Efficiency of a mathematical model in generating CAD/CAM-partial crowns with natural tooth morphology

The "biogeneric tooth model” can be used for computer-aided design (CAD) of the occlusal surface of dental restorations. From digital 3D-data, it automatically retrieves a morphology matching the natural surface left after preparation. This study evaluates the potential of this method for generating well-matched and well-adjusted CAD/computer-aided manufacturing (CAM) fabricated partial crowns. Twelve models with partial crown preparations were mounted into an articulator. Partial crowns were designed with the Cerec 3D CAD software based on the biogeneric tooth model (Biog.CAD) and, for control, with a conventional data-based Cerec 3D CAD software (Conv.CAD). The design time was measured, and the naturalness of the morphology was visually assessed. The restorations were milled, cemented on the models, and the vertical discrepancy and the time for final occlusal adjustment were measured. The Biog.CAD software offered a significantly higher naturalness (up to 225 to 11 scores) and was significantly faster by 251 (±78) s in designing partial crowns (p < 0.01) compared to Conv.CAD software. Vertical discrepancy, 0.52 (±0.28) mm for Conv.CAD and 0.46 (±0.19) mm for Biog.CAD, and occlusal adjustment time, 118 (±132) s for Conv.CAD and 102 (±77) s for Biog.CAD, did not differ significantly. In conclusion, the biogeneric tooth model is able to generate occlusal morphology of partial crowns in a fully automated process with higher naturalness compared to conventional interactive CAD softwar

Local accuracy of actual intraoral scanning systems for single-tooth preparations in vitro

BACKGROUND The authors evaluated the local accuracy of intraoral scanning (IOS) systems for single-tooth preparation impressions with an in vitro setup. METHODS The authors digitized a mandibular complete-arch model with 2 full-contour crowns and 2 multisurface inlay preparations with a highly accurate reference scanner. Teeth were made from zirconia-reinforced glass ceramic material to simulate toothlike optical behavior. Impressions were obtained either conventionally (PRESIDENT, Coltène) or digitally using the IOS systems TRIOS 3 and TRIOS 3 using insane scan speed mode (3Shape), Medit i500, Version 1.2.1 (Medit), iTero Element 2, Version 1.7 (Align Technology), CS 3600, Version 3.1.0 (Carestream Dental), CEREC Omnicam, Version 4.6.1, CEREC Omnicam, Version 5.0.0, and Primescan (Dentsply Sirona). Impressions were repeated 10 times per test group. Conventional (CO) impressions were poured with type IV gypsum and digitized with a laboratory scanner. The authors evaluated trueness and precision for preparation margin (MA) and preparation surface (SU) using 3-dimensional superimposition and 3-dimensional difference analysis method using (95% - 5%) / 2 percentile values. Statistical analysis was performed using Kruskal-Wallis test. Results were presented as median (interquartile range) values in micrometers. RESULTS The authors found statistically significant differences for MA and SU among different test groups for both trueness and precision (P < .05). Median (interquartile range) trueness values ranged from 11.8 (2.0) μm (CO) up to 40.5 (10.9) μm (CEREC Omnicam, Version 5.0.0) for SU parameter and from 17.7 (2.6) μm (CO) up to 55.9 (15.5) μm (CEREC Omnicam, Version 5.0.0) for MA parameter. CONCLUSIONS IOS systems differ in terms of local accuracy. Preparation MA had higher deviations compared with preparation SU for all test groups. PRACTICAL IMPLICATIONS Trueness and precision values for both MA and SU of single-unit preparations are equal or close to CO impression for several IOS systems

Accuracy of digital complete-arch, multi-implant scans made in the edentulous jaw with gingival movement simulation: An in vitro study

