Elimination of Retainers in Anterior Zone in Removable Partial Dentures

Abstract

Partial edentulism, one or more teeth missing, is one of the most common dental problems that can be found in the population. It has multiple resolution possibilities depending on the individual characteristics of the patient. One of these options is the removable partial denture (RPD), which allows for solving complex clinic problems in a faster way and with a considerably lower operational cost compared to other treatment modalities. However, important discontentment has been reported among patients who have opted for this treatment. One of the discomforting factors for users of removable partial prostheses is the presence of visible retainers in the anterior area. In some cases, it could be possible to eliminate the retainers in the anterior sector, providing retention by mechanical friction and the choice of a removal axis.

1. Introduction

Partial edentulism is one of the most common dental problems in the population, which can be solved in many ways, depending on the individual characteristics of each patient. Therefore, the options can be using prostheses on implants, fixed prostheses, and removable prostheses. Restoring with removable partial dentures allows the resolution of complex clinical problems, including extensive edentulous spaces and distal extensions, relatively quickly and at a substantially lower operational cost than other treatment modalities. However, it has been reported that many patients are not satisfied with this type of dental prosthesis [1].

Patients report that one of the most important features is the cosmetic aspect, so if direct retainers are visible when opening the mouth, they are commonly rejected [2].

All direct retainers must provide the following functions in order to be effective and not harm the abutment teeth or tissues of the denture foundation area: support, retention, cross-tooth reciprocation, fixation, and passivity [1, 2]. The literature makes it clear that dissatisfaction can be avoided if the basic requirements of minimal dislodgement during masticatory function are taken into account when making the prosthetic element and if the esthetic demands of the patients can be met. On the other hand, it is not feasible to assume that by achieving an acceptable masticatory capacity with the prosthetic appliance, the esthetic shortcomings of the appliance can be tolerated [3].

On another point, retainers play a key role in removable partial dentures, providing direct retention. However, their esthetic component has been widely recognized as an obstacle to patient acceptance [4, 5]. Also, using a metal clasp in RPD has many disadvantages. They provide sites for bacterial plaque accumulation, can abrade tooth surfaces, and become deformed over time [6]. Therefore, circumferential metal retainers or clasp can challenge esthetic restoration when anterior teeth are present, helping to support and retain removable partial dentures [6].

There are numerous options for the complication caused by visible anterior clasp [7]; however, many of these alternatives do not offer similar retentive characteristics to those provided by circumferential clasps.

Despite the existence of the implant option to replace missing teeth, many patients are not candidates for these solutions. In this group are patients whose economic position does not allow the choice of treatment or whose general systemic health is too deteriorated to undergo dental surgery; and others with psychological or anatomical limitations. Patients in this category may receive a removable partial denture to replace edentulous areas [8].

The removable partial denture (RPD) is an orthopedic appliance intended to restore lost oral structures, which, without causing damage to the remaining components, is installed and dislodged from the oral cavity at the patient’s will, which gives it the character of being removable. It must withstand the forces produced during the physiological acts of chewing, swallowing, and phonation that tend to dislodge it and restore the lost function and esthetic appearance [9, 10, 11]. RPD, for reasons of cost, time of fabrication, and because it is a nonaggressive and reversible treatment, is still widely used in patients who need partial tooth replacement [9, 10, 11, 12].

Because this prosthesis is not rigidly fixed, it is subject to movement in response to loads in function. Since these movements induce stress and displacement of the prosthetic base, retainers are designed to control these possible movements [12]. Many prostheses are fabricated without the essential requirements for proper function to produce a better esthetic appearance. However, they need more stability [13, 14].

Properly fabricating parallel guide planes, well-defined rest seats, and a tangentiograph (surveyor) are important factors to consider for the retention, support and stability of RPD [14]. Although RPDs are a favorable option for restoring many clinical situations, many patients are dissatisfied with this type of prosthesis, especially when they are unstable during mastication [15]. Masticatory function and esthetics should be considered when planning prosthetic-based treatment without overriding one over the other. When anterior teeth must be used to support an RDP, or when they are visible when speaking or smiling, extracoronal retainers are not well accepted by patients for their esthetic demands and can challenge esthetic restoration [16].

With increasing esthetic requirements, more patients ask their dentists to position retainers closer to the gingiva, where the retentive areas tend to be more profound. The rigidity of cobalt chrome retainers makes them impossible to place in these larger retentive areas due to the unacceptable stress that would be placed on the abutments; therefore, their placement cannot be hidden, and they are likely to fit in an area that is still visible when the mouth is opened [17, 18].

2. Classification of removable partial dentures

In oral rehabilitation, classifications have been proposed to allow the application of basic principles in the design of each partial denture so that not only mechanical aspects are taken into account but also the biological conditions of the oral tissues that will be in contact with the denture.

