The basic philosophy of treatment is to plan the best partial denture design for a given arch and to prepare the mouth for an ideal prosthesis. A patient presenting with missing tissues requires preparation and specific treatment. Periodontal disease and unusual configurations of soft tissue and tooth positions can jeopardize long-term success. It is best to resist compromising basic design principles whenever possible by performing proper mouth preparation. It is necessary to have an organized, orderly, and definite design sequence when designing the removable partial denture (RPD) casting. To this end, the elements providing support (rests) that control movement of the partial denture are placed first because their design and position control placement and affect the design of other parts of the prosthesis. The design sequence is accomplished in this order:
1. Rests
2. Major connectors
3. Minor connectors
4. Denture base connectors
5. Retainers
The rationale and principles for use and design of these five parts of the casting have been described and discussed in previ- ous chapters. The design is now outlined on the diagnostic cast, which is a three-dimensional replica of the oral condition of the patient to be treated. The design should be precise and detailed. It will serve as a treatment plan for mouth preparation as well as for communication with other dental specialists involved in dental treatment, including dental laboratory technicians. It will also serve as a patient record for future reference.
The extension, exact position, and width of each rest is precisely and clearly outlined on each tooth that will be engaged by a rest (Fig 7-1). A molar rest on the tooth-borne side should extend at least to the center of the tooth to ensure that occlusal forces are directed along the long axis of the tooth. Further extension of the rest across the tooth is determined by the need to restore the occlusal plane and/or the occlusion. Diagnostic casts mounted with appropriate maxillomandibular records are necessary to determine proper rest design and position.
The rest on the premolar on the extension side is placed on the mesial side of the tooth because of the rotational factors described and discussed in chapter 6. A rounded, ball-and- socket–type rest seat is prepared to allow pure rotational move- ment of the extension portion of the prosthesis during function. The canine rest is positioned where the most positive rest seat can be prepared, compatible with the opposing occlusion. The position of this rest is determined by the mounted diagnostic casts. The central portion of the rest should be open (see Fig 7-1b). This allows the clinician to easily determine whether the rest is properly seating in the rest seat preparation. This design also facilitates cleaning of this portion of the RPD in that the bristles of the denture-cleaning brush can protrude through the opening and keep the rest seat area clear of debris.
Major connectors
The design joins all rests and edentulous areas with a rigid connection to ensure control and maintain the position of abut- ment teeth. The anterior strap of the major connector is placed in the valley of the ridge to reduce bulk and is blended with the rugae pattern. The posterior strap of the major connector should not extend beyond the vibrating line of the palate. The design carries the metal just short of the hamular notch area. The finish line on the tooth-borne side is positioned to permit easy placement of the prosthetic teeth and to allow sufficient bulk of acrylic resin to effectively secure the prosthetic teeth to the metal casting (Fig 7-2).
On the edentulous side, the strap is positioned more toward the middle of the palate to increase the surface coverage of the edentulous ridge. This practice increases the support provided for the RPD and allows easy relining of the edentulous extension area. The anteroposterior palatal strap optimizes the rigidity of the framework, ensuring optimal cross-arch stabilization and distribution of lateral torquing forces during function.
Minor connectors and proximal plates
The minor connectors connected with the proximal plates are extended to cover the tooth-tissue junction and should extend onto the tissue for at least 2 mm (Fig 7-3). The facial extension covers and protects any tissue depressions, and the facial portion extends over the curvature of the ridge. In some instances, such as in the molar region, the minor connector provides the con- nection between the rest and the major connector. The proximal plate portion of the minor connector, contacting the tooth at the tooth-tissue junction, idealizes the bracing (resistance to lateral torquing forces) provided for the prosthesis because this area is closest to the attachment of the tooth to the bone. Note that the casting is designed well away from the tooth-tissue junction on the palatal side of the abutment (Fig 7-4).
Denture base connectors
The denture base connectors are designed to enable appropriate positioning of the denture teeth. Connectors that interfere with tooth placement may lead to foreshortening of the premolar denture teeth, compromising the esthetic outcome. In addition, when the denture teeth are inappropriately reduced in size, it will be difficult to retain them within the acrylic resin of the denture base. To eliminate these problems, the connectors are placed on the crest and the lingual side of the ridge, away from the facial surface (Fig 7-5).
