CAD-CAM for removable partial denture fabrication after marginal mandibulectomy based on a well-adjusted pre-existing denture

Abstract

This clinical report summarizes a workflow that introduces two approaches for fabricating a metal framework removable partial denture for a marginal mandibulectomy patient. First, the pre-existing denture was adjusted according to the piezographic technique to determine the optimal configuration for adhering to the available denture space. Second, computer-aided design-computer-aided manufacturing (CAD-CAM) technology was used to fabricate a new removable partial denture by digitally scanning the oral condition and the well-adjusted denture and then using the obtained data to design and fabricate the new denture. These two approaches can help to reduce chair time not only for maxillofacial prosthetic treatment but also for other denture cases.

Introduction

Marginal mandibulectomy is recommended for the resection of oral neoplasms localized to the residual ridge of the mandible, as it maintains continuity of the mandibular arch and preserves the form and function of the oral cavity greatest extent possible. However, removing part of the mandible can change the movement of surrounding soft tissues, the functional range of the mandible, and occlusal proprioception. In addition, the absence of alveolar bone and teeth and the presence of muscle imbalances can sometimes cause the residual mandible to deviate toward the surgical site, which can severely affect quality of life [1,2,3,4].

When considering treatment options for prosthodontic rehabilitation, the use of computer-aided design and computer-aided manufacturing (CAD-CAM) to fabricate removable partial dentures has multiple advantages, including reduced time, fewer errors, and the elimination of polymerization distortion [5,6,7,8,9].

The denture space, which is the potential space between the lips, cheeks, and tongue that the denture should occupy, must be considered because it is important for denture retention and stability. It can be recorded using various methods, including piezography, the neutral zone procedure, and the flange technique [4,5,10].

This clinical report describes a new method for fabricating a metal framework removable partial denture, using CAD-CAM technology, with reference to a well-adjusted pre-existing denture, in a patient who had undergone marginal mandibulectomy.

Case Report

A 57-year-old Japanese woman presented to the Department of Maxillofacial Prosthodontics at Tokyo Medical and Dental University Hospital with a chief complaint of denture discomfort. Seven years earlier, the patient had undergone marginal mandibulectomy, tracheotomy, and a chemotherapy course using TC-1 100 mg for 3 days a week over a span of 3 weeks. However, this course was discontinued because of bone marrow suppression. A conservative total neck resection for squamous cell carcinoma of the gingiva on the right side of the mandible was performed. Subsequently, the mandibular defect was reconstructed using a titanium mesh tray and an AO (Arbeitsgemeinschaft für Osteosynthesefragen) plate, both of which were eventually removed. Ilium bone was used for the autogenous bone graft. The denture was fabricated after the patient had undergone surgery and completed rehabilitation.

Intraoral (Figs. 1A, B) and radiographic (Fig. 1C) examinations revealed that the right side of the mandible was edentulous, and that the removable partial denture was both uncomfortable and unstable. Mandibular deviation was evident as a result of the surgery, which had led to a change in occlusion of the remaining teeth. Furthermore, the denture was too long and large for the denture space, causing the patient discomfort. Informed consent was obtained from the patient before treatment.

The denture space was evaluated by applying a fit-checking silicone (Fit Checker, GC Corp., Tokyo, Japan) with a changed ratio of 1:2 (catalyst: base) to expand the setting time on the polished surface of the pre-existing denture, and the denture was inserted into the patient’s mouth. The patient was asked to utter several words following piezography [4], after which the denture was adjusted; this process was repeated until stability and comfort were achieved. In addition, a mandibular occlusal ramp was incorporated to the right lower central incisor of the denture to compensate for the malocclusion caused by deviation. Because of the small denture space, parts of the denture became thinner over time, resulting in the denture fracturing twice.

The decision was made to make a metal framework denture, using CAD-CAM technology to reference the patient’s now well-adjusted and comfortable denture. A digital impression of the entire mandibular arch, including the edentulous area and the remaining teeth, was taken using an intraoral scanner (Trios 3D, 3Shape, Copenhagen, Denmark). with (Fig. 2A), and without (Fig. 2B) the denture placed. The pre-existing denture was scanned outside the patient’s mouth using the same intraoral scanner (Fig. 3).

Because of the absence of dentition and the depth of the defect, it was not possible to scan the defect site with the intraoral scanner. Therefore, a laboratory technician used the impression surface of the pre-existing denture to accurately determine the mucosal surface of the defect (Fig. 4), similar to the digitally altered cast removable partial denture impression. The digital design process of the three-part denture involved three software applications. The dental CAD-CAM software (3Shape Dental System, 3Shape) was used to design the metal framework, while the three-dimensional (3D) modeling software (Meshmixer, Autodesk Inc., San Francisco, CA, USA) and (Blender; Blender Foundation, Amsterdam, Netherlands) were used for the other design processes. The cast for the metal framework (Cobarion EX, Shofu Inc., Kyoto, Japan) was fabricated using wax (Castable Wax, Formlabs, Somerville, MA, USA) and a 3D printer (Form3B, Formlabs). The cast was used to fabricate the metal framework, which was finished and polished.

The next step was to fit the metal framework and confirm proper fitting (Fig. 5). The upper and lower trial parts were 3D printed from resin (BioMed Clear, Formlabs). After the artificial teeth were milled (Yamahachi Dental MFG., Co., Gamagori, Japan), they were connected to the upper trial part using paraffin wax (Fig. 6) : and then connected to the metal framework using methyl methacrylate resin. (Unifast, GC Corp) (Fig. 7)

In the dental laboratory, scanned data of the mandible taken of the edentulous area with the remaining teeth both with and without the denture placed were used to predict the occlusal relationship. An auto-polymerizing resin (Palapress vario, Heraeus, Hanau, Germany) was used to separately fabricate the upper and lower acrylic parts. The two parts were then milled using a dental milling machine (DWX-52DC JP, DGShape, Hamamatsu, Japan) and inserted into the pressure chamber (Polymax, Dreve Dentamid GmbH., Unna, Germany) to prevent bubble formation during polymerization. The artificial teeth were milled from a hybrid resin disk (Shofu disc HC, Shofu Inc.).