Partially edentulous arches have been classified by various methods. The primary objective of the classification is to facilitate communication about the combination of missing teeth to edentulous ridges among students, dental practitioners, and laboratory technicians. The most widely accepted classification for partially edentulous patients is that of Kennedy in 1925, who organized the different types of patients into classes numbered from I to IV [9].

Kennedy’s classification of Partially Edentulous Arch

• CLASS I: Bilateral edentulous areas located posterior to the remaining natural teeth.

• CLASS II: A unilateral edentulous area located posterior to the remaining natural teeth.

• CLASS III: A unilateral edentulous area with natural teeth remaining both anterior and posterior to it.

• CLASS IV: A single, but bilateral (crossing the midline), edentulous area located anterior to the remaining natural teeth.

Considering this classification, patients with unilateral edentulous areas and teeth remaining anterior and posterior to them are established as class III [9, 11, 19].

According to the support and transmission of loads to the remaining oral tissues, RDP can be classified as follows:

2.1 Tooth-supported prostheses

These are prosthetic devices whose support or load path depends exclusively on the abutment teeth. For example, this is the case when there are edentulous spans delimited on both sides by teeth [20].

2.2 Tooth mucosa-supported prosthesis

Also called mixed-loaded track prostheses, these are prostheses in which the teeth and the surrounding soft tissues provide support. In this case, there is at least one free end [20].

2.3 Mucosa-supported prosthesis

In this type of prosthesis, the forces exerted by occlusion are transmitted directly to the bone through the alveolar mucosa. Total prostheses and acrylic prostheses generally have this type of support, since they do not have a rest seat for support and are seated directly on the mucosa [20].

3. Components of the removable partial denture (RPD)

The components of an RDP are diverse. The selection and variation of each will depend on the classification of the edentulous arch and the particular characteristics of each clinical case [21].

3.1 Denture base

It is the structure primarily responsible for supporting artificial teeth. This component is located on the soft tissues and provides stability and retention, allowing the transmission of occlusal loads to the biological support structures. In Kennedy class III its function is only to support the artificial teeth, whereas, in a tooth mucosa-supported prosthesis, it must also transmit the loads, provide stability, retention, and, in some cases, lip contouring [22].

3.2 Minor connector

The structure joins the major connector to the anti-rotational elements and the denture base to the clasp assembly. This mechanical element provides mesiodistal stability, since it is attached to the guide plane made in the abutment tooth. It also provides secondary retention to the removable partial prosthesis by means of friction between the guide plane and the minor connector [20, 22].

3.3 Artificial teeth

These elements replace the anatomy, esthetics, and function of missing natural teeth. The most commonly used teeth are acrylic; however, porcelain teeth can also be used [23].

3.4 Major connector

This is the prosthetic structure responsible for joining the RDP components from one side of the arch to the other. All prosthesis components are attached to this element, directly or indirectly. Its function in the tooth-supported prosthesis is only to connect, and in the tooth mucosa-supported prosthesis, it also provides support [24]. The central connector provides support, stability, and retention to the prosthetic appliance. To function correctly, they must meet specific structural requirements, such as rigidity, to prevent torsion, leverage forces on the abutments, and ensure better distribution of forces on the supporting tissues [24].

3.5 Circumferential clasps assembly

This is the RDP unit responsible for resisting the displacement of the prosthesis from its final seating in the supporting tissues on which it rests. This complex involves an abutment tooth on which it will seat, mechanically preventing the prosthesis from shifting out of place in the patient’s functional movements, primarily in the vertical direction [25]. When the prosthesis is at rest, and there are no active vertical displacement forces, the retentive arm is in a passive state relative to the abutment. Therefore, when adjusting a retentive arm for more retention, the tip of the active arm of the circumferential clasp should be positioned as cervically as possible to exert a greater degree of retention [25].

4. Component elements of the clasp assembly

In a conventional RDP, the clasp assembly consists of the retentive arm, a reciprocating arm connected through a minor connector, and a rest (Figure 1) [26].

Figure 1.

Components of clasp assembly.

4.1 Retentive arm

It is the main actor in the retention of an RDP. The shape of the retentive arm must be such as to allow it to be flexible, so the initial two-thirds of the arm is rigid, part of which is located over the prosthetic equator, and a thinner terminal end, which sits in the retentive area under the prosthetic equator as it decreases in diameter toward the tip, its flexibility increases. As a result, it can deform as it passes through the prosthetic equator, offering resistance to prosthesis displacement along its path of insertion and removal, which determines the functional forces [9, 11, 20].

The location of the retentive arm generally corresponds to the visible side of the abutment, that is, in the vestibular area. However, this may vary depending on multiple factors, mainly esthetics, which may be detrimental to its function if necessary [9, 11, 20].

