An In Vitro Three-Dimensional Evaluation and Comparison of The Effect of Different Build Angles on Dimensional Accuracy of Dental Replica Models Made Using Additive Manufacturing

Model and modelling are common terms amongst dental professionals. Perhaps, from the inception of modern dentistry, these terms have been used in many different forms and for various necessities¹. According to the GPT 9, The term model has been used in dentistry since 1572 and is defined as “a facsimile used for display purposes; a miniature representation of something; an example for imitation or emulation ².”

Usually, models used in dentistry are made by impression-making techniques by using impression materials and are commonly transferred to the patient’s mouth through a holder device called a tray. It is generally perceived among dental professions that an impression, or negative likeness of the teeth and surrounding structures, is necessary to obtain a cast which can then be used to make restoration in the laboratory. For more than a century, this technology was taught in dental faculties as a relatively cheap and easy-to-use technique. This technology however necessitates skilful human efforts, i.e., dental technicians, to be able to work on casts/models obtained by impression making to fabricate precision restorations, usually by traditional waxing or plastic forming and investment casting technology. However, it is a reality that the quality and accuracy of the final product depend mostly on the technician/clinician’s subjective judgment. On the other hand, there are several reports regarding inherent impression inaccuracy and casting shortcomings (Gelbard et al., 1994; Johnston et al., 1971; Luthardt et al., 2006; Luthardt et al., 2005; Morey, 1992; Taggart, 1907; Hollenback, 1962) and the inception and fast development of computer sciences and laser technology make a delightful resort ³.The first attempts at computer-assisted production of dental restorations were made in 1971 (Duret et al., 1988), in contrast to the conventional impression techniques³. The introduction of digital technology has increased the options available for dental treatment and drastically improved sciences like restorative dentistry in many aspects and the prosthodontic applications have widened to print accurate fixed dental prosthesis, implant prosthesis, and complete dentures⁴ . The technology has advanced and more concentrations are on data acquisition with accurate 3D details, simplified designing of the prosthesis with software, and printing same with the highest accuracy by utilization in territories that require millimetric accuracy⁵.

To produce restorations from digital data, there are two types of 3D manufacturing processes. Subtractive manufacturing is one of the processes that can produce 3D models⁶ . The other fabrication method being used is an additive manufacturing (AM) also called “Layered manufacturing” or “solid freeform fabrication,” such as 3D printing³

3D printing can be used to produce models by using different techniques. The AM techniques that use light to polymerize resin are stereolithography apparatus (SLA), triple jetting technology (PolyJet), digital light processing (DLP), continuous liquid interface production (CLIP) fused deposition modelling (FDM), selective laser sintering/melting (SLS/SLM), laser engineered net shaping (LENSTM), 3D printing (3DP), direct ink printing (DIP), laminated object manufacturing (LOM)3. AM technologies Using photochemical reactions triggered by light for the shaping of a part, instead of thermal energy (SLS, SLM) or a binder system (3DP), the photopolymerization-based technologies (SLA, DLP) offer several benefits in cases where a higher feature resolution and surface quality are required. A solid part is created on a layer-by-layer basis by photopolymerization of a suspension of ceramic particles in a photosensitive resin⁷ .

The techniques that use light to polymerize resin are digital light processing (DLP), and continuous liquid interface production (CLIP). The DLP technology uses a digital projector as the light source to polymerize the liquid resin layer-by-layer across the entire platform., and has been widely applied in the field of medical products production and development, whereas The CLIP technology is a relatively new method that involves projecting UV images in a continuous sequence.

The advantage of this technology is that the 3D objects grow continuously without interruption by controlling the oxygen flux and it makes a range of features and applications possible for industries as varied as automotive, medical, and consumer electronics⁶ .

Among the various AM techniques, digital light processing (DLP) is gaining increased popularity in the production of dental parts. The DLP process involves a digital micromirror device (DMD) that is used to dynamically define a mask image that is projected on the resin surface. DMDs consist of hundreds of thousands of individually moving micromirrors that control the reflection path of light. Each pixel of the image corresponds to an individual micromirror, the orientation of which can be switched among several degrees based on the geometry of the part to be printed⁸ . Customization of build angle/orientation during the build process is one of the factors that can improve the geometrical accuracy as well as the structural properties of the final 3D-printed part by using the full capabilities of the light source. However, studies in the dental literature investigating the influence of various technical factors involved in the DLP technique on the accuracy of printed dental restorations are lacking. Therefore, the current study aimed to evaluate and compare the effect of different build angles on the dimensional accuracy of dental replica models made using DLP additive manufacturing. The null hypothesis was there will be no statistical difference in the accuracy among the tested groups (p > 0.05).

