26 January 2022

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Digital Versus traditional physical cast for fixed prosthodontics

Edoardo Mancuso

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In fixed prosthodontics, the accuracy of the impression technique and of the definitive casts is essential for proper fabrication of restorations, thus preventing biologic and prosthetic complications. Physical casts are required for the proper contouring, finishing, and polishing of restorations and allow evaluation of the marginal fit, occlusion, and proximal contacts. Therefore, in prosthodontics, all analog, digital, and partial digital workflows still require physical casts. In this context, a definitive cast that is imprecise could lead to discrepancies with the intraoral condition and consequently restoration misfit, compromising the treatment outcome.

The conventional workflow for obtaining gypsum casts is time-consuming, labor-intensive, and dependent on the ability of the operator for accuracy. The impression material thickness, impression handling, cast pouring, and storage procedures may result in dimensional changes. Conversely, the benefits of additively manufactured casts include a reduction of the dental laboratory technician’s time and a standardized and large-scale production of casts with high-wear resistance. 

 the digital workflow for the fabrication of casts foresees the creation of a model from digital impression data using additive manufacturing technology including stereolithography (SLA), digital light processing (DLP), and material jetting (Multijet and Polyjet).  The functioning of SLA and DLP printers are similar in that both use a vat of photosensitive liquid resin polymerized by a scanning laser and a projector light source.  SLA casts are characterized by high accuracy, a smooth surface, and high mechanical strength; however, this method is time-consuming, requiring up to 12 hours for printing at the highest accuracy. DLP has the advantage of faster printing but a more significant roughness of the surface is observed. With Multijet and Polyjet, thinner print layers can be specified than with SLA and DLP printers, improving accuracy and surface quality.

The accuracy of additively manufactured casts greatly depends on the quality of data acquisition and processing, which is influenced by scanner technology, scanning strategies, tooth preparation or implant scan body design, ambient light conditions, operator experience, and scanner calibration. 

Today the digital workflow for the production of cast is well accepted by dental technicians and searched by many dentists, due to its many advantages and technological appeal. However how accurate are additively manufactured casts in comparison with gypsum casts have not been well defined over the literature.

Whit this question in mind Dr. Parize and his team have written a systematic review, published in The Journal of Prosthodontics Research with the purpose to determine the accuracy of additively manufactured casts for tooth- or implant-supported FDPs in comparison with that of gypsum casts. 

MATERIALS AND METHODS
In the review, eight databases were searched in December 2019 and updated in September 2020. Studies evaluating the dimensional accuracy of additively manufactured casts for fixed dental prostheses in comparison with that of gypsum casts were included.

RESULTS
The initial search effectuated resulted in 627 potentially relevant articles, which was reduced to 353 after removing duplicate reports. After meeting the inclusion and exclusion criteria, the research reports that 35 articles were selected for full-text reading, and 15 articles were considered eligible for the review. In the study, all eligible studies were in vitro. In the studies included in the research most of the gypsum casts were obtained by using dual viscosity impression material at room temperature with polyvinyl siloxane materials in a custom tray, and then poured with Type IV with 0.08% setting expansion and mixed under vacuum. 

Most studies included in the review of Dr. Parize reported greater accuracy of gypsum casts for tooth-supported FDPs, compared with that of SLA,  DLP, and Polyjet casts.  Other studies reported SLA  and DLP casts did not differ significantly from gypsum casts. Most studies reported implant-supported FDP casts obtained from SLA, Multijet, and DLP printers with accuracy similar to that of gypsum casts. Two studies reported that Polyjet casts were more accurate than gypsum casts. 

DISCUSSION
In fixed prosthodontics, physical casts are required to obtain restorations in analog and partial digital workflows and to finish and customize restorations in the digital workflow. 

The review reports that among the studies analyzed, gypsum casts showed greater accuracy (trueness and precision), although additively manufactured casts also yielded consistent and precise data, achieving the clinically acceptable range (<120 mm) of accuracy and being recommended for prosthodontic application. No significant difference was reported between gypsum casts and SLA casts for the prepared teeth and for the complete-arch assessment of implant-supported FDP casts. 

Dr. Parize continues assessing that, one of the main drawbacks of the additive manufacturing layer-by-layer buildup technique is the staircase effect, which influences surface smoothness and accuracy, especially in thick layers and with DLP printers. Staircase effect is a phenomenon associated with 3D printing when the layer marks become distinctly visible on the surface of the parts, giving the perception of a staircase. Among the eligible studies included in the review, additively manufactured casts were obtained by different additive manufacturing technologies (SLA, DLP, Polyjet, and Multijet), no meta-analysis could be performed, hindering correct identification of the most accurate technique that could offer a smoother surface.

In the cited review it is then specified that all eligible studies included were in vitro, and for this reason clinical factors that could decrease the accuracy of the casts, such as the presence of anatomic features (tongue, lips, and cheeks), patient movement, saliva, and limited space, were not considered. In addition, it is then noted that, chemical composition, surface morphology, and optical properties of the typodont used as the master cast are different from the intraoral situation. Therefore, the accuracy of gypsum casts compared with additively manufactured casts in clinical conditions remains unknown. 

CONCLUSIONS
The article ends drawing the following conclusions: 

1. In vitro studies showed that additively manufactured casts and gypsum casts have similar accuracy within the acceptable range for the manufacture of tooth-supported and implant-supported FDPs casts. 

2. The quality of scanned data, additive manufacture technology, printing settings, and postprocessing procedures plays an essential role in the accuracy of additively manufactured casts. 

The development of additive manufacturing with different printing techniques for fixed prosthodontics has notably improved time efficiency and enabled the large-scale production of reliable casts. However, more studies and reviews are still needed to definitely abandon the “old fashioned” gypsum casts.

For additional information: Digital versus conventional workflow for the fabrication of physical casts for fixed prosthodontics: A systematic review of accuracy