Note that the location of the posterior portion of the connec- tor on the edentulous side is well to the lingual side and away from the crest of the tuberosity because of the limited space in this area. The double finish lines (internal and external finish lines) on the cast indicate a finish line on the tissue side and the tongue side of the casting that will form a positive union of the acrylic resin and the metal.
Retainers
The location of the retention portion of the retainer is deter- mined by the abutment tooth contour at the retention areas and the pattern of movement of the prosthesis in function. The retention portion is positioned first and designed to cross the tooth-tissue junction at right angles and continues in the direction of the long axis of the tooth (Fig 7-6). This portion extends to the mucogingival junction, where it turns in a hori- zontal direction (Fig 7-7). The retainer is joined to the denture base connector so as not to impair positioning of the denture teeth (see Fig 7-6b).
When viewed from the facial aspect, the tapered design of the retainer can be visualized, with the bulkier portion at the junction with the denture base connector and located at the interproximal area of the denture teeth (Fig 7-8). The junction at this location will not interfere with proper placement of the denture teeth and idealizes the esthetic result by allowing the positioning of teeth of appropriate length and contour (see Fig 7-6b).
The rests provide support and control points between the abut- ment teeth and the prosthesis, so they are positioned first. Where an incisal rest is required (eg, on the right canine), it is designed to cover and restore the entire functional area. The mesial and distal rests on the left premolar may be joined together for increased strength because this is a non-occluding area (Fig 7-9). The rest on the right premolar is placed on the mesial side because this tooth is adjacent to a posterior edentulous extension area. The rest on the molar is extended into the middle of the tooth to direct occlusal forces along the long axis of the tooth.
Major connectors
Rigidity for the major connector is more difficult to provide in the mandible. Greater bulk is required to produce the rigidity necessary between the rests and the edentulous area. When the lingual bar is the design of choice, it should be positioned on unattached mucosa wherever possible and well away from the tooth-tissue junction. The bar can be extended into the lingual area as far as functional anatomical structures permit to gain the required bulk for rigidity. The lingual bar design should be 3 to 4 mm away from the gingival margin, and the bar must be 4 mm in height (Fig 7-10a; see also chapter 4). In some instances, the elevation of the floor of the mouth does not permit the design of a lingual bar with sufficient bulk. In this instance, a lingual plate design is advised (Fig 7-10b).
Where the casting crosses the tooth-tissue junction, it should be at right angles to the junction in order to minimize food impaction between the RPD framework and the tissues and should continue onto the unattached gingiva before the hori- zontal portion begins. The space between the crest of the mucosa and the lingual bar should be as open as possible (see Fig 7-11).
Minor connectors and proximal plates
The proximal plate portions of the minor connectors are po- sitioned to cover the side of the tooth and to extend onto the mucosa from the tooth-tissue junction for a minimum of 2 mm. The design should provide metal contact at all points where the prosthesis contacts the tooth or the tooth-tissue junction. Note that the casting is designed well away from the tooth-tissue junction on the lingual side of the abutment (Fig 7-11).
Denture base connectors
Denture base connectors are positioned on the crest of the ridge and toward the lingual side to minimize interference with denture tooth placement (Fig 7-12). On the side with the edentulous extension, an area of critical concern is the junction with the major connector (Fig 7-13). This area is subjected to flexure and fracture and must be designed with additional bulk. A fan-shaped union will provide maximum strength. Note the position of the external finish line in this area; it is placed well to the distal side of the tooth and curved into the edentulous portion, where there is extensive bone and tissue loss. Placement of the bulky acrylic resin–metal junction in this area restores the contour of lost tissues without infringing on the tongue space. Note that the connectors end abruptly in anticipation of the continuous extension of the acrylic resin of the denture base.
On the tooth-borne side of the arch, the major connector forms the inferior border of the prosthesis to provide maxi- mum strength with minimum bulk and a junction point for the denture base connector as well as the external finish line area (Fig 7-14).
Retainers
The retainers are designed and positioned to provide retention as required and to allow the proper placement of the denture teeth. In the mandibular arch, the contour of the tissues on the facial surface can present extensive undercuts that require a shorter vertical extension of the retainer before the horizontal portion is located (Figs 7-15 and 7-16). A high attachment of the buccal frenum may require the use of a wrought wire retainer or a suprabulge retainer.
