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INDIAN DENTAL ACADEMY
Leader in continuing dental education
www.indiandentalacademy.com

Rete...
Biomechanical principles of
tooth preparation
B OGICAL
IOL

M CH
E ANICAL

E HT
ST E IC

www.indiandentalacademy.com
Biomechanical principles of
tooth preparation
 Preservation of tooth structure.
 Retention and resistance form.
 Struct...
Introduction
 Restoration should remain firmly attached

to tooth structure to meet functional
biological and esthetic re...
Definition (GPT)
 That quality inherent in the prosthesis

acting to resist the forces of dislodgement
along the path of ...
Factors influencing retention
 Magnitude of dislodging forces.
 Geometry of tooth preparation .
 Roughness of the fitti...
Magnitude of dislodging
forces.
 Depends on the stickiness of the food and

the surface area and texture of the
restorati...
Geometry of tooth preparation
 Taper
 Surface area
 Stress concentration.
 Type of preparation.

www.indiandentalacade...
Geometry of tooth preparation
 Most fixed prosthesis depends on the

geometric form of the preparation rather
than on adh...
www.indiandentalacademy.com
Preparation is cylindrical if two horizontal
sections of prepared tooth surface is coincident

 Fig 7-27 and 7-28®

www.i...
A crown depends
on external
retention to
resist removal

An inlay depends
on internal
retention to hold
it within
preparat...
Taper
 Axial walls of the preparation

must taper slightly to permit the
restoration to seat.
 Inclination of the wall.
...
 Parallel walls are impossible to create in the

mouth.
 Preparation walls are tapered
• To visualize preparation walls
...
Relationship between the degree of axial wall
taper and magnitude of retention was first
demonstrated by Jorgensen in 1955...
 To minimize stress in the cement interface

between the preparation and restoration a
taper of 2.5 ° to 6.5 ° is optimum...
Some of the tapers suggested by authors

 Ohm and silness

on vital teeth
19.2° MD and 23 FL

on non vital teeth
12.8 ° M...
 Recommended convergence angle

between the opposing walls is 6 degree
which has been shown to optimize the
retention for...
OPTIMUM DEGREE OF TOOTH
PREPARATION TAPER
ARCH

M/D

F/L

Overall

Anterior tooth

10

10

10

Premolar

14

14

14

Molar...
Too small a taper will lead to unwanted

undercuts.
Too large a taper will not be retentive.

www.indiandentalacademy.co...
Length and surface area
 Occlusogingival length is an important

factor in retention and resistance. Longer
preparation w...
 Walls of shorter preparation should have

as shorter taper as possible.

www.indiandentalacademy.com
 Greater the area of contact of the cement film

bound to the preparation and to the internal detail
of the casting the g...
Doubling the height
of the preparation
would nearly
double the area of
its axial walls

Doubling the diameter
of the prepa...
 Basic unit of retention for a cemented

restoration is two opposing walls with
a minimal taper.
 It may not always be p...
www.indiandentalacademy.com
 Kent etal reported difference between

degree of taper of full crown preparation
18.4 – 22.2 degree and boxes and groove...
Freedom of displacement
 Retention is improved by geometrically

limiting the number of paths along which a
restoration c...
 Full veneer crowns with long parallel axial

walls and grooves will produce such
retention.
 Short over tapered prepara...
Stress concentration
 Stresses not uniform throughout the cement.
 Concentrated around the junction of

occlusal and axi...
Type of preparation
 Different types of preparation have different

retentive values.
 Different types of preparation in...
Retention of different preparation designs
www.indiandentalacademy.com
Roughness of the surfaces
being cemented.
 If the internal surface of the restorations

smooth failure occurs through the...
 Casting must be prepared by the air abrading the







fitting surface by 50 micro m alumina.
It has shown to incre...
Materials being cemented.
 More reactive the alloy more adhesion is with the luting

agent.
 Base metal alloys are bette...
Type of luting agent
 Decision regarding which agent to use

depends on other factors.

www.indiandentalacademy.com
Film thickness of luting agent
 Conflicting evidence about the effect of the

increased thickness of the cement film on
r...
Loss of retention
 Loss of retention should be detected early,

otherwise extensive caries develops.

Detection of loss o...
 If the casting is loose the occlusal motion draws

the fluids to be drawn under the casting and when
the casting is rese...
 If the prosthesis removal reveals lack of adequate

retention as evidenced by the preparation form the
teeth should be m...
Summary
Greater retention
Taper

Parallel

Surface area

Large

Type of
preparation

Molar complete
crown

Surface
texture...
www.indiandentalacademy.com
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Retention in fpd /certified fixed orthodontic courses by Indian dental academy

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The Indian Dental Academy is the Leader in continuing dental education , training dentists in all aspects of dentistry and offering a wide range of dental certified courses in different formats.

