Twenty freshly extracted mandibular second molars were used in this
study. The teeth were randomly divided into 2 groups according to the occlusal veneer material
(n =10 each). The first group in which the teeth were restored with occlusal veneers constructed of Celtra Duo while the second group, the occlusal veneer material was the Vita Enamic. Each group was further subdivided into 2 subgroups (n =5 each) according
to the preparation design either conventional occlusal reduction or reduction of the cclusal surface plus 1 mm axial reduction with rounded shoulder finish line. The vertical marginal gap was measured using stereomicroscope. Results: the results showed that the marginal gap of the two preparation designs of the two
materials were statistically non significant before ageing P≤ 0.05.while after ageing, the subgroups of Celtra recorded, 90.45±13.0 and88.7±11.60µm, for conventional and, rounded shoulder designs respectively, with no significant difference between the two designs, while the subgroups of Vita Enamic recorded 113±7.01µm and 105.2±8.01µm for conventional and, rounded shoulder designs respectively, with high significant difference p≤0.003. Conclusions: The choice of certain preparation design should be associated with meticulous
knowledge of the material of the occlusal veneers properties .