Three-unit fixed partial denture that has become loose

Correct answer: D. Loss of RETENTION AND RESISTANCE FORM from over-tapered, short preparations, so functional stress exceeds what the preparation geometry can withstand

• A. Recurrent caries would be visible radiographically or clinically and would usually loosen one retainer rather than the whole unit; nothing in the findings supports it.
• B. The abutments are described as sound on percussion with no periapical or crestal changes. Periodontal failure would show mobility of the individual abutments, not a prosthesis moving as a unit on intact teeth.
• C. Cement failure is a consequence, not a cause. Luting cements fail when the preparation geometry lets the restoration move — no cement compensates for inadequate form.
• D. CORRECT. Retention resists removal ALONG the path of insertion and depends on near-parallel walls and adequate surface area; resistance resists tipping or rotation under OBLIQUE load and depends on preparation height and rigidity. Short, over-tapered preparations lose both. Under a clencher's lateral loading, the restoration micro-rotates, the cement seal fatigues, and the bridge loosens as a unit.

Note: What this case adds is the failure MECHANISM — how taper and preparation height translate into loss of retention and resistance under function.

Correct answer: D. A span replacing a single missing tooth between two sound abutments

• A. Inadequate occlusogingival reduction leaves insufficient material thickness and a short preparation, compromising both strength and resistance form — also a genuine contributor.
• B. Parafunctional loading is a well-recognised contributor to both cement failure and porcelain fracture.
• C. Excessive convergence directly reduces retention — this is a genuine contributor.
• D. CORRECT as the exception. A SHORT SPAN replacing ONE tooth between two sound abutments is the most favourable configuration there is. Ante's law holds comfortably, and deflection under load is minimal. Deflection rises with the CUBE of span length — a two-pontic span deflects roughly eight times as much as a single-pontic span, which is why long spans fail. Three units is not a long span.

Note: Ante's law: the combined root surface area of the abutments should equal or exceed that of the teeth being replaced. Useful as a screening principle, though modern evidence treats it as a guide rather than an absolute rule.

Correct answer: D. Attrition

• A. Abrasion is wear from a foreign object — toothbrush and dentifrice most commonly — and produces cervical notching or incisal notching in habitual patterns, not flattening of an occlusal surface.
• B. Erosion is chemical dissolution by acid. It produces smooth, cupped, shiny concavities and typically spares restorative margins, which come to stand proud of the surrounding tooth. It does not selectively flatten an occluding surface.
• C. Abfraction describes wedge-shaped CERVICAL lesions attributed to occlusal flexure. It is a cervical phenomenon and does not affect the occlusal table.
• D. CORRECT. ATTRITION is wear produced by TOOTH-TO-TOOTH contact. It generates flat, polished, matching wear facets on opposing surfaces — and in this patient the reported clenching supplies the mechanism. Ceramic is harder than enamel, so a porcelain occlusal surface both wears and abrades the opposing dentition.

Note: ⚠️ SOURCE NOTE. The recall recorded this item with the options attrition, erosion, bruxism and 'unrestorable tooth surface', and no key. Bruxism is not a type of wear — it is the HABIT that causes attrition, so it cannot be the answer to a question asking what the defect IS. Keyed here to attrition, which is what the described appearance represents.