Digital Jaw Relation Record of Edentulous Patients in the CAD-CAM Workflow of the Implant-Supported Full-Arch Prosthesis

In the past two decades, interest in computer-aided design/computer-aided manufacturing (CAD-CAM) technologies for complete fixed denture has increased, such that these technologies are now widely used in dentistry.¹  Important improvements have been made in full-arch rehabilitation using implants, and automated procedures based on CAD-CAM techniques have been developed and applied to various phases of the manufacturing workflow.^2–7  For example, the ability to produce complete dentures using CAD-CAM technology provides high process reliability, and many authors have published reports on this subject.^8–13  Moreover, digitalization enables conversion of the complete denture into a complete-arch interim restoration, as well as transfer of the vertical dimension of occlusion into the final full-arch fixed restoration.^14,15  Other authors developed systems that were successfully employed to create virtual face replicas for comprehensive diagnosis and treatment planning based on esthetic data.^16,17 

However, before beginning any digital process, an evaluation of the provisional complete denture (PD) is needed for assessment of masticatory efficiency, occlusion, and esthetic parameters. After implementing the necessary corrections for improvement of esthetics and occlusion, the PD may be used for the implants guided surgery and for digital design of the final full-arch prosthesis. From a prosthetic point of view, once the clinical comfort of the patient is obtained with the complete denture, the clinical challenge is to transfer all occlusal, functional, and esthetic PD features to the final implant-supported fixed prosthesis. To the best of our knowledge, there are minimal data regarding digital transfer of the occlusion in edentulous patients from the modified PD to the final full-arch prosthesis; moreover, a method to clinically perform corrections on 3D-manufactured resin PD replicas of the definitive prosthesis, and to digitally integrate these modifications into a definitive digital design, has not yet been described. The rationale of this digital procedure is to simplify the clinical steps for the construction of a full-arch fixed prosthesis on implants, by using the preoperative complete denture as a try-in test and copying its final features in the definitive fixed prosthesis.

The main aim of this report is to detail the digitalization process used to transfer data from the PD to the final fixed restoration on implants and to achieve and integrate all occlusal, esthetic, and phonetic parameters into the complete digital workflow.

The workflow is applicable to complete denture wearer scheduled for full-arch prosthetic rehabilitation on implants, and begins with PD revision prior to implant surgery. The base of the complete denture must be relined with a silicone material (Elite HD+ Light Body; Zhermack) or with a soft relining material (Coe-Soft, GC Inc) to account for differences between the osteomucosa and the PD base. To ensure a success with this digital protocol, the accurate record must communicate 6 key factors: midline, centric relation, occlusal vertical dimension (OVD), anterior tooth size, lip support, and incisal edge location. Esthetic parameters are corrected with a build-up of central incisor used as a “guide tooth” (Figure 1): by adding composite material (Tetric Evo, Ivoclar Vivadent) to the incisal edge, the esthetics of the smile line can be evaluated and modified in accordance with the esthetic parameters of the lower and upper lips, as well as the age and chief complaint of the patient. Concomitantly, the occlusal plan may be modified in accordance with the Camper plane: in this case, variations of the occlusal profile must be drawn on the buccal aspect of the denture posterior teeth and measured according to each dental element (Figure 2). Digital impressions of the PD on both mucosal (Figure 3) and buccal surfaces are then made at chairside using an intraoral scanner (True Definition Scanner [TDS]; 3M ESPE).

