Computer Engineering Complete Dentures Workflow: Systematic Techniques Review

Background: The progress in the use of computer-aided design/computer aided manufacturing (CAD/CAM) to fabricat removable prosthodontic prostheses in dental clinics has been exponential. Commercially existing CAD/CAM denture techniques grow every year with increasing benefits to dentists and patients. Study Design: Systematic review. Aims: This review assessed and evaluated the different clinical production protocols of presently accessible CAD/CAM denture systems that offer decision support for dental practitioners. Methodology: Data for the current object were collected by surveys from different companies manufacturing the Computer Engineering Complete Denture (CECDs). All related subjects published at the last 10 years were collected and included in this review. Techniques were arranged in alphabetical order, as follows. AvaDent Digital Dentures (Global Dental Science), Baltic Review Article Moaleem et al.; JPRI, 33(53A): 207-220, 2021; Article no.JPRI.78337 208 Denture Creator System (Merz Dental GmbH), and Ceramill Full Denture System (Amann Girrbach AG) can manufacture denture fabrication in three visits, including a try-in step. DENTCA Digital Dentures (Whole You Nexteeth, Inc) can also perform this in three visits. The Wieland Digital Denture (Ivoclar Vivadent, Inc) can manufacture in four dental appointments. Recently developed VITA VIONIC Digital System (VITA Zahnfabrik, Bad Säckingen, Germany) can perform two-step CECD manufacturing. Results: Most of the systems involved subtractive manufacturing for the fabrication of their dentures and only closed systems. However, Baltic Denture System and VITA VIONIC material types are an open structure that allow users to choose among different handling protocols. It can be shared with several unclosed digital scanner, CAD software options, and milling machines. Conclusions: The six existing CECD fabrication systems offer many advantages. The decision on which technique to use may be contingent on the dentist’s prosthodontic expertise, patient output amount, and requirements concerning denture individualization.


INTRODUCTION
In computer-aided design/computer-assisted manufacture (CAD/CAM), a 3D object is designed using computer software and made-up through a fully automated procedure [1]. CAD/CAM has been used in dentistry since the mid of early 1980s, especially for fixed prosthodontic prostheses, such as crowns or bridges and implant abutments and maxillofacial prosthetic device [1,2].
Patient dissatisfaction, insufficient retention, and inadequate aesthetics were the most common difficulties associated with CECDs. The addition of a trial placement option for CECDs could result in an improved clinical outcome, thereby minimizing the incidence of other complications related to occlusal vertical dimension (OVD), centric relationship, tooth setup, and aesthetics, thereby enhancing patient pleasure and reducing the amount of remakes. The trouble in interpretation the digital performance for an objective calculation earlier to fabricate is a unique complication for CECDs [2,3].
CECD construction offers significant advantages over the conventional processing system described by Kattadiyil 2017, such as minimizing the appointments visits, fitting improvement, and/or retention [4,5,6] and automatic filling [2]. Kattadiyil et al. reported two CECD constructors that used condensed appointment protocols to engineer prostheses and considered these protocols as significantly advantageous [2,3].
For removable dentures, CAD/CAM fabrication is a novelty [1,7,8,9]. The prosthese device base is milled from ready made acrylic resin blocks [6] that were previously polymerized under high heat and pressure. The result if a highly condensed resin that is assumed to release less irritating monomer [10] and to have low numbers of microporosities. Porosities are among the factors measured for the microbial colonization of the denture device base [11], which is often high, particularly among elder ages or dependent patients [12].
The presented CAD/CAM denture assembly techniques provide several returns. The reduced session protocols appeal to dentists and patients and make full-teeth denture device prosthodontics attractive even to young dentists. The choice of which technique to use should depend on the dentist's prosthodontic expertise, requirements for denture individualization, and throughput rate. Although the initial scientific evidence supports the clinical superiority of CECD-made-up full-teeth dentures, evidence regarding material-specific possessions is still scarce [13][14][15][16].
This review aims to comparability the clinical and laboratory protocols of the currently available CAD/CAM full-teeth denture structures' workflow for the fabrication of CDs. The results will give an overview of the different clinical denture adaptation protocols and deliver choice support for dental physicians.

