Lec 2: Biomechanics

Question 1

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.

Answer 1

Biomechanics

Question 2

Types of stress acting on an rpd

Answer 2

1. Vertical
   a) Displacing Force
   b) Dislodging Force
2. Horizontal
3. Torsion

Question 3

The RPD and their associated structures are subjected to various forms of stress. Their ability to resist them depend on:

Answer 3

1) Direction, duration, magnitude and frequency of the stress (force) being applied onto the denture and denture bearing areas.

(2) Capacity of these areas to resist these forces / stress.

(3) Changes due to resistance over time.

(4) The type of resistance generated:
a) Tooth based
b) Tooth and Tissue based

Question 4

Contributes mainly to resisting Horizontal Stress (direct retainers).

Answer 4

Tooth based resistance

Question 5

Contributes to resisting Vertical Stress and Torsion (major connectors and indirect retainers).

Answer 5

Tooth-tissue based resistance

Question 6

Factors contributing to the amount of stress on the rpd

Answer 6

1. The Length of Edentulous Span (example: Kennedy Classification III usually exhibit no lever action).
2. Quality of Ridge Support (example: Wider ridges disperse more stress due to the Snow Shoe Principle).
3. Quality of Oral Mucosa (example: Healthy mucosa are able to withstand much greater force than weak flabby mucosa).
4. Clasp Design (flexibility, length and material used).
5. Occlusal Harmony

Question 7

6 principles of mechanical forces

Answer 7

lever
wedge
inclined plane
screw
pulley
wheel and axle

Question 8

3 mechanical force principles considered within oral cavity

Answer 8

1. lever
2. inclined plane
3. wheel and axle (rotation)

Question 9

• 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

Answer 9

First order lever action

Question 10

• 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.

Answer 10

Third order lever action

Question 11

• Seen wherein the rest will “slip off’ the rest seat.
• However, flattening the rest seat will aid in the retention of the direct retainer on the tooth surface and resist horizontal forces.

Answer 11

Inclined plane principle

Question 12

The partial denture can rotate along 3 planes? and alone 3 axes?

Answer 12

planes:
sagittal
frontal
horizontal

axes:
sagittal
vertical
horizontal

Question 13

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.

Answer 13

Snow show principle

Question 14

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.

Answer 14

Snow shoe principle