Axial shrinkage-stress depends upon both C-factor and composite mass

Objectives To measure and then mathematically model polymerization stress-dependence upon systematic variations of C-factor (bonded/unbonded area ratio) for the Bioman instrument [1], recording stress by free cantilever-beam deflection; compliance 1.5 μm/MPa. Methods A light-cured resin-composite (RZD103; Ivoclar) with 57% (v/v) 450 nm filler was studied. Facing surfaces: glass slab and steel rod-end, constituting the Bioman test chamber, being perpendicular to the measured axial stress-direction, were varied: (a) with rod-diameters (phi), from 1 to 10 mm in 1 mm increments (with 0.8 mm gap height); and then (b) with gap heights (h) in 16 steps from 0.05 to 1.50 mm (with phi = 10 mm). For each h and phi combination, giving C-factors ranging from 0.6 to 100, shrinkage-stress was recorded for 1 h from start of 40 s irradiation at 600 mW cm−2 for photo-polymerization at 23 °C (n = 3). Shrinkage-stress (Sσ) was plotted directly as functions of h, phi, and C and also per unit composite mass, (Sσg−1). ANOVA and Tukeyʹs statistics were applied. Results Series A—diameter variation; with C-factor increasing from 0.6 to 6, gave an exact exponential decrease in Sσ from 45 to 8 MPa. Series B—height variation; with C-factor increasing from 3 to 100, gave increasing Sσ from 1 to 8 MPa. Since composite mass played an equally dominant role, plots of stress-variations per unit composite mass, (Sσg−1) separated these effects, confirming progressive off-axial stress-relief with increasing h. Significance (i) Values of h = 0.8 and phi = 10 mm, recommended [1] for Bioman use, were confirmed as appropriate. Every lab instrument for measuring Sσ necessarily embodies specific C-factors and compliance values in the instrument design. (ii) Configuration (C) factor is recognized as an important parameter affecting manifestation of shrinkage-stress within restorative cavities and luting gaps. However, the restorative mass must equally be considered when translating shrinkage-science into specific clinical recommendations.