The document discusses biomechanics principles for removable partial dentures (RPDs). It describes two main types of RPDs: tooth-borne and extension base. Tooth-borne RPDs transmit forces to abutment teeth, while extension base RPDs share forces between teeth and denture bearing surfaces, causing movement. Proper design aims to counter dislodging and seating forces without overloading tissues. Key factors include rests, connectors, extensions, and occlusal refinement to direct forces optimally.

1. RPD Biomechanics
John Beumer III
and
Ting Ling Chang DDS
Section of Removable Prosthodontics
UCLA School of Dentistry
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2. RPD Biomechanics
Two types of RPD’s
Tooth borne
 Occlusal
forces are transmitted to the teeth
used as RPD abutments
Extension base
 Occlusalforces are shared between the
abutment teeth and the edentulous denture
bearing surfaces. As a result these
prosthesis move or rotate during function.

3. RPD Classification Systems
UCLA - Kratochvil system
 Based on biomechanics
 Three types
 Tooth borne
 Extension base
 Unilateral
 Bilateral
 Periodontal Stabilization
Kennedy – Applegate system
 Based on edentulous spaces

4. Kennedy Classification
Based on locations and number of edentulous areas
Class I – Bilateral edentulous areas located posterior to the
remaining teeth
Class II – A unilateral edentulous area located posterior to the
natural teeth
Class III – A unilateral edentulous area with natural teeth both
anterior and posterior to the area
Class IV – A single but bilateral (crossing the midline) edentulous
area located anterior to the remaining natural teeth
Modification spaces
1. Edentulous areas other than those determining the main classes are
modification spaces and are designated by the number of spaces
present
2. Class IV has no modifications. If more than one space is present in the
dental arch it would fall into one of the other classifications

5. Forces acting on Removable
Partial Dentures
Vertical (dislodging)
 Forces of gravity in
maxillary RPD’s
 Sticky foods
Vertical (seating)
 Forces of occlusion
Horizontal (lateral)
 During the chewing
cycle
The object of RPD design is to counter these forces without
stressing the abutment teeth and the edentulous soft tissue
denture bearing surfaces beyond their physiologic tolerance.

6. 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
Courtesy Dr. G.E. King Courtesy Dr. G.E. King

7. Types of Removable Partial Dentures
Tooth borne – Essentials
of the design
 Rests on molar teeth
should be placed in the
center of the tooth
 Guide planes as parallel
as possible
 Enhance stability
(bracing) unilaterally and
bilaterally (cross arch
stabilization) to resist
lateral forces
 Enhance retention

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

9. Tooth Borne RPD - Molar rests
 When the rest is placed on a marginal ridge of a molar, the bone
associated with th root on that side becomes overloaded
 When the rest extends into the center of the tooth the forces are more
equitably distributed in the alveolar bone
Photoelastic
model

10. Types of Removable Partial Dentures
Tooth borne - Rests
 When the rests extend to the middle of the tooth the forces
are directed down the long axis of the abutment (arrows)

11. Types of Removable Partial Dentures
Tooth borne- Rests
 When the rest extends to the
middle of the tooth the forces
are directed down the long
axis of the tooth
 We confine the rest on
premolars to either the mesial
or distal side of the tooth or
both depending upon the
prognosis of the posterior
molar. Extending the rest
across the transverse ridge
may weaken the tooth
Courtesy Dr. G.E. King

12. 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.
Unilateral Bilateral

13. RPD Biomechanics
Extension base RPD’s
Posterior extension RPD Anterior extension RPD

14. RPD Biomechanics
Extension base RPD’s
Courtesy Dr. A. Davodi
 This patient presents with both anterior and a posterior
edentulous extension areas

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

16. 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

17. Extension Based RPD’s
With improper designs movement of the denture
base during mastication or parafunction is
destructive to the underlying bone and soft tissue

18. Extension Based RPD’s
If the denture base is underextended the alveolar ridge
will rapidly resorb
 During mastication or parafunction (clenching and bruxing)
the periosteum is compressed, the underlying bone
subjected to stress and strain, and a resorptive remodeling
response is provoked.
An extension base RPD of the Mandible must
cover the buccal shelf and the retromolar pad

19. Extension Based RPD’s
Amount of movement is dependent upon:
 The surface area of the mucosal support area
 The compressibility of the bearing surface tissues
Therefore, we must maximize the coverage of the edentulous
extension area with fully extended impressions. Two methods:
 Altered cast impressions
 Fully extended impressions with a custom tray

20. Extension Based RPD’s - Retromolar Pad
One constant, relatively unchanging structure on the mandibular denture bearing
surface is the retromolar pad (dotted line).
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 bone beneath does not
resorb secondary to the pressure associated with denture use. It is one of the
two primary support areas of the mandible.