For the occlusion, if any discrepancy exists between the PD and the correct centric relation, a new intermaxillary registration may be recorded (UTS-CAD, Ivoclar Vivadent) Then, the maxillomandibular relationship of the patient may be scanned as is, using the dental scanner (TDS; 3M-ESPE), and resulting Standard Tessellation Language (STL) file can be sent to the laboratory. The entire procedure may be carried out during the first appointment. When the PD and its occlusion are digitalized in accordance with all corrections and measurements, the digital design of the PD is consequently integrated (Figure 4) into a new project for final rehabilitation (Exocad Dental CAD, Exocad). This project is used to 3D print (Micro, EnvisionTEC) the resin prototype (Prototype polyamide-resin; 3D Systems), and the modified PD replica is used as a clinical try-in during the final check of the esthetics (Figure 5). The vertical dimension of occlusion, Spee arch conformation, Camper plane orientation, smile design, phonetics, and occlusion are finally incorporated into the virtual articulator for subsequent construction of the surgical template for the implants, and for creating successive provisional and final full-arch prostheses on implants (Figure 6). The implant-guided surgery is set up on digital software (Nobel Clinician; Nobel Biocare) for 3D-manufacture of the surgical guide. Flapless surgery is performed and, using the scan bodies of the implants, digital impressions of both the maxillary and mandibular arches are made in the oral cavity with an intraoral scanner (TDS). To register the occlusion while the scan abutments remain in the same position of the impressions, the radiographic guide is applied, and the morphology is modified to allow embracement of the scan abutments when the patient is in occlusion: corresponding to the scan bodies, a hole for each scan body is realized on the radiographic guide, thereby creating a window in the buccal aspect of the resin base to enable the intraoral scanner to clearly visualize the scan body during the digital impression (Figure 7). In this manner, the software (TDS) registers the maxillomandibular relationship of edentulous arches with implants, according to the centric relation previously determined with the PD. This allows for superimposition of the STL file of each single arch on the STL file of the scan bodies in the correct occlusion (Figure 8). Using Geomagic software (Geomagic), three landmarks are created in the scan bodies of both the single arch digital impressions and the bite impression. The center of the screw hole of each scan abutment is used as the landmark, as it is consistently visible in both impressions: this allows superimposition of the STL files (single arch and arches in occlusion) and achievement of an appropriate occlusal position for the virtual articulator (3-Shape Articulator; Zahn Dental). A potential difficulty during bite recording is that scan bodies may interfere with the occlusal plane. If this occurs, customized and reduced scan abutments should be used. The virtual articulator is used to design the complete-arch interim restoration (CIR) for immediate loading on implants, as a copy of the modified PD (Figure 9). A 3-month follow-up is deemed appropriate for optimization of patient comfort according to the new occlusal, functional, and esthetic parameters. When all parameters are adjusted in accordance with the clinical needs and the patient's chief complaint, digital impressions of the CIR are made in the oral cavity using a protocol described previously,¹⁸  and transferred to the final digital design of the framework.

The framework is designed according to the dental positions: using the digital version of the artificial teeth (Bonartic; Candulor), retention pins for acrylic/composite teeth are digitally projected within the metal framework to occupy the standard hole in each artificial tooth. When the design is completed (Exocad Dental CAD), the cobalt/chrome (Co/Cr) framework is manufactured using a milling machine (Program Mill PM7; Ivoclar Vivadent) and clinically checked. As an alternative, as in the exemplificative case presented herein, a direct technique may be used to set the acrylic teeth in the pink resin body of the prosthesis: the pink resin body of the prosthesis is digitally designed and manufactured according to the teeth contour, and the resin teeth are directly cemented using Ivobase CAD Bond (Ivoclar Vivadent) into the specifically designed slots of the pink resin. Then, the pink resin is cemented to the metal framework using Multilink Hybrid Abutment (Ivoclar Vivadent; Figure 10a through 10c) to ensure adequate embrasure and to allow hygienic home care and maintenance. Then, the prosthesis is delivered to the patient (Figure 11).

This protocol allowed for digital transfer of all occlusal, esthetic, and functional parameters from the PD to the CIR for the immediate loading on implants. After the optimization of patient comfort, the CIR parameters were directly transferred to the final digital full-arch implant-supported fixed prosthesis design. This protocol reduced the procedure time, and the laboratory costs (to a maximum of 1200 euros for each full-arch prosthesis), by using a 3D-milling process to manufacture the chrome/cobalt framework and the pink resin body, and by using artificial resin/composite teeth.

In the past decade, CAD-CAM technologies for dentistry have improved the production of full-arch rehabilitations using implants for edentulous patients. The protocol reported in this pilot study described the completion of the digital workflow, using the digital technology to achieve and integrate all steps of prosthesis manufacturing, since the provisional complete denture to the full-arch fixed prosthesis on implants.

Several studies have reported equal or superior results to conventional processes in terms of the marginal fit of implants using metal frameworks.^7,19–22  Conventional procedures (waxing, casting, and refinishing) are increasingly being replaced by modern CAD-CAM techniques; framework costs are reduced by laser-melting, milling, or hybrid technologies, as noble metals are not used for 3D printing. The rapid prototyping process allows milled frameworks of smaller dimensions to be obtained, and a metal structure may be designed more successfully when the framework is created in accordance with tooth positions predetermined by the CIR.

