By Dr. Vishaal Bhat on Saturday, 12 January, 2008 with 0 comments

Philosophies of RPD Design & Stress Distribution

a) Stress Breakers

b) Precision Attachments

c) Broad Stress Distribution - the most commonly subscribed to today


  1. Lateral or torquing forces damage teeth & supporting structures

  2. An RPD can act as an:

    • inclined plane, if not properly supported by well defined and executed rests & tissue support
    • a fulcrum, the length of the fulcrum (lever arm) determines the amount of force transferred to the abutment tooth
  3. Every effort should be made through the design of the RPD to minimize fulcrums and to prevent rotation in the sagittal, vertical, and horizontal planes
  4. Support is more important than retention in the design of RPDs, especially extension RPDs
  5. Fulcrum line- is through the most distal abutment (rest), on either side of the arch, multiple fulcrum lines can exist
  6. Eliminate as many modification spaces as possible, (through fixed,etc.) without creating a unilateral RPD
  • Indirect retainers- resist movement of the RPD away from the tissue base
    • They are rests anterior to the fulcrum line
    • They are placed as far anterior to the fulcrum line as possible
    • Ideally placed, perpendicular to the fulcrum line

  1. Properly prepared guiding planes contribute greatly to the stability and retention of the RPD
  2. Either cover the marginal gingiva completely or stay well away from it
  3. Retention is controlled by
  • depth of the undercut
  • flexibility of the clasp arm
  • Flexibility of clasps is dependant upon:
    • type of metal
    • shape
    • thickness
    • length

Retentive undercuts-

  • NiCr or CrCO - 0.010"
  • Type IV gold - 0.015"
  • Wrought wire - 0.015-0.020"

  • The more flexible the clasp, the less force transmitted to the abutment tooth, but, The more flexible the clasp, the more vertical and lateral forces are transmitted to the residual ridge
  • A retentive clasp must be opposed by reciprocating elements or an element of the framework that encircle at least 180 degrees of the tooth

Reciprocating arm

* Above, but close to the height of contour

* Contacts tooth at the same time or before the retentive arm

* Usually at the junction of the occlusal and middle thirds

Wrought wire - either-

* tack solder to the retentive mesh

* Imbed in the wax prior to acrylic processing

* Imbed in the acrylic post processing

NEVER let a laboratory cast to a wrought wire

Beading (maxillary arch only)

* 6mm from gingival sulcus

* 0.5 - 1.0mm wide

* Prevents food debris from collecting under RPD

* Allows for the tapering of the metal framework, especially

posterior borders

Design Rules

* Rests and guide plates are always adjacent to edentulous areas

* Cross palatal midline at right angles

* Cross gingival sulcus at right angles

* Keep minor connectors 5mm apart

* Stagger internal & external finish lines

Category: Prosthodontics Notes



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