A systematic approach with a step-by-step sequence develops a quick and practical design, which organizes treatment planning and provides for detailed communication with the laboratory.
1. Rests
2. Major connectors
3. Minor connectors
4. Denture base connectors
5. Retainers
The rationale and principles for use and design of these five parts of the casting have been described and discussed in previ- ous chapters. The design is now outlined on the diagnostic cast, which is a three-dimensional replica of the oral condition of the patient to be treated. The design should be precise and detailed. It will serve as a treatment plan for mouth preparation as well as for communication with other dental specialists involved in dental treatment, including dental laboratory technicians. It will also serve as a patient record for future reference.
Maxillary Design Sequence
Occlusal restsThe extension, exact position, and width of each rest is precisely and clearly outlined on each tooth that will be engaged by a rest (Fig 7-1). A molar rest on the tooth-borne side should extend at least to the center of the tooth to ensure that occlusal forces are directed along the long axis of the tooth. Further extension of the rest across the tooth is determined by the need to restore the occlusal plane and/or the occlusion. Diagnostic casts mounted with appropriate maxillomandibular records are necessary to determine proper rest design and position.
The rest on the premolar on the extension side is placed on the mesial side of the tooth because of the rotational factors described and discussed in chapter 6. A rounded, ball-and- socket–type rest seat is prepared to allow pure rotational move- ment of the extension portion of the prosthesis during function. The canine rest is positioned where the most positive rest seat can be prepared, compatible with the opposing occlusion. The position of this rest is determined by the mounted diagnostic casts. The central portion of the rest should be open (see Fig 7-1b). This allows the clinician to easily determine whether the rest is properly seating in the rest seat preparation. This design also facilitates cleaning of this portion of the RPD in that the bristles of the denture-cleaning brush can protrude through the opening and keep the rest seat area clear of debris.
Major connectors
The design joins all rests and edentulous areas with a rigid connection to ensure control and maintain the position of abut- ment teeth. The anterior strap of the major connector is placed in the valley of the ridge to reduce bulk and is blended with the rugae pattern. The posterior strap of the major connector should not extend beyond the vibrating line of the palate. The design carries the metal just short of the hamular notch area. The finish line on the tooth-borne side is positioned to permit easy placement of the prosthetic teeth and to allow sufficient bulk of acrylic resin to effectively secure the prosthetic teeth to the metal casting (Fig 7-2).
On the edentulous side, the strap is positioned more toward the middle of the palate to increase the surface coverage of the edentulous ridge. This practice increases the support provided for the RPD and allows easy relining of the edentulous extension area. The anteroposterior palatal strap optimizes the rigidity of the framework, ensuring optimal cross-arch stabilization and distribution of lateral torquing forces during function.
Minor connectors and proximal plates
The minor connectors connected with the proximal plates are extended to cover the tooth-tissue junction and should extend onto the tissue for at least 2 mm (Fig 7-3). The facial extension covers and protects any tissue depressions, and the facial portion extends over the curvature of the ridge. In some instances, such as in the molar region, the minor connector provides the con- nection between the rest and the major connector. The proximal plate portion of the minor connector, contacting the tooth at the tooth-tissue junction, idealizes the bracing (resistance to lateral torquing forces) provided for the prosthesis because this area is closest to the attachment of the tooth to the bone. Note that the casting is designed well away from the tooth-tissue junction on the palatal side of the abutment (Fig 7-4).
Denture base connectors
The denture base connectors are designed to enable appropriate positioning of the denture teeth. Connectors that interfere with tooth placement may lead to foreshortening of the premolar denture teeth, compromising the esthetic outcome. In addition, when the denture teeth are inappropriately reduced in size, it will be difficult to retain them within the acrylic resin of the denture base. To eliminate these problems, the connectors are placed on the crest and the lingual side of the ridge, away from the facial surface (Fig 7-5).
Note that the location of the posterior portion of the connec- tor on the edentulous side is well to the lingual side and away from the crest of the tuberosity because of the limited space in this area. The double finish lines (internal and external finish lines) on the cast indicate a finish line on the tissue side and the tongue side of the casting that will form a positive union of the acrylic resin and the metal.
