A case series on the basic concept and design of removable partial dentures: support and bracing considerations

Abstract

Background

When designing removable partial dentures, maximizing the effectiveness of support and bracing is necessary to minimize denture movement. Therefore, it is essential to emphasize the importance of providing patients with appropriate, safe, and secure removable partial dentures and have clinicians rerecognize the concept and importance of support and bracing. This study aimed to present extension-base removable partial dentures through six specific clinical case series and describe the effect of support and bracing action on denture design, which is essential for denture movement minimization.

A Case Series Study

Case presentation: The case series highlights the importance of utilizing the contact between the axial surface of the abutment tooth and denture components to provide effective support and bracing action. Furthermore, it emphasizes the need to improve the bracing action by connecting the minor connector and proximal plate with the guiding plane set for multiple teeth (frictional control), control the direction of the denture during the placement/removal (path of insertion), and consider the major connector form to improve the support and bracing actions. Effective support and bracing actions are necessary not only for the retainer but also for the denture components, including the design of the denture base and major connector. Removable partial dentures with “frictional control” and “path of insertion” are expected to reduce denture movement and improve stability.

Conclusions

The denture design described in this study is essential in pre- and postgraduate dental education, and the author believes that it will be helpful for dental students, interns, or residents in clinical practice.

Key Clinical Message

In designing removable partial dentures, the importance of support and bracing effects that utilize the contact between the axial surface of the abutment tooth and the denture structure is required as a specific measure for minimizing denture movement.

Background

In tooth-tissue-supported removable partial dentures, the attachment of the denture to the abutment teeth results in varying amounts of tissue displacement in the periodontal ligament and the residual ridge under the denture base. An even distribution of occlusal pressure across the denture is essential during function. Achieving a harmonious and simultaneous occlusal contact relationship between the denture and the remaining teeth, including the abutment tooth, requires an environment that can compensate for the differences in tissue displacement between the abutment tooth and the supporting elements of the denture [1–3]. In addition, the denture must have the ability to resist occlusal pressure to suppress sinking (support), lateral force to prevent lateral movement (bracing), and vertical force to prevent surfacing (retention) [4]. It is necessary to consider the denture design in terms of support, bracing, and retention [4]. Specifically, the procedure involves the use of rest, denture base, minor connector, proximal plate, major connector, and extracoronal retainer (retentive clasp). As a result, when a functioning removable partial denture is completed, the functional occlusal pressure loading on the denture base is reduced by the supports from the abutment tooth through the occlusal rest and the tissue of the residual ridge [5–7].

Most clinical cases of removable partial dentures involve extension-base removable partial dentures (tooth-tissue-supported removable partial dentures) [8–10]. As such, the denture should be designed in a way that allows the full utilization of the elements of the support and bracing action, with attention to the bracing effect resulting from the contact between the axial surface of the abutment tooth and the denture structure. The author believes that this approach can lead to a minimization of denture movement, resulting in greater stability of the dentures.

In designing a removable partial denture, support and bracing must be considered as fundamental principles. In several studies on the effects of guiding planes and proximal plates on clasp retention, it has been reported that the retentive force was maintained when both were well adapted [11, 12]. Clinicians must acknowledge that denture movement minimization requires increasing support and bracing action. By restricting the movement of the denture, sufficient retention can be expected without requiring excessive force in the undercut portion of the abutment tooth (frictional control).

This study aimed to describe the effect of the support and bracing action on the removable denture design, which is important for denture movement minimization, as well as to explain how these principles can be applied in clinical practice. This study presents six case series of six extension-base removable partial dentures provided from the author’s clinical practice. Therefore, the denture design described in this study is of great importance in pre- and postgraduate dental education, and the author believes that it will be helpful for dental students, interns, or residents.

Case series study

Herein, clinical cases in which it was possible to suppress denture movement by increasing the support and bracing actions of a denture design are described. All the included patients provided written informed consent for the use of personal or clinical details along with any identifying images for publication in this study.

