Comparative analysis of 3D data accuracy of single tooth and full dental arch captured by different intraoral and laboratory digital impression systems

Verification of a digital approach for three-dimensional evaluation of marginal and internal fit

STATEMENT OF PROBLEM

Internal fit is an important aspect of indirect restorations, but methods for the 3-dimensional (3D) measurement of absolute marginal and intaglio fit are sparse.

PURPOSE

The purpose of this in vitro study was to evaluate an innovative 3D measurement method (AIXFit) based on intraoral scanning data for analyzing the fit of dental restorations.

MATERIAL AND METHODS

For the evaluation of AIXFit, 12 monolithic zirconia crowns were fabricated on typodont preparations. The fit was measured digitally with the AIXFit system and compared with the results obtained from an established 2-dimensional (2D) sectional procedure. To compare the values of both methods at identical locations, a common reference system was developed, with each die fixed in a gypsum stand with reference points. Using an intraoral scanner (True Definition), each die with its reference points and the intaglio surface of the finished crown were digitalized as standard tessellation language files. The AIXFit software program, with a specially developed best-fit algorithm, was used to match the intaglio surface of the crown with the surface of the preparation. The virtual cement gap was calculated over the entire surface and returned values for x≥0 µm. A 2D comparison method involved adhesively fixing the crown to the die and sectioning it into 4 parts with a diamond band saw. The thickness of the cement gap was determined under a light microscope at ×100 magnification at 5 defined measuring points per quarter, so that a total of 240 measurements were available for comparison. A software program (Blender Foundation) was used to superimpose the data from the AIXFit system with the data from the 2D method and to compare the cement gaps at the same locations. The agreement between these methods was verified using paired t tests and determine correlation coefficients (α=.05).

RESULTS

The mean ±standard deviation difference between the AIXFit and 2D methods was 6.7 ±29 µm). Two 1-sided tests showed statistical equivalence between the methods of measurement when considering an interval between -20 and +20 µm. The correlation coefficients showed a positive association for both methods (r=.931).

CONCLUSIONS

The AIXFit software program appeared to be accurate for the digital measurement of internal fit when using the True Definition scanner. It enabled a cast-free workflow and allowed the analysis of the entire intaglio surface.

[Assessment of supracontacts size at digital positioning of scans in habitual occlusion]

The study is devoted to the error analysis of digital registration of lower and upper jaws laboratory and intraoral scans in position of habitual occlusion. It was found that in both cases, penetrating contacts (supracontacts) are formed, and their depth is in the range (-50) - (0) microns. The number and area of these contacts is higher for lateral teeth. The number and area of supracontacts is higher for laboratory scanning (3.7±1.81 mm²) than for intraoral scanning (0.5±0.42 mm²). Applying virtual contact optimization does not change the localization of contacts, but rather normalizes them in both cases. It is recommended to perform this preliminary correction before the planning stage in 3D dental treatment.

A quantitative assessment of silicone and PTFE-based stamp techniques for restoring occlusal anatomy using resin-based composites

OBJECTIVES

Publications on stamp techniques for placing resin-based composite (RBC) restorations consist mainly of case studies. Furthermore, comparative studies are rare and no longer relevant to the materials tested today. Thus, two general techniques were investigated in this study.

MATERIALS AND METHODS

Standardized occlusion class I cavities were prepared in twenty-eight extracted caries-free wisdom teeth with unimpaired occlusal surfaces and restored with the RBC material Grandio^®. Light curing of the final layer was performed either after removal of the stamp isolated with PTFE tape or by leaving a stamp made of transparent polysiloxane in place. CEREC scans of the RBC restorations placed (follow-up) were superimposed on scans of the unimpaired occlusal surface (baseline) and quantitatively analyzed with the software OraCheck with regard to volume change and gain or loss of layer thickness in six sectional planes.

