1. RPD Design Philosophies
Ting Ling Chang,
Takahiro Ogawa
and
John Beumer III
Division of Advanced Prosthodontics, Biomaterials
and Hospital Dentistry
UCLA School of Dentistry
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2. 6. RPD Design Philosophies
RPI System (UCLA-Kratochvil)
Mesial rest
Rest Proximal
plate
Proximal plate
“I”-bar Tooth tissue
junction
“I” Bar

3. Types of Removable Partial Dentures
Tooth borne
 Abutment teeth border all edentulous
areas
 Functional forces are transmitted
through the abutments to bone
 It functions like a fixed partial denture

4. Types of Removable Partial Dentures
Tooth borne
 Abutment teeth border
all edentulous areas
 Functional forces are
transmitted through
the abutments to bone
 It functions like a fixed
partial denture
 Parallel guide planes-
proximal plates
provide bracing and
unite the arch

5. Types of Removable Partial Dentures
Tooth borne
 The removable partial denture should provide appropriate
bracing ie. stability (resistance to lateral forces), and
support for all the teeth remaining in the arch
 Ideally and when the occlusion permits rests should be
extended into the middle of molar teeth

6. Types of Partial Dentures
Tooth borne
 Rests can be placed in any position as long as they direct
occlusal forces along the long axis of the abutment tooth
 Exception: When distal abutment is suspect
 Stability, bracing and support should be maximized
 Virtually all of the support derived from the abutments

7. Types of Removable Partial Dentures
 Tooth-Mucosa borne (extension base)
 Exhibits one or more edentulous areas which are not
bordered by abutment teeth
 Functional forces are shared by both the abutment
teeth and denture bearing surfaces in the extension.

8. Types of Partial Dentures
Extension base partial dentures
 Rest position contour critical to the design
 Support shared between the edentulous denture
bearing surfaces and the abutments

9. RPD Biomechanics
Extension base RPD’s
Distal extension RPD

10. Extension Based RPD’s
Challenge
 Mucosal bearing surfaces are compressible. Therefore RPD’s
are displaced and move during function.
 Designs must anticipate the movements of the RPD during
function to prevent overload and loss of the abutments.

11. Extension Based RPD’s
Amount of movement is dependent upon:
 The surface area of the mucosal support area
 The thickness and compressibility of the supporting mucosa
 The adaptation of the denture base to the tissues of the extension base
 Refinement of the occlusal factors (distal extension RPD’s)
 Anterior guidance – Centric only contact posteriorly

12. Types of Partial Dentures
Extension base partial dentures
 Position of the rests determine axis of rotation (fulcrum
line)
 The axis of rotation traverses through the most posterior
portions of the rests adjacent to the edentulous area.

13. Extension Based RPD’s
 The rotation should be pure. There should be no contact with
inclines of teeth that could change the axis of rotation and
cause sliding action during occlusal function.
 The tips of the rests therefore must be contoured on the tooth
as a half sphere (arrows)
 The proximal plates-minor connectors should not bind during
rotation

14. Extension Based RPD’s
Positioning the axis of rotation to idealize support in
the edentulous area
In this example when the rest is placed on the distal side of the
abutment adjacent to the edentulous extension area the forces delivered
are not vertical but almost horizontal in region just adjacent to the
abutment
Occlusal force
Axis of
rotation
(rest)

15. Extension Based RPD’s
Positioning the axis of rotation to provide the best
support for the edentulous area
Occlusal force
Axis of
rotation
(rest)
As a result little or no vertical support is obtained from
edentulous area immediately distal to the abutment tooth

16. Extension Based RPD’s
Positioning the axis of rotation to provide the best
support for the edentulous area
 As you move the rest anteriorly the axis of rotation moves anteriorly, the arc
of rotation becomes greater and the forces are directed more favorably in a
more vertical direction in relation to the edentulous extension area
Occlusal Occlusal
force force

17. Extension Based RPD’s
Positioning the axis of rotation to provide the best support for the
edentulous area
 As you lower the rest and therefore the axis of rotation (for example by placing the
rest on the cingulum of the cuspid) the forces are directed more favorably in a more
vertical direction in relation to the edentulous extension area
Occlusal force
Axis of
rotation
Therefore it is advisable to lower the rotation point (the rest) whenever possible

