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ePaper created 2015-03-06, 14:42:46 | version 1.33.01
to fracture a tooth was the same if the restoration involved only the occlusal surface or included the mesial and distal surfaces as well. Geurtsen, Schwarze and Gunay (2003) agreed that the risk of cuspal fracture increases con- siderably when the isthmus width of a restora- tion is 50 per cent of the intercuspal distance. They stated that amalgam or resin composite restorations should not exceed one-fourth to one-third of the intercuspal distance. The more tooth structure that is removed in cavity prepara- tions, the more the tooth flexes under increasing loads.1 Teeth with cuspal fractures may still be restored; however, the prognosis will be lower and less than ideal because there is less remaining natural struc- ture to retain a crown and withstand the flexing from functional and non-functional forces. These teethmaylastforyears.However,theymayeventu- ally fracture at the gingival crest or below, because of further cracks and propagation of those cracks. Teeth with history of endodontic treatment are at an increased risk of subgingival fracture, render- ingthetoothnon-restorableorwithapoorprogno- sis.2 Therefore,it’simportanttopreventthesecracks from forming at all. I 13 CE article _ biomechanical principles I CAD/CAM 1_2015 Figs. 2a–c_Depth cut bur #3. Fig. 3a_Final depth cuts. Fig. 3b_Final depth cuts, occlusal view. Figs. 4 & 5_Gross occlusal reduction with KS7 #3. Figs. 6 & 7_Gross occlusal reduction with KS7 #4. Fig. 8a_Final occlusal reduction frontal view. Fig. 8b_Final occlusal reduction occlusal view. Fig. 7 Fig. 4 Fig. 2c Fig. 8bFig. 8a Fig. 2bFig. 2a Fig. 3a Fig. 3b Fig. 5 Fig. 6 CAD0115_12-21_Young 02.03.15 12:38 Seite 2 CAD0115_12-21_Young 02.03.1512:38 Seite 2