Laser digitization of casts to determine the effect of tray selection and cast formation technique on accuracy,☆☆,,★★,,♢♢,

https://doi.org/10.1067/mpr.2002.121240Get rights and content

Abstract

Statement of Problem. Several studies have attempted to determine the ideal combination of dental materials and laboratory techniques to produce the most accurate dental cast. Most have made use of 2-dimensional manual measuring devices, which neglect to account for the dimensional changes that exist along a 3-dimensional surface. Purpose. The purpose of this study was to determine the effect of impression tray selection and cast formation techniques on the dimensional change of a dental cast with the use of new, 3-dimensional optical digitizing technology. Material and Methods. Multiple impressions of a machined steel die that resembled a dental arch were made with custom and stock impression trays and vinyl polysiloxane impression material. The impressions were poured in type V artificial dental stone and allowed to set with the tray inverted or noninverted. The steel master die and stone casts were digitized with the Steinbichler Comet 100 Optical Digitizer, which was developed at the Minnesota Dental Research Center for Biomaterials and Biomechanics. Three-dimensional images of the stone casts were aligned to the 3-dimensional image of the master die and analyzed with AnSur-NT software. Multiple measurements of the master die and stone casts were analyzed to determine the accuracy of the 3-dimensional technology and of the impression and cast fabrication techniques. Planar distances between the center of each crown preparation were measured, as were crown heights. Data were analyzed with analysis of variance (P<.05), and root mean square error values were determined Results. Casts were compared with a total of 45 significance tests, of which only 4 yielded P<.05. There was no pattern to these results, which suggests that they were false-positive findings. Conclusion. Results obtained with the use of new optical digitizing technology indicated that neither impression tray type nor cast formation technique affected the accuracy of final casts. (J Prosthet Dent 2002;87:204-9.)

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Material and methods

A total of 56 impressions were made of a machined stainless steel master die. The master die was shaped like a dental arch with 5 metal studs resembling teeth prepared for a fixed prosthesis (Fig. 3).12

. Stainless steel master die representing dental arch. Anterior studs (B-D) had diameter of 6.1 mm at occlusal surface with 8.6 mm wide margin. Two posterior studs (A and E) had similar dimensions, except diameters were 3.1 mm greater.

The occlusal surfaces of the studs were marked with crosshairs

Results

The RMSE for the die measurements for between-stud distances was 92.2 μm. When compared to the RMSE for the casts (Table II), there was not much difference; in fact, some RMSEs for casts were smaller.

. Root mean square error (RMSE) for the 15 measurements

Location measuredRMSE (μm)
A-B100.6 (82.4 with one outlier removed)
A-C120.6
A-D96.7
A-E86.2
B-C106.9 (93.4 with one outlier removed)
B-D84.4
B-E105.7
C-D109.6
C-E113.6
D-E100.3
HA21.7 (9.3 with one outlier removed)
HB131.5 (12.1 with four outliers removed)
HC

Discussion

If the casts had in truth no systematic differences from each other, one would still expect a few of the 45 tests to give P<.05, purely by chance. This fact and the absence of any pattern in the 4 significant tests suggest that these tests may have all been type I errors, or false-positive findings. Specifically, if all 45 null hypotheses were true and the 45 tests were independent, the chance of 4 or more significant tests would be 0.18. Similarly, the lack of consistency or trend of the cast

Conclusions

Within the limitations of this study, in which a 3-dimensional optical digitizer was used for evaluation purposes, neither impression tray type nor cast formation technique affected accuracy. Custom and stock impression trays used in combination with an elastomeric impression material produced equally accurate casts.

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This research was supported by the University of Minnesota School of Dentistry Summer Research Fellowship Program NIH/5T35-DE07098-19, the University of Minnesota School of Dentistry Dental Research Institute, NIH/NIDCR grant P30 DE09737, and the Minnesota Dental Research Center for Biomaterials and Biomechanics. This research was presented at the IADR annual meeting in Washington DC, April 8, 2000.

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aAssistant Professor, Division of Prosthodontics, Department of Restorative Sciences, School of Dentistry.

bAssociate Professor, Division of Prosthodontics, Department of Restorative Sciences, School of Dentistry.

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cDental Student, Summer Research Fellowship, School of Dentistry.

dProfessor, Department of Oral Sciences, Minnesota Dental Research Center for Biomaterials and Biomechanics.

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eSenior Research Associate, Division of Biostatistics.

Reprint requests to: Dr Mary Elizabeth Brosky, Department of Restorative Sciences, 9-450a Moos Tower, 515 Delaware Street SE, Minneapolis, MN 55455, Fax: (612)626-1496, E-mail: [email protected]

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