A review of computer-aided design/computer-aided manufacture techniques for removable denture fabrication.

The aim of this review was to investigate usage of computer-aided design/computer-aided manufacture (CAD/CAM) such as milling and rapid prototyping (RP) technologies for removable denture fabrication. An electronic search was conducted in the PubMed/MEDLINE, ScienceDirect, Google Scholar, and Web of Science databases. Databases were searched from 1987 to 2014. The search was performed using a variety of keywords including CAD/CAM, complete/partial dentures, RP, rapid manufacturing, digitally designed, milled, computerized, and machined. The identified developments (in chronological order), techniques, advantages, and disadvantages of CAD/CAM and RP for removable denture fabrication are summarized. Using a variety of keywords and aiming to find the topic, 78 publications were initially searched. For the main topic, the abstract of these 78 articles were scanned, and 52 publications were selected for reading in detail. Full-text of these articles was gained and searched in detail. Totally, 40 articles that discussed the techniques, advantages, and disadvantages of CAD/CAM and RP for removable denture fabrication and the articles were incorporated in this review. Totally, 16 of the papers summarized in the table. Following review of all relevant publications, it can be concluded that current innovations and technological developments of CAD/CAM and RP allow the digitally planning and manufacturing of removable dentures from start to finish. As a result according to the literature review CAD/CAM techniques and supportive maxillomandibular relationship transfer devices are growing fast. In the close future, fabricating removable dentures will become medical informatics instead of needing a technical staff and procedures. However the methods have several limitations for now.

INTRODUCTION

With continuous developments over several years, present-day technological advancements allow the use of different systems with computer-aided design/computer-aided manufacture (CAD/CAM) technology for the fabrication of removable dentures, including milling and rapid prototyping (RP).[1]

CAD/CAM technology refers to digital design and manufacture. CAD software recognizes the geometry of an object while CAM software is used for the manufacture. The CAD/CAM manufacturing process can either include additive (RP) or subtractive manufacturing (computer numerical control [CNC] machining; milling). RP has been used for industrial purposes and was developed from CAD/CAM technology. It is used to create automatically physical models from computerized three-dimensional (3D) data.[23] RP, also known as solid freeform fabrication or layered manufacturing, has been used for creating 3D complex models in the field of medicine since the 1990s and has recently become popular for the fabrication of removable dental prostheses.[45] CAD/CAM and RP have been used for several years for the fabrication of inlays, onlays, crowns, fixed partial dentures, implant abutments/prostheses, and maxillofacial prostheses.[6] Currently, not only fixed restorations but also removable dentures are manufactured using CAD/CAM and RP.[7891011121314] However, few studies have reported on the use and effectiveness of RP for removable denture fabrication.[4]

Subtractive manufacturing technique is based on milling the product from a block by a CNC machine. The CAM software automatically transfers the CAD model into tool path for the CNC machine. This involves computation that points the CNC milling, including sequencing, milling tools, and tool motion direction and magnitude. Due to the anatomical variances of dental restoration, the milling machines combine burs with different sizes. The accuracy of milling is shown to be within 10 µm.[1516]

The first removable prosthesis based on 3D laser lithography was manufactured by Maeda et al.[12] in 1994. Subsequently, the removable prosthesis duplication technique was improved using CAD/CAM with a computerized numerical control (CNC) system and ball-end mills by Kawahata et al.[11] in 1997. Then, Sun et al.[13] fabricated individual physical flasks using a 3D printer.

Impressions of the edentulous maxilla and mandible or existing dentures are subjected to laser scanning during CAD.[1112] Also, cone beam computed tomography is used for the modification of previous dentures.[7] CNC, laser lithography, and RP are used for the CAM process.[10111213]

AvaDent and Dentca are the two available commercial manufacturers of removable complete dentures with CAD/CAM, using a gadget for transferring the maxillomandibular relation (MMR) to a digital articulator and finalizing the dentures completely with CAD/CAM. In the process used by AvaDent, denture bases are milled using a subtractive technique from prepolymerized denture resin. The Dentca technique uses an additive process, wherein a trial denture can be prepared, if the dentist requires, using RP (stereolithography [SLA]) before the conventional fabrication of a definitive prosthesis.[171819]