21. Extension Based RPD’s - Buccal Shelf
Boundaries of the buccal shelf: Masseter
The external oblique line and the groove
crest of the alveolar ridge (area area
within the dotted lines).
Buccinator
limits the
extension in
this area
The buccal shelf is a prime support area because it is
parallel to the occlusal plane . It is composed of
dense cortical bone and is relatively resistant to

22. Extension Based RPD’s
Amount of movement is dependent upon:
The surface area of the mucosal support area
 Therefore, we must maximize the coverage of the
edentulous extension area with fully extended impressions
 Altered cast impressions
 Fully extended impressions with a custom tray

23. 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

24. 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
Maximize the surface area and cover key
anatomic structures with altered cast impressions

25. 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)
 Anteriorguidance – Centric only contact posteriorly
 This practice will reduce the lateral forces delivered

26. Extension Based RPD’s
Axis of rotation (fulcrum line)
 Axis of rotation
(fulcrum line) is
determined by the
position of the rests
adjacent to the
edentulous extension
area.
 The axis runs
through the deepest
portion of posterior
rests

27. Extension Based RPD’s
 In a posterior tooth the
rotation occurs through
the depest portion of the
rest.
 Therefore this portion of
rest should be contoured
as a half sphere
 We develop this portion
of the rest with a #6 or a
#8 round burr
Proper rest contour

28. Extension Based RPD’s
When and occlusal force is applied in the denture
base extension region:
 The prosthesis rotates towards the mucosa on the extension
base side of the axis of rotation (fulcrum line)
 All parts of the RPD framework anterior to the axis of rotation
(fulcrum line) rotate away from the dentition

29. RPD Biomechanics
Extension base RPD’s
 Fulcrum line (axis of rotation)
When and occlusal
force is applied:
The prosthesis rotates
towards the mucosa on the
extension base side of the
fulcrum line
All parts of the RPD
framework on the dentate
side rotate away from the
dentition

30. RPD Biomechanics
Extension base RPD’s
Fulcrum line (axis of rotation) (dotted line)
When and occlusal
force is applied:
The prosthesis rotates
towards the mucosa on the
extension base side of the
fulcrum line
All parts of the RPD
framework on the dentate
side rotate away from the
dentition

31. RPD Biomechanics
Extension base RPD’s
 By changing the position of the rests you idealize the
forces delivered to the extension areas and minimize
the movement of the extension base
 Most favorable support in the extension areas is
provided when the forces are delivered at right angles
to the edentulous bearing surfaces
Edentulous
extension area

32. RPD Biomechanics
Extension base RPD’s
 By moving the rest towards the mesial the forces are
applied more vertically in the edentulous extension
areas
 As a result there is less rotation around the axis of
rotation (fulcrum line) Edentulous
extension area

33. RPD Biomechanics
Extension base RPD’s
 Fulcrum line (axis of rotation)
Note the rest on the cingulum of the left
cuspid. The rest was positioned here
because the premolar was
periodontally compromised. Note that
the proximal plate on the mesial of the
premolar provides reciprocation for the
retainer as well as stability
Advantages:
 Axis point is lower on the tooth
 The rest is further away from the
edentulous bearing surface resulting in the
occlusal forces delivered in a more vertical
direction to the extension base

34. RPD Biomechanics
Extension base RPD’s
 Fulcrum line (axis
of rotation)
 The RPD rotates
around that
portion of the rest
Extension
that is closest to base
the extension
base area

35. RPD Biomechanics
Extension base RPD’s
 Fulcrum line
(axis of rotation)
Courtesy Dr. A. Davodi

36. RPD Biomechanics
Extension base RPD’s
 Fulcrum line (axis of rotation) for an
anterior base extension RPD

37. RPD Biomechanics
Extension base RPD’s
 This patient presents with
both anterior and a
posterior edentulous
extension areas
 Therefore, depending on
the anterior arch form,
there will be two axis of
rotation as shown
depending whether the
patient is incising with the
anterior teeth or chewing
with the posterior teeth
Courtesy Dr. A. Davodi

38. RPD Biomechanics
Extension base RPD’s
 Fulcrum line (axis of
rotation)
 The tip of the rest on
molar is contoured in
a half circle
 This permits a proper
rotation around the
axis of rotation

39. RPD Biomechanics
Extension base RPD’s
Point of greatest
Position of the retainer mesial distal curvature
 From occlusal view, the
retainer is placed at the
point of greatest mesial-
distal curvature of the Extension
tooth base
 If the retainer is placed
behind the greatest
curvature the retainer
will move forward during
function and torque the
tooth and loosen the
retention

40. Extension Based RPD’s
Indirect retention
When a dislodging force is applied the prosthesis will rotate
around the posterior retainers. The cingulum rest stabilizes
the framework during swallowing and tongue thrusting

