Trueness and adaptation comparison between chairside and laboratory CAD-CAM Zirconia Crowns : an in-vitro study

Abstract

Background: In the presence of tooth structure loss from either carious or non-carious (tooth wear) origin, dental restorative treatment is required to rehabilitate and restore the tooth’s form and function. Restorative treatment can be direct using restorative material, or indirect using dental prostheses fabricated using additive or subtractive methods of manufacturing. These dental prostheses can be temporary or long-term depending on the case and material of choice. Over the years there has been an increase in demand for aesthetic restorations. With superior aesthetics, biocompatibility, and good mechanical properties, zirconia ceramic has become an increasingly popular restorative material for fixed dental prostheses. The fabrication (milling and sintering) of partially sintered monolithic zirconia in a single visit is time-consuming taking up to 12 hours with the conventional laboratory (lab) workflow. With recent advancements, the sintering process can be completed in as little as 15 minutes with high-speed sintering. Regardless of which system the clinician decides to choose, loss of retention and caries are the most common reasons for failure. Both are influenced by the adaptation of the restoration to the prepared surface, expressed as internal and marginal gap measurements. Objectives: The purpose of this in-vitro study is to compare the adaptation and trueness of monolithic zirconia crowns milled using a laboratory workflow (Zirkonzahn, Zirkonzahn GmbH, Gais, Italy) with a chairside workflow (CEREC, Dentsply Sirona). Materials and Methods: A maxillary right second premolar typodont tooth from a mounted prefabricated dental study model was prepared by the CEREC zirconia clinical guidelines with a chamfer finish line. The model’s fully dentate arches with the prepared tooth and bite registration were scanned with an intraoral scanner (Primescan scanner; Dentsply Sirona). A fully anatomical zirconia crown was first designed using the CEREC chairside CAD software with one reference marker on the buccal and lingual aspects. The resulting crown design was exported and used as a reference to create a similar shape and mimic the position of the markers using the scanned arches from the initial scan. The other crown was designed using integrated CAD software (Modellier, Zirkonzahn) for the lab group. Monolithic zirconia crowns were fabricated for both the chairside group (Zirconia+; CEREC, Dentsply Sirona; n=15) and the lab group (Prettau 2; Zirkonzahn GmbH, Gais, Italy; n=15) with the system’s integrated milling and sintering devices (Primemill and SpeedFire, and Zirkonzahn M1 and Zirkonofen Turbo respectively). The internal and marginal gap of each crown was assessed using the triple scan method, scanning the prepared tooth (MS) on a scanning aid, the crown (CS), and the seated scan (SS) after cementation with light-body silicone following timed constant finger pressure. Measurements were taken using 3D software (Zeiss Inspect Optical 3D 2023) on the virtual buccolingual and mesiodistal cross-sectional images. Trueness (3D-deviations between digital wax-up and manufactured crown) of all crowns was evaluated by mesh superimposition of standard tessellation language (STL) files using another software (Geomagic Control X, 3D systems). The root mean squared of the positive and negative deviations of the scanned crown along with a colour map demonstrating these variations were obtained. Statistical analysis was achieved using the Independent Sample t-test for the marginal and internal gap, and the Mann-Whitney U-test for trueness considering that p < 0.05 is significant. Results: Normal distribution was rejected for trueness analysis (p<0.05 with Shapiro-Wilk test) whilst normality was accepted for internal and marginal gap analysis (p>0.05 with Shapiro-Wilk test). The Independent Samples t-test revealed a significant difference in the overall internal and marginal fit between the chairside 81 ± 48.1 μm, 100.7 ± 43.6 μm and lab group 60.9 ± 32.0 μm, 67.8 ± 59.0 μm (p<0.001). No significant difference was found in the axial internal gap (p=0.043). According to the Mann-Whitney U-test the internal trueness of crowns manufactured with the lab workflow was significantly higher (lower deviation) than that of the chairside group with an RMS of 24.2 ± 6.09 μm and 30 ± 7.89 μm respectively (p=0.041). Conclusion: Within the limitations of this in-vitro study, the findings observed herein suggest that both CAD-CAM systems fabricate monolithic zirconia crowns with clinically acceptable cement space and marginal fit. However, the laboratory workflow using the Zirkonzahn software and its respective milling and sintering devices exhibited higher trueness and better marginal and internal fit than the chairside workflow using the CEREC system by Dentsply Sirona.