Current concepts about principles of cavity designs

1. Current concepts about principles of cavity designs • What changes have been made to G.V. Black principles of cavity preparation? • What are the factors influencing cavity design?

2. Principles of cavity design:Review • Establishment of outline form • Establishment of resistance form • Establishment of retention form • Treatment of residual caries • Correction of enamel margins • Cleaning of the cavity (cavity toilet).

3. Outline form • Outline form: of the cavity is the shape which the cavosurface line angle of the cavity assumes after preparation.

5. Outline form • Access: • Gain initial access via the most carious part of the tooth. • Remove all unsupported enamel to expose the entire extent of caries.

6. Outline form • Elimination of the carious lesion • It is the elimination of any infected carious tooth structure left in the tooth after initial cavity preparation. • Caries left in the pulpal/ axial floor is excavated thoroughly making the cavity deeper. 0.75- 1 mm of dentin should cover the pulp. All the infected/ soft dentin should be removed.

7. Outline form • Placement of cavity margins in areas of lessened caries susceptibility • it is not necessary in low caries experience patients • it is advisable to include the pits and fissures in an amalgam cavity in cases of high caries rate patients

8. Outline form • Convenience: • Make the cavity wide enough to admit the instruments you are using. • Extend proximal cavities buccally and lingually just outside the contact area.

9. Resistance form • Resistance form is the design of a cavity in such a way that the remaining tooth substance and the restorative material can withstand masticatory stress.

10. Resistance form • To achieve this ,the prepared cavity should possess the following 6 attributes: • 1.Flat Floor • 2.Adequate bulk of the restorative material • 3.Absence of weak cusps or marginal ridges • 4.Occlusal cavity margins in areas not subjected to excessive occlusal trauma . In practice one-quarter (1/4) of the intercuspal width • 5.Flat floor at right angles to the line of stress • 6.Walls of the cavity parallel to the direction of the stress

11. Resistance form • Absence of weak cusps or marginal ridges • Remove any weak cusp and restore with a metallic material. • remove any weak marginal ridge specially if it is not supported by sound dentine. This way class I becomes Class II

13. Resistance form • Adequate bulk of the restoration • The depth should be enough to take adequate bulk of the restorative material which can withstand forces of mastication. • Amalgam thickness should be 1.5 – 2 mm at least.

14. Occlusal cavity margins in the areas not subjected to excessive occlusal trauma: Occlusal margins of the cavity should not be subjected to heavy stress. Place them about 1/4 of intercuspal distance. Resistance form

15. Resistance form • Flat floor at right angles to the line of stress: • Make the floor of the prepared cavity flat • Make the floor of the cavity at right angles to direction of the occlusal forces. • No sound tooth structure should be removed to achieve this.

16. Resistance form • Walls of the cavity parallel to the direction of stress: • Make the walls of the cavity parallel to the corresponding surfaces.

17. Retention form • refers to those aspects of cavity preparation which ensure that the restoration is not dislodged from the prepared cavity.

18. Retention form • Undercuts: • Dovetail lock • Flat floor and definite internal line angles. • Grooves and boxes • Acid etch technique • adhesives

19. Treatment of residual caries • Amelodentinal junction caries • Pulpal caries • Indirect pulp capping

20. Correction of enamel margins • No undermined enamel rods • Smooth enamel surface • Cavosurface angle 90 • No beveling for amalgam • Composite needs a bevel to provide greater enamel surface area for etching and micromechanical retention.

21. Cleaning of the cavity • The prepared cavity should be free from all debris • No disinfectant should be used to clean the cavity. • Don’t desiccate it.

22. Principles of cavity design • Dr. G.V. BlackChicago, Illinois1836-1915 • "The Grand Old Man of Dentistry" • "The professional man has no right to be other than a continuous student."

