A Comparison of Dimensional Accuracy of Addition Silicone of Different Consistencies with Two Different Spacer Designs - In-vitro Study.
INTRODUCTION
Dimensional accuracy of impression materials is crucial for the production of working casts in Fixed Prosthodontics. The accurate replication of tooth preparations and their arch position requires impression materials that exhibit limited distortion.


METHODS
This study was conducted to comparatively evaluate the dimensional accuracy of additional silicones by comparing two different techniques and spacer designs, by measuring the linear changes in interpreparation distance. The impressions were made from a stainless steel master die simulating a three unit bridge. A total 80 die stone (type IV, Ultrarock) models were obtained from the impressions made using two different parameters. The two different parameters are Multimix and Monophasic technique and different spacer designs.


RESULT
The interpreparation distance of the abutments in the casts was measured using a travelling microscope. Each sample was measured thrice and the mean value was calculated. The results obtained were statistically analysed and the values fall within the clinically acceptable range.


CONCLUSION
The most accurate combination is multimix technique with spacer design which uses less bulk of impression material.
The effect of tray selection, viscosity of impression material, and sequence of pour on the accuracy of dies made from dual-arch impressions.
STATEMENT OF PROBLEM
Dual-arch trays are often used to generate impressions of prepared teeth and of the opposing arch simultaneously. There is concern that accuracy of the casts generated with this technique can be affected by the type of tray, viscosity of the impression material, and sequence of pouring the cast.


PURPOSE
This study compared the accuracy of working dies made from impressions with metal and plastic dual-arch trays, for 2 different viscosities of impression tray material and by altering which side of the impression was poured first.


MATERIALS AND METHODS
Impressions were made of a typodont mandibular arch containing a circular stainless steel crown preparation (standard). There were 3 variables: type of dual-arch tray, impression material viscosity, and order of pour of the impressioned arches. A balanced design with independent samples was used (n=10). Two types of dual-arch trays, plastic (Triple Tray) and metal (COE Impression Tray), and 2 viscosities of addition silicone for the tray were used (Aquasil Rigid and Aquasil Monophase). Type IV gypsum (Fuji-Rock) with a ratio of 20 mL of distilled water to 100 g of powder was hand-mixed for 10 seconds then mixed under vacuum for 40 seconds and poured into the trays while being vibrated. One side of the dual-arch impression was poured with 35 g of stone and allowed to set for 1 hour before the other side was poured with 35 g of stone. The order of pour was randomized, and all casts were allowed to set for 24 hours at room temperature before removal. The dies were measured in 3 dimensions (buccolingual, mesiodistal, and occlusogingival) with a measuring microscope. The gypsum working dies were placed into a custom jig fabricated to permit measurement at a fixed, reproducible position under the microscope. Each dimension of the working dies was measured 3 times, and the mean was used for the sample value. The same 3 aspects of the stainless steel standard were measured multiple times, before and then at the conclusion of measuring all working dies, to arrive at the 3 standard values to which all working die means were compared. The means for the standard used in the statistical analysis were those taken at the conclusion of the study. The intraexaminer variation for measuring the standard was 0.001 mm. A 3-factor analysis of variance was used for the statistical analysis with hypothesis testing at alpha=.05.


RESULTS
Statistically significant differences were found with viscosity selection for the buccolingual and occlusogingival dimensions of the working die. The rigid material produced working dies slightly taller (1 microm) than the standard, and those from the monophase material were 4 microm shorter. Regarding tray selection, metal trays were slightly more accurate in the mesiodistal dimension, and when monophase was used in a plastic tray, gypsum dies were nearly 30 microm smaller in the mesiodistal dimension (P<.05). Differences were not detected for sequence of pouring impressions.


CONCLUSION
Within the limitations of this study, the monophase material, when compared with the rigid impression material, was most accurate for the occlusogingival and mesiodistal dimensions, although not as accurate in the buccolingual. This buccolingual difference (0.002 mm-0.006 mm) would be clinically inconsequential with the application of die spacer. The rigid impression material was also unaffected by tray selection for the mesiodistal, whereas monophase was affected. When a monophase impression material was used, plastic dual-arch trays yielded gypsum dies which were significantly smaller (0.029 mm) than the ones generated from the metal trays (0.006 mm). Thus rigid impression materials can be recommended for use in dual-arch trays; however, the magnitude of the differences would generally not be clinically significant because they could be compensated for with several coats of die spacer.
Traceability strategy for gear-pitch-measuring instruments: development and calibration of a multiball artifact
There is a strong demand to ensure the traceability of gear-measuring instruments (GMIs). We propose a multiball artifact (MBA) for the evaluation of pitch-measuring accuracy. The aim of the MBA is to transfer the minimum uncertainty from a calibrated value at the National Metrology Institute of Japan (NMIJ) to a measured value of GMIs at a shop floor. The MBA is composed of equally spaced high-accuracy balls around an axis. The pitch-measuring accuracy of GMIs is evaluated by measuring the angular pitch deviation of the balls instead of the angular pitch deviation of a gear. We calibrated the angular pitch deviation using a coordinate measuring instrument (CMM) at the NMIJ and adapting a multiple-orientation technique. We proposed a calibration strategy of the angular pitch deviation. The calibration value of the pitch deviation for the MBA at the NMIJ was obtained with a measurement uncertainty (U95) of 0.2 µm. We evaluated the pitch-measuring accuracy of a GMI using the calibrated MBA. Each value of U95 for the cumulative and single pitch deviations for the left and right flanks was less than or equal to 0.5 µm. The small uncertainty was transferred from the NMIJ to the GMI.