In all six cases, no medical, family, psycho-social history, or genetic information was provided. Furthermore, no abnormalities were observed in the periodontal tissue based on the periodontal pocket probing, tooth mobility test, or bleeding on probing of the remaining mandibular teeth and residual mucous membrane. At each visit, intervention adherence and tolerability in terms of denture comfort and problems were verbally evaluated. Subsequently, denture occlusion and denture basal surface conformity tests were conducted each time to check for any abnormalities. Moreover, oral hygiene inspections and maintenance as well as masticatory function tests were carried out to confirm that the patients were able to eat without any problems. The patient was being followed up for maintenance, and no problems have been noted.

Case 1

This case is categorized as Kennedy Class I. A 51-year-old woman visited the university dental hospital with a chief complaint of masticatory disturbance due to inadequately fitting, unstable dentures. An inadequately fitting bridge was attached from the first premolar on the right side of the mandible to the second premolar on the left side as abutment teeth. Radiological examination (orthopantomogram) confirmed the inadequately fitting bridge; there were no abnormal findings in the edentulous ridge. Therefore, the patient was diagnosed with masticatory disorder due to inadequately fitting mandibular dentures and inadequately fitting bridge. Prosthetic treatment of the inadequately fitting bridge and mandibular denture was planned. A porcelain-fused-to-metal restoration, on which rest seats were applied, was used as the abutment tooth to make a bridge (Fig. 1a). After the successful placement of the mandibular bridge on the abutment teeth, a functional impression was taken, incorporating the border molding of the mandibular extension-base removable partial denture. Concurrently, the bridge was secured, and a precise working model was developed. The mandibular major connector was designed as a lingual plate, with rest proximal plate I bar (RPI) clasps as direct retainers on the right side of the mandibular first premolar and left side of the mandibular second premolar as abutment teeth (Fig. 1a, c). A rest was also applied to the left first premolar as an indirect retainer (Fig. 1a, c). For the artificial teeth, composite resin teeth were chosen (Endura posterior; Shofu Japan). The completed final denture is shown after it was inserted and fitted in the mouth (Fig. 1b, d,e, f). To evaluate the movement of the removable partial dentures, occlusal rests that provide support were installed in four places. On the lingual surface of the removable partial denture, the minor connector, proximal plate, and major connector (lingual plate) are essential for maintaining friction (frictional control) with the axial surface of the abutment tooth. These friction-maintaining effects have made it possible to minimize movement during denture function. The design controls the direction of denture placement/removal by maintaining frictional control. This case was followed up for 10 years after the placement of the dentures.

Fig. 1.

Case 1 a: The surveyed crown (the design on the abutment tooth side includes a guide plane) is attached to the definitive cast. b, c, e: Removable partial denture placement with the axial surface of the abutment tooth (surveyed crown). d, f: Lingual view (major connector: metal lingual plate) of the removable partial denture

Case 2

This case is categorized as Kennedy Class I. A 67-year-old man visited the university dental hospital with a chief complaint of missing dentures (Fig. 2a-d). The patient was diagnosed with masticatory disorder in the mandible. Radiological examination (orthopantomogram) confirmed the remaining teeth; there were no abnormal findings in the edentulous ridge. As the treatment was limited by the patient’s wishes, a resin-made mandibular extension-base removable partial denture was designed using acrylic resin. The mandibular right lateral incisor, left canine, and first and second premolars were used as abutment teeth. After applying the guiding plane and rest seat to the abutment teeth, a functional impression with border molding was taken. The completed final denture was a lingual plate major connector made of resin material, wrought wire clasps of the right lateral incisor, double Akers clasps of the left first and second premolars as direct retainers, and embrasure hooks of the left canine as indirect retainers. To correct the occlusal plane of the mandibular left second premolar, a cap clasp made of indirect composite resin was combined with a double Akers clasp that the design controls the direction of denture placement/removal by maintaining frictional control. For the artificial teeth, composite resin teeth were chosen (Endura posterior; Shofu Japan). Furthermore, to prevent bending and breakage of the resin base denture, a reinforcement metal plate (diameter: 3 mm, thickness: 1.5 mm) was inserted to improve the strength. To evaluate the movement of the removable partial dentures, occlusal rests that provide support were installed in four places. On the lingual surface of the removable partial denture, the minor connector, proximal plate, and major connector (a resin lingual plate) are essential for maintaining friction (frictional control) with the axial surface of the abutment tooth. These friction-maintaining effects have made it possible to minimize movement during denture function. The design controls the direction of denture placement/removal by maintaining frictional control. This case was followed up for 8 years after the placement of the dentures.

