7.1.1 History of Complete Dentures and the Development of CAD/CAM Technology

Several materials and techniques have historically been used for the fabrication of complete dentures. The materials include wood, ivory, enameled metal, porcelain, gold, vulcanite, celluloid, Bakelite, and polymethyl methacrylate (PMMA) resin. Wooden dentures were carved by hand and some were inlaid with human teeth. Tacks were placed posteriorly to enhance chewing by functioning like cusps. Ivory dentures where either carved by hand in totality from a block or carved and inlaid with human teeth to improve their natural appearance. Vulcanized rubber (Vulcanite), a highly cross‐linked hard rubber, was introduced as a denture base material in the mid‐1800s. It became the primary material used for denture fabrication for nearly 80 years. The process of making vulcanite dentures was extremely technique‐sensitive in addition to producing dentures that had poor esthetics (dark brown to gray color), a bad taste, and a foul smell (Rueggeberg 2002). However, dentures made from vulcanite were the first functional and affordable dentures.

In 1907, a Belgian chemist Leo Hendrik Baekeland discovered a synthetic resin (phenol formaldehyde) that became known as “Bakelite.” It was tried as a denture base but was not popular because of the lack of dimensional stability in the mouth and by the time it was modified into an acceptable form, PMMA was discovered. PMMA, a heat‐processed material, was introduced in 1936 by Dr. Walter Wright and within four years from its introduction, 90–95% of all dentures were made using this material (Peyton 1975). The physical and esthetic properties along with the handling characteristics of PMMA were an improvement over the previous materials used for removable prosthodontics. However, PMMA has its own disadvantages: a net volumetric shrinkage of about 6–7% causing a lack of optimal fit of the intaglio surface of the denture to the underlying soft tissue, adherence of Candida albicans to the acrylic resin (Berdicevsky et al. 1980, Budtz‐Jörgensen 1974), and the presence of residual MMA monomer; PMMA does not fulfill all the requirements of an ideal denture base material (Murray and Darvell 1993).

Traditionally, five‐appointment visits, in addition to post‐placement visits, have been required for the fabrication of conventional complete dentures. The high number of visits required is one of several limitations that cause some dentists to abstain from treating completely edentulous patients (Christensen 2006). Attempts have been made to shorten the number of appointments needed to make the process faster and less costly while keeping the procedure precise and accurate (Ling 2000, Ling 2004). A recent study quantified the costs of complete dentures fabricated using a simplified technique compared with a conventional one (Vecchia et al. 2013). They found that the fabrication of the conversion denture (CD) using the simplified technique demanded less time and cost 34.9% less.

The successful use of computer‐aided design and computer‐aided manufacturing (CAD/CAM) in the fabrication of fixed, implant, and maxillofacial prostheses (Rekow 1987, Wichmann and Tschernitschek 1993, Sarment et al. 2003, Al Mardini et al. 2005, Mörmann 2004) along with a shortage of qualified dental laboratory technicians encouraged the application of CAD/CAM in the field of removable prosthodontics (Ettinger et al.1984). In 1994, Maeda et al. published the first laboratory study investigating the development of a computer‐aided system for designing and fabricating complete dentures. They used a 3D laser lithography technique to fabricate plastic shells of the dentition and record bases of the denture from photopolymerizing resin. Tooth‐colored composite resin material was used to form the teeth; the two segments were then connected together and filled with colored autopolymerizing composite resin and polished manually. In an effort to improve the esthetics of CAD/CAM dentures, a software program was developed to automatically detect and reconstruct anatomical structures (important for the set‐up of artificial teeth) and suggest an artificial tooth arrangement (Busch and Kordass 2006).

Years later, Kawahata et al. (1997) worked on duplicating a complete denture using the CAD/CAM system. They milled the dentures out of modeling wax using a computerized numerical control (CNC) machine and ball‐end mills. They acknowledged the need for further improvement in the fabrication process. Wu et al. (2010) integrated CAD/CAM technology and laser rapid forming system for the fabrication of a titanium record base for a complete denture. They concluded that the fabrication time and cost involved in the fabrication of metallic denture bases could be reduced with the use of this technology.

