Comparison of digital surface displacements of maxillary dentures based on noninvasive anatomic landmarks

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Abstract

Statement of problem.

Artificial markers called fiducials are commonly used to orient digitized surfaces for analysis. However, when these markers are tangible and placed in the region of interest, they may alter surface topography and influence data analysis.

Purpose.

The purpose of this in vitro study was to apply a modified digital surface fitting method based on anatomic landmarks to evaluate denture accuracy and to use 2 different denture processing techniques to evaluate the method. The goal was to noninvasively measure and describe any surface differences in denture processing techniques at the intaglio and denture tooth levels.

Material and methods.

Twenty standardized maxillary complete dentures were waxed on standardized edentulous casts and processed by using acrylic resin compression (COM, n=10) and injection molding (INJ, n=10) methods. Digital scans were recorded of the anatomic surface of the cast, the intaglio and cameo surfaces of the acrylic resin dentures, and the cameo surface of the wax dentures. Three anatomic fiducials were identified on denture intaglio and cast scans and 4 on the cameo surfaces of waxed and acrylic resin denture scans. These fiducials were then used to digitally align the anatomic with the processed intaglio surfaces and the waxed with the processed cameo surfaces. Surface displacements were compared among processed dentures expressed at specific points (9 tissue landmarks and 8 tooth landmarks). The accuracy of surface displacements was assessed by changes in the number and location of anatomic fiducials. The scanning precision and the intraobserver repeatability in the selection of dental landmarks were also determined. For each landmark, the spatial (x, y, and z) mean differences between the 2 processing techniques were calculated for the intaglio and the cameo surfaces and presented on each orthogonal plane. Statistical nonparametric comparison of these means was analyzed with the Mann-Whitney U test (α=.05). Benjamini-Hochberg corrections for multiple comparisons were used.

Results.

Changing the number and the location of anatomic landmarks had a small effect on the precision of the surface fitting. Repeated scans yielded high precision levels. In contrast, intraobserver repeatability had a larger error. In general, injection-molded dentures showed less displacement after polymerization than did the compression-molded ones. These differences were more substantial at the denture tooth level than on the intaglio surfaces.

Conclusions.

Anatomic noninvasive fiducials chosen at distinct locations of maxillary edentulous areas seem to be reliable markers for the superposition of corresponding digital surface scans. Maxillary dentures processed with the injection molding technique have minimal deformation. Posterior denture teeth displace in 3 dimensions with the compression molding technique.

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