Fig. 2.

Case2 a: Definitive cast after the survey and design. b, d: The removable partial denture is designed with a resin lingual plate for the major connector (b: view of the polished surface of denture, d: view of the tissue surface of denture). c: Placement of the removable partial denture on the axial surface of the abutment tooth

Case 3

This case is categorized as Kennedy Class II-1. A 54-year-old man visited the university dental hospital with a chief complaint of masticatory disturbance due to inadequately fitting, unstable dentures. Inadequately fitting crowns were observed in the mandibular right second molar, first premolar, canine, lateral and central incisors, and left central incisor, which were confirmed by radiological examination (orthopantomogram). Nevertheless, there were no abnormal findings in the edentulous ridge. Therefore, the patient was diagnosed with masticatory disorder due to inadequately fitting mandibular dentures and inadequately fitting crown. A new mandibular denture and connecting resin-veneered restoration was planned with milling on the axial lingual surface. A crown and fused crown with milling were fabricated, to which the guiding plane was applied as the abutment tooth. After the mandibular and fused crowns were applied to the abutment teeth, a functional impression with border molding of the extension-base removable partial denture was taken. Concurrently, a definitive cast of the mandibular and fused crowns was made. The mandibular major connector was designed as a Kennedy bar, and retentive arms were intended for the right second and first premolars as direct retainers. Furthermore, retentive arms were designed for the right canine as indirect retainers. The design controls the direction of denture placement/removal by maintaining frictional control. For the artificial teeth, composite resin teeth were selected (Endura anterior, posterior; Shofu Japan). The completed final denture inserted and fitted in the mouth is presented in Fig. 3a-d. To evaluate the movement of the removable partial dentures, occlusal rests that provide support were installed in three places. On the lingual surface of the removable partial denture, the proximal plate and the design of a Kennedy bar made of metal with a milling technique applied to the major connector are essential for maintaining friction (frictional control) with the axial surface of the abutment tooth. These friction-maintaining effects have made it possible to minimize movement during denture function. The design controls the direction of denture placement/removal by maintaining frictional control. This case was followed up for 10 years after the placement of the dentures.

Fig. 3.

Case3 a: The crown and fused crown with milling are attached to the definitive cast. b, d: The Kennedy bar made of metal with a milling technique applied to the major connector (b: view of the polished surface of denture, d: view of the tissue surface of denture). c: The removable partial denture is placed on the abutment teeth milled on the lingual and mesiodistal axial surfaces

Case 4

This case is categorized as Kennedy Class I. A 72-year-old woman visited the university dental hospital with a chief complaint of masticatory disturbance due to inadequately fitting, unstable maxillary dentures. There were no abnormal findings in the crown fitted from the right maxillary to the left canine. Radiological examination (intraoral radiographic image, orthopantomogram) confirmed the remaining teeth; there were also no abnormal findings in the edentulous ridge. Therefore, the patient was diagnosed with masticatory disorder due to inadequately fitting dentures in the maxilla. Prosthetic treatment of the inadequately fitting maxillary denture was planned. After adding a guiding plane and cingulum rest seat for the canines on both sides, a functional impression with border molding of the extension-base removable partial denture was taken. The maxillary major connector was designed as a palatal plate, with RPI clasps having cingulum rests on the canines as direct retainers on both sides. These clasps were installed to provide frictional retention on the lingual and distal axial surfaces of the abutment teeth. The design controls the direction of denture placement/removal by maintaining frictional control. For the artificial teeth, composite resin teeth were chosen (Endura posterior; Shofu Japan). The completed final denture inserted and fitted in the mouth is presented in Fig. 4a-d. To evaluate the movement of the removable partial dentures, the cingulum rests that provide support were installed in two places. On the palatal surface of the removable partial denture, the proximal plate and the major connector (palatal plate) are essential for maintaining friction (frictional control) with the axial surface of the abutment tooth. These friction-maintaining effects have made it possible to minimize movement during denture function. The design controls the direction of denture placement/removal by maintaining frictional control. This case was followed up for 10 years after the placement of the dentures.