AVAILABLE CECDS SYSTEMS
Although many digital systems have been proposed for digital and milled full-teeth denture construction, the precise digital duplication of an edentulous arch in its functional and dynamic forms with an accurate both-archs relation remains challenging [17,18]. Currently, six systems offered by manufacturing companies are available for the production of CECDs; these are arranged alphabetically [13][14]  VITA VIONIC Digital System (VITA Zahnfabrik, Bad Säckingen, Germany).

AvaDent Digital Denture
Overview: AvaDent uses subtractive manufacturing to fabricate dentures. They offer two types of full-teeth dentures, as follows: milled denture base with bonded teeth; and one-set monolithic prosthesis. These are produced by AvaDent (Global Dental Science Europe BV Inc). The fabrication needs two appointments if a try-in visit to assess aesthetics, phonetics, and function is not involved. The hightranslucency enamel and dentin core give the great natural aesthetics. The other type of prosthesis offers a milled denture base with bonded denture full-setup of teeth. AvaDent device can provide the clinician with immediate CDs, dentures for maxillary or mandibular arch, record bases, radiographic directors, conversion prostheses, confirmation jigs, and definitive hybrid fixed implant prostheses [13,14].

Manufacturing
Appointments: AvaDent dentures can be done in two appointments. If the clinician feels more comfortable gathering a try-in denture to evaluate aesthetics, phonetics, and purpose of the digital CDs can be completed in three dental visits. This system inclouded the use of Anatomic Measuring Device (AMD), which is utilized for archs relation records [3,10,13,[19][20][21]. Fig. 1 shows case manufactured with AvaDent trays and AMD for CD making. Fig. 2 show routine of optical scanning for partial dental arches. Fig. 3 desdribed the use of present dentures of patients as trays.  Second appointment/optional includes try-in denture stage (ATI). This has a milled base with sockets hooked on which denture teeth are held with wax. In this clinic time, denture teeth can be adjusted (if needed) by rearrangement them in trial denture wax.
Third appointment. The CECD delivary is almost identical to the insertion of a conventionally fabricated CD (S). Pressure pointer adhesive or fit inspector is used. Occlusal modification might be important and can be accomplished at the same visit. Severe disparity in occlusal interactions between the dentures can be accustomed next to the clinical remount method [13,14].

Construction Steps:
The existence of teeth on the different dimensions trays (Fig. 4C) permits evaluation of the overall aesthetics, lip support, interocclusal space, and tooth placement. The BD-KEY trays identically replicate the form or outline and size of the denture teeth in the milling blocks; thus, these trays serve as try-in dentures to confirm the patient's approval of the future CDs. BDS protocol composed of an adjustment clinic and the denture insertion visit. Baltic Denture Creator System is produced by Merz Dental GmbH.
Firstly, the vertical occlusal dimension (VOD) is measured. Then, a functional impression is prepered. The trays are adjusted intraorally. Afterward, by supporting the BD upper Key (a bite rim resembling the master upper dental set) with silicone impression material or thermoplastic impression compound, the occlusal level, incisor dimension, and lip support are recorded. When the upper MXDA has been accustomed to the proper position, the BD lower Key (a bite rim approximating the ultimate MUDA) is joined with the BD upper Key by click maneuver and again reinforced with silicone impression material to reach the earlier determined VD and the CR. The BD Keys resemble the final uuper and lower arches. The adjusted keys also serve as try-in dentures, so that the final CDs can be placed at the next dental visit (Fig. 4D).
Next, following the milling of the CD, the insertion is the same as to that of any conventional CD. Occlusal modification can be achieved intraorally or by using a clinical remount step. Otherwise, the usual impression trays are used, and then, the outstanding steps are digitized (Figs. 4A2-G2). [13, 14,22].

