Novel 3D Modeling Technique of Removable Partial Denture Framework Manufactured by 3D Printing Technology

Abstract

The objective of the present study was to digitize the removable partial denture framework construction using a combination of simplified novel modeling technique and a 3D printing prototyping technology. A partially edentulous stone cast was selected, representing mandibular Kennedy class I. The cast was optically scanned using desktop structured light scanner and the generated 3D model data was exported as STL file then imported to universal reverse engineering software. The cast was digitally surveyed according to the selected path of insertion then all undesirable undercuts were selected, removed and blocked by flat surfaces. A stress releasing design for mandibular class I Kennedy was considered then the components were drawn and cut, as a thin shell, from a 3D model duplicate. Each component was offset outside the 3D model surface to a distance equivalent to its required relief. The framework volume was then created by thickening the shell surface followed by smoothening. Finally, the framework was fine-tuned using sculpt tool. The final framework 3D model was generated layer by layer using the 3D printer machine. The final framework was checked on the stone cast for error and fitness. The final outcome of the current technique produced precise and well-fitted removable partial denture framework using simplified, rapid yet accurate technique of modeling.

Full text

ISSN 2320-5407 International Journal of Advanced Research (2014), Volume 2, Issue 9, 686-694
686
Journal homepage: http://www.journalijar.com INTERNATIONAL JOURNAL
OF ADVANCED RESEARCH
RESEARCH ARTICLE
Novel 3D Modeling Technique of Removable Partial Denture Framework Manufactured
by 3D Printing Technology
Dr. Mostafa Omran Hussein1, Dr. Lamis Ahmed Hussein2
1. Assistant Professor of Prosthodontics, Prosthodontic Department, Faculty of Dentistry, Qassim University
Kingdom of Saudi Arabia
2. Assistant Professor of Dental Biomaterials, Dental Biomaterials Department, Faculty of Dentistry, Qassim
University Kingdom of Saudi Arabia
Manuscript Info Abstract
Manuscript History:
Received: 25 July 2014
Final Accepted: 29 August 2014
Published Online: September 2014
Key words:
Digital RPD manufacturing, RPD
3D modeling, 3D printing,
Prototyping.
*Corresponding Author
Dr.Mostafa Omran
Hussein
The objective of the present study was to digitize the removable partial
denture framework construction using a combination of simplified novel
modeling technique and a 3D printing prototyping technology. A partially
edentulous stone cast was selected representing mandibular Kennedy class I.
The cast was optically scanned using desktop structured light scanner and the
generated 3D model data was be exported as STL file then imported to
universal reverse engineering software. The cast was digitally surveyed
according to the selected path of insertion then all undesirable undercuts
were selected, removed and blocked by flat surfaces. A stress releasing
design for mandibular class I Kennedy was considered then the components
were drawn and cut, as a thin shell, from a 3D model duplicate. Each
component was offset outside the 3D model surface to a distance equivalent
to its required relief. The framework volume was then created by thickening
the shell surface followed by smoothening. Finally, the framework was fine-
tuned using sculpt tool. The final framework 3D model was generated layer
by layer using the 3D printer machine. The final framework was checked on
the stone cast for error and fitness. The final outcome of the current
technique produced precise and well-fitted removable partial denture
framework using simplified, rapid yet accurate technique of modeling.
Copy Right, IJAR, 2014,. All rights reserved
Introduction
Removable partial denture is an indispensable treatment option for certain situations. Although various materials and
techniques were developed in the laboratory dental field, the conventional metallic removable partial denture
manufactured by lost wax technique is still used. This old technique is successful however it’s inherited drawbacks.
The conventional technique is time consuming, requires multiple steps and technique-sensitive. It should be noted
that, more steps used, more chance for errors and ill-fitting denture (Phoenix, et al., 2008). (Rudd & Rudd, 2001a,
2001b, 2001c) reviewed and categorized 243 errors possible during the fabrication of a removable partial denture.
Their three-part articles are considered a very good guide for all practitioners, students, and laboratory technicians to
review all possible errors and recognize their solutions. They clarified that avoiding errors by good technique is
easier than their treatment. In addition, they confirmed that errors are cumulative and so may result to denture
remake.