Indian dental academy provides dental crown & Bridge,rotary endodontics,fixed orthodontics,
Dental implants courses.for details pls visit www.indiandentalacademy.com ,or call
0091-9248678078

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  • But only slight increase in the stress
  • Retention in fpd /certified fixed orthodontic courses by Indian dental academy

    1. 1. www.indiandentalacademy.com INDIAN DENTAL ACADEMY Leader in continuing dental education www.indiandentalacademy.com Retention in fixed partial denture
    2. 2. Biomechanical principles of tooth preparation B OGICAL IOL M CH E ANICAL E HT ST E IC www.indiandentalacademy.com
    3. 3. Biomechanical principles of tooth preparation  Preservation of tooth structure.  Retention and resistance form.  Structural durability.  Marginal integrity.  Preservation of periodontium. www.indiandentalacademy.com
    4. 4. Introduction  Restoration should remain firmly attached to tooth structure to meet functional biological and esthetic requirements. www.indiandentalacademy.com
    5. 5. Definition (GPT)  That quality inherent in the prosthesis acting to resist the forces of dislodgement along the path of placement www.indiandentalacademy.com
    6. 6. Factors influencing retention  Magnitude of dislodging forces.  Geometry of tooth preparation .  Roughness of the fitting surface of the restoration.  Materials being cemented.  Thickness of luting agent. www.indiandentalacademy.com
    7. 7. Magnitude of dislodging forces.  Depends on the stickiness of the food and the surface area and texture of the restoration. www.indiandentalacademy.com
    8. 8. Geometry of tooth preparation  Taper  Surface area  Stress concentration.  Type of preparation. www.indiandentalacademy.com
    9. 9. Geometry of tooth preparation  Most fixed prosthesis depends on the geometric form of the preparation rather than on adhesion.  As most cements are non adhesive( act by increasing the frictional resistance between tooth and restoration.  Cylindrical tooth preparation. www.indiandentalacademy.com
    10. 10. www.indiandentalacademy.com
    11. 11. Preparation is cylindrical if two horizontal sections of prepared tooth surface is coincident  Fig 7-27 and 7-28® www.indiandentalacademy.com
    12. 12. A crown depends on external retention to resist removal An inlay depends on internal retention to hold it within preparation www.indiandentalacademy.com
    13. 13. Taper  Axial walls of the preparation must taper slightly to permit the restoration to seat.  Inclination of the wall.  Angle of convergence  Angle of divergence.  Two opposing surfaces with 3° taper would give a taper of 6 °.  Maximum retention is obtained if a tooth preparation has parallel walls . www.indiandentalacademy.com
    14. 14.  Parallel walls are impossible to create in the mouth.  Preparation walls are tapered • To visualize preparation walls • Prevent undercuts. • Compensate for the inaccuracies in the fabrication process • More nearly complete seating of the restoration during cementation. Ward was one of the first to recommend taper of 3 to 12 °respectively. www.indiandentalacademy.com
    15. 15. Relationship between the degree of axial wall taper and magnitude of retention was first demonstrated by Jorgensen in 1955 www.indiandentalacademy.com
    16. 16.  To minimize stress in the cement interface between the preparation and restoration a taper of 2.5 ° to 6.5 ° is optimum.  Slight increase in the stress as the taper increased from 0-15 °  At 20 ° stress concentration was found to increase sharply. www.indiandentalacademy.com
    17. 17. Some of the tapers suggested by authors  Ohm and silness on vital teeth 19.2° MD and 23 FL on non vital teeth 12.8 ° MD and 22.5 ° FL  Mack 16.5 °  Weed worked on dental students preparation 12.7 ° on typhodonts and 22.5 ° in clinical preparations.  Kent and his associates mean of 15.8 ° between mesial and distal walls 13.4 ° facial and lingual walls. Overall mean of 14.3 ° www.indiandentalacademy.com
    18. 18.  Recommended convergence angle between the opposing walls is 6 degree which has been shown to optimize the retention for zinc phosphate cement.  Guyer describes optimal taper to be in range of 2-6.5 °. www.indiandentalacademy.com
    19. 19. OPTIMUM DEGREE OF TOOTH PREPARATION TAPER ARCH M/D F/L Overall Anterior tooth 10 10 10 Premolar 14 14 14 Molar 17 21 19 Maxillary Isthmus 7 Box 7 Mandibular Anterior tooth 10 10 10 Premolar 16 12 14 Molar 24 20 22 Isthmus Box 12 www.indiandentalacademy.com 12
    20. 20. Too small a taper will lead to unwanted undercuts. Too large a taper will not be retentive. www.indiandentalacademy.com
    21. 21. Length and surface area  Occlusogingival length is an important factor in retention and resistance. Longer preparation will have more surface area  Length must be great enough to interfere with the arc of casting pivoting about a point on the opposite side of restoration. www.indiandentalacademy.com
    22. 22.  Walls of shorter preparation should have as shorter taper as possible. www.indiandentalacademy.com
    23. 23.  Greater the area of contact of the cement film bound to the preparation and to the internal detail of the casting the greater the retention of the casting.  Greater the surface area greater the retention.  Total surface area is influenced by the Size of the tooth Extent of coverage of the restoration Features such as grooves, boxes pin holes that are placed in the restoration www.indiandentalacademy.com
    24. 24. Doubling the height of the preparation would nearly double the area of its axial walls Doubling the diameter of the preparation would doubles the area of its axial walls and quadraples the area of occlusal www.indiandentalacademy.com surface.
