The partial edentulous population is increasing because of an increasing aging population, increased life expectancy, and individuals retaining more teeth at an older age. Therefore, the need for fixed and removable partial denture (RPD) therapy will remain high and will continue into the future. RPDs provide minimally invasive, cost-effective, timely care, and are preferred to fixed dental prostheses using teeth or implant therapy in many clinical scenarios. This article discusses RPD classification systems to review basic concepts and special framework design considerations, and explores advancements in the field such as implant-assisted RPD, CAD/CAM RPD, and new polymer framework materials.
Key points
- •
Removable partial denture therapy will remain a cost- and time-effective treatment for partially edentulous patients.
- •
Most basic concepts, framework design, and treatment considerations for metal-framework RPD remain unchanged.
- •
Implant-assisted RPD significantly improves the quality of life and satisfaction of patients.
- •
Computer-assisted design and manufacturing for RPD framework provides high-quality treatment with its accuracy and precision.
- •
More research is necessary to examine changing design principles to better serve the new polymer framework materials.
Introduction
The percentage of complete edentulism has decreased in the aging population because of better understanding and reduction of dental diseases, establishment of effective maintenance and preventive programs, and improvement of dental materials. However, the proportion of the partially edentulous population is increasing because of increased life expectancy, an increasing aging population, and more teeth being retained within this population. The American College of Prosthodontists predicted the number of individuals with partial edentulism could increase to more than 200 million in the United States in the next 15 years, and currently almost 50% of all adults are missing one or more teeth. Therefore, the need for fixed and removable partial prostheses remains high and will continue to grow in the future.
Fixed dental prostheses (FDPs) have traditionally been the gold standard to treat partial edentulism. However, when there is no abutment tooth distal to the edentulous space or in a long-spanning edentulous space, FDP therapy is contraindicated. When these patients would have had no other treatment choice in the past but to receive a removable partial denture (RPD), now they may have a fixed option in the form of dental implants. Frequently, dental implants today provide the best practice alternative. However, financial considerations often contribute to the decision-making process for many of these patients, leading to RPDs being the treatment option selected.
Dental implants come at a cost, both financial and time-related. The financial expense of dental implants is high, especially when additional therapies are required to prepare the implant site. Because the treatment period is generally longer than for nonsurgical interventions, there is an additional expense of time. When compared with an implant placed without site development, bone augmentation procedures can easily double the treatment time. For patients who have a significant medical history or dental anxiety, the surgical therapy may be considered more aggressive and may be contraindicated.
In contrast to implant therapy, RPD treatment is minimally invasive, and allows cost-effective and timely care for partially edentulous patients. When replacing lost hard and soft tissues to provide esthetic support, for long-term transitional prosthesis for a terminal dentition, and when restoring long edentulous spans, it is the best practice therapy for many clinical scenarios ( Figs. 1 and 2 ). Therefore, RPDs will remain an important treatment alternative and viable option for a large proportion of the partially edentulous population.
The purpose of this article is to review RPD classification systems, treatment concerns, and framework design, and to discuss advances in implant-assisted RPD (IARPD), computer-assisted design (CAD)/computer-assisted manufacturing (CAM) in RPD framework design and fabrication, and the use of new materials such as milled polymer frameworks that allow metal-free options.
Removable partial denture classification systems
The most widely used classification system for partial edentulism is the Kennedy method of classification and Applegate’s rule for applying the Kennedy classification. This classification system divides RPD scenarios according to the location of the edentulous region and its popularity. Kennedy classification class I designates bilateral edentulous areas located posterior to the natural teeth, and Kennedy class II describes a unilateral edentulous area located posterior to the remaining natural teeth. Kennedy defined a unilateral edentulous area bound by natural teeth to be class III, and for a single edentulous area crossing the midline located anterior to the remaining natural teeth, class IV was the designation. Applegate provided 8 rules that help with the application of the Kennedy classification in more complex situations. Although the Kennedy and Applegate classifications are useful in visualizing the partially edentulous arch and communicating, they do not address the detailed complexity that is necessary for billing issues and distinguishing complex situations that need referral. The Prosthodontic Diagnostic Index (PDI) classification system for partially edentulous patients was developed to address this issue. This classification distinguishes partially edentulous scenarios according to complexity determined by 5 criteria, namely location and extent of edentulous areas, abutment tooth condition, occlusal scheme, residual ridge morphology, and other conditions, to facilitate consistent and predictable treatment-planning decisions. The descriptive nature of this classification system ( Fig. 3 ) provides a more detailed communication with dental providers, laboratory technicians, and others. The adoption of this system has the potential to benefit dentistry greatly through emphasis of the PDI classification system in dental education and its usage for billing, compensation/reimbursements, and establishing referrals. Unfortunately, this classification system currently has limited use in dentistry because of its complexity, resistance to change, and diagnostic basis in a profession that is largely therapy driven.