RESULTS

Assessing the excess material, there was no difference (p = 0.31) between the silicone technique (0.26 mm ± 0.02) and the PTFE technique (0.22 mm ± 0.02 mm). Nevertheless, the loss of tooth substance was significantly greater (p < 0.001) with the silicone technique (-0.29 mm ± 0.02 mm) than with the PTFE technique (-0.15 mm ± 0.02 mm).

CONCLUSIONS

With the PTFE stamp technique, less healthy tooth structure was removed during the finishing procedure and the stamp was more dimensionally stable.

CLINICAL RELEVANCE

The study shows the advantages and disadvantages of the investigated stamp techniques and helps the practitioner to choose an appropriate technique.

Maxillary expansion or contraction and occlusal contact adjustment: effectiveness of current aligner treatment

OBJECTIVES

To evaluate the precision of aligner (Invisalign®) treatment with the current material (SmartTrack®) in achieving expansion or contraction of the maxilla and occlusal contacts as simulated in the proprietary planning software (ClinCheck®, CC).

MATERIALS AND METHODS

Thirty patients thus treated were retrospectively evaluated. Four maxillary models were analyzed per patient: a pretreatment model, a scan-based CC model, a posttreatment clinical model, and a CC model reflecting the treatment outcome as initially simulated. Thirteen transverse parameters were measured on each model separately by two investigators. Occlusal contacts were also analyzed.

RESULTS

The measuring method was validated by both investigators arriving at similar results for the effectiveness by which the simulated treatment goals had been clinically achieved. Significant differences (p < 0.05; Wilcoxon signed-rank test) were observed for transfer precision from the casts to the planning software and between the simulated and clinical outcomes. Intense occlusal contacts in the simulations materialized less common (≈ 2%) than ideal contacts (≈ 60%) in the clinical outcomes.

CONCLUSIONS

The effectiveness of achieving the simulated transverse goals was 45% and was generally not found to be better with SmartTrack® than with the previously used Ex30® material. Out of 100 simulated occlusal contacts, 40 will never materialize, and achieving around 60 will adequately ensure a clinically favorable contact pattern.

CLINICAL RELEVANCE

With the caveat that any overcorrection will to some extent reduce the precision, it seems perfectly possible to make deliberate use of overcorrection in current aligner therapies for transverse maxillary expansion or contraction.

Accuracy of 3D digital modeling of dental arches

OBJECTIVE

The aim of the study was to verify and compare the accuracy of full-arch digital impressions obtained using two intraoral scanners and three scanning methodologies.

METHODS

A resin model created with dental 3-D printing was scanned by a reference scanner (Zfx Evolution - Zimmer Biomet, Palm Beach Gardens, FL) in order to obtain a 3D reference; the same resin model was then scanned with two different intraoral scanners (Zfx IntraScan and Carestream 3600 - CS 3600®, Carestream, Rochester, NY, USA) using: Technique A (from tooth #27 up to tooth #17); Technique B (from tooth #11 up to tooth #17 and then from tooth #21 up to tooth #27) and Technique C (from tooth #22 up to tooth #17, and then from tooth #12 up to tooth #27 - the MeshLab software v. 1.3.3 was then used to match the two scans). The scans obtained were superimposed over the reference scan by means of a software, and the volumetric discrepancies were calculated.

RESULTS

The mean results for the Zfx Intrascan scanner were: Technique A = 302.47 ± 37.42 µm; Technique B = 180.45 ± 29.86 µm; Technique C = 147.34 ± 28.23 µm. The mean results for the Carestream 3600 scanner were: Technique A = 303.59 ± 40.20 µm; Technique B = 181.53 ± 29.61 µm; Technique C = 142.28 ± 35.33 µm. Technique C, used by both scanners, produced less volumetric discrepancies compared to the other techniques.

CONCLUSIONS

The scanning technique had a statistically significant effect on the quality of the scan (p< 0.0001), whereas the scanner did not present any significant influence (p= 0.91).