5. RETENTION
“the ability of the preparation to prevent displacement
of the restoration in a direction opposite to the path of
insertion”
Classification of Retention:
• PRIMARY RETENTION
• SLEEVE RETENTION
• WEDGE RETENTION
• SECONDARY RETENTION
• BOX
• COVES
• CLEATS
• PINS
• GROOVES
6. Resistance
“the ability of the prosthesis to resist displacement
by forces directed in apical or oblique direction ”
• Prevents movement of restoration under occlusal forces
7. Taper
• Degree taper is inversely proportional to
the retention form
• Zero degree is most retentive
• Sum of degree of taper is degree of
convergence
• Diamond point is designed with a 3-degree
taper. The operator aligns the diamond
parallel to the long axis of the tooth
producing a 6-degree of convergence.
8. Length
• Greater the height of the crown, better the
retention of the restoration
• Height of the crown greater than the tipping
arc of displacement improves retention
• Increase in crown height, increases the area
of cementation and thereby increases the
retention
• Shorter crown height: improve the
retention by incorporating features like
proximal grooves retention pin holes
9. Path of Insertion
• “it is an imaginary line along which the restoration will be placed onto
or removed from the preparation”
• Evaluated by viewing the preparation on a mouth mirror using one
eye (30cm)/ 12 inches away from the restoration
• Mesiodistal inclination/degree of convergence of the preparation
should be parallel to the inclinations of the adjacent tooth
10. Freedom of Displacement
• Taper of the preparation determines the path
of insertion
• Greater the taper, multiple paths of insertion
introduced - freedom of displacement
• Shorter the crown height, greater will be the
freedom of displacement
• Partial veneer crowns will have a greater
freedom of displacement
11. Structural Durability
“ability of the restoration to withstand
destruction due to external forces is
known as structural durability”
• Durability of the restoration is directly
proportional to thickness of the
restoration
• Adequate reduction during
preparation leads to adequate bulk of
the restoration and increase the
durability
• Adequate reduction depends on the
type of material being used
12. Structural Durability
Occlusal reduction
• Metal restorations : functional cusp: 1.5 mm; Non-functional cusp: 1
mm
• Metal-ceramic restorations: FC: 1.5 - 2mm; NFC: 1 - 1.5 mm
• All-ceramic restorations: 2 mm all round reduction
13. Structural Durability
Functional cusp bevel
• Increases the thickness of the occluso-axial junction of the restoration
• Improves the bulk of the restoration and improve resistance to
occlusal forces
14. Structural Durability
Axial reduction
• Adequate reduction increases strength and durability
• Inadequate reduction leads to over-contoured proximal surfaces
• Required tapered planned during axial reduction
• Over reduction leads to loss of retention
• Axial reduction done to parallel the abutments for a single path of insertion
15. Marginal Integrity
• Margins and seating of restoration affects the marginal integrity
• Poor marginal adaptation leads to marginal leakage & secondary
caries
• Casting shrinkage can lead to marginal failures
16. Marginal Integrity
• Supragingival margin
• Sub-gingival/intracrevicular margin
• Equigingival margin
Supragingival margin
• It can be easily viewed and finished
• It can be easily reproduced in an
impression
• Easy to examine during future visits
17. Marginal Integrity
Subgingival margin
• Tooth with short clinical crowns
• Subgingival caries or cervical abrasion
• Contact area is below the gingival margin
• Aesthetics of primary concern – conceal metal-ceramic margins
• Unmanageable root sensitivity
19. Finish Line Configuration
Shoulder margin : Margins at 90-degree to the
axial all anterior restorations where aesthetics
required
Sloping shoulder: Margins at 120-degree to the
axial wall
Radial shoulder
Shoulder with bevel: Margins end with a bevel at
the gingivo-axial surface
20. Finish Line Configuration
Knife edge margin:
Lingual surface of the mandibular posterior teeth
Very convex axial surface
Undercut surface of the tipped tooth
21. Preservation of Periodontium
Margins placed should be clearly visible, accessible for the impression, and
easy to clean and maintain by the patient
Easily reproduced in the impression without any tear
Finish line should be enamel whenever possible
Cavosurface margins should be smooth and continuous for well-adapted
margins