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Nowadays, the CAD CAM technology became one of the most important developments happened in the dental field
at the twenty-one century. The manufacturing of dental restorations and devices using CAD CAM subtractive
behavior was successful in many situations and so used widely in tooth or implant-supported fixed prosthodontics
and operative dentistry. Consequently, all dental labs started to shift their services to the digital manufacturing
where less material consumed, saving time and effort and capability of mass production (Beuer, et al., 2008; Kapos
et al., 2008; Strub, et al., 2006).
Further, the additive rapid prototyping technologies can fabricate organic complex configurations which were very
difficult to be milled with subtractive method. Consequently, it is suitable for human anatomy structures and highly-
detailed prosthetic appliances such as removable partial denture (RPD), complete denture, maxillofacial prosthesis
and implant surgical guide stents (Beuer et al., 2008; Ciocca & Scotti, 2004; Kurtulmus, et al., 2008; Sun, et. al.,
2009). This innovative method relied on what is called “Layered manufacturing” where a 3D standard triangulation
language (STL) file of an object decomposed into cross-sectional layer representations and then an automated
fabrication machine will receive the numerical inputs of these configurations to form the prototype. In this way,
additive methods are more advantageous and many problems, usually accompanied milling, can be easily overcome.
The ability of the additive prototyping technique to create minor details such as undercuts, voids, and complex
internal geometries which is lack even in milling machines with multiple-axes (Azari & Nikzad, 2009).
RPD frameworks could be fabricated by prototyping indirectly using polymer powders through 3D Printing
technology or directly using metal powder through Direct Laser Sintering technology (Bibb, et al., 2006; Venkatesh
& Nandini, 2013). 3D printing is a unique prototyping technology it differs from other rapid prototyping methods in
two important aspects. The first distinction is the relatively low cost of machines and materials. The second major
difference is that 3D printers seamlessly integrate with computer-assisted design (CAD) software and other digital
files like magnetic resonance imaging (Berman, 2012).
To enrich the digital manufacturing of the RPD many companies of the dental products invested a lot of money in
CAD modeling softwares to get their own specialized one and facilitate the digital steps of RPD production for both
dentists and technicians. However, one of the main obstacles that add cost to the digital manufacturing of the RPD
framework is the cost of the modeling software that enable surveying, block-out of the undercut and designing the
components (Schwab, 2014; SensAble, 2014).
(Bibb et al., 2006; Williams et al., 2006) used a specialized CAD modeling software and a haptic device in
conjunction with various prototyping technologies to produce RPD framework. The outcome product was very
promising and resulted in RPD frameworks that are comparable in terms of accuracy, quality of fit and function to
the conventional technique used in the dental technology laboratory. They also mentioned that digital designing and
manufacturing enable excellent values in terms of reproducibility, time saving and reduced materials consumption
than the traditional technique.
(Eggbeer, et al., 2005) succeeded to use common CAD modeling software to design RPD framework over a 3D
scanned cast. They confirmed that from the actual effectiveness, the quality and precision of fabricated RPD
framework could completely meet the clinical needs, though sometimes it required minor adjustment to fit the
patient’s mouth perfectly.
Furthermore, (Han, et al., 2009) extended the previous experience by using a customized new three-dimensional 3D
computer-aided design/computer-assisted manufacturing CAD CAM software package developed specifically for
RPD design for digitally survey, remove undercuts and build virtual patterns for removable partial denture
frameworks. Finally, metal RPD frameworks were fabricated using a selective laser melting technique. They
claimed that their software procedures were simple but suggested further studies to validate and prove the
effectiveness of the software described.
Moreover, (Wu et al., 2012) relied on using CAD and NURBS modeling software to design RPD frameworks on a
modified 3D scanned cast. The model was digitally surveyed, undercut was removed, and then relief space was
generated. Based on their idea, they recommended creation of the universal component library in the future to enable
both dentists and technicians to customize various components that fit their RPD. (Lang & Tulunoglu, 2014)
reviewed many articles represented digital technique and reported that as with any innovative technology, clinical
studies to support its use must be undertaken. They also added that currently no clinical outcomes research has been
published to support the use of CAD/CAM RPDs.

