Combining Digital Technology and Traditional Principles to Optimize Complete Removable Denture Treatment

With a heterogeneous etiology and presentation, edentulism has long been tackled by the dental profession using partial and complete dentures.¹⁰ Edentulism is a public health issue, particularly among geriatric patients, with the capacity to affect overall quality of life.^10-12 Complete edentulism is an irreversible condition that is most prevalent in the elderly population; rates of prevalence vary widely between countries and independent regions within a country.^12,13

In response to this condition, patients often opt to replace their missing teeth with a removable prosthesis. These prostheses can bring about a variety of problems, such as fracture, loss, or even malfunction. Functional problems in dentures can even be responsible for sparking malnutrition, health problems, avoidance of social activities, personal embarrassment, and psychological stress.^14,15

CAD/CAM for Removable Prosthodontics

The success of CAD/CAM in the fabrication of fixed prostheses and implant restorations has prompted the use of its techniques in the area of removable prosthodontics.¹⁶ Traditionally, the fabrication of conventional complete dentures has required five key steps to achieve accuracy and precision^17,18: preliminary impression, final impression, record-taking, artificial tooth arrangement try-in, and delivery of the final prosthesis. The use of CAD/CAM technology in removable prosthodontics presents a variety of advantages, including reducing the number of patient visits to the dental office, better cost efficiency, and a lower risk of cross-contamination. It allows clinical information and data to be stored for fabrication of a spare or even a new denture, if needed.^9,19 Along with reducing the number of clinical appointments and amount of chairtime, the digitization of the edentulous arches can also improve the definition and accuracy of the dental impression as well as eliminate the polymerization shrinkage of polymethyl methacrylate seen during conventional denture processing. In short, CAD/CAM procedures in removable prosthodontics can provide results similar or superior to conventional techniques, offering better fit of the surfaces, improved mechanical properties, and higher patient satisfaction.^9,20

The impression technique plays an important role in the fabrication of removable prostheses, as it is crucial to the subsequent reaction of the supporting tissues. Theoretically, the ideal impression provokes no compression or displacement, incorporates all the edentulous regions, and ensures the adaptation of the denture to the oral structures.^21,22 Traditional impression techniques include the mucostatic, functional, and selective pressure techniques. The clinical process of manufacturing a denture, however, involves a combination of methods rather than the use of each technique independently. Common daily practice includes recognizing each technique as a step in production and using the principles of each technique in combination to achieve the final desired result.

The mucostatic or non-pressure technique registers the tissues in a passive state. This technique relies on the interfacial tension as the sole retentive mechanism of the soon-to-be fabricated denture.^22-24 The functional impression technique, as its name indicates, is taken while the patient is exerting masticatory forces at the desired vertical dimension of occlusion (VDO). This impression technique involves the use of a custom tray that allows the patient to occlude on either the opposing arch dentition or an opposing occlusal rim. As opposed to other techniques, the impression in this technique is made while the tissues and muscles are in function.²⁴ It has been argued, however, that constant compression applied on the tissues is responsible for significantly faster bone resorption.²⁴ Lastly, the selective pressure technique involves elements of the two aforementioned techniques; that is, while some regions are recorded with minimal pressure, others are registered while pressure is applied.^22,25

Unlike the traditional impression techniques, the myofunctional technique is based on an entirely different principle. It involves a tissue conditioning material that is used in a relieved denture and molded by the patient's musculature over time. This approach results in a highly accurate and comfortable replication of the oral structures. A major drawback, however, is that the denture must be surrendered for a rebase procedure and results in a rebased denture of the original product, rendering the denture less durable.

CAD/CAM technology now allows providers to combine the best of each impression technique to achieve a superior, more durable, and reproducible prosthesis for patients.

Treatment Planning for Variable Conditions

The literature is inundated with articles on the use of digital scanning of patients' soft tissues and the fabrication of dentures from scratch. However, patients are not uniformly edentulous. They arrive to dental offices in variable conditions, which can generally be divided into three categories: (1) terminal dentition, which requires edentulation and an immediate-insertion denture; (2) existing, esthetically acceptable denture that is ill fitting and no longer comfortable; (3) existing denture that is unacceptable and can no longer be worn. Rather than "reinvent the wheel" every time a patient needs a denture and start from scratch, a combination digital technology-traditional workflow may be used to treat each of these three common situations and create an efficient denture fabrication process.

First-Time Dentures Are Required

In this instance an immediate-insertion denture needs to be developed. The process entails recording extraoral photographs of the patient smiling as well as lateral views, taking an optical, digital scan of the mouth at rest, and generating an interocclusal (bite) record using the position and shape of the patient's remaining teeth. The rest of the work can be done in the digital laboratory or traditionally on a physical model. The teeth can be digitally removed in the digital laboratory or physically removed on a printed model.

