Statement of problem.
Data comparing the denture tooth movement of computer-aided design and computer-aided manufacturing (CAD-CAM) and conventional denture processing techniques are lacking.
The purpose of this in vitro study was to compare the denture tooth movement of pack-and-press, fluid resin, injection, CAD-CAM-bonded, and CAD-CAM monolithic techniques for fabricating dentures to determine which process produces the most accurate and reproducible prosthesis.
Material and methods.
A total of 50 dentures were evaluated, 10 for each of the 5 groups. A master denture was fabricated and milled from prepolymerized poly(methyl methacrylate). For the conventional processing techniques (pack-and-press, fluid resin, and injection) a polyvinyl siloxane putty mold of the master denture was made in which denture teeth were placed and molten wax injected. The cameo surface of each wax-festooned denture was laser scanned, resulting in a standard tessellation language (STL) format file. The CAD-CAM dentures included 2 subgroups: CAD-CAM-bonded teeth in which the denture teeth were bonded into the milled denture base and CAD-CAM monolithic teeth in which the denture teeth were milled as part of the denture base. After all specimens had been fabricated, they were hydrated for 24 hours, and the cameo surface laser scanned. The preprocessing and postprocessing scan files of each denture were
superimposed using surface-matching software. Measurements were made at 64 locations, allowing evaluation of denture tooth movement in a buccal, lingual, mesial-distal, and occlusal direction. The use of median and interquartile range values was used to assess accuracy and reproducibility. Levene and Kruskal-Wallis analyses of variance were used to evaluate differences between processing techniques (a=.05).
The CAD-CAM monolithic technique was the most accurate, followed by fluid resin, CAD-CAM-bonded, pack-and-press, and injection. CAD-CAM monolithic technique was the most reproducible, followed by pack-and-press, CAD-CAM-bonded, injection, and fluid resin.
Techniques involving compression during processing showed increased positive occlusal tooth movement compared with techniques not involving compression.
CAD-CAM monolithic dentures produced the best combination of accuracy and reproducibility of the tested techniques. The results from this study demonstrate that varying amounts of tooth movement can be expected depending on the processing technique. However, the clinical significance of these differences is unknown. (J Prosthet Dent 2018;119:108-115)
One of the keys to optimal complete denture fabrication is an accurate fitting base. A laboratory study found that milling denture bases using computer-aided design and computer-aided manufacturing (CAD-CAM) improves denture base adaptation compared with conventional processing techniques.1 A clinical study comparing the retention of CAD-CAM and heat-polymerized denture bases showed better retention with the CAD-CAM denture bases than the heat-polymerized denture bases.2 However, to determine which processing technique produces the best overall prosthesis accuracy and reproducibility, an evaluation of denture tooth movement was needed.
Denture teeth move during processing,3,4 with maxillary dentures distorting more than mandibular dentures.4 Tooth movement occurs during both the flasking and the processing stages of the pack-and-press denture fabrication technique.3-8 A correlation between both the water-to-powder ratio of the investing gypsum and the amount of pressure applied during acrylic resin packing has been shown to affect the amount of denture tooth movement.3 Also, tooth movement during investing has been attributed to the setting expansion of the gypsum.4 Grant and Atkinson9 and Antonopoulos10 in separate studies demonstrated that fluid resin complete dentures underwent larger dimensional changes than pack-and-press. Nogueira et al11 showed that the injection molding technique produced a significantly smaller incisal pin opening than the standard compression molding techniques. Additionally, even minimal denture tooth movement occurring in both arches can have a significant effect on the occlusal vertical dimension (OVD).6
CAD-CAM-milled dentures can produce minimal denture tooth movement by eliminating many of the known causes of processing distortion.12 The purpose of this study was to compare the denture tooth movements of pack-and-press, fluid resin, injection, and CAD-CAM (bonded and monolithic denture teeth) techniques of fabricating complete dentures. The null hypothesis was that no differences in denture tooth movement would be found among the 5 groups.
Material and Methods
Five techniques using various denture processing materials were evaluated in this study (Table 1). Assuming a large effect size, based on previously reported findings, a sample size of approximately 10 was needed to have 80% power to detect a significant difference between groups. Therefore, a sample size of 10 was determined to be appropriate. A pilot study was completed to verify the methodology and accuracy of measurements prior to initiating this study. Ten dentures were fabricated for each group (N=50). A master cast was created with American College of Prosthodontists type A classification of residual ridge morphology (Fig. 1A).13 A master denture was fabricated using software (AvaDent Design v.2.74; Global Dental Science, LLC) and milled from prepolymerized poly(methyl methacrylate) (Fig. 1B).
Figure 1. A, Cast with American College of Prosthodontists type A residual ridge morphology. B, Master complete denture used to create putty mold. C, Festooned wax denture made from putty mold.
For the conventional processing techniques (pack-and-press, fluid resin, and injection), a polyvinyl siloxane putty mold (Lab Putty; Coltène) was made of the master denture, in which denture teeth were placed and molten wax (Truwax Baseplate Wax; Dentsply Sirona) was injected (Fig. 1C). This methodology provided consistent base thicknesses and tooth positions. The cameo surface of each wax-festooned denture was laser scanned (iSeries; Dental Wings Inc) (Fig. 2), resulting in a standard tessellation language (STL) format file.
Figure 2. Denture placed in dental wings iSeries scanner.
For the CAD-CAM dentures, 10 dentures were assigned to each of 2 subgroups: CAD-CAM-bonded teeth, in which the denture teeth were bonded into the milled denture base, and CAD-CAM monolithic teeth, in which the denture teeth were milled as part of the denture base. Both types of CAD-CAM dentures were fabricated from separate scans of each of the 10 stone casts assigned to their respective group. Scanned files were sent to Global Dental Science for fabrication, using the same CAD of the master complete denture to keep base thicknesses and tooth positions consistent across all fabrication techniques.
Each denture was then processed according to material manufacturer guidelines. After processing, each denture was hydrated for 24 hours and the cameo surface laser scanned, which resulted in another STL file. The pre- and postprocessing, STL files were superimposed using surface-matching software (Geomagic Control 2014; 3D Systems Inc). Superimposition was attained through the global registration function by finding 10 000 points in common between the pre- and postprocessing files. Using this software, measurements were made at 64 points for all 50 dentures (Fig. 3). In addition, color surface maps were created using the 3D comparison function to visually display the direction and amount of denture tooth movement (Fig. 4). Color spectra were set to have a maximum critical value of ±0.5 mm and a maximum nominal value of ±0.02 mm. The surface-matching and measurements provided the basis for the evaluation of tooth movement in the following directions: buccal, lingual, mesial-distal, and occlusal.
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