Polymerization contraction stress as a function of C-factor

Objective: Polymerization contraction stress (PCS) is determined, among others, by the configuration of restoration. The configuration factor (C-factor) is often used to predict the trend of PCS. A higher PCS is commonly believed to be associated with a higher C-factor, or vice versa. This work was designed to find out if such connection can be generalized and, if not, why.

Methods: A flowable composite (FC) was placed in a cavity formed by an aluminum ring bond to a pliable surface. A strain gage was attached to the ring to measure its shrinkage caused by the contraction of curing composite. Circular inserts of different materials (glass, a hybrid composite, and the FC) and sizes were positioned at the center of cavity to adjust the volume of FC. This varied its C-factor, calculated by dividing the area bonded to the ring and insert by the unbonded area between the two. The FC was irradiated with a halogen curing unit for 30s at 500 mW/cm². The measured strain was converted to PCS based on the configuration and stiffness of the ring material.

Results: Measured PCS (MPa) *

Insert material	Modulus GPa	C-factor
		0.65	1.8	2.7	10	20
None	-	> 20**
Glass	70			19.7 (1.8) a)	17.4 (1.3) a)
Hybrid	15		13.9 (1.0) b)			8.0 (0.5) c)
FC	3		13.6 (1.0) b)			7.9 (1.6) c)

* Numbers with different superscripts are statistically different (p<0.05).

** The PCS exceeded the bond strength that debonding occurred.

The results show that, in these cases, a restoration with a higher C-factor could produce a lower PCS, which contradicts the prediction from the C-factor. They also suggest that, in addition to the C-factor, the size and modulus of substrates (ring and insert) affect the magnitude of PCS as well.

Conclusion: The C-factor approach may fail to correctly predict the PCS when the compliance of substrate makes a significant contribution.