Removable partial denture treatment considerations
Partially edentulous patients must be cautious of the development of caries, periodontal diseases, and resorption of the residual ridge. These disease states may be detrimental to the long-term survival of tissues supporting the RPD. Exposed root surfaces caused by periodontal disease or gingival recession from mechanical brushing are common in the older population, and these root surfaces are more prone to caries. Increased plaque accumulation with the use of RPD prostheses has been well documented, and directly contribute to the high incidences of caries for patients wearing RPDs. When appropriate etiologic factors are present, RPDs may accelerate bone loss and tooth mobility. Therefore, an attentive framework design that minimizes accumulation of plaque and has favorable biomechanical forces is necessary. Continuous maintenance and excellent oral hygiene are necessary for long-term survival of the prosthesis, the abutment teeth, and the periodontium.
A retrospective study found that 39% of RPDs were no longer in use within 5 years of delivery. To be successful, RPDs must be worn by the patient with minimal complications. Therefore, framework design should take the patient’s comfort and esthetic expectations into consideration to ensure patient compliance. Because ill-fitting RPDs transferring detrimental forces to the ridge may accelerate the progression of residual ridge resorption, the fit and occlusion of the RPD must be verified during fabrication of the prosthesis and subsequent maintenance appointments to delay the progression of residual ridge resorption. Partially dentate patients most likely lost their teeth as a result of caries or periodontal disease from poor oral hygiene. Therefore, it is essential that dental care providers help patients improve their oral hygiene habits through proper home-care instructions while enforcing an appropriate maintenance schedule for their removable prosthesis.
The challenge of RPD therapy is to restore function, esthetics, and comfort while minimizing potentially damaging forces to the abutment teeth and supporting tissues. Although the design of the RPD framework is important to minimize detrimental forces to abutment teeth and supporting tissue, most classic RPD concepts are based on empirical observations and philosophic biases of the clinician presenting the concepts. Several concepts are based on attempts to integrate scientific research into the philosophic and biological biases that were acquired during professional practice. The basic concepts of RPD design that are widely accepted are reviewed in the following sections, and are in part based on the properties of conventionally used metal framework.
Review of basic concepts and framework design
Rests
The main function of a rest is to serve as a vertical support for the RPD to resist movement toward the tissue ; it also provides the positional placement and reproducibility for all of the other components of the RPD. All rest preparations should have a positive seat that transmits the occlusal forces to the center long axis of the tooth without slippage of the prosthesis away from the abutment. Any preparation must be free of sharp angles with smooth finishing.
Occlusal rest seats on posterior teeth should be one-third the buccolingual width and one-third the mesiodistal length of premolars or one-fourth the mesiodistal length of molars. These rest preparations should be triangular in shape with dovetails on the marginal ridge to meet the proximal plate of the correct width. A longer rest preparation that extends beyond half of the occlusal surface, mesiodistal width, may be used in special clinical scenarios to transfer forces along the vertical axis, to the center of the tooth, especially in mesially inclined molars.
Cingulum rests can be prepared on anterior teeth with distinct anatomic cingula, which are typically limited to maxillary canines. A cingulum rest seats is prepared into the incisal most extent of the cingulum where there is a bulk of enamel, not above the cingulum ( Fig. 4 ). These rests should have adequate thickness—at least 1 mm at the center and tapered as it moves away from the center. Any undercuts created above the rest preparation should be conservatively removed. When an anterior tooth does not have a prominent cingulum, a lingual ball rest may be used. Because a lingual ball rest is placed on either the mesial or distal part of the marginal ridge, it may transmit tipping forces to the teeth. Careful framework design and a well-fitting framework will help minimize these tipping forces. Although incisal rests have been used in the past, their use has declined in recent years because of unfavorable lever forces and esthetic concerns.
