Digital dentistry : studies on the trueness and precision of additive manufacturing and intraoral scanning

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Digital dentistry : studies on the trueness and precision of additive manufacturing and intraoral scanning

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Publication Doctoral Thesis
Title Digital dentistry : studies on the trueness and precision of additive manufacturing and intraoral scanning
Author Braian, Michael
Date 2018
English abstract
Artificial designs and features usually control production workflows in the industry. The operator has the freedom to adapt designs to achieve the desired function; when the operator is satisfied, mass production of the two objects is possible. The production workflow for prosthetic restorations in dentistry is a fairly complicated procedure that requires several well-controlled processes, and each unit is individually adapted to one unique situation. The aim of the final restoration is to replace damaged or missing soft and hard tissue, and to restore function, phonetics and aesthetics. The restoration has high material property requirements in order to withstand high forces, thermal changes, aging and humidity. If the fit of the reconstruction is insufficient there is a high probability for clinical failures ranging from inflammatory processes to reconstruction fractures. The grading of perfect, sufficient and insufficient fit is unknown although the definition clinically acceptable fit has been used to describe and control a reconstruction that is well seated and controllable by the clinician. Study I in this thesis focuses on the clearance (play) between different implant components in order to achieve a threshold value for how accurate the production in dentistry needs to be. We found that metallic components on external hex connections have a clearance of approximately 50 μm. Not only is every case individually designed and manufactured, but the receiving intraoral part also needs to be replicated into an extraoral part ahead of production, a procedure that has been possible with different impression materials. Subsequently, the production goes through a series of controlled compensations to fit the intraoral situation. The conventional workflow starts by the selection of an impression tray, ranging from custom-made trays to plastic stock trays. The ideal trays are rigid, thereby minimising flexure during the impression taking. There are several types of impression materials with different properties regarding setting time, volume changes and mechanical properties. The next step in the conventional workflow is the casting of the impression. There are various types of gypsum products utilised in dentistry, and they require different amounts of water. The differences depend on the shape and compactness of the crystals. Type IV dental stone gypsum is often used in reconstructive dentistry with a typical setting expansion of 0.10%. For the partial digital workflow the same volume changes can be seen for the conventional impression, the stone model production and the die processing. In order to design the intended construction digitally instead of using wax, the model needs to be digitised in an extraoral scanner, also known as desktop scanner. The fully digital workflow consists of a direct digitisation of the oral cavity utilising intraoral scanning devices. All intraoral scanners have the same goal, to digitise the size, shape and surface of a physical object into a geometrical virtual shape. This acquisition needs to be repeatable, reproducible and accurate. The IOS manufacturers try to achieve these goals with different hardware and software setups. Study IV focuses on the acquisition accuracy of five different intraoral scanners for the digitisation of edentulous and dentated models. The results suggest that the devices had lower accuracy for the digitisation of the edentulous models when compared to the dentated model. Furthermore, Study IV presented observations suggesting that fullarch scans had lower accuracy when compared to shorter arch scans on both models. For the cross-arch measurements on the edentulous scans, the trueness values ranged from 6 μm to 193 μm, and, for the shorter arch measurements, the results ranged from 2 μm to 103 μm. For the dentated cast, the cross-arch trueness values ranged from 6 μm to 150 μm, and, for the shorter arch measurements, the results ranged from 4 μm to -56 μm. The digitised file is then utilised as a virtual model by a computeraided designer in order to virtually design the intended reconstruction. The designed file is then manufactured utilising computer-aided manufacturing, which can be performed either by a subtractive machine (milling) or by additive systems (3D printing). Study II and Study III explore the production tolerances for producing polymeric and metallic objects from additive systems. Study III also contained a subtractive group. The results from these two studies suggest that all tested additive systems for producing polymeric objects were, on average, <20 μm for both precision and trueness, and the additive systems for manufacturing metallic objects ranged from >500 μm to <30 μm in trueness, with precision values of <100 μm. The subtractive system showed trueness values of <25 μm with a precision around 20 μm.
DOI https://doi.org/10.24834/2043/26768 (link to publisher's fulltext.)
Publisher Malmö university
Series/Issue Doctoral dissertation in odontology
ISBN 9789171049407
9789171049414
Pages 142
Language eng (iso)
Subject Medicine
Research Subject Categories::ODONTOLOGY
Included papers
  1. I. Tolerance measurements on internal- and externalhexagon implants. Braian M, De Bruyn H, Fransson H, Christersson C, Wennerberg A. Int J Oral Maxillofac Implants. 2014 Jul-Aug;29(4):846-52.

  2. II. Production tolerance of additive manufactured polymeric objects for clinical applications. Braian M, Jimbo R, Wennerberg A. Dent Mater. 2016 Jul;32(7):853-61

  3. III. Geometrical accuracy of metallic objects produced with additive or subtractive manufacturing: A comparative in vitro study. Braian M, Jönsson D, Kevci M, Wennerberg A. Dent Mater. 2018 Jul;34(7):978-993

  4. IV. Trueness and precision of five intraoral scanners on scanning edentulous and dentated full arch mandibular casts: A comparative in vitro study. Braian M. DDS, CDT, Wennerberg A, DDS, Ph.D. Accepted, journal of prosthetic dentistry

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