41. Extension Based RPD’s
 Retainers placed anterior to the axis of rotation in an
extension situation should not be in undercuts since that
portion of the RPD framework will lift the abutment when a
vertical load is applied posteriorly in the extension area.
 Such retainers should be placed at the height of contour

42. Extension Based RPD’s
This retainer serves two purposes
 Retention – Frictional
 In the event the distal molar is lost
 This retainer can be slightly can be slightly recontoured and
bent to engage the undercut of the canine

43. Forces acting on Removable Partial
Dentures
Vertical (dislodging)
 Forces of gravity in maxillary RPD’s
Horizontal (lateral)
 During bruxing
Vertical (seating)
 Forces of occlusion (clenching)
The object of prosthesis design is to counter these forces without
stressing the abutment teeth and the supporting soft tissue
denture bearing surfaces beyond their physiologic tolerance.

44. Requirements of a Removable Partial Dentures
Design based on:
Support
 Rests
 Major connectors
 Denture bases
Stability (bracing)
 Minor connectors
 Proximal plates
 Rigid portions of retainers
 Lingual plates
 Denture bases
 Rests
Retention
 Direct retainers

45. Requirements of a Removable Partial Dentures
Provided by:
Support
 Cingulum rests
 Extension base
 Major connector
Retention
 Direct
 “I” bars on cuspids
 Proximal plates (when parallel)
 Indirect
 Cingulum rests on incisors
Stability
 Minor connectors-proximal plates
 Lingual plate on anterior teeth
 Cingulum rests
Reciprocation
 Minor connectors
 Cingulum rests
 Proximal plates

46. Requirements of a Removable Partial Dentures
 Retention Provided by:
 Direct
 “I” bars
 Proximal plates
 Indirect
 Cingulum rests
 Stability
 Minor connectors and
proximal plates
 Lingual plate on molar
 Rests
 Support
 Rests
 Extension base
 Major connector
 Reciprocation
 Minor connectors
 Lingual plate
 Proximal plates

47. Requirements of a Removable Partial Dentures
Retention Provided by:
 Direct
 “I” bars premolar
 Circumferential clasp on molar
 Proximal plates (when parallel)
 Indirect
 Cingulum rests on cuspids
 Mesial rest on premolar
Stability
 Minor connectors-proximal plates
 Lingual plate on posterior teeth
 Cingulum rests
Support
 Occlusal rests
 Extension base
 Major connector
Reciprocation
 Minor connectors
 Cingulum rests
 Proximal plates
 Lingual plate

48. Requirements of a Removable Partial
Retention
Dentures
 Direct Provided by:
 “I” bars
 Proximal plates
 Indirect
 Rests – incisal and occlusal
 Lingual plate
Stability (Bracing)
 Minor connectors and
proximal plates
 Lingual plate
 Incisal rest
 Buccal “I” bar on molar
Support
 Rests
 Extension base
 Reciprocation
 Minor connectors
 Lingual plate
 Proximal plates
 Buccal “I” bar on molar

49. Principles of RPD design
 Extension base RPD designs must anticipate and
accommodate the movements of the prosthesis
during function, without exerting pathologic stresses
on the abutment teeth
 Major connectors must be rigid.
 Occlusal rest must direct occlusal forces along the
long axis of the teeth.
 Guide planes are created to enhance stability and
bracing.
 Retention must be within the limits of physiologic
tolerance of the periodontal ligament.
 Maximum support is gained from the adjacent soft
tissue denture bearing surfaces.
 Designs must consider the needs of cleansibility.

50. Preservation of teeth
1.Extension base RPD designs must anticipate and
accommodate the movements of the prosthesis during function,
without exerting pathologic stresses on the abutment teeth.
 Rests on the mesial of teeth adjacent posterior extension area
 Rests on the distal of teeth adjacent to anterior extension area
Courtesy Dr. A. Davodi
Clinical significance: If the RPD designs do not
conform to this idea there is risk that abutment
teeth may be overloaded leading to their premature

51. Principles of RPD design
2. Major connectors must be rigid
Courtesy Dr. A. Davodi

52. Principles of RPD design
3. Occlusal rests must be positive and direct
occlusal forces along the long axis of the teeth

53. Principles of RPD design
4. Parallel guide planes for stability and bracing

54. Principles of RPD design
5. Retention must be within the physiologic tolerance
of the periodontal ligament
 RPI system
 RPA system
 Wrought wire

55. Principles of RPD design
6. Maximum support is gained from the adjacent soft
tissue denture bearing surfaces.
 Altered cast impressions for extension base partial dentures

56. Principles of RPD design
7. Designs must consider the
needs of cleansibility and
food flow patterns.

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