The purpose of this study was to evaluate the fracture strength of porcelain laminate veneer with various lingual extention length when incisal restoring. Sixty recently extracted, intact maxillary incisors were used and stored in a physiologic saline solution from the time of extraction. Seating form was preparation at the labial surface of each tooth with a water-cooled round diamond bur. Standard block was formed with 32 gauge and 24 gauge wax at tooth labial and lingual surface.
Lingual extention length differed according to each group. (group Ⅰ：0.5㎜, group Ⅱ：1㎜, group Ⅲ：2㎜, group Ⅳ：0㎜) All tooth specimens were impressioned with examix(GC Inc., Japan). Refractory cast were maked with refractory die material(Ceramco Inc.,U.S.A.) Laminate porcelain (Ceramco Ⅱ Veneer porcelain, Ceramco Inc.,U.S.A.) was condensed in refractory die cast and baked according to the manufacturer´s recommendations. Each surface was contoured with low speed diamond bur according to guide block. All porcelain specimens were sandblasted and ultrasonically cleaned in distlled water for 3 minutes. then, all porcelain specimen were etched with 8% hydrofluoric acid for 5 minutes. Sixty specimens were bonded with composite resin cement(Choice Porcelain Veneer System, Bisco Inc., U.S.A.) according to manufacture´s directions. The fracture loads of the specimen were measured by Instron universal testing machine. The mean values of fracture loads for the groups were statistically compared by Duncan´s multiful range test. The result were as follows： Mean fracture strengths of each group were 86.95㎫ in no lingual extention group, 44.98㎫ in 0.5㎜ lingual extention group, 27.47㎫ in 1㎜ lingual extention group, 19.61㎫ in 2㎜ lingual extention group. There was a statistically significant difference between all group(p＜0.01).
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