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Radiology · patient case with 3 questions

Radiographic anatomy and removable design

Scenario: Three opening items: one radiographic landmark, one RPD component, one shade-selection decision.

Question 1

A dense radiopaque band is seen running downward and forward from the anterior border of the ramus across the mandibular molar region. Which structure is this?

  1. Nutrient canal
  2. Mandibular canal
  3. Mylohyoid (internal oblique) groove
  4. External oblique ridge
Show answer and explanation

Correct answer: D. External oblique ridge

  • A. Nutrient canals are fine radiolucent lines, most often seen interdentally in the anterior mandible, and are far thinner than this band.
  • B. The mandibular canal is a radiolucent band with corticated superior and inferior borders running from the mandibular foramen to the mental foramen.
  • C. The mylohyoid groove is lingual and appears as a thin radiolucent line running downward and forward from the lingula — radiolucent, not radiopaque.
  • D. Correct. The external oblique ridge is a buccal cortical thickening that continues from the anterior border of the ramus and sweeps anteroinferiorly over the molars, casting a broad radiopaque band.

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Question 2

What is the function of an indirect retainer in a removable partial denture?

  1. Resist horizontal and lateral forces applied to the prosthesis
  2. Direct occlusal load along the long axis of the abutment tooth
  3. Resist rotation of the distal extension base away from the tissue about the fulcrum line
  4. Prevent vertical displacement of the framework toward the tissue
Show answer and explanation

Correct answer: C. Resist rotation of the distal extension base away from the tissue about the fulcrum line

  • A. Horizontal forces are resisted by reciprocal (bracing) arms and rigid minor connectors.
  • B. Axial loading is the job of the occlusal rest seat, which directs force down the long axis.
  • C. Correct. An indirect retainer is placed on the opposite side of the fulcrum line from the extension base, so when the base lifts, the indirect retainer resists that rotation.
  • D. Vertical displacement toward the tissue is resisted by rests and the denture base, not by the indirect retainer.

Note: Indirect retainers are only needed where a fulcrum line exists — Kennedy Class I and II. They are not used in a tooth-borne Class III.

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Question 3

The maxillary anterior teeth have been prepared and an adjacent unprepared tooth is available for reference. Which tooth is used as the reference for shade selection, and which dimension is matched first?

  1. Tooth #11 — chroma
  2. Tooth #10 — value
  3. Tooth #6 — translucency
  4. Tooth #6 — hue
Show answer and explanation

Correct answer: B. Tooth #10 — value

  • A. Chroma is matched after value. A restoration with the right chroma but the wrong value will still read as wrong.
  • B. Correct per the recall key. Value (lightness/darkness) is the shade dimension the eye detects first and the one most responsible for a restoration looking 'off' — so it is matched first, using the closest unprepared neighbour.
  • C. Translucency matters at the incisal third but is refined last, after value and chroma are set.
  • D. Hue is the least discriminating dimension in natural teeth; nearly all teeth fall in the A–B (yellow to yellow-red) families, so hue rarely drives a mismatch.

Note: This item was only partially recalled — the tooth numbers in the original stem were fragmentary. The transferable point is the ranking: value first, then chroma, then hue, then translucency.

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