Effects of water-sorption on creep of resins and macrohybrid layers

Objective: To compare creep (μm) and creep moduli rates (stress/strain/sec) of neat resin beams versus resin-infiltrated beams of demineralized dentin (i.e. macrohybrid layers) under dry conditions vs. 2-wk water sorption.  Methods: Creep was measured by 3-point flexure using the LVDT of a TMA instrument.  Specimen beams 7x1x0.3 mm were created from experimental neat methacrylate blends 1, 2 or 3 or from coronal dentin that was completely demineralized prior to being saturated with ethanol and infiltrated in 2 hr increments in 50% resin/50% ethanol followed by 100% resin, then light-cured.  Deformation was measured at 1% strain while specimens were dry, then repeated after immersion in water for 2 wks at 37°C.  Results: Neat resins had similar dry creep values, with resins 2 and 3 having significantly higher values while wet.  There were no significant differences in the creep values of dry or wet macrohybrid layers (MHL).  The creep rates of neat resins 1 and 2 were similar wet or dry, but were significantly lower in wet resin 3.  All MHL had significantly lower creep rates while wet.   

		Neat Resins			Macrohybrid Layers
		1 (BisGMA/ TEGDMA)	2 (BisGMA/HEMA)	3 (BisGMA/ HEMA/2MP)	1	2	3
Dry	Creep (μm)	4.6±2.0a,b	3.6±1.0b	5.5±1.1a,b	6.5±3.2†	8.0±2.1†	9.0±6.1†
Wet	Creep (μm)	5.2±1.5a	6.1±2.2a	8.7±1.7c	9.7±3.9†	10.7±0.8†	12.5±4.8†
Dry	Creep rate (GPa/s)	5.0±0.3A	5.2±0.5A	5.3±0.9A	5.5±0.3*	6.6±0.8**	7.3±0.4***
Wet	Creep rate (GPa/s)	5.6±0.5A	5.0±0.5A	2.7±0.3B	2.2±0.2****	2.1±0.2****	1.75±0.3****

Different superscript symbols indicate p<0.05.  Conclusion: Immersion in water significantly decreases creep rates in wet macrohybrid layers.  While studying neat resins alone, only resin 3 (the most hydrophilic) showed a significant decrease in creep rate after water immersion.  Supported by R01-DE014911 from the NIDCR (DP, PI).