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The present study was conducted to offer new and simplified RPD modeling technique in order to facilitate digital
manufacturing of the rapidly prototyped denture. Neither special dental software package nor specialized hardware
input devices were required.
Materials & methods
Cast selection and scanning
A silicone-based replica of mandibular class I Kennedy classification (missing 36, 37, 46, and 47 teeth) was poured
by hard stone material using vacuum mixing machine. After cast hardening and removal, the cast was checked for
air bubbles.
The cast was fixed on the scanner table and was scanned using desktop structured-light 3D scanner (Kavo scanner
pro, Kavo Dental, Germany). The measurement precision of the scanner was 20 μm with an absolute ratio 1:1. The
3D model was aligned and the polygon mesh was tuned automatically. Finally, the 3D model was exported as STL
file format, (fig.1A, and B).
RPD Design planning
The simplest design of a class I mandibular RPD was selected with a default strategy of stress-releasing design. The
design included all essential components that fulfill support, retention, bracing, reciprocation, and connection. Two
free-end saddles connected with lingual bar were selected according their structural specifications. In addition, two
RPI clasps were added on the abutment teeth (35 and 45) included mesial rests, proximal plates, and I-bar retentive
arm. Additional rests were added on the distal side of the neighboring teeth (34 and 44). Finally, occlusal rests were
connected to the lingual bar major connector; on each side of the arch, through minor connector.
Cast surveying and modification
The 3D model was imported to a reverse engineering software (Geomagic Studio 2012, Raindrop, Research Triangle
Park, NC, USA) followed by selecting the lateral view to see the side of the model. The path of insertion of the RPD
was selected through anterior tilting of the 3D model in the sagittal plane. The model view was then shifted to top
view where the entire model undercut areas became invisible. Using the select all visible order followed by inverse
selected, the entire undercut areas became selected as red highlighted, (fig.2).
All selected areas were then removed and filled with flat areas free of undercuts except for small areas of undercut
used for clasp retention on the buccal surface of the abutment teeth (specified as desirable undercuts). The cast was
ready at this step to draw the predetermined design directly on the model that was duplicated and be used as a
medium for RPD components creation, (fig. 3, A&B).
Fig. 1: A; stone cast of the selected studied model representing class I Kennedy. B; 3D model of the cast was
scanned using structured-light 3D scanner.

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Fig. 2: Undercut areas relative to the proposed path of insertion were selected.
Fig. 3: A, Top and B, isometric views of the model after undercut blockage with flat areas.
Creation of RPD components
A pen-tablet was used as an input device to facilitate designing process. Using the trim with curve tool, simplified
shaped-saddles were drawn bilaterally and cut from the model as two thin shells, (fig. 4). The formed saddle shells
were then offset outside the model surface by 0.5 mm to represent saddle’s relief (i.e. gauge 24 relief wax), (fig. 5).
Using the same concept the lingual bar was drawn with 4 mm width and 3 mm below the free gingival margin and
from which two minor connectors were cut one between teeth 34 & 35 and the other at 44 & 45. All connectors were
then offset outside the model by 0.25 mm to represent connectors’ relief (i.e. gauge 30 relief wax). The I-bar clasps
were also cut buccaly according to their configuration using 0.25 mm relief offset. Finally, small parts were cut to
connect saddles and lingual bar with extended part forming the proximal plate, (fig. 6).
The next step was converting the surface components into a solid volume. The surfaces were thickened by shell tool
to form 2 mm thickness in outside direction. The RPD framework was formed but with sharp angles (90˚) at
peripheries, (fig. 7). All sharp angles were then smoothed at peripheries while keeping the framework configuration
using smoothing tools, (fig. 8). Finally, some sculpt operations were performed to adjust lower border of the lingual
bar, stopper areas at the saddle end and occlusal rest areas, (fig. 9A).

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Fig. 4: Saddles were cut from model duplicate as thin surface shell.
Fig. 5: Saddle shell (in red color) offset by 0.5 mm outside the model bilaterally.
Fig. 6: Full design of the RPD components (blue color) created as thin surface relieved from the model
surface.

