ABSTRACT
Statement of the Problem: Marginal fitness is the most important criteria for evaluation of the clinical acceptability of a cast restoration. Marginal gap which is due to cement solubility and plaque retention is potentially detrimental to both tooth and periodontal tissues
Purpose: This in vitro study aimed to evaluate the marginal and internal fit of cobaltchromium [Co-Cr] copings fabricated by two different CAD/CAM systems: [CAD/milling and CAD/ Ceramill Sintron]
Materials and Method: We prepared one machined standard stainless steel master model with following dimensions: 7 mm height, 5mm diameter, 90[degree] shoulder marginal finish line with 1 mm width, 10[degree] convergence angle and anti-rotational surface on the buccal aspect of the die. There were 10 copings produced from hard presintered Co-Cr blocks according to CAD/ Milling technique and ten copings from soft non- presintered Co-Cr blocks according to CAD/ Ceramill Sintron technique. Marginal and internal accuracies of copings were documented by the replica technique. Replicas were examined at ten reference points under a digital microscope [230X]. The Student's ttest was used for statistical analysis. p< 0.001 was considered significant
Results: Statistically significant differences existed between the groups [p< 0.001]. The CAD/milling group [hard copings] had a mean marginal discrepancy [MD] of 104 micro m, axial discrepancy [AD] of 23 micro m and occlusal discrepancy of 130 micro m. For CAD/Ceramill Sintron group, these values were 195 micro m [MD], 46 micro m [AD], and 232 micro m [OD]. Internal total discrepancy [ITD] for the CAD/milling group was 77 micro m, whereas for the CAD/Ceramill Sintron group was 143 micro m
Conclusion: Hard presintered Co-Cr copings had significantly higher marginal and internal accuracies compared to the soft non-presintered copings
ABSTRACT
Statement of the Problem: One disadvantage of cement-retained crowns is the lack of predictable irretrievability. This problem can be overcome through designing a screw access hole in the metal substructure of cement-retained restoration and using porcelain stain to define this area
Purpose: This study aimed to evaluate the influence of existence of screw access hole on porcelain fracture resistance of metal-ceramic implant-supported crowns
Materials and Method: Thirty six standardized metal-ceramic crowns were fabricated and divided into 3 groups [n=12]; group 1 conventional cement-retained metalceramic crowns as control group, group 2 cement-retained MC crowns in which porcelain stain was used to define the location of screw access channel, and group 3 cement-retained metal-ceramic crowns in the metal substructure of which a hole and ledge was designed in the location of screw access channel. The specimens were cemented [TempBond, Kerr] to their dedicated abutments. A hole was made in the location of screw access channel in group 2 and 3 and filled with photo-polymerized composite resin [3M; ESPE]. All specimens were thermocycled and loaded in universal testing machine at crosshead speed of 2mm/min until fracture. Mean values of load at fracture were calculated in each group and compared with One-way ANOVA [alpha=0.05]
Results: Mean value of the load required to fracture the restorations was 1947 +/- 487 N in group 1, 1927 +/- 539 N in group 2, and 2170 +/- 738 N in group 3. No statistically significant difference was found between the fracture resistance values of the three groups [p> 0.05]
Conclusion: Presence of screw access channel in cement-retained implant restorations does not compromise fracture resistance