Methodology:

Grouping of the study

A total of twenty-one dental replica models were constructed:

(Group A – Reference model; Group B and C – Test models)

Group A: Seven dental replica models having build angle 1800 were designed by Autodesk Meshmixer

Group B: Seven dental replica models having build angle 900 were designed by Autodesk Meshmixer

Group C: Seven dental replica models having build angle 1350 were designed by Autodesk Meshmixer.

Data acquisition reference models with build angle 1800 and test models with 900 1350 build angle were scanned using a high-resolution optical surface scanner (Shining 3D Ex-pro, Frankfurt Germany) to have a total number of twenty scanned master cast records and forty scanned experimental model records.

Printing of Specimens

The digital data of dental replica models having different build angles designed by Autodesk Mesh mixer software was exported in standard tessellation language (STL) format, where the STL files were again exported to ANY CUBIC Photon slicer V1.3.6 software which was used for the 3D printing of dental replica models using DLP technology.

The dental replica models were fabricated using three different build angles 1800 ,900 ,1350 . The support structure was generated semi-automatically and the software automatically configured any surface that needed support within each build angle. the location of support was generated based on the angle between the building platform and the long axis of the object.

The dental replica models were printed using 3 D printer (ANY cubic, Frankfurt, Germany) with each model fabricated at the centre of the build platform. The DLP printer uses LCD-based technology, an LED light source, a DMD device/chip, a lens, a resin vat, and a build platform moving along the device on the z-axis. The DMD device is composed of several micromirrors that dynamically reflect the light either toward or away from the vat to create light or dark pixels, respectively. The LED source (Fig1) is a high-quality filament having a wavelength of 405nm, with XY axis resolution of 0.051mm 2560*1620(2K) and Z-axis resolution of 0.0 ~ 0.15mm. The printing speed maximum is 80mm/h rated at a power of 55 Watts having a build volume of 130mm(L)*78mm(W)*160mm(H).

Preparing of specimen for analysis:

All the printed dental replica models were soaked into 91% isopropyl alcohol for 5-10minutes and were post-cured in a UV curing machine (Bre. Lux Power Unit 2, Bredent, UK) for 20minutes.

Analysis

All dental replica models were digitally scanned using high resolution dental optical surface scanner (Shining 3D Ex-pro, Frankfurt, Germany). Before scanning, the scanner was calibrated according to the manufacturer’s instructions. The specimens were examined for any manufacturing defects and sprayed with a thin layer of antireflective powder (Fig 2).

The accuracy of printed dental replica models was then evaluated using a digital subtraction technique. All experimental STL (Standard Tessellation Language) files of scanned printed dental replica models were superimposed with the control master STL file via surface matching software Geomagic studio (3D systems), the exported files were aligned to the same coordinate system and a comparison of the accuracy of dental replica models with different build angles was then evaluated through the deviated difference and deviation patterns on color maps (Fig 3). All specimens were scanned and analysed by a trained dental technician.

Statistical analysis:

Data were managed using Microsoft Excel and the statistical analysis was done. All collected data were converted to absolute values for statistical analysis using IBM SPSS Statistics V25 (IBM SPSS Inc., Armonk, NY).

Results:

Mean deviated distance of all the build angles are shown in the table 2,3,4 and figure 4

Discussion

To fabricate a physical prototype (model) in industry and/or medicine; two different approaches have been utilized: subtractive and additive (Liu et al., 2006)⁹. The subtractive technique is usually accomplished by the conventional Numerical Control (NC) machining, generally milling (Petzold et al.,1999)¹⁰. The additive technologies, on the other hand, can produce arbitrarily complex shapes, The key idea of this innovative method which is also called “Layered manufacturing” or “solid freeform fabrication,” is that a solid 3D model of an object decomposed into cross-sectional layer representations and then numerical files in the form of virtual trajectories guiding material additive processes to physically rapid build-up these layers in an automated fabrication machine to form the object called the prototype (Weiss, 1997)¹¹. In this way, the captured 3D data set is rapidly sliced into cross-sections, and constructed layers from the bottom up, bonding one on top of the other, to produce models for applications.

The use of additive manufacturing (AM) in dental branches has many other benefits of which only one of them is medical modelling construction; there are so many useful fields in which AM can be helpful, i.e., mass production of patterns for casting purposes. In this way, time-consuming and/or difficult parts in restoration making can be easily implemented even without human intervention³.