Fig. 4.

Case 4 a: Definitive cast after the survey and design. b, d: The removable partial denture is designed with a metal palatal plate for the major connector (b: view of the polished surface of denture, d: view of the tissue surface of denture). c: Placement of the removable partial denture on the axial surface of the abutment tooth

Case 5

This case is categorized as Kennedy Class II. An 82-year-old woman visited the university dental hospital with a chief complaint of masticatory disturbance due to inadequately fitting, unstable maxillary dentures after tooth extraction in the right maxillary molar region. The patient was also concerned about the appearance of an incompatible left central and lateral incisor crown. Radiological examination (intraoral radiographic image, orthopantomogram) confirmed the remaining teeth; there were no abnormal findings in the edentulous ridge. Therefore, the patient was diagnosed with masticatory disorder due to missing maxillary right posterior tooth and aesthetic disorder due to an inadequately fitting maxillary crown. Prosthetic treatment of the inadequately fitting maxillary crown and maxillary dentures was planned. After two ceramic crowns for the maxillary anterior teeth were fabricated, the cingulum rest seat for the maxillary left canine, occlusal rest seat for the distal second premolar, and occlusal rest seat for the distal first molar and the mesial second molar were installed. After the completed maxillary crowns were applied to the abutment teeth, a functional impression with border molding of the maxillary extension-base removable partial denture was taken. Concurrently, working models of the crowns were developed. The maxillary major connector was designed as a palatal plate and was used to continuously extend to come into contact with the axial surface of the residual tooth. Cingulum rests for the left central and lateral incisors as direct retainers, Akers clasp with cingulum rest for the left canine as indirect retainer, Akers clasp with occlusal rest for the distal second premolar as indirect retainer, and double Akers clasps with occlusal rests for the distal first molar and mesial second molar as indirect retainers were installed. The design controls the direction of denture placement/removal by maintaining frictional control. For the artificial teeth, composite resin teeth were chosen (Endura anterior, posterior; Shofu Japan). The completed final denture inserted and fitted in the mouth is presented in Fig. 5a-d. To evaluate the movement of the removable partial dentures, the cingulum and occlusal rests that provide support were installed in four places. On the palatal surface of the removable partial denture, the minor connector, proximal plate, and major connector (metal palatal plate continuously extended to come into contact with the axial surface of the abutment tooth) are essential for maintaining friction (frictional control) with the axial surface of the abutment tooth. These friction-maintaining effects have made it possible to minimize movement during denture function. The design controls the direction of denture placement/removal by maintaining frictional control. This case was followed up for 6 years after the placement of the dentures.

Fig. 5.

a: The surveyed crown (design on the abutment teeth: two ceramic crowns for he maxillary anterior teeth) is attached to the definitive cast. b, d: The removable partial denture is designed with a metal palatal plate continuously extended to come into contact with the axial surface of the abutment teeth for the major connector (b: view of the polished surface of denture, d: view of the tissue surface of denture). c: Placement of the removable partial denture on the axial surface of the abutment teeth

