Effect of Refrigerated Storage on Bond Strengths of Self-Etch Adhesives

Many self-etch adhesives require refrigerated storage per manufacturer's instructions.  Objective: The purpose of this in vitro study was to compare the shear-bond strength of a composite (Z250, 3M/ESPE) to dentin using a two-step (Clearfil SE, Kuraray) and a one-step (iBond, Heraeus-Kulzer) self-etch adhesive after one- or four-month storage at room or refrigerated temperatures.  Methods: The coronal enamel of 80 extracted human third molars was removed (Isomet, Buehler).  The teeth were mounted in PVC pipe with dental stone, and randomly divided into 8 groups of 10 each (n=10).  Both adhesive types were stored at room (23șC) or refrigerated temperatures (5șC) for one or four months before testing.  To simulate clinical practice, the refrigerated groups were cycled between 8-hours of room temperature and 16-hours at refrigerated temperature.  After each storage time period, the adhesives were placed onto the sanded dentinal surface according to manufacturer instructions.  A delrin insert (Ultradent) was placed over the dentin and the composite was condensed into the insert in two increments.  The composite was light-cured for 40 seconds per increment (Optilux 501, SDS/Kerr).  Specimens were stored in 100% humidity for 24 hours at intraoral temperature (37șC) before testing in shear on a universal testing machine (Model 5543, Instron) at a crosshead speed of 1 mm/min.  Mean shear-bond strengths and standard deviations were determined for each group.  See table.  The data was analyzed with a 3-way ANOVA (alpha=0.05).  Results: A significant difference was found based on bonding agent (p=0.023) and time (p=0.037), but not storage conditions (p=0.225) with no significant interactions (p>0.19).  Conclusions: Clearfil SE had significantly greater shear bond strength than iBond.  Shear bond strengths were significantly lower at four months compared with one month.  There was no significant difference in bond strengths between refrigerated or room temperature storage for the two adhesives.

	Shear Bond Strength  MPa (st dev)
Adhesive Agent	One Month		Four Months
	Refrigerated	Room	Refrigerated	Room
Clearfil SE	24.7 (5.3)	22.3 (4.2)	22.4 (6.1)	22.3 (3.9)
iBond	22.0 (2.2)	22.3 (3.6)	20.3 (4.9)	17.5 (5.5)