The aim of this study was to determine the values of the retentive forces of the telescopic crowns. At the first step of the study, the samples having different heights and angles were compared in relation to retentive force. At the second step, the samples were subjected to removal procedures having a pre-determined speed, force and number. The obtained retention values were compared. Ninety dies were prepared in order to measure the retention of the conus crowns. The removal tests were performed with the insertion/separation device. The speed of the test device was 100 cm min(-1) providing the placement of the pieces with 5 kgf in 100, 200, 300, 500, 1000, 5000 and 10 000 cycles. The mean of the retention values of the changes of the angles were found to be statistically different (P < 0.01). According to the results of the variant analysis, the differences in the changes of the height were statistically significant (P < 0.01). The differences between the changes of the number of procedures of all groups were statistically significant (P < 0.01). The number of procedures, height and angle interactions were statistically insignificant (P = 0.758). According to the results of this study, the effect of the angle and height factors in terms of retention among the groups were not independent of each other. In equal angles, as the height increased, increased retention measurements were obtained. In more acute angles, the increase in retention was more obvious. It was also found that an accessory device was necessary to maintain retention capabilities after a long-term use.

Cervical finish line type has an influence on the marginal adaptation of Y-TZP restorations. Both shoulder and mini-chamfer exhibited the least marginal opening values for zirconia crowns and can be recommended for clinical applications. SUMMARYThe current study evaluated the effect of different cervical finish line designs on the marginal adaptation of a zirconia ceramic. Four different marginal finish lines (c: chamfer, mc: mini-chamfer, fe: feather-edge and s: rounded shoulder) were prepared on phantom incisors. Die models for each preparation group (N=28, n=7 per finish line design group) were made of epoxy resin. Y-TZP (ICE Zirkon) frameworks were manufactured by a copy-milling system (Zirconzahn) using prefabricated blanks and tried on the master models for initial adaptation of the framework; they were then sintered, followed by veneering (Zirconzahn). The finished crowns were cemented with a polycarboxylate cement (Poly F) under 300 g load and ultrasonically cleaned. The specimens were sliced and the marginal gap was measured, considering absolute marginal opening (AMO) and marginal opening (MO) for each coping under a stereomicroscope with image processing software (Lucia). The measurements were statistically analyzed using the Kruskal Wallis, Mann Whitney and Wilcoxon Signed Ranks tests at a significance level of α α=0.01. Means of AMO measurement (µm) for the feather-edge finish line (87 ± 10) was significantly lower than that of the chamfer (144 ± 14), shoulder (114 ± 16) and mini-chamfer finish line types (114 ± 11) (p<0.01). Means of MO measurements was the lowest for feather-edge finish line (68 ± 9) (p<0.01) and then, in ascending order, shoulder (95 ± 9), mini-chamfer (97 ± 12) and chamfer (128 ± 10). The cervical finish line type

SUMMARYThis study evaluated the stress distribution on an endodontically treated maxillary central incisor restored with different post-and-core systems by using a three-dimensional finite element analysis model. Seven three-dimensional finite element models were created. Each model contained cortical bone, cancelous bone, periodontal ligament, 3 mm apical root canal filling, post-andcore and all-ceramic crowns. Two different prefabricated zirconia ceramic post systems, a glass fiber-reinforced post system and a titanium post system were modeled. As a control, an all-ceramic crown on an endodontically treated maxillary central incisor without a post-and-core was modeled. Each model received a 45°oblique occlusal load at a constant intensity of 100 N. In each model, the ratio of Von Mises stress distribution was compared. The greatest stresses were observed in the coronal third of the roots on facial surfaces. The ratio of Von Mises stress distribution in dentin for the zirconia ceramic post According to FEA analysis, under a simulated occlusal load, the cervical area of endodontically treated and crowned maxillary central incisors was the most stress concentrated area. Zirconia ceramic posts create slightly less dentinal stress concentration than titanium and glass-fiber posts. 90Operative Dentistry stress concentrations in dentin created by two different zirconia ceramic post systems were nearly the same. The zirconia ceramic post systems created slightly less stress concentration in dentin than the glass fiber-reinforced and titanium posts.

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The removable partial dentures supported by the telescopic crowns are an alternative for directly retained removable partial dentures. The stress distribution on the retainers and the surrounding tissues created by the telescopic and conus crowns of different sizes (4, 5, 6 mm) and taper (0 degrees, 2 degrees, 4 degrees, 6 degrees ) was investigated with the finite element analysis (FEA) method. The stress values obtained were evaluated either as strain or tensional forces. The loosening force of the secondary crown being determined as 5 N, the increase in tension of the dentine, metal structure, alveolar bone, periodontal ligament and the pulp were determined by the increasing height and taper. The reason for the increase in tensional forces with increasing taper was the result of the constant loosening force of 5 N applied in all experimental models. The strain was more effective than the tension with the highest stress being in the cervical region of the metal structure. The aim of this study was to determine the force exerted on the teeth and surrounding tissues by the loosened secondary crown.

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