23. G.V. Black • “The complete divorcement of dental practice from studies of the pathology of dental caries, that existed in the past, is an anomaly in science that should not continue. It has the apparent tendency plainly to make dentists mechanics only”

24. Critique of his principles • It does not include root caries • It does not include secondary caries • It does not include non-carious lesions which are treated in the same way as carious lesions

25. The objectives of restoring teeth • To remove diseased tissue as necessary • To restore the integrity of the tooth surface • To restore the function of the tooth • To restore the appearance of the tooth

26. What determines cavity design? • 1. The dental tissues • 2. The disease • 3. The properties of the restorative material • I want you to think

27. 1. The dental tissues - Enamel • Enamel is the hardest tissue in the body, inelastic and brittle. Tend to split along the line of rods • Unsupported enamel vs. composite and GIC • Unsupported enamel vs. amalgam • Unsupported enamel and tightening a matrix band

28. 1. The dental tissues - dentine • Dentine is softer than enamel, more porous and sensitive. • Dentists can use pins • Dentine and pulp are considered one unit. • Why? • Diseases or operative procedures that affect the dentine may also affect the pulp

29. 1. The dental tissues - pulp • Pulp size and secondary dentine • The preparation is designed to avoid pulp • Avoid physical, chemical or thermal trauma to the pulp during cavity preparation and placing the restoration

30. 1. The dental tissues – the gingiva • The cavity design and restoration should have minimal encroachment on the gingiva • The margins of the restoration should be smooth and not retentive to plaque • The margins should be cleansable as possible

31. 2. The disease • Two points to be considered in the spread of caries: pulpal and lateral spread

32. 3. The properties of restorative materials - Amalgam • Mechanical retention • Weak in thin sections • Is not tooth colored • corrosion forms at the tooth amalgam interface • Needs force to be condensed into cavity

33. Properties - composite • It is attached physically to the tooth by means of acid etch • It is strong in thin sections • Can be attached to dentine by chemical bonding or via intermediate layer of GIC • Shrinkage is a problem

34. Properties - GIC • Chemical adherence to enamel and dentin • It releases fluoride slowly • Margins can be left in areas which are difficult to clean • Weak and not abrasion resistant • Cermet contains silver to make abrasion resistance and radiopaque • They destroy the tooth like appearance

35. New principles of cavity design • Gain access • Remove caries • Look, think and design • Complete the restoration

36. Gaining access – class II • Remove the marginal ridge • Via the buccal or lingual surfaces • Tunnel through from the occlusal surface leaving the marginal ridge intact • Direct access when the adjacent tooth is missing

37. Gaining access • Access cavity

38. Gaining Access • Class II cavity

39. Gaining access • Marginal ridge is removed

40. Gaining access in tunnel • Initial access

41. Prepared cavity • Conventional cavity

42. Look, think and design • The final choice of material • Retention • Protection of remaining tooth structure • Optimize strength • The shape and position of cavity margins

43. Retention in 5 directions • Arrows

44. Design features to protect remaining tooth structure • Remove unsupported enamel • Remove weakened cusps • Support the weak structure by bonding it to other parts of the tooth with GIC • Have it crowned

45. Design features to optimize the strength of the restoration • Remove sufficient tooth tissue to make room for the material taking into consideration its properties • Round the occlusal-proximal line angle

46. Position of cavity margins • Place cavity margins supragingivally • Don’t extend beyond the contact area unnecessarily • Do not leave sharp edge of an amalgam restorations

47. Protection of pulp • Pulpal injury can result due to: • 1.Heat generated by injudicious cutting. • 2.Restorative material with good thermal conductivity. • 3.Chemical from the restorative materials. • 4.Galvanic currents • 5.Microleakage.

48. Cavity lining and chemical preparations • 1. Linings: e.g. calcium hydroxide, zinc oxide eugenol and GIC. These are protective, therapeutic and structural. • 2. Varnishes: natural or synthetic resins in a solvent and are applied to cavity walls in an attempt to improve the marginal seal of an amalgam restoration