The Crown That Keeps Falling Off

Correct answer: A. Excessive occlusal loading from the deep overbite and parafunction

• A. Correct. A deep overbite with minimal overjet places heavy, obliquely directed loading on the maxillary anterior teeth during excursive movements. Combined with nocturnal parafunction, the shear stresses at the cement interface exceed its bond strength, and the crown repeatedly debonds. The examination points to this directly: wear facets, masseter hypertrophy, and morning soreness.
• B. Incorrect. Cement allergy is exceedingly rare and would present as a mucosal inflammatory reaction adjacent to the margin, not as loss of retention.
• C. Incorrect. The stem specifies an adequate 2 mm ferrule, which is the accepted minimum for resisting functional and parafunctional load.
• D. Incorrect. She has 4 mm of attached gingiva with no recession. Attached gingiva relates to periodontal health and soft tissue response, not to retention of a crown.

Correct answer: A. Occlusal analysis using mounted diagnostic casts

• A. Correct. The failure is a force problem, so it must be diagnosed before it can be corrected. Mounted casts, an evaluation of centric and excursive contacts, and an assessment of anterior guidance identify what is overloading the restoration. Recementing without this simply resets the clock on the same failure.
• B. Incorrect. Stronger cement addresses a symptom rather than the cause. Three failures already indicate the problem lies elsewhere.
• C. Incorrect. There is no pulpal diagnosis and no symptoms suggesting pulpal involvement. Endodontic treatment is not indicated.
• D. Incorrect. Crown lengthening is indicated for inadequate ferrule or biologic width violation. Neither applies here.

Correct answer: A. Free gingival graft at #8

• A. Correct — this is the exception. A free gingival graft is indicated to augment a deficient band of keratinized tissue or to arrest progressive recession. She has 4 mm of attached gingiva and no recession, so there is nothing to graft and no benefit to be gained.
• B. Incorrect — this is appropriate. An occlusal guard protects the restoration from nocturnal parafunctional forces, which cannot be eliminated by adjustment alone.
• C. Incorrect — this is appropriate. Reassessing the preparation geometry, the resistance and retention form, and the choice of material is a logical part of preventing a fourth failure.
• D. Incorrect — this is appropriate. Selective adjustment redistributes force away from the restored tooth and can substantially reduce the load driving the failure.

Correct answer: B. Fit-checking vinyl polyether silicone

• A. Incorrect. Zinc oxide-eugenol paste is used for final impressions on edentulous ridges and as a bite registration medium. It sets too rigidly and thickly for this purpose.
• B. Correct. Fit-checking materials are formulated as very thin, low-viscosity films that record contact points on the intaglio surface. Where the film is displaced or perforated, an interference exists and can be relieved precisely.
• C. Incorrect. Irreversible hydrocolloid is a preliminary impression material with poor dimensional stability and no capacity for the fine detail this application requires.
• D. Incorrect. Heavy-body impression material is far too viscous to record fine internal contacts. It is designed to carry a tray, not to detect microscopic interference.

Note: Two different materials for two different jobs: fit-checking silicone or disclosing wax evaluates the internal surface, while articulating paper or shimstock evaluates the occlusal surface.

Correct answer: C. An excessively tight proximal contact on the distal

• A. Incorrect. Excessive taper reduces retention and resistance form but makes seating easier rather than harder. It does not create an open margin at try-in.
• B. Incorrect. An undercut prevents the crown from seating at all, or the crown would not have been fabricated, since the die would have been blocked out in the laboratory.
• C. Correct. A proximal contact that is too tight prevents the crown from seating fully on that side. The restoration tips away from the tight contact, lifting the margin on that surface. Marking the contact with articulating paper and adjusting until the crown seats confirms the diagnosis.
• D. Incorrect. Insufficient occlusal reduction produces a high restoration in occlusion or an overcontoured crown, not an open margin on a proximal surface.

Correct answer: B. Monolithic zirconia

• A. Incorrect. Feldspathic porcelain has the lowest strength of these options and is unsuitable for posterior occlusal loading, let alone in a bruxer.
• B. Correct. Monolithic zirconia has the highest flexural strength and fracture toughness of the available ceramic options, and because it has no veneering layer there is no porcelain to chip. That combination makes it the standard choice for a posterior tooth in a bruxing patient.
• C. Incorrect. A porcelain occlusal surface remains vulnerable to chipping and will abrade the opposing dentition. A metal occlusal surface would be far more durable, but the patient’s esthetic concerns and modern zirconia performance make monolithic zirconia the better answer.
• D. Incorrect. Lithium disilicate is strong in monolithic form, but a layered design introduces a veneering porcelain that becomes the weak point under parafunctional load.

Note: Note the trade-off in monolithic zirconia: polish the occlusal surface thoroughly after any adjustment. Unpolished, roughened zirconia is markedly abrasive to opposing enamel; well-polished zirconia is comparatively kind to it.