Retainers
The location of the retention portion of the retainer is deter- mined by the abutment tooth contour at the retention areas and the pattern of movement of the prosthesis in function. The retention portion is positioned first and designed to cross the tooth-tissue junction at right angles and continues in the direction of the long axis of the tooth (Fig 7-6). This portion extends to the mucogingival junction, where it turns in a hori- zontal direction (Fig 7-7). The retainer is joined to the denture base connector so as not to impair positioning of the denture teeth (see Fig 7-6b).
When viewed from the facial aspect, the tapered design of the retainer can be visualized, with the bulkier portion at the junction with the denture base connector and located at the interproximal area of the denture teeth (Fig 7-8). The junction at this location will not interfere with proper placement of the denture teeth and idealizes the esthetic result by allowing the positioning of teeth of appropriate length and contour (see Fig 7-6b).
Mandibular Design Sequence
Occlusal restsThe rests provide support and control points between the abut- ment teeth and the prosthesis, so they are positioned first. Where an incisal rest is required (eg, on the right canine), it is designed to cover and restore the entire functional area. The mesial and distal rests on the left premolar may be joined together for increased strength because this is a non-occluding area (Fig 7-9). The rest on the right premolar is placed on the mesial side because this tooth is adjacent to a posterior edentulous extension area. The rest on the molar is extended into the middle of the tooth to direct occlusal forces along the long axis of the tooth.
Major connectors
Rigidity for the major connector is more difficult to provide in the mandible. Greater bulk is required to produce the rigidity necessary between the rests and the edentulous area. When the lingual bar is the design of choice, it should be positioned on unattached mucosa wherever possible and well away from the tooth-tissue junction. The bar can be extended into the lingual area as far as functional anatomical structures permit to gain the required bulk for rigidity. The lingual bar design should be 3 to 4 mm away from the gingival margin, and the bar must be 4 mm in height (Fig 7-10a; see also chapter 4). In some instances, the elevation of the floor of the mouth does not permit the design of a lingual bar with sufficient bulk. In this instance, a lingual plate design is advised (Fig 7-10b).
Where the casting crosses the tooth-tissue junction, it should be at right angles to the junction in order to minimize food impaction between the RPD framework and the tissues and should continue onto the unattached gingiva before the hori- zontal portion begins. The space between the crest of the mucosa and the lingual bar should be as open as possible (see Fig 7-11).
Minor connectors and proximal plates
The proximal plate portions of the minor connectors are po- sitioned to cover the side of the tooth and to extend onto the mucosa from the tooth-tissue junction for a minimum of 2 mm. The design should provide metal contact at all points where the prosthesis contacts the tooth or the tooth-tissue junction. Note that the casting is designed well away from the tooth-tissue junction on the lingual side of the abutment (Fig 7-11).
Denture base connectors
Denture base connectors are positioned on the crest of the ridge and toward the lingual side to minimize interference with denture tooth placement (Fig 7-12). On the side with the edentulous extension, an area of critical concern is the junction with the major connector (Fig 7-13). This area is subjected to flexure and fracture and must be designed with additional bulk. A fan-shaped union will provide maximum strength. Note the position of the external finish line in this area; it is placed well to the distal side of the tooth and curved into the edentulous portion, where there is extensive bone and tissue loss. Placement of the bulky acrylic resin–metal junction in this area restores the contour of lost tissues without infringing on the tongue space. Note that the connectors end abruptly in anticipation of the continuous extension of the acrylic resin of the denture base.
On the tooth-borne side of the arch, the major connector forms the inferior border of the prosthesis to provide maxi- mum strength with minimum bulk and a junction point for the denture base connector as well as the external finish line area (Fig 7-14).
Retainers
The retainers are designed and positioned to provide retention as required and to allow the proper placement of the denture teeth. In the mandibular arch, the contour of the tissues on the facial surface can present extensive undercuts that require a shorter vertical extension of the retainer before the horizontal portion is located (Figs 7-15 and 7-16). A high attachment of the buccal frenum may require the use of a wrought wire retainer or a suprabulge retainer.
A systematic approach with a step-by-step sequence develops a quick and practical design, which organizes treatment planning and provides for detailed communication with the laboratory.
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