dentine and remove the caries
- Box walls occlusally convergent, with no unsupported enamel left on the margin walls
- Occlusal lock about 2 mm deep, following the fissure pattern, with only a slight dovetail
- A thin lining on the axio-pulpal wall, if one is placed
The same Class II criteria cover MO, DO and MOD cavities, in a premolar or a molar.
Source: ORE Part 2 Dental Manikin (DM) Guidance, version 2.0, effective 1 March 2026, p. 9–10. Values marked “about” are approximate in the guidance. Check the current version before your exam. How to reach these, step by step, is in the course.
Candidate questions
How deep should each step go?
Only as deep as it takes to open the gingival contact by 0.5 mm. Check at the lower end of the range before cutting further — the tooth narrows below the contact, and one stroke too many over-opens it.
How do you remove the proximal enamel shell safely?
Thin it with gentle buccal-palatal strokes until it breaks away on its own, or push it off with the hatchet once thin. Never lever on it mesially against the adjacent tooth.
How is the double matrix band inserted when contacts are tight?
Wedge first to separate the teeth, slide the band through, remove the wedge, then re-wedge between the band and the non-working tooth. The wedge then presses the band against the working tooth and flares the view open.