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Aesthetic Considerations for the Treatment of Partially Edentulous Patients with Removable Dentures

Treatment with removable partial dentures (RPDs) is an inexpensive solution for the prosthetic rehabilitation of patients who have a functional or aesthetic need for replacement of posterior teeth (Kennedy-Applegate classification Class I and II). An RPD may also be indicated in Class III, IV, or V situations when the edentulous space is too large for a fixed prosthesis or when alveolar bone loss is so important that it is necessary to replace the missing bone with a resin flange to support the middle third of the face. The principle disadvantages of RPDs are the risk of caries and periodontal disease adjacent to the abutment teeth and resorption of the residual ridge.1-3 In addition, the wear of RPDs may be associated with complaints regarding the appearance of the dentition as elements of the denture framework or the acrylic resin becomes visible.4,5 In general, the aesthetics and function of fixed partial dentures (FPDs) are superior to that of RPDs.6 In numerous instances, however, treatment with RPDs may be the only feasible solution due to financial restrictions, biological considerations (eg, residual ridge resorption), or doubtful prognosis of potential abutment teeth for an FPD. Various clinicians have previously considered principles of framework design in order to improve the comfort of patients wearing RPDs.7 In this article, several concepts for treatment with RPDs that could improve the aesthetic result of treatment will be outlined.

Maintenance of Anterior Residual Ridge Profile

When lost anterior teeth are to be replaced by an FPD or RPD, it is important to maintain the anterior residual ridge profile in order to fit the gingival aspect of the dentures directly against the residual ridge. In this manner, a labial acrylic denture flange -- which often compromises the aesthetic result -- can be avoided. Following extraction of anterior teeth, however -- particularly if they have been periodontally involved -- the remodeling of the alveolar bone in the region will often produce a marked reduction of the residual ridge. To establish lip support and a tooth length that corresponds to the natural remaining anterior teeth, it is generally necessary to provide the denture with a base that includes a labial flange. In this instance, aesthetics may be compromised if the acrylic part of the denture is exposed by the patient. It is well-accepted that roots beneath overdentures prevent bone resorption, improve load distribution, and maintain the sensory feedback of periodontal receptors (Figure 1).8,9 In partially edentulous patients, the maintenance of roots in the anterior region provides the clinician with an efficient manner to maintain the residual ridge profile. While such conditions may warrant restoration with posts and cores and ceramic crowns, patients' financial situations often prohibit this treatment. Alternatively, an RPD with retentive clasps may override more suitable treatment with the roots of the natural teeth serving as abutments for the denture teeth (Figure 2).

Appropriate Framework Design

The aesthetic considerations for RPD framework design are primarily concerned with its concealment. In Class I and II situations, the denture should be designed aesthetically and mechanically to achieve retention with two retentive clasps and with the diagonal retention close to the fulcrum line: no retentive clasps are positioned anteriorly to the fulcrum line.10 Consequently, the tooth just mesial to the extension base should serve as an abutment tooth, if possible, and the occlusal rest should be placed mesially to avoid the risk of posterior tilting of the abutment tooth. To ensure proper aesthetics, a roach clasp can be used, or the retentive clasp arm should emerge from the minor connector, be placed distally, and extend mesially (Figure 3). Biomechanically, this is a fail-safe design.7 Thus, when the denture base is loaded, the denture can rotate around the fulcrum line, connecting the rests on the maxillary right canine and left second molar without exerting torque on the abutment, because the bracing arm, the shoulder of the retentive clasp, and the minor connector are positioned on the survey line; only the retentive aspect of the clasp is placed gingival to the survey line. In this design, no retentive clasps on the maxillary left canine and second premolar are situated anterior to the fulcrum line. The diagonal of retention is thus situated distally to the fulcrum line (Figure 4). When the free-end denture base is charged and the denture has the potential to rotate slightly around the fulcrum line, the retentive elements will move gingivally without exerting any torque on the abutment teeth. In Class III, IV, V, and VI cases, aesthetics can be improved with conventional clasp-retained RPDs by reducing the number of retentive clasps to two and, if possible, obtaining a posterior placement of the diagonal of retention (Figure 5). Provided an entirely tooth-supported denture comprising two large denture bases is prepared with lingual rests on teeth #10(22) and #6(13) after the preparation of shoulder-shaped rest seats, mastication forces will be transported along the axis of the teeth. Although no retentive clasp is placed on the lateral incisor for aesthetic reasons, there is no risk of orthodontic movement of this tooth as the tooth-supported denture bases are secured by the distinct rest seat preparations. When premolar teeth have been lost, placement of retentive clasps on the canine will compromise the aesthetic result (Figure 6). The premolar/molar teeth could also be replaced by conventional FPDs or implant-supported reconstructions, if the patient is not opposed to surgical intervention or tooth preparation. By fabricating an RPD with retentive clasps on the molar teeth, the restorative team is able to provide an acceptable aesthetic solution (Figure 7). In this design, the diagonal of retention has an appropriate position that supports the RPD. Thus, the occlusal rests on the mesial and distal aspects of the second molars provide support as well as adequate indirect retention. Replacement of teeth in the anterior or premolar region with bounded denture bases can be achieved by the use of precision attachments or bar attachments, which will be addressed subsequently. Nevertheless such solutions are generally much more expensive and the treatment more invasive.

