An in vitro Investigation of the Effects of Glass Inserts on the Effective Composite Resin Polymerization Shrinkage

We placed an MOD preparation in each of 12 permanent molars, then restored each tooth with a posterior composite resin by means of six different application techniques (I-polymerization as one complete unit; II-polymerization as one complete unit with glass inserts; IIIpolymerization in gingivo-occlusal increments; IV-polymerization in gingivo-occlusal increments with glass inserts; V-polymerization in bucco-lingual increments; and VI-polymerization in a gingival increment with glass inserts, then bucco-lingual increments). A precision strain gauge was attached to the buccal surface of each tooth and balanced at zero. After each increment was polymerized, the strain appearing on the strain gauge indicator was recorded. Each tooth was restored by use of all techniques; two teeth started with each technique. Results demonstrated the average microstrain units to be 127-1, 102-11, 105-III, 86-IV, 72-V, and 66-VI. A randomized block design was the format used for data evaluation. Scheffé's Test indicated that composite resin placement and polymerization in bucco-lingual increments (V) created significantly less cuspal deflection than polymerization as one complete unit, with or without glass inserts (I and II), p<0.001, and gingivo-occlusal increments (III), p<0.05. Placement and polymerization in a gingival increment with glass inserts, then bucco-lingual increments (VI), also created significantly less internal deflection than polymerization as one complete unit, with or without glass inserts (I and II), p<0.001, and gingivo-occlusal increments (III), p < 0_005.