Prosthodontics
Prosthodontics on the OSCE is almost entirely visual recognition. Fourteen impression errors, each with a diagnostic feature on the photograph. Some are reusable; some demand a remake. Knowing which is the highest-yield distinction in this chapter.
The 14 Impression Errors
A photograph can show 2-3 errors at once. Read systematically: tray fit · periphery · vestibule · finish line · surface texture · contour.
Incorrect Tray Size
Too small: incomplete capture of the arch — anatomy missing at periphery.
Too large: uneven bulk of material, distorted borders.
Incorrect Tray Position
Too anterior: insufficient material/space at front teeth.
Too posterior: excess material anteriorly, posterior under-extended.
Insufficient Capture of Teeth
Crown anatomy missing including gingival portions. Teeth appear truncated; only partial occlusal/incisal capture.
Overextension
Excessive capture of anatomy, thick over-extended border. Material extends beyond the functional vestibule.
Underextension
Insufficient capture of anatomy; thin or under-extended borders that don't reach the functional vestibule.
Overtrimmed
Border has been excessively trimmed. Tray plastic shows or anatomy is missing.
Incorrect Border Moulding
Borders are over- or under-extended; lack a smooth, rounded vestibular roll.
Underextension Past Finish Line
No "fin" of material apical to the prep margin. Finish line continuous with tissue — cannot identify the prep margin.
Excessive Contact (Show-Through)
Tray material visible through the impression — not enough material between tray and tissue.
Tray Separation
Material has lifted/peeled off the tray — visible at the edges. Compromised integrity.
Inadequate Polymerization
Soft, tacky impression with loss of fine detail and a glossy/shiny surface (didn't fully set).
Voids
Pits and craters of various shapes and sizes in the impression material.
Critical area (finish lines, occlusal): remake.
Non-critical area: usable.
Tears
Partially attached flaps in thin areas of sulcus or interproximal spaces.
Attached to finish line: remake. Off margin: may be usable.
Distortion
Stretched and elongated anatomy with smearing and rounded/blurred edges. Often from premature removal or improper storage.
Critical vs Non-Critical Zones
A void or tear in a critical zone always demands a remake. A defect in a non-critical zone may be acceptable.
Critical zones (REMAKE)
Defects here = remake.
- Finish lines (crown / inlay / bridge prep margin)
- Occlusal surfaces of indirect restorations
- Edentulous ridge crest (denture)
- Posterior palatal seal area
Non-Critical zones (usable)
Defects here are tolerated.
- Vestibule (away from border)
- Hard palate, central body
- Interproximal soft tissue away from prep margin
Maxillary critical zones — schematic
How to Read an Impression Photo
Six checkpoints, in order. Run them every time.
① Tray fit
Is the arch fully captured? Is material thickness even? Is the tray centred?
② Periphery
Are borders smooth and rounded? Over- or under-extended? Anatomy missing?
③ Vestibule
Smooth vestibular roll? Frenal notches present? Material reaches functional depth?
④ Finish line
Visible "fin" of material apical to prep? Margin clearly demarcated all around?
⑤ Surface texture
Glossy / tacky = under-set. Tray showing through = insufficient material.
⑥ Contour defects
Voids (pits) — where? Tears (flaps) — where? Distortion (smearing/elongation)?