Digital denture with mucostatic base and functional borders: A cast‐free digital technique

Abstract

Digital complete dentures fabricated using a mucostatic impression, like intraoral scans, are desirable for their beneficial effect on long‐term residual ridge stability but may have less retention than those fabricated with a mucocompressive impression. Border molding procedure may improve initial retention, thus favoring adaptation to new dentures, especially in cases where neuromuscular dysfunctions may diminish the patient's ability to cope with adapting to existing motor patterns or learn new ones. However, a mucocompressive impression may cause higher residual ridge resorption, thus resulting in a retention decrease over time. A desirable combination would be a digital denture with a mucostatic intaglio surface obtained from intraoral scans and functional borders. A cast‐free digital workflow for that is demonstrated in this technical report.

The adoption of digital technologies for removable prosthodontics has recently become more common as it provides clinical ¹ and financial ² advantages over analog treatment workflows. From a practical point of view, the concept of digital complete dentures cannot be codified or described by a single technique or specific workflow; on the contrary, it encompasses a wide spectrum of combinations of clinical and laboratory techniques, computer‐aided design and computer‐aided manufacturing (CAD‐CAM) technologies, and prosthetic materials. ³ These potential different combinations can be organized into manufacturer‐dependent systems, ⁴ or protocols based on open technologies. ⁵

Regardless of specific CAD‐CAM technologies, workflows for digital denture fabrication generally start with either a digitized conventional impression or an intraoral scan (IOS) of the edentulous arches. ⁶ While these two clinical approaches can be employed, they inherently imply the adoption of a different impression philosophy, either mucocompressive or mucostatic, respectively, with their own principles, foundational concepts, advantages, and disadvantages.

Outlining the differences between these two philosophies is beyond the scope of the present paper. Briefly, though, the mucocompressive technique relies, in its conceptual framework, on the application of pressure on the edentulous mucosa, while a peripheral seal and adequate retention are obtained through functional border molding and the creation of a postpalatal seal area. This impression technique can improve initial denture stability as well as patient acceptance and comfort, however, the resultant tissue pressure has been demonstrated to cause residual ridge resorption. ⁷ In addition, it is a common clinical experience that, while residual ridge resorption, over time, will cause misfit of the denture base, ⁸ a patient may not readily report to the dentist's office for an eventual reline, because they may compensate for the loss of fit by adapting muscle activation patterns. ⁹ This misfit can then potentially cause tissue impingement and result in pressure spots or traumatic ulcers.

A mucostatic impression technique is not only related to the adoption of an intraoral scan. The technique was introduced in 1946 ¹⁰ and it is generally advocated when thin, sharp, or flabby ridges are present ¹¹ due to its recognized ability to preserve residual ridge tissues. ⁷ Under the mucostatic impression conceptual framework, denture retention is mainly based on the surface tension forces that occur between two precisely congruent wet surfaces, such as the edentulous mucosa and intaglio surface of the denture base. Consequently, the precise definition and functional capturing of the vestibular depth and width is not mandatory. Not surprisingly, considering that every impression material, by definition, applies some pressure on the edentulous tissue, mucostatic impression techniques have received less credit than mucompressive approaches. However, the advent of digital technologies in removable prosthodontics, and specifically the adoption of an intraoral scan of the edentulous tissues, ¹² provides a 100% mucostatic impression. In addition, the adoption of CAD‐CAM workflows critically provides design and manufacturing tolerances, in other words, trueness of the intaglio surface of digital dentures, close to zero. ¹³ , ¹⁴ The coupling of IOS and digital design and manufacturing, therefore, conceptually provides for a more realistic adoption and implementation of the mucostatic concept.

