Facially Driven Digital Workflow for Maxillary and Mandibular Milled Implant-Retained Overdentures Using Two Different Unsplinted Attachment Systems: Case Report

Overdentures represent a valuable and reliable alternative treatment for completely edentulous patients. Several quality-of-life studies have supported the mandibular overdenture as a standard-of-care treatment for edentulous patients.^1-3 Implant overdentures offer improved retention while enhancing comfort and taste in the maxillary arch by allowing a palateless design.^4,5 Significant improvement in biting force and chewing efficiency has been reported.^6,7 Implant overdentures have also been shown to improve speech, self-confidence, and social interaction, and the removable nature of the prosthesis facilitates access for oral hygiene.^8-10

Various overdenture attachment systems, which may be splinted or unsplinted, are available. Splinted attachments involve connecting multiple implants with a bar. Bar-retained implant overdentures are reported to provide good retention and patient satisfaction.^11,12 However, this type of overdenture is challenging to repair, requires additional restorative space, and is comparatively expensive. In addition, it is associated with high plaque accumulation, resulting in relatively high incidences of peri-implant mucositis and peri-implantitis and lower implant survival.^13-16 Unsplinted attachments involve the use of separate dental implants that are not connected by a bar. This approach is known for its cost-effectiveness, improved cleansability, and reduced need for manual dexterity intervention.¹⁴ In a recent study, four unsplinted maxillary implant overdentures were associated with high mid-term implant survival and high patient satisfaction with the palateless design.¹⁷

The most common complications of implant overdentures are associated with a significant decrease of retention force over time, the need to replace attachments with those that are retentive resilient, and the need for multiple adjustments.¹⁸ Theoretically, eliminating the lack-of-resilience factor will reduce maintenance, cost, and the time burden for dentists and patients by providing a rigid attachment system that relies on friction for retention. Originally, this conometric attachment-style system was intended to be used on four implants for immediate implant loading for overdentures.^19,20 These attachments have been investigated for implant overdentures only infrequently, with recent randomized controlled clinical trials reporting promising results with high implant survival rates and patient satisfaction.²¹

Computer-aided design/computer-aided manufacturing (CAD/CAM) systems for complete dentures have evolved, with several digital methods of fabrication becoming available.²² The advantages of complete dentures manufactured in a subtractive manner are unlimited denture teeth design, excellent retention, chairside time reduction, a 3-dimensional view of the prosthesis during design, cross-sectional dimensional analysis, and improved physical properties compared to conventionally processed ones.^22,23 Conversely, additively manufactured removable prostheses are currently recommended as a trial or immediate denture due to a lack of long-term data on permanent prosthesis performance.²⁴ While reports have been published on the evolving digital processes for complete denture fabrication,²⁵ only a few studies have described a digital workflow for implant overdentures.^26,27

This case report describes a digital workflow for maxillary and mandibular implant overdentures with a maxillary palateless design.

A 61-year-old female patient presented with existing maxillary and mandibular complete removable dental prostheses that were inserted immediately after extractions of the remaining dentition 1 year prior. The patient had no medical comorbidities that prevented dental care but was a regular smoker (more than 10 cigarettes per day). She was generally unsatisfied with the retention and appearance of her dentures and interested in a natural, customized appearance. Clinical evaluation revealed that the immediate prostheses were unretentive but were at the appropriate occlusal vertical dimension with adequate restorative space. After a review of alternative treatment options, the patient elected to have maxillary and mandibular implant-retained overdentures.

The existing dentures were scanned using an intraoral scanner (CEREC^® Primescan, Dentsply Sirona, dentsplysirona.com). All surfaces of the prostheses were scanned-intaglio, cameo, flange, and borders-creating a digital file of the dentures in Standard Tessellation Language (STL) file format. The file was sent to a 3D printer (D20+, Rapid Shape, rapidshape.de), and a duplicate of the patient's existing prostheses was printed and used as a radiographic stent for the dual-scan technique with radiopaque markers (Figure 1). A cone-beam computed tomography (CBCT) scan was obtained, and data were imported into virtual implant planning software (DTX Studio^™, Nobel Biocare, nobelbiocare.com). Prosthetically driven implant planning was conducted virtually, taking into consideration anterior-posterior spread, anatomic limitations, and restorative space,²⁸ and a pilot guide was fabricated via the 3D printer (D20+) to be used for implant placement (Figure 2 and Figure 3).

