Support for Distal Extension Bases

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Support for the Distal Extension Denture Base

• Dr. Shujah A Khan
• MDS Resident Prosthodontics
• DIKHOS, DUHS

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Distal Extension Removable Partial Denture

• The distal extension removable partial denture
  does not have the advantage of total tooth
  support because one or more bases are
  extensions covering the residual ridge distal to
  the last abutment.
• It therefore is dependent on the residual ridge
  for a portion of its support

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• The distal extension removable partial
  denture must depend on the residual ridge
  for some support, stability, and retention.
• Indirect retention, to prevent the denture
  from lifting away from the residual ridge,
  should also be incorporated in the design.

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Factors Influencing Support of a Distal Extension
Base

• Support from the residual ridge becomes
  more important as the distance from the last
  abutment increases and will depend on the
  following several factors
• Contour and quality of the residual ridge
• Extent of residual ridge coverage by the denture
  base
• Type and accuracy of the impression registration
• Accuracy of the fit of the denture base
• Design of the removable partial denture framework
• Total occlusal load applied

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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.

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• 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 region.

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• Buccal shelf area seems to be better suited for
  the primary stress bearing area.
• Slopes of residual ridge ? primary stress bearing
  region for resistance of horizontal and
  off-vertical forces

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The dotted portion outlines the crest of the
residual ridge, which should be recorded in its
anatomic form in impression procedures.
Buccal shelf regions are outlined by a
herringbone pattern, and selected additional
pressures may be placed on these regions for
vertical support of the denture base
Lingual slopes of the residual ridge(cross-hatched
) these regions principally resist the
horizontal rotational tendencies of the
denture base and should be recorded by the
impression in undisplaced form.
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• Immediate crest of the bone of the maxillary
  residual ridge may consist primarily of
  cancellous bone
• The crestal area of the residual ridge will
  become the primary stress bearing area to
  vertically directed forces
• Some resistance maybe obtained by the immediate
  buccal and lingual slopes of the ridge

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• Palatal tissues between the medial palatal raphe
  and the lingual slope of the posterior edentulous
  ridge are readily displaced and cannot be
  considered as primary stress bearing areas

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The crest of the maxillary residual ridge
(herringbone pattern) is the primary supporting
region for the maxillary distal extension denture
base
The dotted portion outlines the incisive papilla
and the median palatal raphe. Relief must be
provided for these regions, especially if tissues
covering the palatal raphe are less displaceable
than those covering the crest of the residual
ridge
Buccal and lingual slopes may furnish limited
vertical support to the denture base. Their
primary role is to counteract the horizontal
rotational tendencies of the denture base.
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Extent of Residual Ridge Coverage by Denture Base

• The broader the residual ridge coverage the
  greater is the distribution of the load ?
  Snowshoe principle?
• Kaires maximum coverage of the denture-bearing
  areas with large, wide denture bases is of utmost
  importance in withstanding both vertical and
  horizontal stresses

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Types and Accuracy of Impression Registration

• Residual ridge
• Anatomic Form
• Surface contour when it not supporting an
  occlusal load
• Functional Form
• When the ridge is supporting a functional load

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Accuracy of the Fit of the Denture Base

• Support of the distal extension base is enhanced
  by intimacy of contact of the tissue surface of
  the base and the tissues that cover the residual
  ridge.
• The tissue surface must optimally represent a
  true negative of the basal seat regions.

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Design of the Removable Partial Denture Framework

• Some rotation movement of a distal extension base
  at the distal abutment is inevitable under
  functional loading.
• The greatest movement takes place at the most
  posterior extent of the denture base.
• Retromolar pad of the mandibular residual ridge
• Tuberosity region of the maxillary residual ridge

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• Steffel and Kratochvil have suggested that as
  the rotational axis is moved from a
  disto-occlusal rest to a more anterior
  location, more of the residual ridge
  receives vertically directed occlusal forces
  to support the denture base.

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• Occlusal rests may be moved anteriorly to better
  use the residual ridge for support without
  jeopardizing either vertical or horizontal
  support of the denture by occlusal rests
  and guiding planes

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Total Occlusal Load Applied

• Patients with distal extension removable
  partial dentures generally orient the food bolus
  over natural teeth rather than prosthetic teeth.
• More stable nature of the natural dentition, the
  proprioceptive feedback it provides for
  chewing, and the possible nocioceptive
  feedback from the supporting mucosa.
• This has an effect on the direction and
  magnitude of the occlusal load to the
  removable partial denture, and thus on the load
  transferred to the abutments.
• The number of artificial teeth, the width of
  their occlusal surfaces, and their occlusal
  efficiency influence the total occlusal load
  applied to the removable partial denture.

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Anatomic Form Impression

• One stage impression using an elastic impression
  material that does not represent the functional
  relationship between various supporting
  structures of the partially edentulous mouth
• Only represent hard and soft tissues at rest.

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• A partial denture fabricated from a one stage
  impression, places more of the masticatory load
  on the abutment teeth and that part of the bone
  that underlies the distal end of the extension
  base.
• Effects
• Traumatic load to the underlying bone
• Bone loss and loosening of abutment tooth

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Selective Tissue Placement Impression Method

• The goal is to maximize soft tissue support while
  using the teeth to their supportive advantage,
  the framework fitted to the teeth while soft
  tissue support is registered provides a means of
  coordinating both.
• A secondary impression for the distal extension
  mandibular removable partial denture is made in
  individual trays attached to the denture framework

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Altered Cast Method

• The altered cast method of impression
  making is most commonly used for the mandibular
  distal extension partially edentulous arch
  (Kennedy Class I and Class II arch forms).
• A common clinical finding in these
  situations is greater variation in tissue
  mobility and tissue distortion or
  displaceability, which requires some
  selective tissue placement to obtain the
  desired support from these tissues.

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