An IOS has also been demonstrated to have clinical advantages, such as increased patient comfort, avoidance of gag reflex or allergic reaction to impression materials, ¹ as well as practice‐based ones, such as less chair time, fewer appointments and procedures, reduced supply cost, and no need to manage handling, shipping, and storage of casts. ² The major criticism directed at the use of the IOS for removable complete denture impression, besides the inevitable learning curve, is that border molding is not possible and, consequently, the denture borders cannot incorporate functional movements. ¹⁵ , ¹⁶ Attempts to overcome this potential issue led to the proposal of techniques based on the relining of occlusion rims ¹⁵ or trial dentures ¹⁷ designed on IOS, which are effective in recording the neutral zone ¹⁵   and/or obtaining functional borders, ¹⁷ which are potentially useful for patients with neuromuscular disorders, cognitive impairment, ¹⁸ or age‐related neuroplasticity reduction ¹⁹ that may diminish the patient ability to cope with denture adaptation. ²⁰ However, all such approaches inevitably lead to a mucocompressive denture base with its inherent disadvantages related to ridge stability. ⁷

Therefore, it would be desirable to develop impression techniques and workflows that combine the advantages of both mucocompressive and mucostatic impression techniques. The purpose of this technical report was to demonstrate a combined, novel workflow for the fabrication of complete dentures based on mucostatic IOS of edentulous arches with the capturing and development of functional denture borders.

TECHNIQUE

1. Take an IOS of the edentulous arches using an intraoral scanner (iTero Element Flex; Align Technology, Inc.) with instruments for adequate retraction and stabilization of tissues (Lo Russo Retractors; ELDO srl). Process and export the scans (Figure 1).

2. Design and 3D print baseplates to fabricate occlusion rims (OR).  Use the workflow for “offset coping” in the Exocad software program (exocad GmbH) and set the cement gap to zero so that the baseplates are completely adapted to the mucosa (Figure 2). Transfer the standard tessellation language (STL) files of the designed baseplates to the software application of the 3D printer (Prusa i3 MK3; Prusa Research) and then print with 0.2‐mm‐thick material (Prusament PLA; Prusa Research).

3. Add wax (Bite Wax Rims; Henry Schein, Inc) to the baseplates for the definitive OR.

4. Clinically assess the completed OR for adaptation and accuracy, then proceed with the border molding procedure using a thermoplastic material (Iso Functional; GC Corporation). Care should be taken to apply a thin layer of the thermoplastic material on the top of the border of the baseplates; this will favor a smooth transition from the border to the intaglio surface of the baseplates. Make a maxillo‐mandibular relation record. Record the vertical dimension, occlusal plane, lip support, maxillary incisor length, and midline. Obtain a centric relation record, verify that it is a repeatable position, and record it in the occlusion rims using a recording material (Registrado X‐tra; VOCO GmbH).

5. Keep maxillary and mandibular OR sealed together; scan them extraorally as a single object with an intraoral scanner (iTero Elemen Flex; Align Technology, Inc.). Small surface irregularities can be created on the wax in order to optimize the scanning. Process the file and export the scan (Figure 3).

6. Create a new project in Exocad for “copy of an existing denture”; this makes possible a workaround to obtain the digital master casts. To this end, import, in the created project, the scan obtained at step 5. The same scan will be used for the maxillary and mandibular arches and the wax rim. From two of them, digital master casts will be obtained. The third copy (wax rim) is just intended to complete the project setting.

7. Start the CAD workflow. The software program will propose a trimming line to segment the intaglio surface of the upper OR baseplate (Figure 4). Make sure it includes the molded borders (or modify it, if necessary). Once the segmentation is accepted the intaglio surface is automatically extracted and transformed into the upper digital master cast (Figure 5). The same is performed with the intaglio surface of the lower OR baseplate (Figure 6), to obtain the lower digital master cast (Figure 7). Since the intaglio surfaces of the baseplates are the negative of IOS, the created master casts are congruent with the IOS, except at the borders, where they include the precise reproduction of the molded borders (Figures 5 and 7).

8. Export digital master casts obtained at step 7 and use them in a new project for a regular double arch complete denture workflow in Exocad. Being extracted from the intaglio surface of the OR, the digital master casts are already in the position as per the registered maxillo‐mandibular relationships (Figure 8), and the wax rim (Figure 9) will provide all information for tooth arrangement as detailed in step 4. The occlusal plane, lip support, and maxillary incisor length are included in the shape of the wax rim, whereas other relevant data, for example, midline, references for canine position, or smile line, are marked on the wax (Figure 3).

