This document discusses the biomechanics of removable partial dentures. It outlines how removable partial dentures are prone to movement under functional loads, which can exert stresses on supporting teeth and structures. The goal of biomechanics in design is to minimize these potentially destructive forces to within the physiological tolerance of tissues. Simple machines like levers are discussed, and how their forces should be avoided in design. Key considerations for minimizing damaging movements include obtaining maximum tissue support, minimizing cantilevers, and positioning retainers and guides to resist various movements. The use of implants can help restrict movements in removable partial dentures.

1. Mc Cracken’s
Removable Partial Prosthodontics
Edition - XII
Seminar Created By
Dr. Joel Koshy Joseph MDS

2. Chapter 4
Biomechanics of Removable Partial
Dentures

3. Chapter Outline
1. Biomechanics and Design Solutions
2. Biomechanical Considerations
3. Possible Movements of Partial Dentures
4. Impact of Implants on Movements of Partial
Dentures.

4. Removable partial dentures are not rigidly
attached to teeth, so it has a potential for lots of
movement under functional loads.
As a result, huge stresses are exerted on to the
tooth and other supporting structure.
Hence, it is important that the stress does not
exceed physiologic tolerance limit.

5. FORCE (During Mastication/
Functional Movements)
Removable Partial Denture
Abutment tooth and other supporting
structures
Supporting Structures like Periodontal
Ligament, alveolar bone etc. experiences
Strain.
The Study of this
phenomena is called
BIOMECHANICS
For the Goal of
providing and
maintaining stable
prosthesis

6. Biomechanics and Design Solutions
Every design in RPD has been developed
through a series of Open Ended Problem and
ended with Ill Structured Solution.
Open Ended Problem: Problem which has more
than one solution
Ill Structured Solution: Its not the result of
standard mathematical formulas.

7. Design process includes (with Example):
1. Need:
2. Definition of Problem:
3. Objective:
4. Background Information:
5. Choice of solution for
application:
Tooth replacement in Distal
Extension cases
Extra mucosal movement of 1-
3mm in distal extension cases
results in torqueing stress on
primary abutments.
Limited Functional movement of
Distal extension RPDs within tooth
tissue tolerance
Use of stress Breaker/ RPI system
clasp/ Altered Cast fabrication.
Observations made from clinical
experience, clinical research,
concepts from school and
textbooks.

8. Biomechanical Considerations
As Maxfield states:
“Common observation clearly indicates that the
ability of living things to tolerate force is largely
dependent upon the magnitude or intensity of
forces”
Support structures of RPD:
ABUTMENT TEETH
RESIDUAL RIDGES

9. Depending factors include:
1. Type of Forces
2. Duration and intensity of forces
3. Capacity of the teeth and mucosa to resist
these forces (Health of the supporting
structures)
4. Material used and application influencing
this teeth-tissue resistances
5. Whether resistance changes over time.

10. Main objective is to:
‘Reduce the potentially destructive forces to
minimum, so that the physiological tolerances
of the supporting structures are not exceeded
and pathologic change does not occur.’
How to achieve this:
‘…..by appropriate design of removable partial
denture, which includes the selection and
location of components in conjunction with a
harmonious occlusion….’

11. However, understanding the working of simple
machines is crucial for designing of Removable
Partial Dentures to accomplish the objective.
6 SIMPLE MACHINES

12. These simple machines are used all over
Dentistry, Eg:
During Extraction of tooth Simple Wedge System

13. Elevators being used as a lever
to lift out tooth from socket
Lever system

14. Anterior bite plane appliances Inclined plane system

15. Type of tooth extraction
instrument
Pulley system

16. Dental Implants Screw system

17. Wheel and axle systemDuring Extraction procedures

18. Among simple machines, forces generated
by Lever, the wedge, wheel and axle and
the inclined plane systems should be
avoided in the design of removable partial
dentures.
Possible movements of Partial
Dentures

19. Lever mechanism
There are three Class in lever mechanism:

20. In RPD:
1st Class lever mechanism Eg 1. :
Resistance
Fulcrum
Masticatory
load
Abutment tooth suffers heavy torsional stress in distal extension cases. Thus to
avoid this while designing RPD, RPI system of clasp should be used in cases of
distal extension cases.

21. Fulcrum
Fulcrum
Resistance
arm
Load arm
Masticatory
load
Resistance
Masticatory
load
Longer the load arm, more destructive force would be applied on abutment tooth.
Thus, while designing RPD, care should be taken to minimize the extend of
cantilever.
Eg 2.

22. Eg 3.
Masticatory
load
Resistance
Fulcrum
With heavy masticatory force, harmful stress would be imparted on canine region
upward and posteriorly.
Thus, while designing RPD, care should be taken to minimize the extend of
cantilever and obtain maximum tissue support.

23. The most beneficial design in cases of distal
edentulous cases is to use Dental implants in
conjunction with removable partial dentures.
Tooth can better tolerate vertical forces than non
vertical forces. This is because more periodontal
fibres are activated to resist vertical forces than non
vertical ones.

24. • Clasp placed closer to the occlusal/ incisal surface have a
greater likelihood of imparting tipping forces to the
abutments.
• An abutment tooth will better tolerate non vertical forces if
these forces are applied as near as possible to the horizontal
axis of rotation of the abutment.
Horizontal axis of rotation

26. Key to minimize damage from First
Order lever movements
• minimize the extend of cantilever.
• obtain maximum tissue support.
• non vertical forces to be applied as near as
possible to the horizontal axis of rotation of
the abutment.
• use Dental implants in conjunction with
removable partial dentures whenever
possible

27. 2nd Class lever mechanism
Resistance
Fulcrum
Force by sticky
food
Thus to obtain adequate retention, the RPD should be secured by proper
positioning of direct retainer and guide planes.

28. Key to minimize damage from Second
Order lever movements
• By proper positioning of direct retainer and
guide planes.

29. 3rd Class lever mechanism
Resistance
Fulcrum
Load
(Gravity)
Thus to prevent the anterior part of the denture from being displaced tissue wards,
appropriate placement of indirect retainers is required.

30. Key to minimize damage from third
Order lever movements
• By appropriate placement of indirect
retainers.
• minimize the extend of cantilever

31. Rotation About axis through the most posterior abutment: Fulcrum line
Wheel and axil movements

32. • This vertical tissue wards or away from tissue movement
occurs because of occlusal load while mastication or sticky
pull from food respectively.
• Movements which are towards the tissue are best resisted
by components like Rest and indirect retainers.
• Movements which are away from the tissue is best resisted
by clasp function of direct retainer.

33. • This rotational movement around the long axis is seen
mostly in distal extension cases.
• This movement is resisted primarily by major connectors
(rigid) and minor connectors and their ability to resist
torque.

34. • This rotational movement around the horizontal axis is seen
when horizontal support from ridge and other structures is
compromised.
• This movement is resisted primarily by stabilizing
components like reciprocal clasp arm and minor connectors
that are in contact with the vertical tooth surfaces.
Stabilizing components of one side will stabilize the
horizontal forces of the other opposite side.

35. Impact of implants on movements of
RPD
• Use of implant in RPD is for restricting the
movements of it. Rather than to provide a role
as an abutment.
• They provide
Stability, support
and retention for
the removable
prosthesis.

36. References
1. Mc Cracken Removable Partial
Prosthodontics Edition 12.
2. Picture source: Google search engine.