18. Extension Based RPD’s
Direct effect of rest placement on abutment teeth
Open contact
 If the rest is placed on the
distal of the tooth adjacent
to the edentulous extension
area, when a posterior
force is applied the tooth is
tipped towards the
edentulous area, resulting
in an open contact with the
adjacent anterior tooth
 This tends to isolate the
tooth making it more
susceptible to bone loss
and periodontal breakdown

19. Extension Based RPD’s
Direct effect of rest placement on abutment teeth
 In this example when the rest is
placed on the surface away from
the edentulous extension area the
force tends to move the tooth
towards the adjacent tooth mesially
 Thus, the adjacent teeth anterior to
the abutment absorbs some of the
forces of occlusion.
 The remaining teeth function as a
unit and as such help brace and
stabilize each other during
mastication

20. Extension Based RPD’s
Design and positioning of the retainer
 The direction of retainer movement is determined by the location of the
axis of rotation. Note the direction of movement of the retainers on both
sides of the rotational axis (side A and side B)

21. Extension Based RPD’s
Design and positioning of the retainer
 Positioning the retainer posterior to the rest produces a
downward and forward movement of the retainer further into the
undercut disengaging the retainer and preventing the tooth from
being exposed to tipping forces

22. Extension Based RPD’s
Design and positioning of the retainer
 If the axis of rotation is placed between the retainer and the
edentulous extension area, the retainer moves upward, engaging
and torquing the tooth when an occlusal force is applied in the
edentulous extension area

23. Extension Based RPD’s
Design and positioning of the retainer
 The retainer must not be placed behind the greatest curvature of the
tooth in an extension base RPD because when an occlusal force is
applied in the edentulous extension area, the retainer will be displaced
anteriorly, torquing the tooth as shown (arrows).

24. Extension Based RPD’s
Design and positioning of the retainer
 The retainer contact is placed at the greatest point of mesial-distal
curvature of the tooth. The retainer will then disengage when an
occlusal force is applied to the edentulous extension

25. Extension Based RPD’s
Design and positioning of the retainer
 Rest position
(mesial or distal)
controls the axis of
rotation, which in
turn influences the
direction of the
movement of the
retainer

26. Extension Based RPD’s
Design and positioning of the retainer
 Retainers should not be placed in retentive (undercut) areas anterior to
the axis of rotation
 In this example, if a retainer is placed on the cuspid (point A) in an
undercut, when an occlusal forces is applied in the edentulous
extension area, a lifting or extraction force will be applied to this tooth.
The retainer should be placed on the height of contour

27. Extension Based RPD’s
Movement of Proximal Plates and Minor Connectors
 When an occlusal force is applied to the edentulous extension area,
in this example the proximal plates engaging the distal surfaces of
the abutment, will engage the tooth surface, producing a torquing
action. Moving the rest down and forward lessens this problem
 To avoid this problem, the RPD is “physiologically adjusted”

28. Extension Based RPD’s
Movement of Proximal Plates and Minor Connectors
 When an occlusal force is applied to
the edentulous extension area, in this
example the minor connectors will
engage distal surfaces of the tooth
anterior to the abutment preventing
pure rotation around the rests.
 To avoid this problem, the RPD is
“physiologically adjusted”

29. Extension Based RPD’s
Physiologic Adjustment
 A disclosing medium (gold rouge and chloroform) is applied to the
undersurface of casting. Pressure is applied in the extension areas. As
the casting rotates the rouge will be rubbed away from the casting.
 These areas are adjusted with a high speed hand piece until the casting
freely rotates around the axis of rotation as designed without lifting.

