Fabrication of Metal-Reinforced Complete Dentures Using the CAD-CAM Technique

The digital manufacture of complete dentures would greatly simplify the workflow; however, the metal-reinforced complete dentures production method has not been well established. This article describes a technique of fabricating metal-reinforced complete dentures in a milling machine using the geometry guide, a negative impression of the occlusal surface of the metal-reinforced complete denture designed using the computer-aided design software. The geometry guide supports and stabilizes the artificial teeth and metal framework in the correct position in the surrounding resin. Fabrication of metal-reinforced dental prosthesis using the digital technique is possible with this method.


Introduction
With the wide applications of digital technology in dentistry, research to establish a completely digital workflow for the fabrication of complete dentures has been underway since 1994 [1]. Currently, complete dentures are being fabricated using computer-aided design and computer-aided manufacturing (CAD-CAM) via milling or rapid prototyping methods. The clinical feasibility of these dentures has been proven [2][3][4][5]. Additionally, various advantages have been reported, such as better fit, retention, easy reproduction, and decreased manipulation [6,7]. However, fabricating metal-reinforced complete dentures using the digital technique is different, as these dentures are fabricated using at least two other materials.
Metal-reinforced complete dentures are associated with improved retention, reduced probability of fracture, and greater patient satisfaction in terms of comfort and pronunciation compared to conventional resin-based complete dentures [8]. Metal denture bases, fabricated using selective laser melting, milling, stereolithography apparatus, or digital light processing techniques, have been found to be feasible for clinical use [9,10]. Digital fabrication of metal-reinforced complete denture bases using the current technology is difficult.
To overcome this limitation, considerable research has been conducted. Oh et al. [11] introduced a method to combine the digital technique and conventional flask procedure to fabricate a removable partial denture. Moreover, Park et al. [12] fabricated a digital denture by bonding the functional cusps of artificial teeth manufactured using a 3D metal printer with the resin denture base. Nevertheless, a fully digital workflow for the fabrication of metal-reinforced complete dentures, preserving the basic structure, has not been developed. Hence, in this technical article, we report a novel method to add a metal framework structure to the complete denture milling procedure. A geometry guide, an indentation for the complete denture, was designed to determine and replicate the correct positions of the artificial teeth and metal framework.

1.
Digitize the master casts and the occlusal relation record by scanning using a desktop scanner (T500; Medit, Seoul, Korea); 2.
Design the metal frameworks with the attachments and complete dentures for both arches separately in the CAD software (Exocad DentalCAD 2.4 Plovdiv; Exocad GmbH, Darmstadt, Germany) using the acquired data; 3.
Replace the denture base with the corresponding tissue surface of the dental cast in the CAD software (Meshmixer; Autodesk, Inc., San Rafael, CA, USA) in the imported standard tessellation language (STL) files. Combine the denture and designed metal framework ( Figure 1); GmbH, Darmstadt, Germany) using the acquired data; 3. Replace the denture base with the corresponding tissue surface of the dental cast in the CAD software (Meshmixer; Autodesk, Inc., San Rafael, CA, USA) in the imported standard tessellation language (STL) files. Combine the denture and designed metal framework ( Figure 1); 4. Extrude the metal portion by 1 mm and export the modified metal-reinforced complete denture data; 5. Design the geometry guide for the metal-reinforced complete dentures using the "Boolean command" in the CAD software (Meshmixer; Autodesk, Inc., San Rafael, CA, USA) that contains the negative impression of a part of the metal-reinforced complete dentures ( Figure 2);

4.
Extrude the metal portion by 1 mm and export the modified metal-reinforced complete denture data; 5.
Design the geometry guide for the metal-reinforced complete dentures using the "Boolean command" in the CAD software (Meshmixer; Autodesk, Inc., San Rafael, CA, USA) that contains the negative impression of a part of the metal-reinforced complete dentures ( Figure 2);

Technical Note: Designing Process on the CAD Software
1. Digitize the master casts and the occlusal relation record by scanning using a desktop scanner (T500; Medit, Seoul, Korea); 2. Design the metal frameworks with the attachments and complete dentures for both arches separately in the CAD software (Exocad DentalCAD 2.4 Plovdiv; Exocad GmbH, Darmstadt, Germany) using the acquired data; 3. Replace the denture base with the corresponding tissue surface of the dental cast in the CAD software (Meshmixer; Autodesk, Inc., San Rafael, CA, USA) in the imported standard tessellation language (STL) files. Combine the denture and designed metal framework ( Figure 1); 4. Extrude the metal portion by 1 mm and export the modified metal-reinforced complete denture data; 5. Design the geometry guide for the metal-reinforced complete dentures using the "Boolean command" in the CAD software (Meshmixer; Autodesk, Inc., San Rafael, CA, USA) that contains the negative impression of a part of the metal-reinforced complete dentures ( Figure 2);

6.
Modify the geometry guide design by discarding the labial, mesial, and distal surfaces of the teeth sockets ( Figure 3). Export the STL file of the geometry guide.
6. Modify the geometry guide design by discarding the labial, mesial, and distal surfaces of the teeth sockets ( Figure 3). Export the STL file of the geometry guide.