An electronic search was conducted in the PubMed/MEDLINE (National Library of Medicine, Washington, DC), ScienceDirect, Google Scholar, and Web of Science databases for identifying English articles using the following key word combinations:

“CAD/CAM and complete dentures”

“CAD/CAM and removable partial dentures (RPDs)”

“CAD/CAM and removable dentures”

“CAD/CAM and removable prosthesis”

“RP and complete dentures”

“RP and RPDs”

“RP and removable dentures”

“RP and removable prosthesis”

“Digitally designed and removable dentures”

“Digitally designed and complete dentures”

“Digital complete dentures”

“Digital removable dentures”

“Rapid manufacturing and removable dentures”

“Milled,” “machined,” “computerized,” and “removable dentures.”

Articles about removable dentures fabricated using CAD/CAM and RP that were published from 1987 to 2014 were selected. These included reviews and laboratory and clinical reports. Articles published in non-English languages that included identified search terms in the title or abstract were excluded. The search process was executed in three phases as searching of titles, analysis of abstracts, and identification of full-text articles. Also, Google search was conducted for available commercial manufacturers of CAD/CAM prostheses and their processing techniques. The identified developments (in chronological order), techniques, advantages, and disadvantages of CAD/CAM and RP for removable denture fabrication are summarized in Table 1.

Table 1

The table of published articles about CAD/CAM techniques for removable denture fabrication from 1994 until 2015

TECHNIQUES AND MATERIALS USED FOR DENTAL COMPUTER-AIDED MANUFACTURE

CAM includes subtractive and additive manufacturing techniques [Figure 1].

Figure 1

Overview of computer-aided design/computer-aided manufacture systems for dental use

Early CAM systems are based on substractive method that was relied on cutting the restoration from a prefabricated block using burs, drills, or diamond disks. Subtractive manufacturing includes CNC machining used for the manufacture of crowns, posts, inlays, and onlays. The subtractive production methods include spark erosion and milling. The spark erosion can be defined as a metal substractive process using continuing sparks to erode material from a metal block according to the CAD under required conditions. Milling techniques are diamond grinding and carbide milling which are now found together in chairside and inLab CAD/CAM devices together and as the latest transferred technology from manufacture industry to dental use is laser milling, which was announced in first quarter of 2015. Milling techniques are mostly dependent on the device properties such as the dimensional approach and possibilities of working axis: 3 spatial direction X, Y, and Z which refers to 3 axis milling devices while 3 spatial direction X, Y, Z and tension bridge refers to 4 axis milling device, and finally 3 spatial direction X, Y, Z, tension bridge with milling spindle is classified as 5 axis milling device.[30]

Additive 3D printing techniques include SLA, digital light projection (DLP), jet (PolyJet/ProJet) printing, and direct laser metal sintering (DLMS)/selective laser sintering (SLS).

The SLA technique uses ultraviolet (UV) laser for layer-by-layer polymerization of materials. The technique is used for the manufacture of dental models from UV-sensitive liquid resins. DLP uses UV laser and visible light for polymerization and is used for the manufacture of dental models, wax patterns, removable partial frameworks, and provisional restorations from visible light-sensitive resins, wax, and composite materials. After the material is printed, it is cured using a light-emitting diode light source or lamp.[31] Also, polymethyl methacrylate (PMMA) is used in the DLP technique.[32] Jet (PolyJet/ProJet) printing uses a series of ink-jet print heads and tiny pieces of material jetted onto support material and create each layer of the part. Then, each jetted layer is hardened using a UV lamp, light source, or heating. This technique is used for the manufacture of dental models, surgical drill guides, aligners, wax patterns, and removable frameworks from dental resin and waxes. DLMS/SLS is a powder-based technique wherein high-power laser beam hits the powder, resulting in melt and fusion of the powder particles. This technique is used for the manufacture of dental models, copings, and surgical guides from cobalt-chrome, palladium chrome, and nylon.[31]

MANUFACTURING PROCESS OF REMOVABLE PROSTHESIS WITH COMPUTER-AIDED DESIGN/COMPUTER-AIDED MANUFACTURE AND RAPID PROTOTYPING