The digital data which is most accurate in the present generation is provided by various intraoral as well as desktop scanners and the integrated software aids in the tessellation of the captured images. The data obtained are commonly stored as an STF file. The abbreviation STF is also used for standard triangle language (STL), stereolithography (SLA), Standard Tessellation Language, or standard triangle program. The names are derived from the process involved. The information of an object is always broken down and stored as triangles giving the process of STL. Tessellation is the process of linking the surface with geometric shapes to avoid overlaps and gaps. Stitching the triangle files leads to the process of Standard Tessellation Language. The STL file has all information required for the 3D modelling process that is required for printing. When the STL files are linked with any 3D slicers it allows for printing⁴ .

Many AM technologies have been employed for making medical prototypes in medicine and dentistry, among them two common types of 3D printing methods: The nozzle-based and the light-based 3D printing. The nozzle-based 3D printing includes extrusion printing and inkjet printing. In these printing methods, the printed materials are extruded or jetted and deposited onto the platform. The light-based 3D printing including digital light processing (DLP) is gaining increased popularity in the production of dental parts.

DLP-based 3D printing technology comes from the image projection technology developed by Texas Instruments in the 1980s. This method uses a set of chipsets based on optical micro-electromechanical technology to process working light sources to photosensitive materials. The main functional part is a digital micromirror device (DMD) which consists of a group of micron-sized, controllable mirrors. The mirrors rotate to control the path of light and then project it onto the photosensitive resin during working. The ordinary arrays have a large number of mirrors, from nearly a million mirrors to more than 2 million. On the other hand, the pixel spacing of the micromirror is only a few microns or a dozen microns. The resolution of the DLP-based 3D printing depends on the projection plane adjusted by DMD and lens. Thus, the DLP printing technique has a relatively high resolution, which is usually at the micron scale¹² . Zhang J, Hu Q, Wang S, et al., in 2020, in their study on Digital light processing-based 3D printing for medical applications have stated that the accuracy improvement is related to the printing equipment, materials, and various process parameters¹². In the same context, a study by Aharbi et al evaluated the influence of build angle and support configuration (thick vs thin) on the dimensional accuracy of full coverage restorations printed using SLA (Stereolithography) technology¹³ .

However, similar studies in the dental literature investigating the influence of various technical factors involved in the DLP technique on the accuracy of printed dental restorations are lacking. Therefore, this study aimed to evaluate and compare the effect of different build angles on the dimensional accuracy of dental replica models made using additive manufacturing and Geomagic surface matching software. Twenty-one dental replica models were 3-D printed (Fig5)

using three different build angles 180 degrees, 90 degrees, and 135 degrees. To minimize all potential handling and processing errors several measures were taken. Each dental replica model was printed individually in the centre of the build platform and post cured (Fig 6)

by a trained dental technician. The models were designed virtually to eliminate any error encountered from the digital model of the original dental replica model. The results were interpreted taking into consideration the RMSE values and the deviation pattern on the color maps (Fig 7).

Areas of yellow and red determine the expansion of the models when compared with the control model. The area of light blue to dark blue represents the shrinkage of the tested model. The pattern of deviation was heterogeneous. Maximum, minimum, and average deviation values were not considered when the accuracy was evaluated. The deviation values can be either positive or negative, which when averaged to provide the arithmetic mean can preclude any existing actual difference.

In this study, the minimum deviation value was 0.047mm for a 135-degree build angle whereas the maximum value was 0.073mm for a 90-degree build angle. This can be explained by the fact that angulation of the dental replica model with a 135-degree build angle offered the most self-supporting geometry in comparison to other build angles. This assumption can be confirmed by observing the increase and decrease pattern of the deviated distance values as well as the pattern of deviation on the colour map.

However, the results of this study should be carefully interpreted when using different DLP systems, other printing materials, different model designs of the printed parts, and thickness of the printed model. Other Factors to be evaluated in the future and having limitations in this study include the depth of cure/ light intensity and the position of the part within the build platform. further drawbacks involve inherited errors associated with any digital procedure, scanning, and slicing errors. An additional consideration was the effect of the quality of the printer. In this study, only one brand of the printer was representative of technology. The accuracy of the 3D printed model may affect by the brand of the printer regardless of the technologies of the printer. Future studies might include this topic to investigate the effect of the printer on the accuracy of the 3D printed dental replica models.

Conclusion

Within the limitations of this study, the recommended build angle using the current DLP system was 135 degrees. Among the selected build angles, it offers the highest dimensional accuracy and self supporting geometry throughout the building process.

Financial support and sponsorship: Nil

Conflicts of interest: There are no conflicts of interest