30. RPI System -Posterior Extension Based RPD’s
Summary
 Restore the integrity of the arch with guide planes – proximal plates
 Guide planes extend and cover the gingiva margin (tooth tissue junction)
 Rests should direct occlusal forces axially during function
 Rest on of the side of the tooth away from the edentulous extension area
 Retainers in undercuts must be designed to disengage under functional
conditions
 Retainer on the teeth anterior to the fulcrum line should be placed on the height
of contour, not in an undercut

31. RPI design principles
Rest position
 On mesial side of tooth adjacent to distal extension defects
 On distal side of tooth adjacent to anterior extension defects
Retainer position
 Retentive portion of the retainer should not engage an undercut anterior
to the axis of rotation
 From occlusal view, the retainer is placed at the point of greatest mesial-
distal curvature of the tooth

32. Other RPD designs
Conventional Akers design - Distal rest with a cast
circumferential retainer
 When an occlusal force in delivered in the edentulous extension area the
RPD rotates around the distal rest, the retainer engages and the tooth and
is tipped distally

33. Other RPD designs
The RPA system
 The rest is moved to the mesial. As a result, when an occlusal force is applied to
the edentulous extension area, the tip of the retainer disengages by rotating
further into the undercut
 The portion of the retainer above the height of contour is relieved. Otherwise this
portion of the retainer will deliver a torquing force to the abutment tooth

34. Other RPD designs - The RPA system
 For the RPA system to work properly the retainer must not contact the
tooth surfaces above the height of contour.
 When occlusal forces are applied in the extension area the tip of the
retainer will disengage.
 Shim stock has been used to ensure that the portion of the retainer
above has been properly relieved

35. Other RPD designs - RPA System
Indications For Use
 Bulbous gingival contours
 Significant recession
 Lack of access for an I-bar clasp due to a shallow vestibule
Significant periodontal recession Lack of vestibular depth for an I-bar

36. Other RPD designs - RPA System
Indications For Use
 High frenum attachments
 Short teeth with poor or no guide planes
and minimal undercuts
 Tilted teeth
High frenum attachment Short tooth

37. Other RPD designs
Wrought Wire Circumferential Clasp (18 gauge wire)
One can also use PGP
(Platinum, Gold, Palladium)
wire and solder it to frame-
work prior to processing of
the denture base.
This type of retainer is very flexible and minimizes the
torquing of the abutment when an occlusal force is
delivered in the edentulous extension area.

38. RPD Design – Wrought Wire Retainers
(Brudvik)
RPI and RPA system using wrought wire
 Rests away from the extensions base
 On the mesial if distal extension
 On the distal if anterior extension base
 Altered cast impressions for extension
bases
 Physiologic adjustment of RPD framework

39. RPD Design – Wrought Wire Retainers
(Brudvik)
 Guides for its application:
 If the distance from its origin when it leaves the acrylic
resin base is 7mm or less use PGP wrought wire clasp
gauge # 20
 If the distance from its origin when it leaves the acrylic
resin base is greater than10mm use PGP wrought wire
clasp gauge 18
 For I-bar clasp: Use PGP wrought wire clasp gauge #19

40. Designing RPD’s
Planning sequence for RPD patients
 Diagnostic assessment and preliminary
impressions
 Diagnostic casts mounted in centric relation
 Draw the ideal RPD design (on paper)
 Survey the study casts and determine the
most advantageous position (MAP) of the
designed RPD path of insertion and
withdrawal
 Revise and finalize the RPD design

41. Designing RPD’s
Planning sequence for RPD patients
 Diagnostic assessment and preliminary impressions
 Diagnostic casts mounted in centric relation

42. Designing RPD’s
Planning sequence for RPD patients
 Draw the ideal RPD design (on paper)

43. Survey the Cast
 Transfer the design to the study cast and determine the most
advantageous position (MAP) for path of insertion and
withdrawal and treatment position

44. Survey the Cast
 Determine the most advantageous position (MAP) of the
designed RPD path of insertion and withdrawal and
treatment position

45. Prepare guide planes and rests

46. Impressions and Casts
Submit to lab
 Detailed drawing of design on the study cast
 Detailed drawing of design on paper

47. Lab Prescriptions
Good Design Prescriptions will have -
 Sharp, smooth outlines
in identical color
 Design outlines
proportionately drawn
 Bead seals clearly
marked
 Retention areas
indicated
 Guide plane tissue
contacts marked
 Resin - metal finish
lines clearly marked

48. Lab Prescriptions
Good Design Prescriptions will have
 Sharp, smooth outlines in identical color
 Design outlines proportionately drawn
 Bead seals clearly marked
 Retention areas indicated
 Guide plane tissue contacts marked
 Resin - metal finish lines clearly marked

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