Shrinkage Stress Compensation in Composite-Restored Teeth: Relaxation or Hygroscopic Expansion?

Objectives: Polymerization of a composite restoration initially causes shrinkage, which deforms and thus stresses the tooth. This shrinkage has been shown to decrease in as little as four weeks. This study investigated whether shrinkage stress compensation occurred due to hygroscopic expansion or relaxation of the composite/tooth complex.

Methods: Extracted molars were mounted in rigid stainless steel rings with four spherical reference areas. Twelve molars were prepared with large mesio-occluso-distal slots, etched, bonded, and restored in two horizontal layers using Filtek Supreme composite (3M ESPE). Ten intact molars were used as controls. The mounted molars were digitized after preparation (baseline), restoration (0-week), and after 1, 2, and 4 weeks of storage. Half of the molars were stored in deionized water; half were stored in silicone oil. Following digitization, reference areas were fit with the baseline using Cumulus software. Tooth deformation was calculated from lingual and buccal volume change/area using Cuspflex software. Statistical analysis was performed with ANOVA followed by Student-Newman-Keuls post-hoc tests (p=0.05).

Results: Cuspal deflections (microns) are shown in the table (mean±standard deviation). Different letters indicate significant differences between groups (uppercase) or time intervals (lowercase). Intact teeth had minimal deformation, whereas polymerization shrinkage caused contraction of the restored crowns (negative deflections). Contraction decreased significantly after 4 weeks in water. Contraction also decreased when stored in oil, but not significantly.

 

	0-Week	1-Week	2-Weeks	4-Weeks
Restoration (water)	-13.4±2.6 A,a	-12.3±4.5 A,ab	-11.6±4.0 A,ab	-7.3±3.2 A,b
Restoration (silicone oil)	-14.1±4.4 A,a	-13.2±2.6 A,a	-15.0±3.1 A,a	-11.4±5.0 B,a
Control (water)	-0.4±0. 9 B,a	-0.9±0.7 B,a	-0.6±1. 1 B,a	2.1±1.3 C,b
Control (silicone oil)	-0.4±1.4 B,a	-0.3±1.6 B,a	-3.0±2.5 B,a	-1.6±1.1 C,a

Conclusions: Storing composite-restored teeth in oil diminished compensatory expansion. Thus, shrinkage stress compensation occurred mainly due to hygroscopic expansion.

Acknowledgment: Supported, in part, by the UT College of Dentistry Alumni Student Research Training Program.