Following these early attempts, several investigators presented various techniques for CAD/CAM complete denture fabrication in an effort to improve and facilitate the CAD/CAM denture fabrication (Maeda et al. 1994, Kawahata et al. 1997, Goodacre et al. 2012, Kanazawa et al. 2011, Inokoshi et al. 2012, Sun et al. 2009). Kawahata et al. (1997), Kanazawa et al. (2011), and Goodacre et al. (2012) used subtractive manufacturing (such as CNC machining) for the fabrication of their dentures. Kanazawa et al. (2011) scanned a patient’s preexisting complete denture and a set of artificial teeth using three‐dimensional cone beam computed tomography (CBCT) to acquire data of the mucosal surfaces and the jaw relationship records. A 3D CAD software was used to design the virtual dentures. CNC machining was then used to mill a complete denture base where the teeth were manually bonded. They also measured the fabrication accuracy between the 3D master image they obtained from scanning and the 3D data from the fabricated acrylic denture. They found that the occlusal surface presented a 0.5 mm average deviation from the 3D image. Goodacre et al. (2012) reported the first clinical trial placement of a CAD/CAM denture base milled from prepolymerized PMMA in which recesses were created into which denture teeth were bonded.

Other authors (Maeda et al. 1994, Inokoshi et al. 2012, Sun et al. 2009) used additive manufacturing (such as rapid prototyping (RP) or 3D printing) to fabricate their CAD/CAM dentures. Sun et al. (2009) designed a complete denture following the scanning of edentulous casts and occlusal rims. They developed a special program to aid them in virtual teeth arrangement and the fabrication of virtual flasks. The flasks were then printed using RP, the artificial teeth inserted into the printed flasks and the traditional laboratory procedures of packing, processing, and polishing were used to finalize the complete dentures.

Clinicians have long realized the importance of the trial placement appointment in the fabrication of conventional complete dentures (Stephens 1969, Payne 1977). Similarly, for CAD/CAM complete dentures, clinicians recognized the importance of a trial placement appointment and suggested 3D printing or RP, also referred to as additive manufacturing, for the fabrication of CAD/CAM trial dentures (Kanazawa et al. 2011, Goodacre et al. 2012, Bilgin et al. 2015). Inokoshi et al. (2012) evaluated the suitability of using a 3D CAD software loaded with prototype dentures to fabricate trial placement dentures. Trial dentures were fabricated using the RP technique and compared with the conventional method. They concluded that the digitally fabricated trial dentures were as accurate as the conventional ones. They concluded that the adjustments of trial dentures (i.e. arrangement of teeth) required significantly longer chair time than the conventional ones. However, both the patients and the treating prosthodontist rated esthetics and stability of the conventionally fabricated trial dentures significantly higher than the RP trial dentures.

Bilgin et al. (2015) combined CAD/CAM and RP technologies with the conventional techniques of impression and jaw relation records to determine their feasibility and applicability. The milled or printed teeth were attached to the occlusal rims using conventional waxing and flasking techniques to fabricate the dentures. The authors acknowledged that enhancements were necessary relative to equipment, production, and the toxicity of the materials used.

Recently, Katase et al. (2013) evaluated the accuracy of a method that simulates the face after changing the artificial teeth arrangement in complete dentures fabricated by RP. They simulated the faces of 10 edentulous patients with integrated facial and denture data. They compared the facial data of a patient wearing dentures fabricated by RP to the simulated ones. No significant differences were found between simulated faces and actual faces with RP dentures. They concluded that their method could be useful for clinicians who design complete dentures with a computer.