9. Proceed with the denture design workflow. The extension of the denture bases will correspond to the trimmed surfaces of OR (Figure 9), and the denture bases will precisely reproduce the molded borders (Figure 10).

10. Make a rapid prototype of the trial dentures, ²¹ and clinically assess it with the patient. Then, proceed with a CAM process and materials of choice for denture fabrication and finalization (Supplementary Video S1).

FIGURE 1.

Intraoral scans: maxilla (left side), mandible (right side).

FIGURE 2.

Baseplates for occlusion rims completely adapted to mucosa surface.

FIGURE 3.

Scan of registered occlusion rims with border molding.

FIGURE 4.

Segmenting intaglio surface of upper occlusion rim.

FIGURE 5.

Upper digital master cast automatically obtained from segmented surface of upper occlusion rim.

FIGURE 6.

Segmenting intaglio surface of lower occlusion rim.

FIGURE 7.

Lower digital master cast automatically obtained from segmented surface of lower occlusion rim.

FIGURE 8.

After extraction from the intaglio surface of occlusion rims, digital master casts are already in the registered maxillo‐mandibular relationships.

FIGURE 9.

Wax rims will also provide information for tooth arrangement.

FIGURE 10.

Denture borders as defined by segmented surfaces from occlusion rims are the negative of molded borders of occlusion rims.

DISCUSSION

Using IOS of edentulous arches offers the opportunity to fully digitalize removable dentures. Nonetheless, it has been reported that border‐molded impression‐making for recording functional denture borders is preferable for improved retention of digital complete dentures when compared with IOS. ³

In the short term after the insertion of a new denture, or replacement of a denture with a new form and function, denture retention facilitates the adaptation process, as denture kinetics rely less on motor skills, ²² thus compensating or overcoming the issues related to a potentially compromised ability to learn new motor skills or adapting existing motor patterns, caused by a reduced neuroplasticity. ¹⁹ However, if retention is obtained by applying pressure on the mucosa, as it happens with a mucocompressive impression, this may cause harm to denture‐bearing tissues by significantly increasing the long‐term residual ridge resorption. ⁷ As a consequence, the retention will decrease over time, and if the denture base is readapted after a new mucocompressive impression, a vicious cycle is created, which is harmful to the residual ridge and, ultimately, to denture stability.

The cast‐free workflow presented in this article was developed to exploit and combine the benefits of mucostatic and mucompressive impression techniques into a digital workflow for complete denture fabrication. The proposed technique, therefore, makes it possible to obtain a denture characterized by a mucostatic intaglio surface, and peripheral borders obtained by border molding and the creation of a postpalatal seal area. The mucostatic intaglio surface is beneficial for tissue preservation, whereas the functional border molding may increase denture retention, which is a desirable feature, especially in older patients with neurological dysfunctions.

The entire workflow, as described, can be realized within a single digital ecosystem (equipment and software programs), nonetheless, it is not system‐dependent and can be implemented within any open system with similar functions. This technique is also applicable to the design and fabrication of removable partial dentures. It can also be tailored and incorporated with other treatment philosophies, such as the neutral zone, by adding suitable material on the occlusion rims.

The intaglio surface of the denture base is the negative of digital casts, which were obtained by segmenting intaglio surfaces of OR baseplates, which in turn were the negative of IOS; thus, the intaglio surface of the denture base should be considered the negative of IOS, net of manufacturing accuracy of OR baseplates. For this reason, careful selection of the manufacturing technology and equipment of OR baseplates is mandatory. The 3D printing technology used in the present technique has been demonstrated to reach a very high accuracy, with a close to zero tolerance for intaglio surfaces of trial dentures ²¹ realized with this same protocol; hence, the above assumption (denture base is the negative of IOS) seems to be reasonable.

CONCLUSIONS

The described technique incorporated concepts from mucostatic and mucocompressive impression techniques in a single cast‐free workflow. Its potential benefits should encourage further validation, especially with long‐term follow‐up.

DISCLOSURE

This research did not receive any specific grant from funding agencies in the public, commercial, or not‐for‐profit sectors.

CONFLICT OF INTEREST STATEMENT

The authors declare no conflicts of interest.

Supporting information

Supporting Information