02-10-2012, 05:25 PM
Tooth Surfaces
Tooth Surfaces.ppt (Size: 1.34 MB / Downloads: 27)
Forces exerted during occlusion:
The tooth surfaces are curved or at an incline. These forces are not only vertical but other forces are also exerted
The cuspal planes are taken as inclined planes
The forces are not in equilibrium when acting on inclined planes
The equilibrium can be maintained if more than one force is exerted on tooth or forces are resolved in both directions
The effect of friction:
Friction is the resistance to a sliding motion of one body to another
Normally two or more inclined planes of one tooth are in contact with buccal or lingual cusps of opposing tooth
If contact is not normal it may account for displacements of restoration or fracture of teeth. This effect is called wedging effect
The horizontal components of normal force are responsible for this wedging effect
Forces acting on tooth:
1.In centric occlusion
2.During chewing
Centric occlusion: there exists contact points on the surface of tooth
Thus equilibrium can be maintained if more than one force is exerted or forces are resolved in both directions
restoration:
Forces acting on class 2 restoration:
Mesioocclusal preparation:
As load is applied over teeth, stresses are distributed both parallel and perpendicular to long axis of tooth
The MO preparation is an example of cantilever beam as the resultant force is supported from one end
In MO preparation bending moment is at axio pulpal line angle
Thus gingival retention is required
Moment of force= Force × perpendicular distance
Total retentive force=F×L/l
L=depth of axial wall
If we take depth of gingival wall(d) into account, then R and d will be in same direction
Moment of force will be zero