Objective: In this in vitro study, the resistance to fracture of occlusal enamel supported by a bonded tooth-colored restorative material was compared to that of unsupported enamel and enamel supported by sound dentin. Methods: 80 extracted human lower molars were sorted into 5 groups of 16 teeth each. Lingual cusps were removed. In four groups, the dentin was removed from the facial cusps leaving a shell of enamel. In Gp 1, dentin was not removed. Gp 2 remained unrestored. The groups in which a restorative material was inserted to replace missing dentin were as follows: Gp 3 - bonded resin composite (Scotchbond MP/Z250 [A2], 3M); Gp 4 - resin-modified glass-ionomer (Fuji 2 LC [A2], GC), Gp 5 - conventional glass ionomer (Fuji IX [A2], GC). Specimens were thermocycled (1500 cycles, 6^o–60^o C, dwell 30 sec.), then mounted in die stone with lingual inclines of facial cusps horizontal. The lingual inclines were flattened slightly using a horizontally-mounted separating disk. Specimens were loaded evenly on flattened inclines in an Instron with a flat rectangular rod, crosshead speed 5 mm/min. Results: Data analysis was w/ one-way ANOVA and Student-Newman-Keul's test (F=50.30, p <0.0001).

Gp	Description	Mean Failure Load (KN)	SD
1	Supported by dentin	2.880*	0.965
2	Unsupported	0.615**	0.206
3	Bonded resin composite	1.199***	0.303
4	Resin-Modified GI	1.093***	0.248
5	Conventional GI	1.054***	0.298

Restorations provided significantly less enamel support than natural dentin. Conclusion: There was no difference in the support provided by the three restorative materials. Research supported by Student Training Fellowship NIH grant DE07160.

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