ABSTRACT
<p><b>OBJECTIVE</b>To determine the effect of composite restoration on reinforcement of weakened tooth structure and the possible mechanism.</p><p><b>METHODS</b>Sixty freshly extracted non-carious maxillary premolars were collected and divided into 6 groups with 10 specimen in each group. MOD cavities (buccolingual width: 2.8 to 3.2 mm; palatal cusp width: 2.0 mm; cusp height: 5.0 mm) were prepared individually. Group 1 was prepared and not restored (control). The other 5 groups were restored with silver amalgam alloy (group 2), Z250 without bonding (group 3), F2000 (group 4), Z250 (group 5) and Z350 nanocomposite (group 6) (3M ESPE) respectively. The fracture resistance of the tested teeth was determined by applying a vertical splitting load through a specially shaped steel rod at a crosshead speed of 1 mm/min. The data were analyzed by ANOVA.</p><p><b>RESULTS</b>The average fracture resistance of the 6 groups was: (245.29 +/- 39.49) N (group 1), (255.09 +/- 42.14) N (group 2), (267.34 +/- 31.56) N (group 3), (293.90 +/- 33.42) N (group 4), (337.81 +/- 32.63) N (group 5) and (349.08 +/- 32.93) N (group 6). There was no significant difference between the group 1, group 2 and group 3. The fracture resistance of group 4, group 5 and group 6 was higher than that of group 1 and group 2 (P < 0.05). Significant difference was noted between group 5 and group 3 (P < 0.01). The fracture resistance of group 4 was much lower than that of group 5 and group 6 (P < 0.01). No significant difference was found between group 5 and group 6.</p><p><b>CONCLUSIONS</b>The use of composite increased the fracture resistance of the tooth with an MOD restoration. This effect was related to the adhesive force, polymerization shrinkage stress and the elastic modulus of the composite.</p>