The same milling machine was used to trim the acrylic parts. The finishing line surface of the metal framework was treated by air abrasion with 50-µm alumina particles (Kulzer., Tokyo, Japan), using an air abrasion machine (Jet Blast III, Morita Inc., Osaka, Japan), followed by the application of a bonding agent. Finally, a methyl methacrylate resin was loaded into a syringe and injected between the metal framework and the acrylic parts to bond them together. The denture was placed in a high-pressure chamber (Polymax, Dreve, Inc.) and subsequently polished, finished, and delivered to the patient (Fig. 8A-D).

Fig. 1 Clinical and radiographic presentation

(A) Intraoral view of edentulous area. (B) Intraoral view of old denture. (C) Panoramic radiograph showing a marginal mandibulectomy defect on the right side of the mandible

Fig. 2 Intraoral scanned data

(A) Intraoral scanned data with denture. (B) Intraoral scanned data without denture

Fig. 3 3D scan of the pre-existing denture outside the patient’s oral cavity

Fig. 4 Superimposition step using 3Shape software

Fig. 5 Intraoral view of the metal framework

Fig. 6 Three-piece denture: metal framework and the trial parts

Fig. 7 Bonding the acrylic trial parts to the metal framework

Fig. 8 Definitive removable partial denture

(A-C) Intraoral view of the denture. (D) Outside the patient’s oral cavity

Discussion

Prosthodontic rehabilitation after surgical procedures is crucial to boost patients’ confidence and self-esteem as well as improve their oral functions and quality of life.

Marginal mandibulectomy usually leads to changes in the denture space resulting from changes in the anatomic landmarks caused by the surgery, and the tension in the scar tissue that occurs after the surgical procedure can affect the retention and stability of the denture [4].

Making these modifications fit the denture space helped to rearrange and modify the position of the teeth accurately, thereby achieving better stability and retention. Having a well-adjusted pre-existing denture was extremely beneficial because the scanned data of the mucosal surface of the pre-existing denture provided the only way to determine the mucosal surface of the defect. Despite advances in intraoral scanners, it is still difficult to detect the mucosal surface, and obtaining denture space using the intraoral scanner was not possible in this case. However, the scan of the denture mucosal surface worked well, providing the mucosal structure for the definitive cast. As a result, it was possible to fabricate a new prosthesis with the same design as the previous denture. On the other hand, to maintain the scanning technology consistency and integrate it seamlessly with the existing intraoral scan data and dental CAD software workflow, the pre-existing denture was scanned outside the patient’s mouth using the same intraoral scanner. A streamlined digital workflow without import and export of scan data across different systems was used.

This process also allowed the patient to continue wearing the existing denture during the fabrication of the new denture, which helped ensure patient satisfaction throughout the treatment. The use of digital technologies saves time by reducing the number of visits and chair time, in addition to reducing the cost of treatment, wastage of materials, and discomfort to the patient compared with conventional methods. This improves the procedure for both the patient and dentist.

The presented technique can be used for patients with trismus and limited mouth opening or who have dentures that need to be refabricated. The patient in this report expressed satisfaction with the prosthesis fabricated using the presented technique. Prostheses fabricated using conventional methods require a great deal of experience and professionalism from the prosthodontist but often fall short in terms of precision and customization and have a high likelihood of errors.

However, the presented technique has some limitations which need to be addressed. For instance, this workflow was implemented in a single case and the obtained results could not be generalized. The incorporation of advanced technologies and materials leads to higher costs, which may not be feasible for all patients. Furthermore, the transition to new digital techniques may require additional training for dental professionals, potentially affecting the adoption rate and proficiency. Finally, having a well-adjusted pre-existing denture proved useful in fabricating a new denture more efficiently and effectively with the help of CAD-CAM technology, resulting in optimizing patient satisfaction and minimizing discomfort.

Abbreviations

AO: arbeitsgemeinschaft für osteosynthesefragen; CAD-CAM: computer-aided design and computer-aided manufacturing; 3D: three-dimensional

Ethical Statements

The agreement sheet from the patient about the case report publication was obtained.

Conflicts of Interest

The authors have no conflict of interest.

Funding

This research was partly supported by the Tokyo Metropolitan Government in April 2023 to March 2025 with a number of 2023-2025.

Author Contributions

BLA: investigation, writing – original draft preparation, visualization; YY: methodology, visualization, investigation, and resources – dental laboratory design/fabrication; NP: writing – review and editing, investigation; YIS*: methodology, supervision, conceived and performed treatment, edited the manuscript, and secured funding; NW: conceptualization, validation, supervision, and reviewing. All authors read and approved the final version of the manuscript.

ORCID iD

¹⁾BLA: [e-mail], https://orcid.org/0000-0002-3056-0207

²⁾YY: [e-mail], https://orcid.org/0009-0003-4180-702X

¹⁾NP: [e-mail], https://orcid.org/0009-0005-0090-7600

^3,4)YIS*: [e-mail], https://orcid.org/0000-0003-3982-8369

¹⁾NW: [e-mail], https://orcid.org/0000-0002-0517-6756

Data Availability Statements

Due to patient confidentiality and privacy considerations, the data underlying this case report cannot be publicly shared.

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