Manufacturing steps for complete dentures

First, models can be prepared using conventional impression or intraoral digital impression. When digital impression is considered, practitioner will need for high speed, high density, small size, and multifunctional device which has driven the development of 3D imaging.[33] The precision of digital impression has been studied by several researchers and found out that use of digital models is a relatively new technique that has an accuracy of up to 10 µm, and the models have been found to be as reliable as traditional stone casts.[34] Nalcaci et al.[35] found out statistically significant differences between measurements obtained for width of 6 anterior teeth and 12 overall teeth using plaster and digital models; however, these differences were not within the clinically significant range (~0.27–0.30 mm). Therefore, casts are scanned using digital scanner for conventional technique. After taking impression, the next step is making MMR transfer during complete prosthesis fabrication using CAD/CAM. There are three options for MMR transfer during complete prosthesis fabrication using CAD/CAM: The MMR can be transferred using conventional impression and transfer techniques, the AvaDent system kit, or the Dentca system kit.[1819]

Two clinical appointments are required for the manufacture of removable complete dentures using the Avadent and Dentca systems. In the first appointment, impressions are recorded using special trays provided in the AvaDent or Dentca system. Then, the jaw relation is recorded using an anatomical measuring device. The occlusal vertical dimension (OVD) is determined using conventional methods. Subsequently, the centric relation is recorded, and teeth are selected. The last step of the first appointment is the delivery of the final impression to the manufacturer (AvaDent or Dentca).

At the laboratory, the denture borders are first defined and marked using the system's computer software. Then, the teeth are virtually set, and the prosthesis base is milled from traditional denture resin material. A trial denture can be prepared as per the dentist's request.

In the second clinical appointment, the dentures are delivered and any occlusal adjustments made. These steps are similar to those for conventional prosthesis delivery. Only the AvaDent technique of denture base manufacture is not conventional.[171819]

Manufacturing steps for framework of partial prosthesis

Designing of the RPD framework generally consists of four parts as base, plate, clasp, major, and minor connector of the framework. Every part of the RPD framework must be done proper design and thick value in the designing process.[23] Because of the variety of RPD parts and their irregular forms, 3D designing of RPD framework is taking much time and complicated. For this reason, researchers investigated proper CAD/CAM method and software for 3D designing of RPD framework for many years.[20213637]

Basically, steps for manufacturing of framework of partial prosthesis with CAD/CAM and RP are:First, dental casts are prepared using conventional impression method or digital impression. Casts are scanned using digital scanner for conventional technique. Path of insertion of the RPD is defined digitally, and then shape of the components of the framework is designed 3D by dentists or laboratory technicians. Finally, digitally designed metal RPD frameworks are produced with RP.[36]

Advantages of digital fabrication of dentures

Decreased number of appointments

Shrinkage of acrylic base caused by milling of prepolymerized acrylic resin with an increase in the strength and fit of dentures

Decreased duration of prosthesis manipulation

Decrease in the risk of microorganism colonization on the denture surfaces and consequent infection

Advances in standardization for clinical research on removable prostheses

Easy reproduction of the denture and manufacture of a trial denture using stored digital data

Superior quality control by clinicians and technicians.[38]

Limitations and disadvantages of digital fabrication of dentures

Manufacturing challenge caused by impression-taking and OVD-recording procedures, MMR transfer, and maintenance of lip support, which are all similar to the procedures used in the conventional process

Inability to define the mandibular occlusal plane

Expensive materials and increased laboratory cost compared with those for conventional methods

Lack of trial denture manufacture by the Avadent system,[18] which precludes the evaluation of dentures by patients and dentists before final denture fabrication.

CONCLUSIONS

Since the fabrication of the first modern removable dentures using PMMA, no significant changes in fabricating techniques were introduced until CAD/CAM techniques came into the picture in the 1990s. Current innovations and developments in dental technology allow the fabrication of removal dentures using CAD/CAM technologies from start to finish, thus decreasing the chair side and working time for patients and dentists and providing superior or satisfactory functional and esthetic outcomes. The development of a digital face simulator using imaging techniques with lower effective doses of radiation in the close future will be another milestone for removable denture fabrication with digital OVD recording and MMR transfer before finalization with CAM.

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest