Restorative & Materials · patient case question
Why is a Class V composite restoration particularly vulnerable to polymerisation shrinkage stress?
Patient case: Cervical lesions in a care-home resident
Patient: Male, 84 years old
Chief Complaint: Brought by his caregiver — “His teeth are wearing away at the gumline.”
Background: Lives in residential care with an all-day caregiver. Mild cognitive impairment; cooperates for short appointments only. Takes four medications, three of them anticholinergic. Limited manual dexterity.
Vital Signs: BP 142/80 mmHg · HR 70 bpm
Findings: Generalised recession with multiple soft, leathery root-surface Class V lesions across all quadrants. Markedly reduced salivary flow. Isolation is difficult and he cannot tolerate rubber dam or long appointments.
Show answer and explanation
Correct answer: B. Because of its high configuration (C) factor
- A. Pulpal proximity determines whether a liner is needed and influences post-operative sensitivity, but it has no bearing on the shrinkage geometry of the preparation.
- B. CORRECT. The CONFIGURATION FACTOR is the ratio of BONDED to UNBONDED (free) surfaces in a preparation. The higher that ratio, the less the composite can flow to relieve polymerisation shrinkage, and the more stress is transferred to the adhesive interface — producing marginal gaps, debonding, post-operative sensitivity and white lines at the margin. A Class V is a box bonded on five surfaces with only one free surface, giving a C-factor of about 5, the highest of the cavity classes. Class I is similarly unfavourable. A Class IV, bonded on only one or two surfaces with several free, sits around 0.5 to 1.
- C. A LOW configuration factor is the favourable situation — it means the composite has ample free surface to flow from and can relieve shrinkage stress as it polymerises.
- D. The gingival margin of a Class V often does lie in dentine or cementum rather than enamel, and that genuinely does weaken the bond there — but it is a separate problem from the geometry of shrinkage stress, which is what the question asks about.
Note: Mitigation: place in small INCREMENTS rather than bulk, use a flowable or stress-absorbing liner at the base of the preparation, and consider soft-start curing. Note how this connects to the previous question — it is a further reason GLASS IONOMER suits a Class V, since it sets by an acid-base reaction and does not generate free-radical polymerisation shrinkage in the same way.
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