Technical Note: Fabrication of Dental Prosthesis
1. Mill the artificial teeth from a double cross-linked resin disk (SR Vivodent CAD; Ivoclar Vivadent AG, Schaan, Liechtenstein). Fabricate the metal framework using the lost-wax casting technique after milling the wax pattern (Mazic Wax; VERICOM Co., Ltd., Chuncheon, Korea); 2. Mill the geometry guide from the gypsum poured on the metal holder attached to the milling machine. Mill the designs in a milling machine (Rainbow Mill-Zr 2nd; Dentium Co., Ltd., Seoul, Korea) after calculating the toolpath; 3. Bond the artificial teeth and the metal framework to the geometry guide ( Figure 4);

Pour autopolymerizing resin (Retec PRESS LT; Retec Kunststofftechnik GmbH,
Rosbach, Germany) to fill the space above the geometry guide. Cure the resin using a dental polymerization unit (Palamat elite; Kulzer GmbH, Hanau, Germany) as per the manufacturer's instructions; 5. Set the metal holder that contains the geometry guide, artificial teeth, and metal framework in the milling machine. Mill the denture according to the acquired modified denture data, in which the metal portion is extruded by 1 mm ( Figure 5);  Mill the geometry guide from the gypsum poured on the metal holder attached to the milling machine. Mill the designs in a milling machine (Rainbow Mill-Zr 2nd; Dentium Co., Ltd., Seoul, Korea) after calculating the toolpath; 3.
Bond the artificial teeth and the metal framework to the geometry guide ( Figure 4); Appl. Sci. 2021, 11, x FOR PEER REVIEW 3 of 6 6. Modify the geometry guide design by discarding the labial, mesial, and distal surfaces of the teeth sockets ( Figure 3). Export the STL file of the geometry guide.

Technical Note: Fabrication of Dental Prosthesis
1. Mill the artificial teeth from a double cross-linked resin disk (SR Vivodent CAD; Ivoclar Vivadent AG, Schaan, Liechtenstein). Fabricate the metal framework using the lost-wax casting technique after milling the wax pattern (Mazic Wax; VERICOM Co., Ltd., Chuncheon, Korea); 2. Mill the geometry guide from the gypsum poured on the metal holder attached to the milling machine. Mill the designs in a milling machine (Rainbow Mill-Zr 2nd; Dentium Co., Ltd., Seoul, Korea) after calculating the toolpath; 3. Bond the artificial teeth and the metal framework to the geometry guide ( Figure 4);

Pour autopolymerizing resin (Retec PRESS LT; Retec Kunststofftechnik GmbH,
Rosbach, Germany) to fill the space above the geometry guide. Cure the resin using a dental polymerization unit (Palamat elite; Kulzer GmbH, Hanau, Germany) as per the manufacturer's instructions; 5. Set the metal holder that contains the geometry guide, artificial teeth, and metal framework in the milling machine. Mill the denture according to the acquired modified denture data, in which the metal portion is extruded by 1 mm ( Figure 5);

4.
Pour autopolymerizing resin (Retec PRESS LT; Retec Kunststofftechnik GmbH, Rosbach, Germany) to fill the space above the geometry guide. Cure the resin using a dental polymerization unit (Palamat elite; Kulzer GmbH, Hanau, Germany) as per the manufacturer's instructions; 5.
Set the metal holder that contains the geometry guide, artificial teeth, and metal framework in the milling machine. Mill the denture according to the acquired modified denture data, in which the metal portion is extruded by 1 mm ( Figure 5); 6.
Remove the attachments, gypsum, and resin covering the metal base. Finish the prosthesis by polishing ( Figure 6). 6. Remove the attachments, gypsum, and resin covering the metal base. Finish the prosthesis by polishing ( Figure 6).