Four maxillary implants (NobelReplace Conical Connection NP, 3.5 mm x 10 mm, Nobel Biocare) and two mandibular implants (NobelReplace Conical Connection RP, 4.3 mm x 10 mm) were placed based on guidance provided by a pilot surgical guide (Figure 4 and Figure 5). A 4-month healing period was allowed for conventional healing, osseointegration was confirmed clinically during the second stage of treatment, and healing abutments were connected (Figure 6).

The replicated existing dentures were used as custom trays, border molding was performed using heavy-body polyvinyl siloxane (PVS) (Aquasil^® Ultra, Dentsply Sirona), and final wash implant-level open-tray impressions were made with light-body PVS impression material (Aquasil Ultra). The replicated dentures were reinserted, the occlusal vertical dimension was evaluated, and centric relationship and protrusive records were made using PVS bite registration material (Blu-Mousse^®, Parkell, parkell.com). Implant analogs were connected, and master casts were poured into type IV dental stone (high strength, low expansion). Afterward, the master casts were mounted on a semi-adjustable articulator, and protrusive records were used to program the condylar inclinations (Figure 7 and Figure 8).

The master casts and the replicated dentures were scanned using a laboratory scanner (inLab CEREC, Dentsply Sirona) to build a 3D model. All STL files were exported and transferred into dental design software (DentalCAD 3.0 Galway, exocad, exocad.com). During model preparation, the maxillary master cast was digitally scored around 0.5 mm to represent the border extension of the maxillary overdenture in the palatal area using software (Meshmixer, Autodesk, meshmixer.com). This small groove is intended to prevent food entrapment under the prosthesis and enhance its strength. The virtual articulator was selected with a compatible analog semi-adjustable articulator, and the condylar inclinations were entered based on the protrusive records (Figure 9).

The digitized replicated dentures were superimposed with the facial smile photograph during design preparation. First, facial landmarks were determined in the facial smile photograph in preparation for facially driven teeth design. Then, basic esthetic parameters were applied for virtual teeth set-up; these parameters included mold selection, incisal tooth display, smile curve following the mandibular lip curvature, incisal plane parallel to the inter-pupillary line, the midline matching the facial midline, and sufficient buccal corridor (Figure 10). Lastly, lateral excursive movements were simulated virtually, and adjustments were conducted in free mode to obtain bilateral balanced occlusion.

Full border extension denture bases were designed for the mandibular and maxillary arches with a palateless design. The design files were sent to the 3D printer (D20+) to fabricate trial dentures. The clinical try-in of the maxillary and mandibular printed resin dentures was conducted to confirm the occlusal plane, esthetics, phonetics, occlusal vertical dimension, and centric relationship and to obtain the patient's approval (Figure 11 and Figure 12). Any additional desired modifications were noted for incorporation into the final design.

The STL files were sent to the implant manufacturer (Atlantis, Dentsply Sirona) to design the conometric abutments (Atlantis^® Conus). The custom abutments were designed to be as parallel to each other as possible, and, finally, the prostheses were milled at a commercial laboratory (Figure 13).

The delivery visit for the milled prostheses (Figure 14) involved fit assessment using pressure-indicating paste and occlusal evaluation using articulating paper and shimstock to achieve multiple bilateral symmetric occlusal contacts in centric occlusion and balance occlusion in excursive movements. For the maxillary arch, Conus abutments were inserted using a provided printed positional guide and torqued following the manufacturer's recommendations. Radiographs were taken to ensure the passive fit of the abutments. The copings (SynCone^®, Dentsply Sirona) were placed, and the denture was adjusted to provide adequate space. The intraoral pick-up was conducted using autopolymerized acrylic resin material (Jet^™ Denture Repair Acrylic, Lang Dental Manufacturing Co., langdental.com) with the prostheses in centric relation (Figure 15 and Figure 16).

For the mandibular arch, two prefabricated abutments (LOCATOR^®, Zest Dental Solutions, zestdent.com) were inserted and torqued following the manufacturer's recommendations, and radiographs were taken to ensure their passive fit. The metal housings were placed, and final pick-up was conducted using the autopolymerized acrylic resin material with the dentures in centric relation.

The digital design met the required esthetic and functional goals of treatment and the patient's expectations (Figure 17 through Figure 19). The patient was given oral and denture hygiene instructions with an emphasis on the potential risks of peri-implantitis and denture mucositis.

