A maxillary model of the maxilla (Prosthetic Restoration Jaw Model, Nissin, Kyoto, Japan) with two abutments (second premolar and second molar) and a pontic space at the site of the first molar was used in this study. The soft tissue of the resin model was designed by the additional silicon specifically designed for this purpose (Gingival Mask, Feguramed GmbH, Buchen, Germany) (Figure 1 ). The finish line of the second premolar was located 0.5 mm subgingivally, and the second molar tooth was at the level of the gingiva. The prepared teeth had a chamfer finishing line.
The original model was scanned by an intraoral scanner, and a three-unit metal framework was designed using the CAD/CAM system and milled using a cobalt-chromium block. Next, the metal framework was seated on the original model, and its clinical adaptation was evaluated by a technician. A reference point with some distance from the margin was marked in the mesial, distal, lingual, and buccal surfaces of the abutment teeth not to damage the margin.
The soft tissue designed to simulate the gingiva was removed from the original model. Next, the metal framework was placed on the abutments in the original model with gentle finger pressure and fixed with the putty material. The vertical marginal misfit was then evaluated using a light microscope (SZX16, Olympus, Japan) at ×10 magnification under a direct LED lamp in the midbuccal, midmesial, and midlingual around the second premolar abutment as a standard (Figure 2 ). Considering the presence of a pontic space, it was not possible to measure the vertical marginal misfit at the distal surface of the second premolar and the mesial surface of the second molar. The vertical marginal misfit at the designated points was analyzed using software (Carl Zeiss AxioVision Microscopic Imaging Software Release 4.8, Germany).
According to a previous study [12 (link)], the minimum sample size was calculated to be 9 in each study group, using the two-samplet-test power analysis (SPSS 19), assuming alpha = 0.5, beta = 0.2, a mean difference of 13, and standard deviations of 10.25 and 7.06. In order to increase the reliability of our study, twenty-four quadrant impressions were made with additional silicon using the one-step putty/light body (#12) and two-step putty/light-body (#12) techniques. Prefabricated perforated plastic trays were used for impression making.
In the one-step technique, the putty and light body (Duo sil, Bukwang, Busan, Korea) were used simultaneously. Both putty and light body were mixed simultaneously according to the manufacturer's instructions. The putty material was applied into a tray, and the light body was injected directly around the abutments using an automixing gun dispenser. The tray was placed on the cast and kept in place with hand pressure for 10 minutes. The impression was then removed from the cast. In the two-step putty/light-body technique, a putty impression was made. For this purpose, the putty material was prepared according to the manufacturer's instructions and applied to the tray. The tray was placed on the original model with hand pressure and compressed for 10 minutes until the material set and was removed from the cast. The putty was cut out by 2 mm at the marginal area of the second premolar, and the light body was then injected around the abutment teeth. The putty impression was placed again on the model, and a 12-minute time was allowed to set.
All impressions were kept at 25°C temperature for one hour prior to pouring. They were then poured with type IV dental stone (Welmix G30, Asia Chemi Teb, Tehran, Iran). To prepare the dental stone, 50 g of gypsum was mixed with 10 mL of water according to the manufacturer's instructions. It was first mixed manually and then placed on the auto-mix vacuum (Auto mix II, KFP-Dental, Tehran, Iran) in order to eliminate voids. The impressions were poured, one-hour time was allowed for the primary setting, and the casts were separated from the impressions. After 24 hours, the casts were evaluated under a light microscope to assess the vertical marginal misfit.
Vertical marginal misfit was evaluated for each of the 24 plaster casts three times. The amount of vertical marginal misfit in each of the measures was subtracted from the vertical marginal misfit of the original model in the two impression techniques. The mean and standard deviation values were calculated and analyzed using the independent t-test.
The original model was scanned by an intraoral scanner, and a three-unit metal framework was designed using the CAD/CAM system and milled using a cobalt-chromium block. Next, the metal framework was seated on the original model, and its clinical adaptation was evaluated by a technician. A reference point with some distance from the margin was marked in the mesial, distal, lingual, and buccal surfaces of the abutment teeth not to damage the margin.
The soft tissue designed to simulate the gingiva was removed from the original model. Next, the metal framework was placed on the abutments in the original model with gentle finger pressure and fixed with the putty material. The vertical marginal misfit was then evaluated using a light microscope (SZX16, Olympus, Japan) at ×10 magnification under a direct LED lamp in the midbuccal, midmesial, and midlingual around the second premolar abutment as a standard (
According to a previous study [12 (link)], the minimum sample size was calculated to be 9 in each study group, using the two-samplet-test power analysis (SPSS 19), assuming alpha = 0.5, beta = 0.2, a mean difference of 13, and standard deviations of 10.25 and 7.06. In order to increase the reliability of our study, twenty-four quadrant impressions were made with additional silicon using the one-step putty/light body (#12) and two-step putty/light-body (#12) techniques. Prefabricated perforated plastic trays were used for impression making.
In the one-step technique, the putty and light body (Duo sil, Bukwang, Busan, Korea) were used simultaneously. Both putty and light body were mixed simultaneously according to the manufacturer's instructions. The putty material was applied into a tray, and the light body was injected directly around the abutments using an automixing gun dispenser. The tray was placed on the cast and kept in place with hand pressure for 10 minutes. The impression was then removed from the cast. In the two-step putty/light-body technique, a putty impression was made. For this purpose, the putty material was prepared according to the manufacturer's instructions and applied to the tray. The tray was placed on the original model with hand pressure and compressed for 10 minutes until the material set and was removed from the cast. The putty was cut out by 2 mm at the marginal area of the second premolar, and the light body was then injected around the abutment teeth. The putty impression was placed again on the model, and a 12-minute time was allowed to set.
All impressions were kept at 25°C temperature for one hour prior to pouring. They were then poured with type IV dental stone (Welmix G30, Asia Chemi Teb, Tehran, Iran). To prepare the dental stone, 50 g of gypsum was mixed with 10 mL of water according to the manufacturer's instructions. It was first mixed manually and then placed on the auto-mix vacuum (Auto mix II, KFP-Dental, Tehran, Iran) in order to eliminate voids. The impressions were poured, one-hour time was allowed for the primary setting, and the casts were separated from the impressions. After 24 hours, the casts were evaluated under a light microscope to assess the vertical marginal misfit.
Vertical marginal misfit was evaluated for each of the 24 plaster casts three times. The amount of vertical marginal misfit in each of the measures was subtracted from the vertical marginal misfit of the original model in the two impression techniques. The mean and standard deviation values were calculated and analyzed using the independent t-test.
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