    25. 25.  Basic unit of retention for a cemented restoration is two opposing walls with a minimal taper.  It may not always be possible to use opposing walls for retention.  It may be desirable to leave a surface uncovered for a partial veneer.  Features like boxes grooves and pinholes can be incorporated to enhance the retention www.indiandentalacademy.com
    26. 26. www.indiandentalacademy.com
    27. 27.  Kent etal reported difference between degree of taper of full crown preparation 18.4 – 22.2 degree and boxes and grooves in axial surface 7.3degreee  Taper of these features nearly same as the taper of the instruments used to cut them. www.indiandentalacademy.com
    28. 28. Freedom of displacement  Retention is improved by geometrically limiting the number of paths along which a restoration can be removed from the tooth preparation.  Maximum retention can be achieved by only one path .. www.indiandentalacademy.com
    29. 29.  Full veneer crowns with long parallel axial walls and grooves will produce such retention.  Short over tapered preparation would be without retention because the restoration can be removed along an infinite number of paths www.indiandentalacademy.com
    30. 30. Stress concentration  Stresses not uniform throughout the cement.  Concentrated around the junction of occlusal and axial walls.  Changes in the geometry of the tooth preparation (rounding the internal line angles) may reduce stress concentrations and thus increase the retention. www.indiandentalacademy.com
    31. 31. Type of preparation  Different types of preparation have different retentive values.  Different types of preparation include Type of coverage of the restoration. Inclusion of internal features. www.indiandentalacademy.com
    32. 32. Retention of different preparation designs www.indiandentalacademy.com
    33. 33. Roughness of the surfaces being cemented.  If the internal surface of the restorations smooth failure occurs through the cement restoration interface  Worley and Smith in their study conclude that the retention of the restoration can be increased by roughening or grooving the restoration. www.indiandentalacademy.com
    34. 34.  Casting must be prepared by the air abrading the     fitting surface by 50 micro m alumina. It has shown to increase the retention by 64 % Deliberately roughening of the preparation hardly influences the retention and it is not recommended as this adds to difficulty of impression making and waxing. Oilo and Jogerson found retention of castings on cemented with zinc phosphate cement on test dies with 10 degree taper to be twice as great on preparations with 40 micro m scratches as on those with 10 micro m scratches. Smith found no significant difference in castings cemented on 14 degree taper preparations whose roughness varied by a factor of 24 from smoothest to roughest. www.indiandentalacademy.com
    35. 35. Materials being cemented.  More reactive the alloy more adhesion is with the luting agent.  Base metal alloys are better retained than less reactive high gold content metals. Effect of adhesion on different core materials.  Chan kc etal in his study found that cement adhered better to amalgam than to composite resin or cast gold  De wals JP etal found that higher value of retention was found with composite rather than amalgam cores. It may be due to dimensional changes of the core materials although the clinical implications are not clear. www.indiandentalacademy.com
    36. 36. Type of luting agent  Decision regarding which agent to use depends on other factors. www.indiandentalacademy.com
    37. 37. Film thickness of luting agent  Conflicting evidence about the effect of the increased thickness of the cement film on retention of the restoration.  It may be important if a slightly oversized casting is made. www.indiandentalacademy.com
    38. 38. Loss of retention  Loss of retention should be detected early, otherwise extensive caries develops. Detection of loss of retention  Patients awareness to looseness or sensitivity to temperature or sweets.  Periodical examination which includes attempt to seat prosthesis by lifting retainers up and down while they are held between the hands and a curved explorer placed under the connector. www.indiandentalacademy.com
    39. 39.  If the casting is loose the occlusal motion draws the fluids to be drawn under the casting and when the casting is reseated with the cervical force the fluid is expressed generally producing bubbles as the air and liquid are simultaneously displaced.  When the retainer becomes loose the retainer should be removed so that the abutment teeth can be evaluated.  If the restoration can be dislodged from the other prepared teeth without damage and no caries present it is possible to recement the restoration. www.indiandentalacademy.com
    40. 40.  If the prosthesis removal reveals lack of adequate retention as evidenced by the preparation form the teeth should be modified to improve their retention and resistance.  Some fixed partial dentures may become loose even when maximal retentive preparations have been developed . It may be caused by excessive span length or heavy occlusal forces. www.indiandentalacademy.com
    41. 41. Summary Greater retention Taper Parallel Surface area Large Type of preparation Molar complete crown Surface texture Rough Film thickness Effect uncertain Luting agent Adhesive resin Lesser retention 6 degree Excessive Small Premolar complete crown Partial crown Intracoronal restoration Smooth Glass ionomer Zinc oxide Polycarboxylate eugenol Zinc phosphate www.indiandentalacademy.com
    42. 42. www.indiandentalacademy.com

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