Case 6

This case is categorized as Kennedy Class I-1. A 55-year-old man visited the university dental hospital with a chief complaint of masticatory disturbance due to inadequately fitting, unstable dentures. An inadequately fitting crown was attached to the maxillary right first molar, and an inadequately fitting bridge with the maxillary left canine, first premolar, and first molar as abutment teeth was observed. Furthermore, the bilateral maxillary tubercles were remarkably bulging, and it was difficult to extend the denture base. Radiological examination (intraoral radiographic image, orthopantomogram) confirmed an inadequately fitting bridge; there were no abnormal findings in the bone quality of the edentulous alveolar ridge. Therefore, the patient was diagnosed with masticatory disorder due to inadequately fitting maxillary dentures and inadequately fitting maxillary crown and bridge. A new maxillary denture, a right maxillary complete metal crown with milling on the axial lingual surface, and a left maxillary porcelain-fused-to-metal restoration with milling on the axial lingual surface were planned. A crown and a bridge, milled with a guiding plane applied to the abutment tooth, was fabricated. After the placement of the maxillary crown and bridge on the abutment teeth, a functional impression with border molding was taken. Concurrently, the crown and bridge were secured, and a precise working model was developed. The maxillary major connector was designed as a palatal plate, with Akers clasps for the right molars as direct retainers, a cingulum rest for the left canine as a direct retainer, and Akers clasp for the left first premolar as an indirect retainer and that for the first molar as a direct retainer (Fig. 6a, c). The design controls the direction of denture placement/removal by maintaining frictional control. For the artificial teeth, composite resin teeth were chosen (Endura anterior, posterior; Shofu Japan). The completed final denture inserted and fitted in the mouth is presented in Fig. 6a-d. To evaluate the movement of the removable partial dentures, the cingulum and occlusal rests that provide support were installed in five places. On the palatal surface of the removable partial denture, the minor connector, proximal plate, and major connector (metal palatal plate with a milling technique applied to the major connector) are essential for maintaining friction (frictional control) with the abutment tooth milled on the lingual and mesiodistal axial surface. These friction-maintaining effects have made it possible to minimize movement during denture function. The design controls the direction of denture placement/removal by maintaining frictional control. This case was followed up for 10 years after the placement of the dentures.

Fig. 6.

a: The crown and bridge (milled on the lingual and mesiodistal axial surfaces with a guiding plane) are attached to the definitive cast. b, d: The removable partial denture is designed with a metal palatal plate and a milling technique applied to the major connector (b: view of the polished surface of denture, d: view of the tissue surface of denture). c: The removable partial denture is placed on the abutment teeth milled on the lingual and mesiodistal axial surfaces. The bracing clasp (reciprocal clasp) as a direct retainer is fully seated, and the lingual contour of the abutment teeth is restored

Practice denture design based on support action

Support action

To alleviate the impact of occlusal loading on the abutment teeth due to the pressure exerted on the denture base during function, it is important to minimize the soft-tissue displacement beneath the denture base during occlusion and reduce the difference between the displacement under the denture base and the abutment teeth [13–18]. Through this, simultaneous occlusal contact between the denture base and the abutment teeth can be achieved during function [6, 7, 19–21] (Fig. 7). As a result, it is possible to disperse the functional occlusal force so that the transmission during function is in the axial direction of the abutment tooth and the vertical to the alveolar ridge mucosa [19–21] (Figs. 1, 2, 3, 4, 5 and 6).

Fig. 7.

Abutment tooth and alveolar ridge mucosa with different amounts of tissue displacement. It is clinically important to compensate for the difference in tissue displacement between the abutment tooth and residual ridge using the functional impression method for the removable partial denture. Consequently, the functional force can be dispersed in such a way that the transmission direction during the function is in the axial direction of the abutment tooth and the vertical direction of the residual ridge

Practice denture design based on bracing action

Maintaining friction on parallel surfaces (frictional control)

To minimize denture movement, it is essential to utilize the bracing action by establishing contact between the axial surface of the abutment teeth (guiding plane) and the minor connector, proximal plate, and major connector between the lingual and palate axial surfaces of the abutment teeth [22–24]. In addition, it is necessary to use bracing clasps designed for effective bracing action.

Figure 6 (a, c) presents a clinical case (surveyed crown) where a part of the crown contour was processed using a milling device to ensure parallel alignment of the lingual and mesio-distal axial surfaces of each abutment tooth of the maxillary dentition [25–27]. Multiple rests and guiding planes were set parallel to the direction of denture placement/removal to regulate its movement direction [28]. This improves the bracing action through contact with the axial surface of the abutment tooth and multiple rests to ensure denture stabilization during function [29].