4.2 Reciprocal or containment arm

This element is responsible for opposing the forces of the retentive arm on the abutment to prevent damage to the abutment. It exerts an opposing force on the retentive arm when the prosthesis moves out of place and neutralizes it. It is generally located on the opposite side of the retentive arm and toward the occlusal of the prosthetic equator line. Its thickness is uniform throughout and thicker than the retentive arm. In all load-bearing track cases, this element is rigid due to its construction’s shape and volume, allowing it to fulfill its primary function. The rigidity also allows it to contribute to prosthetic stability [9, 11, 20].

4.3 Rest

Its rigid structure comes from the minor connector and rest seat milled in the abutment teeth. In posterior teeth, it will be on the occlusal face, while in anterior teeth, it will be in the cingular or incisal area. The rest seat is a preparation on the tooth or restoration created to receive occlusal, cingular, incisal, or root support [5, 21]. It is responsible for transmitting functional forces to the abutment teeth acting on the prosthesis along the axial axis of the prosthesis, preventing movement of the prosthesis into the soft tissues (intrusion), maintaining the positional relationship of the retentive arms concerning the prosthetic equator and also contributes to stability [20, 22]. This structure must be rigid; therefore, it must have a minimum volume that fits into the milled support and does not interfere with the antagonist’s occlusion [20, 22]. The rest is considered one of the most important components because, in addition to providing support and controlling the prosthetic position of the oral tissues, the support also helps to restore the occlusal plane [27].

5. Retention in RPD

In RDP, retention of prosthetic appliances is generally not a factor of concern. Through direct retentive arms, the parallelism of guide planes and indirect retention concepts, adequate retention is achieved, which is demanded by patients [28].

5.1 Direct retainers

There are two classes of direct retainers:

1. Intracoronal: These are mechanical devices placed in the casting of a full crown. They are usually reserved for RPD therapies that require exceptional effort to produce ideal esthetics. There are precision attachments on the one hand and semi-precision attachments on the other.

2. Extracoronal: They engage an external surface of an abutment in a natural undercut or in a prepared depression. There are two categories of extracoronal: 1) Clasp and 2) extracoronal attachment. Clasp are classified as circumferential or ackers (suprabulge) and bar clasp (infrabulge).

Retention is derived by placing a clasp arm into an undercut area so that it is forced to deform upon vertical dislodgement. Resistance of the clasp to deformation generates retention. Resistance is proportionate to the flexibility of the clasp arm. Nonflexible portions of clasp arms must be placed occlusal to the height of contour (suprabulge area) [29].

5.2 Indirect retainers

They are those elements of a removable partial denture, which prevent the rotational displacement of the prosthesis on the supports of the main abutment teeth. Indirect retainers usually take the form of rests, on the opposite side of a fulcrum line.

The retention and stability of an RDP depend on the retentive arms, the mesial bedding component for support, the rest seat within its bedding, and the contact of the minor connector against the guide planes. The tooth-to-base metal relationship must be maintained to allow positive contact.

Noted authors agree that the factors that determine the amount of retention, or if preferred, the tensile strength of an RDP, are given by the seating of the base, the number and distribution of teeth, the design, the material and quality of the retainer, the point of force application, the cervical convergence angle or depth of the retentive area, the location of the active arm tip at the convergence angle, its diameter, cross-sectional shape and the texture of the surfaces on which the active arm of the retainer is placed [28].

The total resistance of the indirect retainer to removal reaches its most significant magnitude before the retentive arm reaches the largest diameter of the tooth, that is, the prosthetic equator, and decreases as it moves toward the occlusal surface of the abutment after reaching this position. The maximum force value reached corresponds to the retention force of the retainer, which in some studies has been determined to be around 500 gf (4.903 Newton) [30].

6. In vitro study for retention of RDP without anterior retainer

Rochefort et al. evaluated the retention and settlement of the RPD in an in vitro study assessing the possibility of removing anterior retainers in patients classified as Kennedy class III subdivision 1 (Figures 2 and 3).

Figure 2.

Removal of clasp from the anterior sector.

Figure 3.

Front view of metal bases positioned on the plaster model without the anterior clasp.

In the study, they used ivory artificial teeth provided by Nissin dental products Inc. as prosthetic abutments in maxillary dental models in plaster type IV, where the main characteristic of these is to include in the design retainers in anterior abutment teeth that compromise esthetics (canine teeth or first premolars), which were subjected to tensile tests to evaluate retentive strength (Figure 4).

Figure 4.

Side view of metal bases positioned on the plaster model without the anterior clasp.

7. Conclusions

Given the tensile tests performed in the study, it was determined that it is possible to remove the anterior retainers without detriment to the retention of the prosthetic appliance while maintaining its functionality in the in vitro study. However, it may be a therapeutic alternative for patients showing abutments in anterior teeth that represent an esthetic requirement (Figures 5 and 6).

Figure 5.

Pulling device installed on the metal bases for testing.

Figure 6.

Pulling device installed on the metal bases for test execution.