An under-extended denture is then fabricated utilizing the extrapolated position of the original teeth, using the original vertical dimension, and retaining the original shape or, if desired, incorporating a new, more attractive tooth mold. Chairside, upon extraction of the teeth, the new dentures are delivered with soft reline material and the patient is sent home with the transitional prosthesis as is done in traditional immediate denture fabrication.

The patient must go through accommodation to the new denture, and the soft tissues and bone need to be allowed to heal. The original scan of the patient is likened to a mucostatic impression because it captures tissues in an uncompressed state. The soft-tissue reline, however, is likened to a functional impression technique, which uses a closed mouth to mold the borders of the denture. Once healing takes place, the denture is relined with a tissue conditioner/functional impression material and the patient is sent home to function as usual. Once the final contours of the denture are established, the denture is then scanned, and a final denture can be milled and delivered to the patient.

Patient Presents With Acceptable Complete Dentures With Some Discomfort

These cases typically involve either an experienced denture wearer with an old denture or a patient who requires a final denture following previous treatment with an immediate-insertion transitional denture, as described in the first scenario. In this case, the acceptable denture is hollowed out and relined with tissue conditioner impression material, such as Hydro-Cast^® (Sultan Healthcare, sultanhealthcare.com) or Lynal^® (Dentsply Sirona, dentsplysirona.com). The patient will then function for a period of time as recommended by the manufacturer of the tissue conditioner/functional impression material, resulting in an excellent, well-fitting myofunctional impression.

However, contrary to traditional protocol of surrendering the denture for a few days for the laboratory to rebase the denture, the myofunctionally molded denture is scanned chairside, and a final denture is milled. The advantage of this is twofold: the patient retains the original denture for future emergency use and production of a new denture, and more importantly, the new, digitally milled denture avoids the problems associated with the chemical incorporation of the rebase material and the shrinkage upon curing associated with the process, and a much more durable result is produced. It has been shown that "there is less residual monomer in the CAD/CAM milled resins than conventional heat-cured resins."²⁶

Patient Presents With Unacceptable Complete Dentures

Patients often present with ill-fitting dentures, including those that are esthetically and/or phonetically unacceptable. In these cases, rather than starting from scratch, the clinician can use the old dentures to obtain proper borders and take records. This can be done using conventional heavy-body impression material or chairside soft reline material. The dentures with the corrected borders can be used to obtain the proper VDO and to register the proper position of the teeth. Rather than taking the corrected dentures from the patient, they can be scanned and virtually articulated. The clinician can then idealize tooth position, adjust the denture base extension, and duplicate again once the fit and function are idealized, as discussed in the second scenario.

In all three of these procedures the denture never needs to travel to the laboratory, thus reducing contamination risk. The denture travels only from the point of production to the patient. In addition, the cost of milling a new denture is substantially less than the cost and labor involved in producing a conventional laboratory reline.

Case Descriptions

Case 1

This case represents a situation that would meet either of the first two scenarios; in this instance a first-time denture wearer was delivered immediate-insertion complete dentures, and once the patient healed, these dentures were used to fabricate the final new set of dentures.

In April 2017, a 74-year-old female patient presented with a chief complaint of having two flippers in her mouth and that "everything is loose" (Figure 1 and Figure 2). Her medical history included high blood pressure and breast cancer treatment in 2002 with a left mastectomy; based on consultation with the patient's physicians, however, there were no contraindications for dental treatment. Her medications included Avapro^® and Pravachol^®. She was allergic to penicillin.

The patient reported severe fear of dental treatment, which contributed to the poor condition of her dentition. She presented with maxillary and mandibular removable partial prostheses to replace missing teeth, along with neglected remaining dentition with advanced periodontitis, caries, and tooth mobility. A panoramic x-ray was taken, and all teeth were deemed nonrestorable (Figure 2). Considering the patient's financial limitations and fear of surgical treatment involving implant placement, transitional immediate-insertion dentures using a digital workflow and production were deemed to be the optimal treatment choice. Once bone and tissue healing and remodeling was complete, the transitional dentures would be replaced with final removable complete dentures. Because of her fear of dentistry, the patient did not return for treatment until nearly 3 years later.

Since all teeth were periodontally involved, traditional alginate impressions for custom trays were impossible to perform without the risk of all teeth being removed with the impressions themselves. In addition, traditional methodology calls for the initial removal of all posterior teeth and initial tissue healing so that denture bases can be used with wax rims to register the VDO, try-in the posterior tooth arrangement, and, only then, complete the fabrication of immediate-insertion dentures, which would be delivered upon removal of the remaining teeth. The traditional process requires considerable chairtime and waiting time for the patient. Moreover, the patient must go partially edentulous for a prolonged period, and a long, drawn-out treatment could further exacerbate a fearful patient's apprehension.