Discussion
In this technique, the denture base designed in the CAD software was first replaced with the corresponding tissue surface of the dental cast to obtain an ideal denture border and enhance the retention and stability of the denture [13]. The geometry guide, specially designed through this technique, functions as a connector to maintain parts of the denture in the correct position. To maintain a stable position, three hemispheres were attached to the maxillary flat metal base and three bars were attached to the mandibular metal framework. The customized design of the artificial teeth ensures ideal teeth arrangement and interocclusal relationship as per individual requirements. In addition, milling individual teeth from a double cross-linked resin block according to the CAD data provides adequate esthetics and accuracy.
Oh et al. [11] utilized the CAD-CAM technique to mill the denture base and artificial teeth from a wax disk. After the placement of the wax structure and metal framework, conventional flasking and injection molding were performed. Thus, the possibility of inevitable errors in these conventional flasking and packing procedures cannot be excluded. However, in our technique, unexpected errors can be eliminated as the conventional manual procedures were replaced by the geometry guide and subtractive milling process. Additionally, polymerization shrinkage of the base resin will not affect the fit of the denture, as the resin is cured before the milling procedure [7,14].
The limitation of this method is that additional efforts are required compared to the commonly used commercial techniques, due to the lack of automatic function in the CAD software for replacing the palatal part of the denture base with the metal base. To protect the milling burs from breakage while in contact with the metal base, the corresponding region should be extruded by 1 mm while designing. Thin remnant parts of pink resin and gypsum should be removed after the milling process. 6. Remove the attachments, gypsum, and resin covering the metal base. Finish the prosthesis by polishing ( Figure 6).

Discussion
In this technique, the denture base designed in the CAD software was first replaced with the corresponding tissue surface of the dental cast to obtain an ideal denture border and enhance the retention and stability of the denture [13]. The geometry guide, specially designed through this technique, functions as a connector to maintain parts of the denture in the correct position. To maintain a stable position, three hemispheres were attached to the maxillary flat metal base and three bars were attached to the mandibular metal framework. The customized design of the artificial teeth ensures ideal teeth arrangement and interocclusal relationship as per individual requirements. In addition, milling individual teeth from a double cross-linked resin block according to the CAD data provides adequate esthetics and accuracy.
Oh et al. [11] utilized the CAD-CAM technique to mill the denture base and artificial teeth from a wax disk. After the placement of the wax structure and metal framework, conventional flasking and injection molding were performed. Thus, the possibility of inevitable errors in these conventional flasking and packing procedures cannot be excluded. However, in our technique, unexpected errors can be eliminated as the conventional manual procedures were replaced by the geometry guide and subtractive milling process. Additionally, polymerization shrinkage of the base resin will not affect the fit of the denture, as the resin is cured before the milling procedure [7,14].
The limitation of this method is that additional efforts are required compared to the commonly used commercial techniques, due to the lack of automatic function in the CAD software for replacing the palatal part of the denture base with the metal base. To protect the milling burs from breakage while in contact with the metal base, the corresponding region should be extruded by 1 mm while designing. Thin remnant parts of pink resin and gypsum should be removed after the milling process.

Discussion
In this technique, the denture base designed in the CAD software was first replaced with the corresponding tissue surface of the dental cast to obtain an ideal denture border and enhance the retention and stability of the denture [13]. The geometry guide, specially designed through this technique, functions as a connector to maintain parts of the denture in the correct position. To maintain a stable position, three hemispheres were attached to the maxillary flat metal base and three bars were attached to the mandibular metal framework. The customized design of the artificial teeth ensures ideal teeth arrangement and interocclusal relationship as per individual requirements. In addition, milling individual teeth from a double cross-linked resin block according to the CAD data provides adequate esthetics and accuracy.
Oh et al. [11] utilized the CAD-CAM technique to mill the denture base and artificial teeth from a wax disk. After the placement of the wax structure and metal framework, conventional flasking and injection molding were performed. Thus, the possibility of inevitable errors in these conventional flasking and packing procedures cannot be excluded. However, in our technique, unexpected errors can be eliminated as the conventional manual procedures were replaced by the geometry guide and subtractive milling process. Additionally, polymerization shrinkage of the base resin will not affect the fit of the denture, as the resin is cured before the milling procedure [7,14].
The limitation of this method is that additional efforts are required compared to the commonly used commercial techniques, due to the lack of automatic function in the CAD software for replacing the palatal part of the denture base with the metal base. To protect the milling burs from breakage while in contact with the metal base, the corresponding region should be extruded by 1 mm while designing. Thin remnant parts of pink resin and gypsum should be removed after the milling process.
With the geometry guide providing support and stability for all components in the right position, this technique can be applied in edentulous cases. Furthermore, additional clinical studies are needed to comprehensively evaluate the metal-reinforced complete den-tures fabricated using this method. In addition, research can be conducted for the adaption of this technique in the manufacturing of removable partial dentures and overdentures.

Conclusions
This technique enables the fabrication of metal-reinforced complete dentures via the CAD-CAM technique using a specially designed geometry guide. The digital technique offers the possibility of fabricating dental prostheses using more than 2-3 materials.