Among the denture components, including Figs. 1, 2, 3, 4, 5 and 6 of clinical cases, the minor connector and proximal plate provide frictional control [7, 29–31] by maintaining contact with the guiding plane on the axial surface of the abutment teeth, which in turn controls the movement direction and minimizes movement during denture function. Furthermore, by restricting the direction of placement/removal (path of placement [or insertion] and removal) [6, 7, 12, 29, 32], the denture becomes less likely to be lifted and separated (Fig. 8a, b,c).

Fig. 8.

Bracing effect enhanced by minor connectors and proximal plates through friction on the parallel surfaces (frictional control). a: The abutment tooth side with a guide plane (the black lines denote the parallel surfaces). b, c: Restriction of the placement/removal direction on the guide plane (path of insertion)

Figure 1 (a, b,c, d) shows an example of a design that controls the direction of denture placement/removal by maintaining friction on parallel surfaces (frictional control). The abutment tooth side is a surveyed crown [33–39] designed with a guide plane and mesial rest (Fig. 1a) to maintain friction on surfaces parallel to the denture components (minor connector, proximal plate, and major connector) (Fig. 1c). The denture design incorporates the concept of structural design (Fig. 1c), with the major connector designed as a lingual plate due to the distance from the gingival margins to the floor of the mouth being less than 7 mm (Fig. 1a, c). The continuous contact of the major connector with the tooth surface regulates the direction of denture placement/removal, as can be seen from the lingual view (Fig. 1d, f).

Reciprocation

Figure 9 presents three cases of reciprocation in the retentive and bracing clasps (reciprocal clasps). The bracing clasp opposing the retentive clasp on the buccal side can be designed to have a platelike shape with vertical width, as shown in Fig. 9a, to improve the bracing action and suppress the lateral movement of the denture. Similarly, a plate-shaped bracing clasp with a vertical width can be designed in combination with the denture base (metal based or acrylic resin based) to suppress lateral movement, as shown in Fig. 9b. From the viewpoint of the vertical and lateral reciprocation of denture placement/removal, the bracing clasp is desirable in the platelike shape with a vertical width [6, 7, 22]. Furthermore, the bracing clasp can be designed as a lingual plate, the major connector illustrated in Fig. 9c, that opposes the buccal retentive clasp and extends on the lingual axial surface toward the occlusal side.

Fig. 9.

Reciprocation: The association between the retentive clasp and the bracing lasp. The lingual portions of figures a, b, and c indicate the action of bracing. a: Design of the bracing clasp; buccal: action of retention, lingual: action of bracing. b: The bracing clasp can be designed in combination with the denture base; buccal: action of retention, lingual: action of bracing. c: It is possible to design a lingual plate that can be extended on the lingual axial surface toward the occlusal side as a bracing clasp; buccal: action of retention, lingual: action of bracing

Therefore, including Figs. 1, 2, 3, 4, 5 and 6 of clinical cases, it is necessary for the retentive and bracing clasp arms to simultaneously come into contact with the axial surface to protect the abutment tooth and prevent buccolingual movement [6, 7, 22]. This phenomenon, known as reciprocation, is an important design requirement for clasp elements and plays a pivotal role during denture placement/removal.

Major connectors

The major connectors effectively and appropriately distribute the functional load applied on the denture by extensively covering the residual lingual surface (palatal surface). The lingual and palatal plates have a continuous and wide contact with the axial surfaces of the abutment teeth that resist the lateral force applied to the denture, thereby providing effective bracing action.

In daily clinical practice, cases with multiple missing teeth in the anterior and molar regions of the maxillary and mandibular arches are frequently encountered (Figs. 1, 2, 3, 4, 5 and 6). In these cases, minimizing denture movement using direct or indirect abutments alone is difficult. Therefore, it is desirable to provide support and bracing action by the major connector [7, 37]. In the mandibular region, as illustrated in Figs. 1 and 2 and a plate-shaped major connector is designed to be continuously extended to come into contact with the axial surface of the remaining tooth. Essentially, the methodology for designing the major connector in the maxillary region is the same as that of the mandibular region. As shown in Figs. 4 and 5, to prevent lateral movement of the denture and lifting or separation at the posterior end of the denture base, a plate-shaped major connector is used to continuously extend and contact the axial surface of the residual tooth. These are important considerations when designing the form of the major connecters. In addition, as shown in Figs. 3 and 6, milling on the axial surface of the abutment tooth may enhance the bracing effectiveness of the major connector design in some cases.