Ceramill Full Denture System
Overview: The Ceramill denture system is parallel to the AvaDent digital system. It is twovisit structure and industrial by Amann Girrbach (AG), Germany. It uses subtractive manufacturing for the construction of upper or lower CDs [13, [23][24].
Manufacturing visits: First visit. The existing CDs are used as custom trays to record the final impression of the edentulous arches via the closed mouth system by means of a light-body VPS material. The patient is educated to make functional movements of the mouth to registered a dynamic impression of the buccal and lingual vestibule (Fig. 5A). Then, the upper and lower are detached from the patient's mouth after complete setting of the impression material. The dentures are digitized with an intraoral scanner. In this step, all the denture surfaces, including the impression, intaglio, and cameo surfaces, are recorded digitally. The scanning of the upper impression starts at the distobuccal area on one side and passages toward the anterior area, the soft palatal areas, and finally, the distobuccal area on the opposite (Fig. 5B). For lower impression, scanning includes moving the scanner head in a slow zigzag manner, beginning at the distal area on one side and following the ridge crest to the other side (Fig.  5B). The dentures are placed back in the patient's mouth, and their stability is ensured (Fig. 5C). Next, the CR is documented with an intraoral scanner. At the end of this appointment, the alignment function of the scanning software is used to automatically align the two digitized denture images to CR by means of the bite scan area (Fig. 5D) [13, 14,[22][23][24].
At the laboratory, the scanned upper and lower denture duplicate format is saved and transferred to a 3D image control computer software program. The entire scan image is segmented into three parts, namely, the upper impression, the lower impression, and the dentition portions. These parts are saved separately in the software format. Subsequently, the digital impressions of the intaglio surface of the denture are inverted to create a digital model of the upper and lower arches by using the software. In this stage, the digital models of the edentulous arches are digitally constructed in the correct inter-arch jaw relationship and are ready to be used for designing the denture (Figs. 5D-E). The digital upper and lower edentulous arches, as well as the dentition part, are transferred into a dental design software to design the denture base and arrange the artificial teeth for the new CD (Fig.  3F). The design includes arrangements of the teeth on the existing arches in CR. Lastly, the setting up of teeth are completed.

DENTCA Digital Denture/Whole You Nexteeth
Overview: DENTCA's concept is parallel to that of AvaDent, except that DENTCA uses a proprietary impression tray with a detachable and re-connectable two-piece upper tray and a threepiece lower tray with a built-in intraoral tracer. The tray can register a final impression and an interocclusal record simultaneously.
The DENTCA system uses one set of impression trays, whereas the AvaDent system requires one set of trays and one AMD [13][14]. The DENTCA technique was presented to the dental schools, centers, and clinics by Whole You, Inc and DENTCA system, Inc, Mitsui Chemicals Group [25].
Manufacturing procedures: Provided upper and lower two-piece trays are selected. These are used to make the definitive impressions and record the two-arches relationship. A #15 surgical blade is used to slice through the impression material on both arches impressions. The anterior sections of the trays are relocated in the mouth for jaw relation records [14,25].
During the first patient visit, anatomical alginate impressions are generated using conventional impression or centric trays ( Fig. 5a  and b). In addition, the VD and provisional archs relation are registered using the so-called centric tray, and the occlusal plane is determined provisionally using a UTS CAD transfer arch (Fig.  5c). In the laboratory, milled upper and lower custames trays are arranged (Fig. 5d and e). The UTS CAD device is used to produce key digital model positioned on a virtual articulator (Fig. 5f) [14].
During the second visit, optionally, functional upper and lower silicone impressions are taken ( Fig. 5f and g) with milled individual impression trays that already contain the information about the vertical jaw relation and the occlusal plane. If necessary, both impressions can be adjusted by observing the compensation values registered by the UTS CAD transfer arch. The gothic arch is then registered using the functional impressions. A click-in set called gnathometer CAD, which is a cutback, is integrated to leave sufficient space for the intra-oral center-point recording system ( Fig.  5h and i). In the laboratory, the gnathometer CAD can be easily clipped to tray impressions and does not require retention ( Fig. 5K and l). Gysi Gothic arch paths meet at an equilibrium area used as a reference during the inter-arch relationship recording step. Then, Gysi Gothic arch is registered with Gnathometer® ( Fig. 5m  and n). The embedded functional impressions are scanned, and a drawing of the coming CD base is gained. Artificial teeth arrangements are proposed by the 3-shape software with posterior teeth positioned in an ideally bilateral balanced occlusion concept. Then, virtual wax final step was digitally accomplished. Finally, the manufactured upper and lower templates are presented on a white PMMA disc (Fig. 5).
Third appointment. The CECD insertion is almost identical to that of a conventionally made CD. Pressure indicator paste is used. Intra-oral occlusal adjustment might be important and could be performed. Severe disparity in occlusal contacts between the CDs can be corrected after a clinical remount laboratory step. [13,14,25].

