11. On of the most common violation of
this principle is seen in discriminate use
of full porcelain coverage in a situation
when partial veneer coverage can be
used
12. Also you should be careful about;
1- Adjacent tooth
- by using matrix band of the adjacent tooth
- cutting in the enamel of prepared tooth with
fine tapered stone
2- Soft tissues
- by using mirror or the flange of saliva ejector
15. Avoidance of over contour
• Gingival inflammation is commonly associated with crowns and
FPD abutments having excessive axial contours, probably because it
is more difficult for the patient to maintain plaque control around
the gingival margin. A tooth preparation must provide sufficient
space for the development of good axial contours.
• Under most circumstances a crown should duplicate the contours
and profile of the original tooth (unless the restoration is needed to
correct a malformed or malpositioned tooth). If an error is made, a
slightly under contoured flat restoration is better because it is
easier to keep free of plaque; however, increasing proximal contour
on anterior crowns to maintain the interproximal papilla may be
beneficial.
• Sufficient tooth structure must be removed to allow the
development of correctly formed axial contours, particularly in the
interproximal and furcation areas of posterior teeth, where
periodontal disease often begins
16.
17. Margin placement
Whenever possible, the margin of preparation
should be supragingivally, where the supragingival
margin is prepared on sound enamel, while the
subgingival margin is often prepared on cementum
advantages:
1- Easier to prepare accurately without trauma to
soft tissue.
2- Prepared on hard enamel.
3- Easy to finished.
4- More easily to kept clean.
5- Impression are more easily made.
6- Restoration can be easily to evaluated at recall
appointment.
18. Indications of subgingival finish line
1- Dental caries, cervical erosion, restoration extend
subgingivally.
2- Proximal contact area extend near gingival crest.
3- Short tooth and additional retention is required.
4- The margin of metal-ceramic restoration is to be
hidden subgingivally.
5- Root sensitivity cannot be controlled by
conservative procedures.
19.
20.
21. Marginal Geometry
Ideal requirements of finish lines:
1.
They must fit closely to the finish line of the
preparation.
2.
They must have sufficient strength to
withstand the force of mastication.
3. Whenever possible, finish line should be
located in areas where the dentist can finish
and inspect them, and the patient can clean
them
29. Retention & Resistance
What is the Retention?
The ability of the preparation to prevent
removal of the restoration along the path of
insertion.
What is the Resistance?
The ability of the preparation to prevent
dislodgment of restoration by force directed
in an along apical oblique and horizontal.
31. I. Retention Form
Factors affecting retention:
1- Taper.
2- Total surface area
3- Roughness of tooth structure.
4- Film thickness
5- Type of luting agent
32. Taper
The more nearly parallel the opposing walls of preparation
the greatest will be the retention.
The optimum degree of taper is 2.5 to 6.5
degrees
The recommended degrees of taper are: 3 : 5,
6, 10 : 14
the degree of taper
stress in the
cement interface between the preparation
and restoration
the degree of taper
retention
41. Film Thickness
there is a doubt (no certain data)
Type of luting agent
1- Resin cement
2- Glass ionomer cement
3- Polycarboxylate cement
4- Zinc phosphate cement
5- Zinc oxide eugenol cement
42. II. Resistance form
Mastication and parafunctional activity may
subject a prosthesis to substantial horizontal ,
oblique or apical forces. These forces are normally
much greater than the ones overcome by retention
Factors affecting resistance to displacement:
1. Magnitude and direction of the dislodging forces
2. Geometry of the tooth preparation
3. Physical properties of the luting agent
43. Magnitude and direction of the dislodging forces
In a normal occlusion, biting force is distributed
over all the teeth; most of it is axially directed. If a
fixed prosthesis is carefully made with a properly
designed occlusion, the load should be well
distributed and favorably directed . However, if a
patient has a biting habit such as pipe smoking or
bruxing, it may be difficult to prevent fairly large
oblique forces from being applied to a restoration
44. Geometry of the tooth preparation
What is the definition of Tipping Path?
It is the path along which the
restoration, could be displaced
under the displacing occlusal
force
49. Path of insertion
Definition
That direction through which the restoration
could be precisely seated on the corresponding
tooth or teeth
Parallel to the long axis of the tooth
Parallel to the incisal 2/3 of the tooth