The design of the preparation for composite inlays/onlays could influence the prognosis of the restoration. Objective: This study was carried out to determine the tensile stresses in different cases of cavity preparation for composite inlay/onlays, which develop at marginal area when subjected to vertical and horizontal loads. Methods: Finite element models of upper first molar with different type of preparation were generated. As a composite material for indirect restoration Sculpture (Jeneric Pentron) was used. The load of 100 N was applied.vertically. The following three exercises with FEM (Ansys 5,3 Software) were studied: - inlays with different width of remaining vestibular/palatal walls (1,5mm and 3,0mm), - onlays with different type of cusp coverage (shoulder 0,75mm; shoulder1,5mm), - inlays with different preparation of occlusal margin (coincide or not coincide with occlusal contacts). Results: In case of inlays with 3,0mm width of remaining walls the maximal recorded tensile stress was 31,5 MPa and in case of 1,5mm width – 34,2 MPa. In case of onlays with 0,75mm shoulder, maximal tensile stress of 34 MPa was recorded and in case of 1,5mm shoulder – 15,6MPa. In case of inlays, where occlusal margin coincided with occlusal contacts maximal tensile stress of 20,7 Mpa was recorded and in case of no occlusal contacts – 13,6 Mpa. Conclusions: The restoration margins should not coincide with occlusal contacts. In case of unsufficient width of remaing walls, cuspal coverage should be taken into consideration. In case of cuspal coverage, shoulder preparation should be sufficiently thick.

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