The previously described techniques used for the fabrication of CAD/CAM dentures are promising. However, they still require impressions to be made or casts to be fabricated for the manufacturing process. Patzelt et al. (2013) evaluated, in vitro, the feasibility of using intraoral scanners (IOS) to digitize edentulous jaws to eliminate the need for conventional impression‐making. Intraoral scanners have been successfully used in fixed prosthodontics and studies by Ender and Mehl (2013), Mehl et al. (2009), and Luthardt et al. (2005) have shown that they are accurate. Patzelt et al. (2013) verified the accuracies of the data sets obtained by scanning edentulous jaws with different commercially available IOSs. They concluded that digitizing the edentulous jaws was feasible. However, they did not recommend the use of IOS in vivo to digitize the edentulous jaws due to the high level of inaccuracy. A study evaluated the three‐dimensional variations existing between scans of a conventionally fabricated complete dentures and IOSs of an edentulous maxilla made for the same patient. No significant difference was found between the two scans. They found that the palatal area presented with the least deviation while the anterior area presented the highest incidence of discrepancy (Masri et al. 2020). Another study assessed the feasibility and accuracy of computerized optical impression‐making of edentulous jaws in an in vivo setting. They concluded that IOS were not currently able to fully replace a conventional impression for the fabrication of a complete denture (Hack et al. 2020).

Different techniques that used IOS to record mucosal morphology, maxillomandibular relationship, and centric relation records have been described (Goodacre et al. 2018, Mai and Lee 2020, Fang et al. 2019). They all agree that this technique produces mucostatic impressions, increased patient comfort during the impression, and faster turnaround time for the fabrication of the dentures. However, it was noticed that the maxilla is easier to scan than the mandible and scanning requires two people, an assistant to retract and the clinician to scan the edentulous jaw.

7.1.2 Advantages of CAD/CAM Dentures

1. It is advantageous to be able to provide dentures in two visits. This reduction in the number of patient visits when fabricating a CAD/CAM denture is advantageous for older patients, those who live in nursing homes or assisted living facilities and have difficulty in commuting for multiple appointments to the dental office (Bilgin et al. 2016). The clinical information (impressions, interocclusal records, and tooth selection) is recorded in one appointment of approximately one to two hours depending on the clinician’s experience and the dentures placed in a second appointment.
2. The reduced clinical chair time required for the fabrication of complete dentures makes complete denture treatment more cost‐effective by decreasing the clinician’s overhead (Srinivasan et al. 2019).
3. All 3D images and collected data involved in the CAD/CAM denture fabrication are saved digitally. The stored data can be used to produce a spare denture or a replacement denture if the patient loses their denture(s) (Srinivasan et al. 2019). These new fabricated dentures will have the same form as the previous dentures, thereby eliminating or minimizing adaptation time for the patient. A surgical or radiographic template can be constructed using the same data to facilitate the treatment planning and placement of dental implants in the future (Bilgin et al. 2016).
4. The prepolymerized acrylic resin used for the fabrication of the denture base provides a superior fit and strength when compared with conventionally processed bases. The prepolymerized acrylic resin, since it is milled, undergoes no polymerization shrinkage, which usually eliminates the need for incorporating posterior palatal seal, especially for those patients with firm residual ridges (Goodacre et al. 2016).
5. The prepolymerized acrylic resin used for the fabrication of CAD/CAM dentures has improved physical properties. For example, the milled prepolymerized acrylic resin contains less residual monomer and exhibits better density compared with conventional denture base resins (Ayman 2017, Steinmassl et al. 2017); the resin is more wettable (hydrophilic) (Gesser and Castaldi 1971, Monsénégro and Proust 1989), has a smoother surface (Arslan et al. 2018), and exhibits better resistance to surface staining (Al‐Qarni et al. 2020). In addition, it demonstrates a higher flexural strength, modulus of elasticity, and fracture toughness (Steinmassl et al. 2018, Al‐Dwairi et al. 2020).
6. Several advantages present for the dental technician. CAD/CAM denture fabrication requires a shorter time for fabrication compared with the conventionally fabricated complete dentures. There is no need for a gypsum cast, articulated casts, teeth set‐up, flasking, and processing. The CAD/CAM denture fabrication is faster and provides high‐quality dentures that fit more accurately.