Weiland Digital Denture
Overview: In the laboratory, the preliminary impressions and the interocclusal record are scanned. Camper line and interpupillary line values are entered into the design software, which then produces virtual models of the edentulous jaws and determines the patient-specific occlusal plane. Customized impression trays with integrated occlusion plates are designed with a uniform offset to allow for the application of impression material and with a recess to allow the stabilization of the Gnathometer CAD (Ivoclar Vivadent, Inc), which is a device that allows the tracing of the gothic arch and records CR.
Intraorally, the customized impression trays are border molded, and then, a definitive impression is made using a PVS material. The UTS CAD is used again to verify the occlusal plane. The Gnathometer CAD is close to the customized trays. The patient's midline, smile line, and canine-to-canine line are established, and the VDO and CR are resolute with traditional systems.
In the laboratory, the records and the functional impressions are scanned, and the occlusal plane is recorded. At this step, the denture teeth are chosen from the software library of denture teeth, and the design program advises a virtual tooth setup. The tooth setup can be modified according to the demands of the clinician and patient or finalized by adding the gingival portion of the dentures if no changes are requested. If the clinician sensations more calm, then ordering a try-in denture to evaluate aesthetics, phonetics, as well as function to allow improvements to the try-in denture if it is necessary, after-that, the dental technician can mill a monolithic PMMA tryin CD. A prepolymerized disk is used to mill the gingival portion of the denture bases. The bases include milled recesses, in which the denture teeth will be bonded with the use of a positioning jig.
Clinically, after the milling of the CD, the placement is performed. Assignment is analogous to that of any conventional CD. Occlusal corrections can be achieved intraorally or by using a clinical remount appointment [13-14,26].

VITA VIONIC VIGO (VITA Zahnfabrik)
The VITA VIONIC VIGO technique offers materials for open CECd systems. The digital proposal and construction can be facilitated by non-system-inherent scanners, software, and milling machines. The system supports whatever adjustment protocol to which the user is familiarized. Hence, a five-step conventional CD production procedure can be applied. Otherwise, a reduced-session protocol with only three sessions (anatomical impression, functional impression plus determination of vertical and maxillomandibular jaw relation, and denture placement) can also be useful. The impressions, casts, or registrations are generated conventionally and digitalized, thereby entering the CECD workflow. If a try-in session is chosen, the try-in dentures can be milled from wax discs provided by VITA system [13, [27][28]. VITA VIONIC VIGO is the first denture tooth that has been perfected for the digital workflow. The tooth has already been preconditioned and shortened basally. After the additive or subtractive fabrication of the denture bases, it is simply removed from the blister pack and fixed into the designed alveoli using the special VITA VIONIC BOND adhesive. The bond is extremely thin and clean. Moreover, it saves time because of the precise and stable fit. Any person can fit it.
Sometimes, just a small detail is missing to make this technology work for everyday laboratory use. Fig. 7 shows the procedures for the construction of CECDs [27][28].