Discussion

This study presents the principles of the design of removable partial dentures using clinical cases from the author’s clinical practice (Figs. 1, 2, 3, 4, 5 and 6), highlighting the crucial role of support and bracing action in minimizing denture movement. Therefore, the basic procedure for designing dentures is as follows: rest/denture base (support action), followed by the minor connector/proximal plate/major connector (bracing action), and finally, the extracoronal retainer (retentive clasp, retentive action).

As demonstrated in the clinical examples presented in this study (Figs. 1, 2, 3, 4, 5 and 6), removable partial dentures with clasps applied to the retainer belong to the category of rigid connections. By enhancing the effect of support and bracing action, it is possible to improve the connection strength and achieve a condition close to a rigid connection, thereby minimizing denture movement. This study highlights the role of a plate-shaped major connector that comes into contact with the axial surface of the abutment tooth. This enhances the bracing action, thereby contributing to denture movement minimization. This approach incorporates the concept of structural design into removable partial denture prostheses. While the paper cited in this study discusses the design of removable partial dentures, few include actual clinical cases such as the ones presented here. Therefore, the author believes that this case series provides valuable insights and is clinically important, potentially enhancing prosthetic treatment for removable partial dentures.

When designed to provide sufficient support and bracing action, such dentures exhibit adequate stability in the mouth. The establishment of multiple contact points between the guiding plane of the abutment teeth and the denture components (frictional control) enhances the bracing action. It elucidates the placement/removal direction (path of placement and removal) of dentures (Fig. 9).

In this study, the author proposes a denture design, that emphasizes bracing action through contact with the axial surface of the abutment tooth, particularly for major connectors and denture bases among denture components with bracing action (Figs. 1, 2, 3, 4, 5 and 6). Contact with the axial surface of the tooth, parallel to the placement/removal direction of dentures, not only suppresses lateral movement when the denture is in a fixed position but also governs the placement/removal direction (path of placement and removal) of dentures [6, 7].

From the perspective of minimizing denture movement, movement direction regulation is essential in denture design.

Preventive dentistry is one of the factors to consider when designing removable partial dentures [38–40]. When designing the mandibular major connector, either the lingual bar or lingual plate should be selected based on the distance from the gingival margin of the remaining tooth to the floor of the mouth [6, 7]. If there is no issue with the position of the gingival margin of the remaining tooth, the lingual bar, which exhibits excellent self-cleaning action, is considered as the first choice. However, there are many clinical cases in which the superior support and bracing action of the lingual plate are more important than the cleanability and self-cleaning action of the lingual bar [7, 29, 37]. Thus, when applying a plate-shaped major connector, it is necessary to ensure that the patient does not have extensive caries or advanced periodontal disease and is capable of reliably and effectively controlling dental and denture plaque during home care maintenance. Furthermore, if it is impossible to secure enough retainers to minimize denture movement or if the residual ridges have poor support, it may be preferable to choose a lingual plate to compensate for these factors (Figs. 1, 2 and 5).

In recent years, clinical research using crossover studies with lingual bars and plates has been conducted. The present study elucidates that lingual plates do not directly facilitate the growth of bacteria that cause periodontal disease when oral hygiene management is adequately performed [41]. In addition, a 30-year retrospective cohort study of fitted removable partial denture design at the University of Montreal School of Dentistry found that adequate oral hygiene management and instruction as well as a planned maintenance system ensured appropriate application of the lingual plate and maintenance of good oral health [42]. Based on the evidence presented in this section, the application of plate-shaped major connectors is an effective treatment strategy for removable partial dentures when strengthening of the support and bracing action is the top priority.

