The relationship of Mechanical Forces with biological aspects acting on a Dental Removable Partial Denture within the oral cavity.

1. TASEEF HASAN FAROOK, BDS (final year)

2. What is Biomechanics in Prosthodontics?
Application of mechanical principles on biological tissues while
studying the biology from a functional viewpoint and then using these
principles to design a stable prosthesis.

3. Biomechanical considerations:
The RPD and their associated structures are subjected to various forms of
stress. Their ability to resist them depend on:
Direction, Duration, Magnitude and frequency of the Stress (Force) being
applied onto the denture and denture bearing areas
Capacity of these areas to resist these forces/stress
Changes due to resistance over time
The type of Resistance Generated: 1. Tooth based
2. Tooth and Tissue based

4. Types of Stress (Force) acting on an RPD
within the oral cavity:
These stresses can be classified into:
1. Vertical -
a) Displacing Force
b) Dislodging Force
2. Horizontal
3. Torsion
Resistance to stress can be divided into:
1. Tooth based resistance contributes mainly to resisting Horizontal Stress
(direct retainers)
2. Tooth-Tissue based resistance contributes to resisting Vertical Stress and
Torsion (major connectors and indirect retainers)

6. Factors contributing to the amount of Stress on the RPD:
• The length of edentulous span (example: Kennedy Classification III
usually exhibit no lever action)
• Quality of Ridge Support (example: Wider ridges disperse more stress
due to the Snow Shoe Principle)
• Quality of Oral Mucosa (example: Healthy mucosa are able to
withstand much greater force than weak flabby mucosa)
• Clasp Design (flexibility, length and material used)
• Occlusal Harmony

7. Principles of Mechanical forces (According to McCraken):

8. Mechanical Force Principles to be considered within the oral cavity:
1. Lever Principle (Further divided into Orders I, II and III)
2. Inclined Plane Principle
3. Wheel and Axle (Rotation)
Are of prime concern while designing an RPD
LEVER PRINCIPLE
INCLINED PLANE PRINCIPLE

9. WHEEL AND AXLE PRINCIPLE (along horizontal axis)
WHEEL AND AXLE (along Saggital Axis)

10. Types of lever Action:
There are 3 types of
mechanical lever action
based on:
1.the position of the fulcrum
2.The location of the load
along the fulcrum line.
3.The area from which the
effort to displace is exerted

11. First Order Lever Action (Class I):

12. Example of 1st Order Lever Action
In cantilever type of
Removable Partial Denture
where There is Distal
Extension. If there is bone
Resorption of the residual
alveolar ridge under the
distal extension, it will result
in an effort leading to first
order lever movement along
the fulcrum line.

13. Second Order Lever Action (Class II)

14. Third Order Lever Action (Class III)

15. Example of third order Lever action:
Usually seen in the tooth supported RPD. Upon consuming sticky food,
the food exerts pulling effort on the prosthetic teeth while the natural
teeth and retainers exert counteracting forces from both sides.

16. Biomechanics of Inclined Planes:
The rest
will ‘slip
off’ the
inclined
rest seat
However, flattening the rest seat
will aid in the retention of the
direct retainer on the tooth
surface and resist horizontal
forces

17. Wheel and Axle Principle:
(Rotation)
The Partial denture can
rotate along one of 3 planes:
A. The saggital plane
B. The frontal plane
C. The horizontal Plane
And along one of 3 axes:
1. Saggital axis
2. Vertical axis
3. Horizontal axis.

19. Rotation of Frontal Plane along
Saggital Axis
Rotation of Horizontal Plane along Vertical
Axis

20. Saggital Plane and Horizontal Axis Rotation

21. Frontal Plane Rotation Along the saggital Axis

22. Snow Shoe Principle
The basis of the principle is to
distribute stress/forces onto as
large an area as possible in
order to counteract the stresses
applied to a partial denture.

23. Snow Shoe Principle
In order to overcome the forces acting against the RPD, the prosthesis
must take full advantage of all the primary and secondary stress
bearing areas.
Ex. Buccal shelf area are the primary stress bearing areas in the
mandible because of their position on the occlusal plane. Not
extending the denture to the shelf area will result in displacing forces
and horizontal forces during mastication.

24. References:
• McCraken’s Removable Partial Prosthodontics , 11th and 12th editions
• Stewart’s Clinical Removable Prosthodontics, 3rd edition
• Basic Principles of removable partial denture, Abbasi Begum, online
journal, slideshare.net, 06-06-2016
• RPD biomechanics, Foundation for Oral Facial Rehabilitation, online
journal, ffofr.org
• Biomechanics of removable partial denture, Dr Ann Winchy, Dept. of
Oral Health and rehabilitation, University of Louisville, 17-05-2014.