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Article

An In-Vitro Evaluation of Articulation Accuracy for Digitally Milled Models vs. Conventional Gypsum Casts

Department of Restorative Dentistry and Biomaterials Science, Harvard School of Dental Medicine, Boston, MA 02115, USA
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Author to whom correspondence should be addressed.
Dent. J. 2022, 10(1), 11; https://doi.org/10.3390/dj10010011
Submission received: 25 November 2021 / Revised: 4 January 2022 / Accepted: 6 January 2022 / Published: 11 January 2022
(This article belongs to the Special Issue Feature Papers in Dentistry Journal in 2021)

Abstract

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With the advent of a digital workflow in dentistry, the inter-occlusal articulation of digital models is now possible through various means. The Cadent iTero intraoral scanner uses a buccal scan in maximum intercuspation to record the maxillomandibular relationship. This in-vitro study compares the occlusion derived from conventionally articulated stone casts versus that of digitally articulated quadrant milled models. Thirty sets of stone casts poured from full arch polyvinyl siloxane impressions (Group A) and thirty sets of polyurethane quadrant models milled from digital impressions (Group B) were used for this study. The full arch stone casts were hand-articulated and mounted on semi-adjustable articulators, while the digitally derived models were pre-mounted from the milling center based on the data obtained from the buccal scanning procedure. A T-scan sensor was used to obtain a bite registration from each set of models in both groups. The T-scan data derived from groups A and B were compared to that from the master model to evaluate the reproducibility of the occlusion in the two groups. A statistically significant difference of the contact region surface area was found on #11 of the digitally articulated models compared to the master. An analysis of the force distribution also showed a tendency for a heavier distribution on the more anterior #11 tooth for the digitally articulated models. Within the limitations of this study, the use of a digitally articulated quadrant model system may result in a loss of accuracy, in terms of occlusion, the further anteriorly the tooth to be restored is located. Care must be taken to consider the sources of inaccuracies in the digital workflow to minimize them for a more efficient and effective restorative process.

1. Introduction

Multiple techniques and materials have been used for the articulation of conventional dental models in their desired maxillomandibular relationship. Commonly, a recording medium such as wax or polyvinylsiloxane is used to transfer the patient’s maxillomandibular relationship to the casts for mounting. However, it is not always possible to achieve the desired accuracy clinically. Possible reasons for this include the dimensional instability of the recording material as well as the inability to fully seat the record due to inaccurate casts. Multiple studies have shown both vertical and horizontal discrepancies with conventional recording materials. These materials can undergo dimensional distortion under different temperature and moisture conditions [1,2,3], as well as over time [3,4,5,6]. Due to the dimensional instability of interocclusal registration materials, when adequate stable occlusal stops are present, the hand articulation of the casts has been shown to be a more accurate means of locating the proper interocclusal relationship [7,8]. The dimensional stability and accuracy of dental casts has been studied by multiple authors but can vary greatly depending on factors such as the impression and stone material used, impression technique, and storage time [9,10].
The popularity of computer-aided design and computer-aided manufacturing (CAD-CAM) generated restorations is on the rise as the technology and materials continue to evolve. Recent studies on these types of restorations have shown comparable clinical outcomes, and the digital workflow was proven to be the more efficient and the preferred method by both clinicians and patients [11,12,13,14,15]. The outcome of the restorations fabricated by CAD-CAM is dependent on the accuracy of the scan, casts, articulation and fabrication process through milling or 3D printing. Several studies have shown that the digitally fabricated casts and digital articulation process utilizing buccal scans are comparable to or more accurate than conventional methods [16,17,18]. However, there is a lack of information regarding the accuracy of the occlusal relationship derived from the digital articulation of digital models, specifically quadrant scans and models.
In order to assess the accuracy of the articulation of two mounted casts, a precise measurement of the occlusal contacts and their corresponding forces is necessary. Various methods for recording areas of occlusal contact have historically been used, including articulating foils, paper, waxes, silicone impressions, and photocclusion [3,19]. However, barring complex photoanalytical processes, most of these methods leave the analysis of the markings up to a subjective operator interpretation rather than outputting a quantifiable value. The T-scan III is a digital occlusal analysis instrument which uses a pressure sensitive sensor to allow the simultaneous registration and imaging of the relative distribution of occlusal forces and the contact time sequence. Furthermore, the results produce an immediate output of quantifiable data values as percentages of the load registered by the sensor as opposed to occlusal markings that must be qualitatively interpreted. Previous studies have shown contradictory results regarding the accuracy and reproducibility of previous generations of this system, with some showing an excessive variability in readings and a questionable reliability [20,21]. This may be attributable to the variable positioning of the digital pressure sensor as well as the sensitivity of the sensor. Others have found a very high level of precision [22,23,24], but recent literature has shown the T-scan III to have an improved precision and reliability when it comes to the measurement of the relative force distribution and number of contacts [25,26]. In this study, the digital occlusal markings from the digital pressure sensor were filtered through an imaging software in order to eliminate the artifacts from its sensitivity and precisely isolate the occlusal contacts.
The aim of our in-vitro study was to determine the accuracy of the digital articulation of milled polyurethane quadrant models using a buccal scan versus that of conventionally mounted stone casts using a digital occlusal analysis device. The null hypothesis is that there will be no difference in the accuracy of the occlusion of digitally articulated milled models and conventionally articulated stone models.

2. Materials and Methods

2.1. Study Design

Mounted custom typodont master models were both conventionally impressed and digitally scanned by 30 different participants at the Harvard School of Dental Medicine. The resultant 30 sets of stone casts poured in type III stone (Microstone, Whip Mix Corp, Louisville, KY, USA) from full arch polyvinyl siloxane impressions (Aquasil, Dentsply Sirona, Waltham, MA, USA) (Group A) and thirty sets of polyurethane quadrant models milled from digital impressions (Itero Cadent) (Group B) were used for this study.
In Group A, the full arch stone casts were hand-articulated and mounted on semi-adjustable articulators (Artex CR, Amann Girrbach AG, Pforzheim, Germany) with a low expansion mounting stone (Mounting stone, Whip Mix Corp, Louisville, KY, USA) uniformly in relation to the hinge-axis by the use of a mounting jig (Figure 1a–c). In Group B, the digitally derived quadrant models were mounted on a prefabricated articulator (Itero Cadent Articulator, Itero, Carlstadt, NJ, USA) from the milling center based on the data obtained from the buccal scanning procedure. A T-scan (T Scan III, Tekscan Inc., Boston, MA, USA) bite registration was obtained at a uniform position on the T-scan sensor with a weight of 1.5 lbs placed over the upper member of the articulator (Figure 2a,b) for the master model and for each pair of models in Group A and Group B. The same sensor was used between the paired groups. The relative occlusal pressures for teeth #11, 12, and 14 within each group were recorded