Digital technology allows for streamlined treatment with the removal of all the offending teeth and delivery of dentures right away. Using an optical scanner (TRIOS^®, 3Shape, 3shape.com), the teeth and hard and soft tissues were scanned and an interocclusal (bite) record was obtained (Figure 3). A CAD/CAM digital denture protocol (3Shape) was followed, artificial tooth arrangement was developed, and short-bordered dentures were fabricated. All necessary information as to the VDO and position of the teeth was obtained from the 3D optical scan and the interocclusal record (bite scan) acquired at the first visit. All clinical information checklists were completed and sent electronically along with patient photographs to the lab processing center. Two weeks later the extraction of all teeth was performed (Figure 4 and Figure 5) and the immediate complete prostheses with a soft reline (COE-SOFT^™, GC America, gc.dental/america) were delivered (Figure 6).

At this point, a traditional immediate denture protocol was followed with follow-up visits and adjustment of the reline material performed as needed.

Once soft-tissue healing was deemed complete and satisfactory, which is typically within 6 months, the soft reline material was removed from both prostheses, and both dentures were relined with Hydro-Cast, a short-term soft liner tissue conditioner (tissue treatment) that is used as a functional impression material. The patient was instructed to wear the dentures and allow her muscles to mold the tissue conditioner. After 2 weeks, the excess Hydro-Cast material was removed and CAD/CAM scans of the maxillary and mandibular prostheses were completed for duplication (Figure 7).

The process of transitional and final denture fabrication was initiated with 3D intraoral optical scans in mid-February 2020 and completed during the COVID pandemic with final dentures delivered in early March 2021 (Figure 8 and Figure 9). The patient traveled to the clinician's office a total of 10 times, including two separate surgical visits (one for each arch) and all postoperative and adjustment visits. This also included the fabrication and delivery of transitional dentures, soft- and hard-tissue healing, functional impressions, and delivery of the final dentures. For comparison, traditional staged extraction of posterior teeth, record-taking, and conventional impressions for transitional dentures would generate far more visits and the patient would have had to endure the indignity of functioning without most of her teeth for a prolonged period while the transitional dentures were being fabricated.

The patient was pleased with the esthetic and functional result and also indicated that her fear of dentistry was abating and she was considering implants for a mandibular overdenture. She was enthusiastic about the reduced number of visits and risk of contamination. The basal seat of the dentures was considered exceptional, occlusion was stable, and the esthetics were deemed more than satisfactory. The patient was especially pleased that a copy of the dentures was available in case of an emergency.

Case 2

In this case, an example of a patient who had ill-fitting dentures that could not be used, a 74-year-old man presented with an extensive history of health issues that included chronic obstructive pulmonary disease with a heart stent and arthritis. Medications included Brilinta^®, metoprolol, atorvastatin, lisinopril, and prednisone; however, based on consultation with the patient's physicians, there were no contraindications for dental treatment. The clinical examination revealed resorbed ridges with minor keratinized mucosa. The patient had a pair of ill-fitting, highly unesthetic maxillary and mandibular partial dentures, which were fabricated many years prior and could not be reused. The treatment plan involved proceeding with a new full denture using the CAD/CAM technology (3Shape) digital denture protocol.

The patient's maxillary and mandibular arches were digitally scanned in an analogous process to obtain a pure mucostatic impression (Figure 10 and Figure 11). Short-bordered denture bases were fabricated on printed models and delivered with wax occlusal rims. The VDO, esthetics, and phonetics were evaluated in a traditional manner, registering all of these aspects in the wax rims. A traditional tooth arrangement try-in was performed and the dentures delivered with functional impression material. After 1 week, a liquid plasticizer was applied on both dentures' short-term soft liners (tissue conditioner) (Hydro-Cast) to facilitate molding as per the manufacturer's protocol (Figure 12). After another week, the final digital scan of the molded dentures was performed using the TRIOS scanner and sent electronically to the laboratory for duplication. Of course, in this scenario the option of surrendering the dentures for a rebase is available but not necessary because the original dentures used for fabrication can be used over again to obtain a myofunctional impression at a future date when new dentures are needed.

Denture delivery was efficient and consisted of evaluating the basal seat, occlusal relationship, phonetics, and esthetics (Figure 13). The dentures were inserted with a reassuring sound of suction, on both arches. Both dentures were highly stable. Pressure-indicating paste revealed no pressure spots. Occlusion was evaluated both visually and with articulating paper. Contacts were equal and well distributed. Evaluation of phonetics revealed some slight lisping, but it was not considered excessive and proved to be nearly nonexistent at the 3-day recall.