STATEMENT OF PROBLEM The use of computer-aided design and computer-aided manufacturing (CAD-CAM) technologies is widely established, with single restorations or short fixed partial dentures having similar accuracy when generated from digital scans or conventional impressions. However, research on complete-arch scanning of edentulous jaws is sparse. PURPOSE The purpose of this pilot in vitro study was to compare the accuracy of a digital scan with the conventional method in a workflow generating implant-supported complete-arch prostheses and to establish whether interference from flexible soft tissue segments affects accuracy. MATERIAL AND METHODS An edentulous maxillary master cast containing 6 angled implant analogs was used and digitized with mounted scan bodies by using a high-precision laboratory scanner. The master cast was then scanned 10 times with 4 different intraoral scanners: TRIOS 3 with a complete-arch scanning strategy (TRI1) or implant-scanning strategy (TRI2), TRIOS Color (TRC), CEREC Omnicam (CER), and CEREC Primescan (PS). The same procedure was repeated with 4 different levels of free gingiva (G0-G3). Ten conventional impressions were obtained. Differences in implant position and direction were evaluated at the implant shoulder as mean values for trueness and interquartile range (IQR) for precision. Statistical analysis was performed by using the Kruskal-Wallis and post hoc Conover tests (α=.05). RESULTS At G0, position deviations ranged from 34.8 μm (IQR 23.0 μm) (TRC) to 68.3 μm (12.2 μm) (CER). Direction deviations ranged from 0.34 degrees (IQR 0.18 degrees) (conventional) to 0.57 degrees (IQR 0.37 degrees) (TRI2). For digital systems, the position deviation ranged from 48.4 μm (IQR 5.9 μm) (PS) to 76.6 μm (IQR 8.1 μm) (TRC) at G1, from 36.3 μm (IQR 9.3 μm) (PS) to 79.9 μm (IQR 36.1 μm) (TRI1) at G2, and from 51.8 μm (IQR 14.3 μm) (PS) to 257.5 μm (IQR 106.3 μm) (TRC) at G3. The direction deviation ranged from 0.45 degrees (IQR 0.15 degrees) (CER) to 0.64 degrees (IQR 0.20 degrees) (TRC) at G1, from 0.38 degrees (IQR 0.05 degrees) (PS) to 0.925 degrees (IQR 0.09 degrees) (TRI) at G2, and from 0.44 degrees (IQR 0.07 degrees) (PS) to 1.634 degrees (IQR 1.08 degrees) (TRI) at G3. Statistical analysis revealed significant differences among the test groups for position (G0: P<.001; G1: P<.05; G2: P<.001; G3: P<.001) and direction (G0: P<.005; G1: P<.001; G2: P<.001; G3: P<.001). CONCLUSIONS Without soft tissue interference, the accuracy of certain digital scanning systems was comparable with that of the conventional impression technique. The amount of flexible soft tissue interference affected the accuracy of the digital scans

Accuracy of complete-arch dental impressions: a new method of measuring trueness and precision

STATEMENT OF PROBLEM: A new approach to both 3-dimensional (3D) trueness and precision is necessary to assess the accuracy of intraoral digital impressions and compare them to conventionally acquired impressions. PURPOSE: The purpose of this in vitro study was to evaluate whether a new reference scanner is capable of measuring conventional and digital intraoral complete-arch impressions for 3D accuracy. MATERIAL AND METHODS: A steel reference dentate model was fabricated and measured with a reference scanner (digital reference model). Conventional impressions were made from the reference model, poured with Type IV dental stone, scanned with the reference scanner, and exported as digital models. Additionally, digital impressions of the reference model were made and the digital models were exported. Precision was measured by superimposing the digital models within each group. Superimposing the digital models on the digital reference model assessed the trueness of each impression method. Statistical significance was assessed with an independent sample t test (α=.05). RESULTS: The reference scanner delivered high accuracy over the entire dental arch with a precision of 1.6 ±0.6 µm and a trueness of 5.3 ±1.1 µm. Conventional impressions showed significantly higher precision (12.5 ±2.5 µm) and trueness values (20.4 ±2.2 µm) with small deviations in the second molar region (P<.001). Digital impressions were significantly less accurate with a precision of 32.4 ±9.6 µm and a trueness of 58.6 ±15.8µm (P<.001). More systematic deviations of the digital models were visible across the entire dental arch. CONCLUSIONS: The new reference scanner is capable of measuring the precision and trueness of both digital and conventional complete-arch impressions. The digital impression is less accurate and shows a different pattern of deviation than the conventional impression

The effect of zirconia sintering temperature on flexural strength, grain size, and contrast ratio

Objective: This study investigated the effect of sintering temperatures on flexural strength, contrast ratio, and grain size of zirconia. Materials and Methods: Zirconia specimens (Ceramill ZI, Amann Girrbach) were prepared in partially sintered state. Subsequently, the specimens were randomly divided into nine groups and sintered with different final sintering temperatures: 1,300°C, 1,350°C, 1,400°C, 1,450°C, 1,500°C, 1,550°C, 1,600°C, 1,650°C, or 1,700°C with 120min holding time. Three-point flexural strength (N = 198; n = 22 per group) was measured according to ISO 6872: 2008. The contrast ratio (N = 90; n = 10 per group) was measured according to ISO 2471: 2008. Grain sizes and microstructure of different groups were investigated (N = 9, n = 1 per group) with scanning electron microscope. Data were analyzed using one-way ANOVA with Scheffé test and Weibull statistics (p < 0.05). Pearson correlation coefficient was calculated between either flexural strength or contrast ratio and sintering temperatures. Results: The highest flexural strength was observed in groups sintered between 1,400°C and 1,550°C. The highest Weibull moduli were obtained for zirconia sintered at 1,400°C and the lowest at 1,700°C. The contrast ratio and the grain size were higher with the higher sintering temperature. The microstructure of the specimens sintered above 1,650°C exhibited defects. Sintering temperatures showed a significant negative correlation with both the flexural strength (r = −0.313, p < 0.001) and the contrast ratio values (r = −0.96, p < 0.001). Conclusions: The results of this study showed that the increase in sintering temperature increased the contrast ratio, but led to a negative impact on the flexural strength. Clinical Relevance: Considering the flexural strength values and Weibull moduli, the sintering temperature for the zirconia tested in this study should not exceed 1,550°