DISCUSSION
Currently, the six available CECD fabrication systems offer a number of advantages, and the decision on which technique to use may be contingent on the dentist's prosthodontic expertise, patient output amount, and requirements concerning denture individualization. Nowadays, the progress in the use of CECDs to produce RP or complete dentures has been exponential in dental centers, private clinis and specialized hospitals, and the number of commercially existing CAD/CAM denture systems increases yearly [14,29]. Fig. 9 gives a summery of the steps of the 6 avaliable systems.
Mai and Lee., 2020 described a well-organized digital workflow for recording functional edentulous arch images with precise inter-arch relationship (centric relation). In addition, exact tooth setup in CAD programs is established by using intraoral scanner for an existing denture and digital denture scanning. The new denture can be planned competently and predictably [24]. Goodacre et al., 2018 described a technique by using intraoral scanning to capture CD impressions. This scanner can capture an accurate mucostatic impression and results in a good mucosal adaptation and stability of the CECD milled prostheses. This system can record the CR and is efficient in the clinical steps. It minimizes the need to transport conventional records to the dental technicians [20].
Steinmassl et al., 2017 stated that the existing CAD/CAM denture fabrication systems offer a variation of advantages, and the choice on which technique to use should rely on the prosthodontic and dental technicians' expertise, patient amount rate, and necessities concerning denture individualization [13].

Fig. 9. Summery of the steps of the different CECDs construction systems
Srinivasan et al., 2019 published a technical statement describing the workflows for manufacturing CECDs by using a novel, custommodified tray to successfully fabricate CAD-CAM milled CDs and resin interim removable partial denture [21]. Also, AlHelal et al., 2017 and Batisse et al., 2021 tried to overcome the difficulties regarding digital performances that can result in additional appointments and costly remakes of CEM-CDs. They presented a good technique that resulted in good aesthetics, fitting, and retention [4,30].
Pacquet et al., 2021 said that composite resin and lithium disilicate glass-ceramic material reveal acceptable dimensional accuracy, and milling glass-ceramic before crystallization significantly enhances dimensional accuracy [31].
Every system has a drawback. For example, in AvaDent, the adjustment steps are made in one session, which may be stressful for CAD/ CAMdenture learners. However, the system was graded as "easier to perform" than the other systems. Failure can be adjusted during the try-in step [13][14]. In addition, this system is the most used for fabricating different type of prostheses and can accommodate many steps and instruments or materials from other types of procedures (Figs. 1-3). BDS cannot fabricate a single-arch denture; dysgnathia also represents a limitation; and training is needed before it can be applied to patients [13][14]22]. Some digital systems do not offer a try-in step, which can cause complications during delivery. DENTCA DD can be used in the presence of severe bone ridge resorption.
The system used does not offered all schemes of upper and lower teeth relationship during function and is only available with a lingualized occlusion opportunity. No long-term clinical research outcomes and good results have been published. Current commercial protocols should be validated through sound clinical case controal and long-term laboratory invistigations and improved if required to overcome the numeros disadvantages listed. Clinician feedback is critical if the manufacturers wish to progress the current treatment protocols.
Six systems exist, and others are being developed. Clinical and laboratory investigations have been conducted, but not on all of the available systems. Nevertheless, current research indicates that this talented digital workflow benefits both the dental technician and the dentists or clinicians.

CONCLUDING REMARKS
Based on the findings of this systematic review of CECDs, the following conclusions are drawn:  Techniques and machines are continually changing and improving in mechanical and surface properties to overcome and/or minimize the patient's overall dissatisfaction in terms of aesthetic, bulkiness, and inadequate retention.
 In all techniques, post insertion adjustments are performed in the conventional way. The systems can accept the using of old dentures if they are utilized in the try-in for the new CECD fabrication. Most of the systems accept external staining for denture teeth.  Try-in for the virtual dentures is highly recommended in all techniques, because it minimizes most of the negative outcomes in various aspects, such as aesthetics, retention, size of selected teeth, vertical dimension of occlusions, centric relation verification, and overall profile of the patient face. In most of the techniques, the white acrylic resin is used.  Issues in the interpretation of the digital preview was recognized as a unique difficulty related to the fabrication of CECDs.  Each technique has its own advantages, disadvantages, and limitations, but all techniques preserve a digital record, which is a great advantage for older adults with limited access to dental care.  Other advantages of CECD fabrication techniques over conventional ones are as follows: elimination of the use of stone, flasking, and processing techniques; and absence of monomer usage and its effects.

CONSENT
It is not applicable.

ETHICAL APPROVAL
It is not applicable.