Successfully reported this slideshow.
We use your LinkedIn profile and activity data to personalize ads and to show you more relevant ads. You can change your ad preferences anytime.

Retention in fpd /certified fixed orthodontic courses by Indian dental academy


Published on

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 ,or call

Published in: Education
  • Be the first to comment

Retention in fpd /certified fixed orthodontic courses by Indian dental academy

  1. 1. INDIAN DENTAL ACADEMY Leader in continuing dental education Retention in fixed partial denture
  2. 2. Biomechanical principles of tooth preparation B OGICAL IOL M CH E ANICAL E HT ST E IC
  3. 3. Biomechanical principles of tooth preparation  Preservation of tooth structure.  Retention and resistance form.  Structural durability.  Marginal integrity.  Preservation of periodontium.
  4. 4. Introduction  Restoration should remain firmly attached to tooth structure to meet functional biological and esthetic requirements.
  5. 5. Definition (GPT)  That quality inherent in the prosthesis acting to resist the forces of dislodgement along the path of placement
  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.
  7. 7. Magnitude of dislodging forces.  Depends on the stickiness of the food and the surface area and texture of the restoration.
  8. 8. Geometry of tooth preparation  Taper  Surface area  Stress concentration.  Type of preparation.
  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.
  10. 10.
  11. 11. Preparation is cylindrical if two horizontal sections of prepared tooth surface is coincident  Fig 7-27 and 7-28®
  12. 12. A crown depends on external retention to resist removal An inlay depends on internal retention to hold it within preparation
  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 .
  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.
  15. 15. Relationship between the degree of axial wall taper and magnitude of retention was first demonstrated by Jorgensen in 1955
  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.
  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 °
  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 °.
  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 12
  20. 20. Too small a taper will lead to unwanted undercuts. Too large a taper will not be retentive.
  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.
  22. 22.  Walls of shorter preparation should have as shorter taper as possible.
  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
  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 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
  26. 26.
  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.
  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 ..
  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
  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.
  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.
  32. 32. Retention of different preparation designs
  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.
  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.
  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.
  36. 36. Type of luting agent  Decision regarding which agent to use depends on other factors.
  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.
  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.
  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.
  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.
  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
  42. 42.