Rotational Path Partial Denture Design

The concept of a rotational or dual path of placement of the denture has been described in the literature by several authors.11 With a conventional RPD, a straight path of placement is used to ensure that all rests and clasps are seated simultaneously. The incorporation of a dual, curved, or rotational path of placement permits one portion of the framework to be seated first, followed by the remainder of the framework (including the retentive clasps). This type of design may be indicated in Class III and IV cases. In Class III situations, the gingival extension of the minor connectors adjacent to the distal surface of the anterior abutments serves as rotational centers or axes for the denture. The gingival extension of the minor connectors serves as rigid retentive elements when the denture is rotated in position (Figure 8). The anterior section of the denture is retained by minor connectors, which engage the undercut portions of the abutment teeth, and the posterior of the denture is clasp retained. Thus, no clasps have been placed in visible regions. In Class IV situations, appropriate cingulum rest seats are prepared on the canines, and the minor connectors engage the undercut area distogingivally for the canine teeth (Figure 9). When the posterior section of the denture is rotated in correct position, retention is achieved anteriorly by the undercut zones and posteriorly by the clasps. The acrylic resin of the anterior denture flange covers the minor connectors. While this technique is simple to apply and provides reliable results, it does raise two concerns. Since the denture engages the dentogingival area of the abutment tooth, a risk factor is introduced with regard to plaque accumulation, periodontal health, and caries. In addition, the acrylic resin that contacts the abutment tooth does not always provide an optimal aesthetic result (Figure 10).

Clasp Designs

The preparation of guiding planes lingually or proximally on the abutment teeth is an important means to secure retention of the RPD; this simultaneously obviates the need for visible retentive clasp arms. The prepared, vertically parallel surfaces on the abutment teeth should be oriented to guide the placement and removal of the RPD. If it is possible to prepare a guide plane mesially on the abutment tooth, the retentive clasp arm can be placed in contact with the distofacial aspect of the tooth surface in a less visible area (Figures 11 and 12).

Design of the Denture Base

It is a well-known experience that the covering of the gingival areas adjacent to the abutments may have a detrimental effect on their periodontal status and increase the risk of caries.3,12,13 In fact, the wearing of RPDs increases the salivary counts of mutans streptococci. The negative effect of wearing RPDs can be reduced using the "RPD mini design," which reduces the extension of the denture base and the major connectors (Figures 13 and 14). The smaller denture base replacing left maxillary premolars and first molars can be designed without a buccal flange to improve the quality of the denture biologically and aesthetically. While superior aesthetic results could have been obtained with a roach L-clasp engaging the distobuccal surface of the maxillary left canine, a visible gold clasp has less negative aesthetic effect in this particular situation.

Auxiliary Spring-Loaded Attachments

The mesiodistal clasp engages the mesial and distal surfaces of the abutment tooth, thus avoiding the use of a visible buccal clasp arm. Retention is gained through parallelism and frictional resistance of the clasp assembly against the natural tooth. As a consequence, the abutment teeth have to be prepared so that their proximal surfaces are parallel or have a slight convergence to one another. While this design exhibits adequate supporting and bracing qualities, it is potentially traumatic to the abutment tooth when used on distal extension RPDs. If the abutment tooth is or has to be crowned, it is generally more appropriate to incorporate a plunger attachment into the RPD design (Figures 15 and 16). The mesial surface of the crown is prepared with a semiprecision rest, such as a channel with tapering sides (mounted as far gingivally as possible) and a mesial rest. The lingual aspect of the crown is provided with a shoulder preparation or a plane contour to allow sufficient space for the lingual part of the clasp and the mesially located minor connector. The plunger is generally incorporated in the RPD, as this simplifies fabrication and makes subsequent adjustments more convenient. In the case of an Ipsoclip attachment, the plunger is spring-loaded. A dismantling screw is placed opposite the plunger so the spring or plunger can be changed if they become inactivated, fractured, or worn. When the denture is placed, the correct position of the plunger versus the distal surface of the abutment crown is indicated in a thin layer of wax that is applied to the surface of the crown. A small depression corresponding to the mark from the plunger is then ground into the crown. The advantage of using a semiprecision attachment (eg, Ipsoclip) is that slight movement of the denture is possible before the abutment tooth is engaged. This retentive system is particularly indicated in situations with few remaining teeth and large extension denture bases, when conventional clasps are contraindicated for aesthetic reasons.