7.1.3 Disadvantages of CAD/CAM Dentures

1. Commercially available CAD/CAM complete denture systems where carded teeth are manually bonded to the milled bases and not adjusted on an articulator, a clinical remount procedure will be often required to balance the occlusion.
2. Even though the CAD/CAM denture fabrication process facilitates denture fabrication and reduces the appointment visits, there is a learning curve for the inexperienced clinician initially that could lead to disappointment and unsatisfactory results.
3. When the laboratory is not near the dental office, the dentist needs to use a scannable impression material that is dimensionally stable and temperature resistant to resist distortion during shipping.
4. The lack of trial placement appointment could create an increased chance for less than an ideal outcome and missed opportunity for minor adjustments. Wax trial denture with a processed base can be provided by using a three‐appointment process.
5. Laboratory fees and material cost could be higher than conventionally fabricated complete dentures in some parts of the world.

7.3.1 Step‐by‐Step Procedures for the Fabrication of Complete Dentures Using the AvaDent® System

7.3.1.1 Appointment 1

Four popular techniques can be used to order an AvaDent® complete denture: (i) if the patient presents wearing existing dentures, the dentures can be duplicated, border molded with the use of polyvinyl siloxane (PVS) impression material followed with a wash and a light‐body PVS; an interocclusal record will be made to record the occlusal vertical dimension (OVD); (ii) definitive impressions are performed in the traditional manner (Figure 7.1a–f). The length of the maxillary lip is measured with a lip ruler. The measurement is made between the incisal papilla and the inferior border of the maxillary lip (Figure 7.2). The AvaDent‐Wagner EZ guide protocol uses printed bases with printed maxillary and mandibular anterior teeth set in rims formed of a special material that resembles pink baseplate wax called the Wagner try‐in (Figure 7.3). The bases and teeth are provided by the manufacturer in the form of standard tessellation language files (STL files); (iii) an IOS can be used to perform an intraoral scan of the edentulous jaws obtaining STL files that could be used to fabricate Wagner EZ guides for the trial placement appointment; (iv) conventional record bases and wax occlusion rims fabricated from definitive impressions could be used to set the teeth, have a wax trial appointment, then scan the wax trial dentures, and send the data to the laboratory for fabrication of the digital complete dentures.

7.3.1.2 Appointment 2

Prior to the fabrication of the final digital denture, the clinician can request four types of trial dentures to allow for evaluation of esthetics, phonetics, and functional components of the denture in the patient’s mouth. One option is the advanced try‐in denture (milled base with denture teeth set in wax rims) that allows a limited amount of denture teeth movement and adjustments that can be made to the trial denture by repositioning the teeth in the wax to meet the needs of the patient. The second option is the Wagner EZ guides (Figure 7.4a–e). The third is a trial denture where the base and the teeth are milled of white resin. The fourth option is a printed resin trial denture in a monochromatic shade with both the base and the teeth having the same shade.

7.3.2 AvaDent Conversion Denture for Immediate Loading of a Complete Arch Implant Prosthesis

It is possible to fabricate a digital conversion prosthesis that has a unique design for easy modification to an immediately loaded interim fixed complete denture on implants placed using a guided surgical protocol. The technique saves significant clinic time by allowing easy pick up and conversion of the digital denture to an interim fixed complete denture.

7.3.3 Clinical Procedures

1. The first step in the complete denture (CD) protocol is the fabrication of provisional dentures following the previously described steps involved in the fabrication of the digital complete dentures (Figure 7.6a,b). The provisional monolithic dentures are fully milled (including both the denture base and teeth) using a proprietary technique.
2. A radiographic template is fabricated by AvaDent^TM that is a duplicate of the mandibular provisional denture to which fiduciary markers (gutta‐percha placed into spherical indents made on the facial cameo surface) are placed (Figure 7.7). A CBCT scan is made of only the radiographic template and then the radiographic template is positioned intraorally using an index and a CBCT scan made of the patient following the NobelGuide™/NobelClinician protocol (Figure 7.8). The surgical planning for implant placement is performed using NobelClinician Software based on the orientation of the denture bases and teeth to the existing bone and vital anatomical structures so the most appropriate locations are identified for each implant (Figure 7.9a–d).
   

