In dentistry, the configuration factor(C-Factor) was first introduced by Davidson CL,1986, and later quantitavly extended Feilzer et al. in 1987 and refers to the number of bonded surfaces to the number of un-bonded surfaces in a dental restoration. For example, for a class I preparation there would be 5 bonded surfaces and only 1 un-bonded surface; the net result would be a c-factor of 5. With an increasing C factor the developing curing contraction stresses of bonded restorations (resin composites) increases too (Feilzer et al. 1987). The developing curing contraction in a bonded restoration generate stress on the bonded interface that are in competition with the developing bond strength of the setting restorative to the cavity surfaces, which may result in (partial) debonding, marginal leakage and post-operative pain (Davidson et al. 1984).
Internal stress can be reduced in a restoration subject to potentially reduce high disruptive contraction forces by using:
- "soft-start" polymerization instead of high-intensity light curing
- incremental layering to reduce the effects of polymerization shrinkage; and
- a stress-breaking liner, such as filled adhesive, flowable composite, or resin-modified glass ionomers
- the application of non or low shrinking restorative materials
In case of adhesive inlays such as indirect composite or ceramic inlays, the C-factor of a class II cavity luted with resin cement will reach 207.74 ± 37.98 with a discrepancy of 200 mm (space for the cement). The clinical performance of these restorations has still not been understood considering their unfavorable C-factor. The probable reason may be the inclusion of voids/porosity in the cement that may lower the C-factor. Another reason may be that the C-factor may be more favorable at the cavosurface margins where flow can somewhat compensate for the polymerization shrinkage of the setting cement. In case of dual cure cements, this flow is blocked by the light curing of the margins immediately after placement. The restoration can thus be said to be held in place mainly by a collar of superficial adhesive luting cement.