Implant overdentures are associated with a high need for repair, frequent tooth fracture, denture base fracture, and the need for periodic attachment insert replacements.¹⁸ Furthermore, denture stomatitis was reported to be the most common biological complication for implant overdenture wearers.¹⁷ A major factor related to the prevalence of denture stomatitis, besides an ill-fitting denture base and poor oral and denture hygiene, is smoking.^29,30

The digital workflow described in this article was adopted to enhance the physical properties of a milled denture base, which may reduce the risk of denture base fractures, eliminate the need for metal reinforcement, improve denture base adaptation, and lower the risk of denture stomatitis through decreased adhesion of Candida albicans on CAD/CAM-milled denture bases.^23,31,32 In addition, as described in this article, rigid attachments may be used for the maxillary arch to reduce the need for insert replacement and enhance retention.²¹

Digital replication and 3D printing of existing dentures as trials can streamline the restorative process and facilitate the fabrication of the new prosthesis.³³ The use of digital technology in complete implant overdenture fabrication is similar to a conventional complete denture workflow and facilitates efficiency and superior esthetics while saving clinical time. The present digital workflow does not require the use of special devices and allows the clinician to follow steps without any significant compromise in conventional steps for overdenture fabrication. The virtual design workflow provides the opportunity to incorporate limitless digital elements, such as choosing from a wide library of denture teeth molds or even customized ones. The virtual scoring of the master cast that was presented in this case report was traditionally done only in an analog cast.

Limitations of the described digital workflow include the need for an acceptable existing prosthesis to scan and extended chairside time for a single visit to perform border molding, final impression, maxillomandibular relationship records, and esthetic evaluation. The recommended try-in appointment and the cost of a 3D-printed trial denture may be considered additional limitations. However, the authors suggest that the benefit of receiving the patient's approval before the final restoration outweighs the potential cost and time drawbacks.

This case report presented the fabrication of maxillary and mandibular implant overdentures using CAD/CAM technology and 3D design software. For both the maxilla and mandible, the master casts and replicated existing dentures were digitized using a laboratory scanner to facilitate the design. During the fabrication process, printed try-in dentures were used to validate esthetics and phonetics and obtain patient approval. Attachments were connected via chairside pick-up. The outcome met the required esthetic and functional goals of treatment and the patient's expectations.

Dr. Sourvanos was supported by the National Institute of Dental & Craniofacial Research of the National Institutes of Health grant number T90DE030854 and the Center for Innovation & Precision Dentistry at the University of Pennsylvania.

Abdulrahman Almalki, BDS, MS
Postgraduate Student, Department of Preventive and Restorative Sciences, University of Pennsylvania School of Dental Medicine, Philadelphia, Pennsylvania; Faculty, Department of Prosthetic Dental Science, Prince Sattam Bin Abdulaziz University, SaudiArabia; Fellow, American College of Prosthodontists; Fellow, Royal College of Dentists of Canada

Dennis Sourvanos, DDS, CTR
Postgraduate Student, Department of Periodontics, and Postdoctoral Fellow, Center for Innovation and Precision Dentistry, University of Pennsylvania School of Dental Medicine, Philadelphia, Pennsylvania

Noor Kutkut, DDS, MS
Postgraduate Student, Department of Preventative and Restorative Sciences, University of Pennsylvania School of Dental Medicine, Philadelphia, Pennsylvania; Fellow, Royal College of Dentists of Canada

Markus B. Blatz, DMD, PhD
Professor of Restorative Dentistry, Chair, Department of Preventive and Restorative Sciences, and Assistant Dean, Digital Innovation and Professional Development, University of Pennsylvania School of Dental Medicine, Philadelphia, Pennsylvania

Vu Dang La, DMD
Clinical Assistant Professor, Department of Periodontics, University of Pennsylvania School of Dental Medicine, Philadelphia, Pennsylvania; Diplomate, American Board of Periodontology

Joseph P. Fiorellini, DMD, DMSc
Postdoctoral Director of Periodontics, Professor of Periodontics, Department of Periodontics, University of Pennsylvania School of Dental Medicine, Philadelphia, Pennsylvania; Diplomate, American Board of Periodontology

Rodrigo Neiva, DDS, MS
Chair, Clinical Professor of Periodontics, Department of Periodontics, University of Pennsylvania School of Dental Medicine, Philadelphia, Pennsylvania; Diplomate, American Board of Periodontology

Evanthia Anadioti, DMD, MS
Adjunct Associate Professor, Department of Preventative and Restorative Sciences, University of Pennsylvania School of Dental Medicine, Philadelphia, Pennsylvania; Fellow, American College of Prosthodontists