4 Additional Features for Class-II Preparation
CL-II tooth restoration may displace four ways.
1. Proximal displacement of entire restoration
When force is applied obliquely to the tooth, it has two components.
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i. Horizontal component (H)
ii. Vertical component (V)
Vertical component will seat restoration into the tooth but horizontal component will tend to rotate the restoration proximally at gingival cave surface margin.
This is prevented by facial and lingual retention grooves and occlusal dovetail.
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2. Proximal displacement of proximal portion
If restoration is considered as being L-shaped, long arm will remain occlusally and short arm remains proximally.
When the long arm is loaded by vertical force (v), it will seat the restoration more into the pulpal floor will change the location as indicated by dotted line in.
The short arm of L will move proximally which is prevented by facial, lingual and/or gingival grooves.
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3. Lateral rotation around hemispherical floor
Lateral rotation is prevented by definite line angles, point angles and ledges.
It is prevented by directing occlusal force in such a way that it will seat the restoration
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These four displacements are repeated thousands of times per day and to counteract these movements all parts of the cavity preparation should be self-retaining.