   

3. The virtually determined implant positions are finalized using the NobelClinician Software and the required data is sent to Nobel Biocare for fabrication of a NobelGuide™ surgical template (Figure 7.10). The virtual implant positions are then used by Global Dental Science to fabricate an AvaDent^TM CD (Figure 7.11). This CD has channels milled through the denture base at the appropriate positions where temporary copings would be located after their attachment to the implant abutments. It also has a pre‐milled slot located around the denture base with a small number of struts that connect the peripheral denture base to the central portion of the CD that will function as the immediate provisional fixed CD. The presence of the channels and slot facilitates easy conversion of the denture to a fixed prosthesis while using the positional stability of the peripheral denture base to accurately orient the prosthesis in the patient’s mouth during attachment of the denture to the temporary copings.
   

   

4. The NobelGuide™ surgical template is then used to place the selected implants (NobelReplace® Conical Connection) as planned virtually (Figure 7.12a,b). Nobel Biocare multi‐unit abutments of appropriate height are attached to the implants and the abutment screws torqued to the manufacturer’s recommended values (Figure 7.13). Nobel Biocare Temporary Copings for multi‐unit abutments are then attached to the multi‐unit abutments (Figure 7.14).
5. The AvaDent™ CD is positioned over the temporary copings with the denture base and occlusion guiding the appropriate position (Figure 7.15a–d). The denture is connected to the temporary copings by flowing autopolymerizing acrylic resin between the channels in the denture and the temporary copings and allowing the resin to polymerize (Figure 7.16). The temporary coping screws are then loosened and the prosthesis is removed from the mouth (Figure 7.17).

The struts are sectioned to separate the peripheral section of the denture base from the central portion that will serve as the immediate fixed CD (Figure 7.18). Autopolymerizing resin is flowed as needed between the denture base and the temporary copings where voids were present to attain a smooth transition between the denture base and copings (Figure 7.19). The converted mandibular provisional fixed CD is finished, polished, and attached to the implant abutments using the temporary coping screws (Figure 7.20 and 7.21). The occlusion is finalized against the previously fabricated maxillary provisional CD (Figure 7.22a–c). Post‐operative medications and instructions are given to the patient followed by periodic reevaluation appointments.

7.3.4 Technique Description for the Fabrication of a Digital Definitive Fixed Complete Denture

Once the implant healing phase is completed, the definitive phase for the fabrication of a mandibular fixed complete denture begins. The steps involved in the fabrication include: Final impressions and records.

Based on the information obtained from the Nobel Clinician software and the surgical guide used for the guided surgery, Global Dental Science provides an AvaDent implant record device (AIRD) (Figure 7.23) and AvaDent verification jig (AVJ), which are designed to simplify the impression‐making process (Figure 7.24). The AIRD is a duplicate of the complete denture but has an occlusal opening that fits over the verification jig. It functions as an impression tray.

The resin AVJ fits simultaneously over and around either impression copings or temporary copings (Figure 7.25).

Either of these copings can be used for the final impression. The AVJ is positioned so there is a 2–3 mm space between the inferior surface of the jig and the mucosa. It is then secured to the copings by injecting flowable composite resin between the jig and the copings (Figure 7.26a–c).

The implant record device has an opening in the denture base that is large enough to fit around the AVJ that has been connected to the copings while also resting on the posterior edentulous ridge (Figure 7.27a,b). If any portion of the device comes in contact with the AVJ, the device is adjusted sufficiently to remove the contact to allow easier and accurate placement.

The final impression is then made using the AIRD as an impression tray. Modeling base plate wax can be used to cover the open portion of the AIRD after it is coated with impression adhesive and then heavy‐body impression material is placed in the device (Figure 7.28).