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Fig. 7: RPD volume creation from surface using shelling tool, while arrows showed offset direction.
Fig. 8: Smoothening of the sharp angles of the designed framework after creating the volume.
3D printing of the framework
Before 3D printing of the framework, the 3D model was sliced into layers (0.06 mm layer thickness) using the
software controlling machine (EOS PSW, EOS RP Tools, EOSTATE, Germany) then production process started by
the 3D printing machine (EOS P 396, EOS GmbH, Germany). The CO2 laser type generated framework from
polymer power at 70 watt to sinter the polymer into a solid form with layer-by-layer sequence. The solidified
framework was then removed and cleaned, (fig. 9B) ("EOS system data sheet EOS P 396,").
Fig. 9: A; representative of the final 3D model before import to the machine’s software. B; 3D printed
framework placed on the stone cast

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Results & Discussion
The outcome product of the current technique is an accurately well-fitted RPD framework. All procedures used were
simple and did not require experienced operator. In addition, the whole steps were timesaving and reduce overall
materials consumed during RPD framework production. If a 3D printing service was used, the cost of the RPD
prepared from 3D printed framework was approximately equal to those prepared by conventional technique. It
should be noted also that 3D printing technology is much cheaper than other rapidly prototyping techniques,
(Berman, 2012).
The use of digital surveying was a simple, fast, and precisely determined operation. The role of surveyor and
conventional tools like analyzing rod, carbon marker, and wax trimmer were combined into two simple steps;
selecting undercuts and blocking them with flat surfaces. Both arbitrary and paralleled block-outs were performed
automatically as one-step. Moreover, no need for shaped block-out as its functions becomes unnecessary. This
finding coincides with (Wu et al., 2012) and (Han et al., 2009), as they applied digital surveying and block-out and
excluded the need of shaped and arbitrary block-out. (Wu et al., 2012) typically used the same reverse engineering
software for digital surveying and block-out process while (Han et al., 2009) used a specifically developed CAD
modeling software package for the same function. In addition, (Williams, et al., 2004) developed a new plugin
written using MATLAB softwares and especially designed for this purpose. On the other hand, (Eggbeer et al.,
2005) neglect both procedures during building of their digital framework.
Moreover, the RPD framework components were customized manually through hand drawing. This option is not
available as a tool for commercial software (Schwab, 2014; SensAble, 2014). Although this option could require
more time to start building components, it enables users to finalize the whole framework in a few minutes. It also
facilitates good merging of the designed components.
The use of controlled surface offset using shell tool was used to exchange the conventional virtual creation of relief
wax and enable visualization of the relieved space between the cast and the framework. Accordingly, this method
facilitated relief process and enabled effective relief at no time. Finally, the use of sculpt and smoothening tool
matched the function of adding and removal of wax in the conventional commercial softwares (SensAble, 2014)
(Schwab, 2014) (Williams et al., 2006).
The main difference between the current technique of modeling and the (Wu et al., 2012) and (Han et al., 2009)
techniques’ was the direct use of the 3D cast mesh as a medium for modeling and building the RPD framework in
the current technique. On the other hand, their technique required conversion of the 3D model mesh into CAD
surface before using a universal CAD modeling softwares. Moreover, they also relied on using surface modeling
with their usual building commands like (sweep, loft, extrude .etc.) using sketch planes. This manner of modeling is
suitable for solid engineering models and not so, for models with organic or complex shapes that requires more time
and effort. Unless the components library; using drag and drop, are considered as in the commercial software, the
use of their technique will be impractical and time-consuming.
Based on these findings, the current technique offers a smooth and easy way for simple user. The main concern
during using this technique will be the correct and accurate drawing of the RPD components based on the good
prosthetic knowledge regarding the mechanical and biological considerations of components configuration and
location.
Further studies concerned with clinical application of the present technique are recommended, especially with more
complicated designs and cases, and compared to the commercial CAD/RP techniques. Another recommendation is
fully digitizing the RPD framework production by the use of direct laser sintering technique (DLS). Although (DLS)
technique will be timesaving, it will add more cost to the outcome. Currently, prototyping using metal powders
using Titanium, Chromium Cobalt, and Stainless steel are available for rapid prototyping technology. Although, the
use of such metal powders for human, through prototyping manufacturing, was not supported by biocompatibility
tests (Lang & Tulunoglu, 2014).
Additionally, (Kibi et al., 2009) extended their research to include stress-strain analysis for the CAD RPD before
their fabrication which enriched the overall value of the digital manufacturing method. Consequently, once this tool
enabled as an ordinary step during digital manufacturing, this technology will be indispensable.

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Conclusion
The integrated CAD/RP technique for RPD framework manufacturing becomes a popular successful alternative to
the conventional technique. The use of digital model as a base for modeling frameworks after their modification is a
successful technique for RPD framework manufacturing. Accordingly, the current technique offers a simple, fast,
and precise method for RPD manufacturing. Further clinical trials should be considered to validate their clinical use.
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