In the clinical cases presented herein (Figs. 1, 2, 3, 4, 5 and 6), the effect of denture movement suppression was demonstrated in all denture designs. In each case, the author explained the importance of the remaining teeth to the patient and provided instructions on oral hygiene before starting denture treatment. In addition, the author explained the importance and handling of dentures to the patient, and the patients’ understanding was confirmed during treatment and after wearing the dentures; moreover, consent was obtained before the treatment. After the treatment, the patient’s motivation for oral hygiene and the understanding of dentures improved compared with that before treatment, thereby positively impacting the long-term prognosis after wearing the dentures and improving the quality of life [38, 42]. There have also been recent reports that the masticatory function can be improved by prosthodontic rehabilitation wearing removable prosthesis of a hyperhidrotic ectodermal dysplasia patient [43]; the mixing ability test was capable of detecting improvement in masticatory function with new removable partial dentures (type of denture base: Kennedy Classes I and II) [44]. In the clinical cases in this study (Figs. 1, 2, 3, 4, 5 and 6, type of denture base: Kennedy Classes I and II), no problems with masticatory function were recorded in a masticatory ability test using gummy jelly [45]and interviews with the patients. Also, during the follow-up period, the author checked the components of the dentures, condition of the denture basal surface, occlusion, maintenance of motivation for oral hygiene, and maintenance and stability of masticatory function at regular checkups.

This study has several limitations that need to be acknowledged. In recent years, with the rise of the superaging society, the number of individuals with remaining teeth has been increasing annually at a considerable rate [46]. Removable partial denture treatment for missing teeth is an essential prosthetic dental treatment in everyday clinical practice, and its demand is expected to further increase [47]. Despite the limitations in using data to elucidate the association between various factors in superaging societies and the importance of removable partial dentures, the findings of this study are of utmost importance. The long-term case in which the patient demonstrated remarkably stable functional and preventive dental outcomes, coupled with the importance of the denture design principles outlined in this study, highlights the value of our work in the prosthetic treatment of removable partial dentures for missing dentition. In the context of this paper, each case will be discussed based on the previously described design principles of the removable partial dentures. However, in today’s superaging society, patient satisfaction with removable partial denture treatment is higher than ever. Thus, there is a strong demand for individualized treatment for each case (number of missing teeth, distribution of missing teeth, properties of remaining tissues, etc.). With the continuously increasing diversity of removable partial dentures, the author believes that their importance will remain the same. To achieve this, the author thinks it is important to emphasize the need to return to the basics of clinical cases and reconsider the fundamentals and design principles of removable partial dentures. Therefore, the author has considered cases involving extension-base removable partial dentures. In addition, the denture design described in this study is significant in pre- and postgraduate dental education, and the author believes that it will be helpful for dental students, interns, or residents in clinical practice.

Conclusion

When designing removable partial dentures, maximizing support and bracing action is required to minimize denture movement. This study highlights the importance of support and bracing effects that utilize the contact between the axial surface of the abutment tooth and the denture structure. These can be summarized as follows:

1) Improving the bracing action by contacting the minor connector and proximal plate with the guiding plane set for multiple teeth (frictional control) is crucial. Furthermore, it is important to control the direction of the denture during placement/removal (path of insertion).

2) It is essential to consider the design of major connectors to improve the support and bracing action.

Therefore, in prosthetic rehabilitation that involves the use of removable partial dentures, the combined action of support and bracing is required not only for the retainer but also for other denture components, including the denture base and major connector. Moreover, it is important to consider the equitable distribution of force during function. Strategically designed removable partial dentures with frictional control and a defined path of insertion help minimize denture movement and enhance stability.

Author contributions

Jun Takebe: Dental treatment physician, investigation, methodology, project administration, validation, visualization/photography/clinical case management, manuscript writing/review and edting.

Funding

This work was supported by a grant from JSPS KAKENHI (grant no. JP20K10082 and 24K13016).

Data availability

The datasets used and/or analyzed during the current study are available from the corresponding author on resonable request.

Declarations

Ethics approval and consent to participate

The patient provided written informed consent, and this case report was approved by Ethics Committee School of Dentistry, Aichi Gakuin University (the committee’s reference number 729). Informed consent for the use of photographic materials in dental education and research papers was obtained from the patient and recorded in the medical records.

Consent for publication

All patients in this study provided written informed consent for the use of personal or clinical details along with any identifying images for publication in this study.

Competing interests

The authors declare no competing interests.