Statement of problem:Although endocrowns have proven their clinical success in many situations, the intra-chamber endocrown extension and the proportion between the clinical crown height and the radicular stump and their effects on their fracture resistance and mode of failure have not yet been verified.Purpose: Assess the effect of intra-chamber extension of lithium disilicate endocrowns on the fracture resistance and failure modes in endodontically treated molars after thermo-mechanical loading.Material and Methods: Forty extracted molar teeth were selected and divided into 5 groups (n=8) as follows: unprepared natural teeth (NAT; control); post and core supported crowns (PCC); teeth with 4 mm intra-chamber extension (EC4); 2 mm intra-chamber extension (EC2); 1 mm intra-chamber extension (EC1). Endocrowns were subjected to 5000 thermal cycles then 1,200,000 chewing cycles. Surviving specimens were vertically loaded in a universal testing machine until fracture. Data were analyzed by 1-way ANOVA, followed by pair-wise comparaisons with the Bonferroni post-hoc test (p ≤ 0.05). The mode of failure was determined by visual inspection.Results: Natural teeth had statistically significantly lower fracture resistance than groups with crowns (p= 0.002). However, there were no statistically significant differences between the crown groups with mean fracture resistance ranging from 2502.5 N to 2843.8 N. Endocrowns exhibited mainly unfavorable fracture failure. The amount of teeth destruction related to the group EC4 was less than that related to the group EC1. Conclusions:Endocrowns with greater intra-chamber extension provided insignificantly higher fracture resistance but more protection of underlying tooth structure than endocrowns with lesser extension.

The effect of nitrogen fertilization levels (60, 75, 90 and 105kg N/fed) and sources (urea and Ammonium sulphate) on yield and its components of two wheat varieties (Giza 164 and Sakha 69) was investigated at the Agriculture Experimental and Research Center, Faculty of Agriculture, Cairo University at Giza, Egypt during 1996/97 and 1997/98 seasons .Differences in grain and straw yields as well as their related variables due to changing N levels were significant in both seasons. Application of the highest N level (105 kg N/fed) produced 2.3 (13.4%) and 1.8 ard/fed (10.2%) more than the application of N at the recommended dose (75 kg N/fed) in the first and second seasons, respectively.With regard to the N sources, the highest values resulted from application of N as urea, while the lowest one produced by ammonium sulphate in both seasons.Varieties had a significant effect on plant height, number of grains/spike and grain yield/fed in both seasons and number of tillers/m 2 , number of grains/spike and 1000-grain weight in one season only. As an average of both seasons, Sakha 69 outyielded Giza 164 in grain yield by 1.6 ard/fed and in straw yield by 0.6 ton/fedThe interaction of varieties x N levels had a significant effect on plant height, and 1000-grain weight in both seasons and on spike length and number of grains/spike in one season. Moreover, varieties x N levels x N sources interaction had significant effect on either grain or straw yield/fed in both seasons. As an average of both season the highest grain yield/fed (26.55 ard) was obtained by using Sakha 69 variety and 105 kg N/fed as urea.Consequently, it could be concluded that under the conditions of this study the highest wheat production from either grain or straw can be produced with using Sakha 69 variety and application of 105 kg N/fed as urea.