A common cause of framework failure is rest fracture at the junction between the rest seat and the proximal surface. A 1.5-mm minimal thickness of metal is advocated to prevent this type of a complication. At times it is difficult to prepare a posterior occlusal rest seat on sound enamel structure without penetrating into dentin when a minimal preparation of 1.5 mm is required. Therefore, it is critical that the patient’s occlusion be evaluated before initiating any treatment, either through clinical evaluation or by articulating opposing diagnostic casts. If the opposing tooth does not directly contact the marginal ridge area or if the patient is edentulous on the opposing arch, a more conservative rest seat preparation may be allowed without compromising the integrity of the metal framework as long as it does not interfere with patient’s static and dynamic occlusion. In this clinical situation, the metal can be designed to meet the minimum thickness and not interfere with opposing occlusion.
Major Connectors and Minor Connectors
The major connector joins components of the prosthesis on one side to those on the opposite side of the same arch. The marginal gingiva of teeth is highly vascular and susceptible to injury from pressure. Therefore, the major connector must maintain a safe distance of 6 mm away from the gingival margin of maxillary teeth and 3 mm away from that of mandibular teeth.
The most important requirement of a major connector is rigidity for cross-arch stability, resulting in effective distribution of occlusal forces to supporting teeth, bone, and soft tissue. A flexible major connector is detrimental because it will concentrate forces on individual teeth and areas of edentulous residual ridges, and not allow other components of the prosthesis to fulfill their purposes. Ben-Ur and colleagues characterized major connectors based on their rigidity. For maxillary RPDs, anteroposterior palatal straps on different planes were deemed most rigid, followed by a palatal strap and U-shape. Campbell investigated patient preferences for the various major connector designs, rank ordering them by ability to speak, chew, swallow, overall comfort, and satisfaction. Based on patient perceptions, the preferred rank order was broad strap, anteroposterior palatal straps (no anterior plating), anteroposterior strap (with plating), and full palatal coverage. It is no surprise that both studies rated lingual bar higher than lingual plating in the mandible. The most commonly used maxillary major connectors are the single palatal strap for short-span tooth-supported scenarios and the anterior-posterior strap for most partially edentulous scenarios. Complete palatal coverage may be used in compromised anatomic situations and a U-shaped palatal major connector may be used when a palatal torus or a sensitive gag reflex is present. The use of the bar type of maxillary major connector is decreasing because of discomfort created in patients from the bulk required for rigidity as it impinges on the tongue space and coincides with the natural anatomy of the palate. The most commonly used mandibular major connectors are the lingual bar and plate, when there is limited vestibular space. Other major connectors with limited use include sublingual bar, lingual bar with cingulum bar, cingulum bar, and labial bar. The best type of major connector must be selected to fit individual patients and their needs.
Minor connectors are the components that unite other components of the prosthesis to the major connector. These components include proximal plates, the rests, indirect retainers, clasps to the major connector, tissue stops, and the open lattice or mesh that joins the denture base to the major connector. The purpose of minor connectors is to transfer functional forces on the RPD to the abutment teeth and transfer the effects of retainers, rests, and stabilizing components throughout the prosthesis. Therefore, rigidity is the primary requirement for minor connectors, as it is for major connectors.
Although the concepts proposed by Kratochvil and Krol are relevant to distal extension clasp assemblies, it is important to mention their differences and discuss the length of the proximal plate. Kratochvil advocated the use of long proximal plates to maximize contact with the abutment tooth to improve stabilization, prevent food impaction, and prevent hypertrophy of soft tissue in the area. Krol advocated the use of a short proximal plate to provide physiologic relief, and disengagement of the proximal plate from the abutment tooth during function, thereby avoiding torquing of the abutment tooth. It is widely accepted to use a proximal plate somewhere in between, that is, one-half to one-third of the clinical crown height. The width of the proximal plate should be one-third of the buccal lingual width of the tooth and follow the natural curvature of the tooth.
Retainers
Direct retainers are used to provide retention for the RPD; resisting the movement of the prosthesis away from the tissue surface. The terminal tip of a direct retainer (retentive clasp) passes below the height of contour (HOC) to engage a 0.01-inch or 0.02-inch undercut. Extracoronal retainers are the most widely used and taught in dental curriculums. There are 2 types of clasps, suprabulge and infrabulge. Because the suprabulge clasp approaches the undercut from above the HOC, it is crucial to have a low HOC to allow sufficient room for the approach arm of the clasp to originate from the rest or the proximal plate ( Fig. 5 ), thereby providing improved flexibility and esthetics. An infrabulge clasp approaches the undercut from below the HOC. Its use is not recommended when there is a significant amount of soft tissue undercut or a shallow vestibular depth.