The patient was extremely pleased with the esthetic result and reported total comfort and satisfaction on the day of delivery (Figure 14). The 2-week recare appointment was uneventful, although one small sore spot was adjusted. The patient was happy with the experience of using the CAD/CAM digital denture protocol. The basal seat of the dentures was considered exceptional, occlusion was stable, and the esthetics were deemed more than satisfactory. Moreover, the patient was glad to know that a copy of the dentures was available for future use if needed.

Discussion

With an aging population, the prevalence of edentulism worldwide is increasing.^27,28 While conventional methodology for removable prosthodontic fabrication was established many decades ago, CAD/CAM technology offers promising advances. Besides fewer patient visits, less clinical treatment time, and reduced contamination from material handling, particularly with dental impressions, digital technology has created an opportunity for a change in prosthodontic protocols.

Many removable prosthodontic impression techniques have demonstrated great success for recording edentulous ridges. An important aspect of removable prosthodontic impression-making is the reaction of supporting tissues, particularly in complete dentures.^29,30 Today it is important to use a technique that is amenable to the patient and able to record the anatomic ridge landmarks. The use of CAD/CAM technology can reduce the number of office visits and improve cost efficiency.^31,32

The selective pressure technique, first reported by Boucher in 1950, mainly concentrates on the principle of impression-making that is based on the belief that the mucosa over the ridge is best able to tolerate pressure.^29,30 These tissues are recorded under slight placement of pressure while other tissues are recorded at rest or relieved with minimal pressure in a position that offers equilibrium between the resilient and non-resilient tissues. This kind of impression involves an interaction between tissues and impression material using the traditional denture making of the selective pressure technique.

Traditional impression techniques have been described as either mucocompressive (recording oral tissue in a functional and displaced form) or mucostatic (impression taken when tissue is at rest). Yet, none of these types of techniques have been classified in terms of chairtime commitment. Traditionally, the selective pressure technique has received much more attention in literature and dental education. However, it typically requires the five main visits, identified earlier, and the procedure is more involved if extractions and other issues need to be addressed.^17,18 Conversely, a myofunctional impression technique that can be performed with the patient wearing the existing dentures with a tissue conditioner impression material presents a significant advantage.

By using the myofunctional impression technique, the custom border molding of preliminary impression trays is unnecessary. The most complicated and time-consuming phase of treatment, the border molding and final impression, is bypassed. It is replaced with a more user-friendly and patient-friendly process and results in a far more accurate impression of the supporting structures, as well as a more accurate extension of the flanges as the muscles mold the denture to the proper position. Logistically, this shifts the impression-making process on to the patient functioning at home rather than spending the time at the office. Most importantly, it allows the dentist to decide whether to reline the denture or just scan and proceed with fabrication of a more durable duplicate denture. Economically, the digital duplication of a denture is more cost-efficient than the traditional steps of labor-intensive laboratory denture fabrication.

Most notably, the technique described in this article solves the main problem of the digital impression technique: the capture of the functional borders of the denture. While a digital scan can capture the hard palate or the keratinized portion of the mandibular denture, it is not possible to capture the functional borders of the denture since they are movable. Utilizing the combination of a predictable myofunctional impression and digital technology gives providers the ability to deliver a predictable, superior, and cost-effective treatment to patients and should be considered as the future standard operating procedure for CAD/CAM removable prosthodontic dentistry.^31,32

Conclusion

The integration of CAD/CAM technology into removable prosthodontics marks a significant advancement in the treatment of edentulism, offering both clinicians and patients a more efficient, accurate, and cost-effective approach to denture fabrication. By reducing the number of patient visits, minimizing clinical treatment chairtime, and improving the precision of impressions and denture fit, digital workflows are revolutionizing traditional denture-making protocols. The combination of digital scanning with the traditional myofunctional impression techniques provides a versatile solution to the challenges of denture fabrication, ultimately delivering superior results with greater patient satisfaction.

About the Authors

Zev Kaufman, DDS
Clinical Assistant Professor, Director of Advanced Implant Restorative Concepts Lecture Course, Department of Periodontology and Implant Dentistry, New York University College of Dentistry, New York, New York; Private Practice in Prosthodontics and Implant Surgery (Surgically Trained Prosthodontist), New York, New York

Klenise S. Paranhos, DDS, MS
Clinical Assistant Professor/Assistant Group Practice Director, Department of Cariology and Comprehensive Care, New York University College of Dentistry, New York, New York; Diplomate, International Congress of Oral Implantologists; Private Practice, Chappaqua, New York

Ludovic F. de Carle, DDS
Prosthodontics Resident, University of Washington School of Dentistry, Seattle, Washington

Mary Kang, DDS
Clinical Associate Professor, Department of Periodontology and Implant Dentistry, New York University College of Dentistry, New York, New York; Private Practice in Prosthodontics and Implant Surgery (Prosthodontist), New York, New York