All ceramic restorations are known for needing high circular preparation removal to achieve enough mechanical strength. This is true for silicate all ceramic systems but there does not exist a evidence based preparation rule for zirconia-frame-based restoration. Objectives: Different preparation designs should be tested in term of mechanical strength. Methods: 50 crowns were manufactured by a CAD/CAM-system working with presintered zirconia (Everest, KaVo EWL/Leutkirch, Germany) using the following 5 preparation designs: Tangential, discreet chamfer, accentuated chamfer, step and bevel shaped step. All the sharp edges were removed. Ever zirconia frame was manufactured on an own master die and was 0.4 mm thick. They were adapted until their best possible marginal fit by an experienced dental technician using the stereo microscope. The crowns were fixed on a metal cast by glas ionomer cement. After 24 hours the restorations were tested till their failure in the testing machine (Zwick/Ulm, Germany). The fracture parts were examined by the scanning electron microscop. Results: The tensile strength of the distinct chamfer crowns was 1208 N and 1905 N for the discreet chamfer restoration. The tangential preparation showed an average of 2166 N the step 1763 N and the bevel shaped step1266 N. Discussion: The best performance of the tangential praparated crowns is easy to explain. The cement is a kind of buffer and if the force on the crowns is increased the cement is compressed and the restoration moves further in the apical direction. The other preparation designs with their border stop the crown from moving. The result is the crowns break earlier. To proof this another cement should also be tested. Conclusion: A tangential preparation is obsolete in terms of periodontical reasons not in technological ones. But the huge circular removal described for silicate ceramic systems is not necessarry for zirconia based restorations.

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