Attachment-Retained RPDs

In Kennedy-Applegate Class III and IV cases, bar attachments or precision attachments can be used to avoid visible retentive clasp arms for the RPD. Such treatments are generally very complex and expensive to realize as splinting of the abutment teeth is necessary. While connecting a group of teeth reduces their mobility, such designs do provide load-sharing possibilities. As the removable reconstruction is generally designed to be firmly attached to the fixed restoration, however, there is a risk that strain may give rise to cementation failure or abutment fracture. The consequence of a mechanical failure is usually that the entire reconstruction must be replaced. Today, a more relevant and safe solution will be replacement with a fixed reconstruction based on implants and natural teeth as abutments.

Bar Attachments

Retention based on bar attachment may be a relevant solution in Class III and IV cases, when there is an indication for restoration of the entire abutment tooth crown with a fixed restoration. In this situation, the bar can either be soldered to full-coverage ceramic crowns (bar-retained RPD) or cast dome-shaped copings (bar-retained removable partial overdenture). With any bar type used, it is important to relieve the gingival margin by 1.5 mm to 2 mm in order to consider periodontal health. A laboratory-fabricated bar (sleeve bar) provides excellent retention and horizontal stabilization of the denture. In practical terms, the only indication for this solution is a Class III mandibular case with severe resorption of the residual ridge, as the bar needs significant vertical space (8 mm). Retention is supplemented by one or two auxiliary attachments. While the prefabricated rigid Dolder bar is rectangular and cannot be bent to conform with the shape of the ridge or arch, it is possible to solder segments of the bar to establish a form that follows the contour of the residual ridge. The sleeve can be activated to secure retention; this bar provides excellent retention and stability of the denture in Class III and IV cases. Resilient Ackermann or Dolder bars can be applied in Kennedy-Applegate Class II modification I cases. The bar is placed between the abutments for the tooth-supported denture base, whereas a conventional clasp or an auxiliary attachment is used for retention and support of the free-end denture base (Figures 17-18-19-20-21). In general, however, adequate retention can be obtained by two clasps.

Precision Attachments

In order to avoid visible retentive clasps, precision attachments can be used to retain Class III and IV RPDs, since such reconstructions become very expensive compared to conventional RPDs (as mentioned previously). A reconstruction based on implants and natural tooth abutments may provide a more suitable alternative. The use of precision attachments in Class I and II cases to avoid visible retentive clasp arms is contraindicated, as the firm connection between a distal extension free-end denture base and the abutment increases the risk of mechanical failures.14,15 In this situation, treatment with a posterior FPD supported by implants has provided durable and predictable results.

Implant-Retained RPDs

The use of implants to retain and support a mixed RPD is a relatively rare prosthetic solution that can be proposed to patients in whom treatment with an implant-supported FPD is not feasible for economic or technical reasons. The rationale for this treatment is to place a limited number of implants capable of sustaining an entirely implant- and tooth-supported RPD without visible retentive elements. In a Class III case with important tooth loss, a retentive clasp on the canines can be avoided by bilateral placement of one implant in each premolar region (Figure 21). The rests placed on the canines and molar teeth provide support for the denture, whereas two spherical attachments mounted on the implants provide adequate retention of the RPD. In a Class II case with a large denture base, three implants in the canine/premolar region connected by a bar are sufficient to provide retention for the denture base (Figure 22). An occlusal rest should be placed on the central incisor tooth to secure additional periodontal support for the denture base. In a Class I case with the absence of one canine, an implant placed in the canine region can provide support and retention for a bilateral extension base RPD (Figure 23). In a Class IV case with extensive tooth loss in the anterior and the premolar regions, two implants in the canine regions can provide support and retention for the large anterior denture base. Finally, in a Class V case with a large edentulous zone, the placement of three implants in the canine/premolar region will be sufficient to achieve an entirely tooth- and implant-supported RPD without visible retentive elements. In this circumstance, it is recommended for the clinician to use a bar connecting the implants as an element of retention and stabilization, which guarantees comfort and permits satisfactory distribution of the occlusal forces.At present, implant-supported RPD designs have not been verified by sufficient clinical and scientific experience to establish the long-term success. In a situation with doubtful prognosis for the natural dentition, it is important to place the implants in a position that is optimal for a future full FPD or complete denture supported by implants. Furthermore, periodontal health adjacent to the natural teeth should be well controlled to reduce the risk of peri-implantitis. It seems that treatment with tooth and implant-supported RPDs might have particular relevance in instances where the interarch space is too limited for aesthetically and technically acceptable metal-ceramic FPDs.

Conclusion

Lost anterior or posterior teeth can be inexpensively and predictably restored with RPDs. This presentation has described the various framework and retentive concepts that enable the restorative team to provide functional, aesthetic care to contemporary dental patients. These designs may also involve implant therapy, although such treatment must anticipate future restorative criteria. Removable partial dentures allow the attending clinician to simultaneously address patients' aesthetic and socioeconomic concerns while ensuing functional success.

 

*Professor, Division of Gerodontology and Removable Prosthodontics, University of Geneva, School of Dentistry, Geneva, Switzerland.

References    

 

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