Moreover, use of a digital protocol improves accuracy during the creation of impressions. For digital impression data, Ender et al^23,24  reported mean accuracy at the framework/implant interface to within 40.3 ± 14.1 to 58.6 ± 15.8 μm, depending on the system used. Patzelt et al²⁵  reported accuracy using the digital impression technique to within 38 to 333 μm, depending on the oral scanner used. Meanwhile, Guth et al²⁶  noted 13/17 μm minimum/maximum differences in measurements between oral scanners, and Flugge et al²⁷  reported a 50-μm mean gap at the implant interface. Gimenez et al^28–31  investigated the accuracy of full-arch impressions of scan abutments using an intraoral scanner, and found a mean error of less than 70 μm. Ciocca et al⁷  reported a mean 3D-positioning error of between 41 ± 23 and 82 ± 30 μm when using a single digital impression system. Other factors, such as the scanning protocol²⁶  and operator skill,³¹  may reduce the final accuracy of the impression. Milling technology enables production of more accurate metal frameworks than conventional lost-wax techniques.^2,5,18,32,33  Several studies have provided evidence of an effect of misfit of prototype frameworks at the interface of the implant platform on long-term outcomes.^34–40 

The digital workflow presented in this study describes the use of digital technology to transfer occlusal, esthetic, and functional parameters from the PD to the CIR, and then to the final prosthesis of upper and lower totally edentulous patients using implants, thereby reducing the working time and cost of full-arch rehabilitation. The intraoral scanner simplifies conventional implant impression procedures: application of a resin splint between transfer abutments is no longer necessary, and no impression or casting material is used to generate the model, thereby eliminating the influence of material bias on the accuracy of the final result.

Digital recording of the centric occlusion is the main challenge when making digital impressions of implants of edentulous upper and lower arches. The interocclusal record of the final fixed restoration should copy the occlusion determined by reference to the PD and CIR occlusion; this often requires switching to a conventional analogic procedure using a facial bow and stone models, with devices being constructed by the dental technician. To the best of our knowledge, few studies have described a digital method to register the occlusion of two opposing completely edentulous arches when a full-arch restoration using implants is scheduled.^14,15 

In this study, a new digital approach was proposed for registering occlusion, using the same diagnostic radiographic guide adapted for scan abutments on implants. The template was modified to permit construction of open windows in its occlusal and buccal aspects, to in turn allow fitting even when the scan abutments remain screwed in the same position of the digital impression of the single arch (Figure 7). This modification of the radiographic guide, which represents a copy of the PD, facilitates the digital record of the occlusion: the digital record of the scan abutments in the buccal aspect is sufficient to detect the landmarks necessary for the superimposition. These landmarks are the centers of the screw holes of the scan abutments, which can be geometrically determined and manually marked on the digital impressions (single arches and occlusal registration STL files). The Geomagic software (3D Systems) then finds the best-fitting superimposition tool and the upper and lower single arch impressions are positioned in occlusion on the digital articulator.

The occlusion, esthetic, and functional parameters of the CIR teeth are transferred from the provisional fixed prosthesis to the final design, according to the protocol described by Monaco et al.¹⁸  If deemed clinically necessary, the 3D printing prototyping process allows reproduction of the resin replica for the final clinical try-in, so all esthetic and functional parameters can be tested before manufacture of the definitive prosthesis. A further advantage of this protocol is the time and cost reductions that it affords: a reduced number of clinical appointments are required and the maximum total cost of the CAD-CAM procedures (ie, design and manufacture of the final prosthesis) is 1,200 euros for each full-arch prosthesis.

A potential disadvantage of this protocol is the need for specialized instruments to create the digital impression (ie, an intraoral scanner) and a specialized manufacturing process (ie, SLM, milling, or hybrid technology). Further studies should focus on the digitalization of pink resin manufacturing processes for full-arch restorations, and on improvement of teeth-mounting in the virtual articulator.

Within the limitation of this pilot study, CAD-CAM technologies provide a viable complete digital workflow for full-arch rehabilitation using implants for edentulous patients. A preoperative PD is constructed with consideration of the final esthetic, functional, and occlusal requirements of the fixed full-arch prosthesis. Digital technology allows accurate transfer of occlusion parameters from the PD to the CIR, and finally to the design of the metal framework of the fixed full-arch prosthesis, thereby reducing time and costs.

Abbreviations

CAD-CAM:

computer-aided design/computer-aided manufacturing

CIR:

complete-arch interim restoration

PD:

provisional denture

STL:

Standard Tessellation Language

TDS:

True Definition Scanner

3D:

three-dimensional

Authors declare no conflict of interest.