Support for the distal extension denture base Flashcards
Tooth supported RPDs:
- Rigid framework transfers occlusal forces to abutment teeth through the occlusal rests
- This is regardless of the edentulous span
- Residual ridge does not contribute to support of the prosthesis
- Ridge shape, degree of resorption and resiliency of the mucosa are not important considerations
- The denture base in the saddles should prevent food entrapment and offer support for the facial tissues (e.g. lips), but an under extended base in these cases is usually not detrimental
Tooth & mucosa supported RPDs:
- Are dependent on the residual ridge for part of the support
- No tooth support and no direct retention at the distal end
- Indirect retention becomes an important consideration
- Some movement of the prosthesis must be accepted
Conclusion:
- Maintaining a distal abutment tooth should be our priority!
Factors that influence the support of a distal extension base:
- Residual ridge contour and quality
- Extent of residual ridge coverage by the denture base
- Impression technique and accuracy
- Accuracy of fit of the denture base
- RPD framework design
- Total occlusal load applied
Contour and Quality of the Residual Ridge:
The ideal residual ridge to support a denture base would consist of:
- cortical bone that covers relatively dense cancellous bone, with a broad rounded crest with high vertical slopes, and is
- covered by firm, dense, fibrous connective tissue.
Such a residual ridge would optimally support vertical and horizontal stresses placed on it by denture bases.
How did Atewood (1971) describe Residual Ridge Resorption?
Atewood (1971) described Residual Ridge Resorption RRR as “MAJOR ORAL DISEASE ENTITY” characterized by loss of oral bone after the extraction of teeth.
- The size, shape and tolerance of residual ridges provides the basis of stability, retention, support of Partial and Complete denture.
- Residual ridge resorption (RRR) is a term that is used to describe the changes which affect the alveolar ridge following tooth extractions, which continue even after healing of the extraction socket.
Unfavourable residual ridge:
- Lack of attached mucosa – highly mobile mucosa due to advanced resorption
- Sharp crest of the ridge (‘knife-edge’ ridge) and sharp mylohyoid ridge
- Thin, atrophied, easily traumatised mucosa
Consequences of severe ridge resorption:
- Loss of sulcus width and depth,
- Displacement of the muscle attachment closer to the crest of the residual ridge,
- loss of VDO,
- Reduction of lower face height,
- Anterior rotation of the mandible,
- Increase in relative prognathia,
- Morphological changes such as sharp, spiny, uneven residual ridge,
- Resorption of the mandibular canal wall and exposure of the mandibular nerve,
- Location of the mental foramina close to the mandibular residual ridge
Unfavourable residual ridge:
- Easily displaceable tissue will not adequately support a denture base, and tissues that are interposed between a sharp, bony residual ridge and a denture base will not remain in a healthy state.
- Crest of the bony mandibular residual ridge is most often cancellous therefore cannot be a primary stress bearing area.
- Buccal shelf area seems to be better suited for the primary stress bearing area.
What can an Unfavourable residual ridge lead to?
Due to an easily displaceable tissue the consequent exerted pressure on the mandibular residual ridge usually result in irritation, leading to chronic inflammation. Therefore the crest of the mandibular residual ridge cannot be considered as a primary stress-bearing region
Extent of residual ridge coverage by the denture base:
Snowshoe principle:
- This principle is based on the distribution of forces to as large an area as possible. Like in a snow shoe which is designed to distribute forces on the entire base area of the shoe, a partial denture should cover maximum area possible within the physiologic limits so as to distribute the forces over a larger area.
- In a given constant occlusal force, a broader denture bearing area decreases the stress unit area under the denture base
- It decreases tissue displacement and reduces denture base movement
Comparison of two removable partial dentures for the same patient:
- The denture on the right has severely underextended bases. Its replacement, with properly extended bases, is on the left. Occlusal forces are more readily distributed to denture-bearing areas by the replacement denture.
- Maximum coverage with large wide denture bases withstands both vertical and horizontal stresses
Denture base extension:
Where does the support come from?
In the tooth supported section (regardless the length of the edentulous span), support comes entirely form abutment teeth. Despite that underextended underextended denture base leads to food entrapment and inadequate facial contours. The distal extention saddle depends its support from the residual ridge. The broader the residual ridge coverage the is the distribution of the load which results in less load per unit
Load bearing areas:
The Load bearing areas are the surfaces of oral structures that resist forces, strains, or pressures brought on them during function. They are divided into primary, secondary and non stress bearing areas:
- The primary stress-bearing areas generally have thicker mucosa and /or underlying bone that is less subject to resorption because it is cortical bone.
- The residual ridge is generally a secondary stress-bearing area as it is made up of cancellous bone
Load bearing areas: maxilla
- Primary supporting areas: Horizontal portion of hard palate.
- Secondary supporting areas: Anterior ridge (Rugae area),maxillary tuberosity, All ridge slop.
- Non stress bearing areas: Labial and lingual inclines
Load bearing areas: mandible
- Primary supporting areas: Buccal shelf, Posterior ridges, Retromolar pad area
- Secondary supporting areas: Anterior ridge, All ridge slops.
- Non stress bearing areas: Ridge slopes
Load bearing areas: Buccal shelf area (area within the dotted lines):
Buccal shelf is a prime support area, because it is parallel to the occlusal plane and the bone is very dense. These two factors make it relatively resistant to resorption. The greater the access to the buccal shelf the more support is available for the denture.
Load bearing areas: Retromolar Pad:
- Retromolar pad is one of the primary support areas. It is a constant relatively unchanging structure on the mandibular denture bearing surface.
- The bone beneath it does not resorb secondary to pressure associated with denture use.
- The pad contains glandular tissue, loose areolar connective tissue, the lower margin of the pterygomandibular raphe, fibers of the buccinator and superior constrictor, and fibers of the temporal tendon.
The residual ridge may be said to have two forms:
- The anatomic form: The anatomic form is the surface contour of the ridge when it is not supporting an occlusal load
- The functional form: The functional form of the residua ridge is the surface contour when it is supporting a functional load
