Tooth Preparation and Principles of Tooth Preparation

Characteristics of Tooth Tissue: Teeth do not possess the regeneration capability found in other tissues. Enamel or dentin lost due to decay, trauma, or wear is compensated with restorative materials to restore form and function. Five Basic Principles of Tooth Preparation According to Shillingburg: Preservation of Tooth Structure Retention and...
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Tooth Preparation and Principles of Tooth Preparation
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Characteristics of Tooth Tissue:

  • Teeth do not possess the regeneration capability found in other tissues.

Enamel or dentin lost due to decay, trauma, or wear is compensated with restorative materials to restore form and function.

Five Basic Principles of Tooth Preparation According to Shillingburg:

  1. Preservation of Tooth Structure
  2. Retention and Resistance
  3. Structural Durability
  4. Marginal Integrity
  5. Preservation of the Periodontium

Preservation of Tooth Structure

Preserving Remaining Tooth Structure:

  • The restoration should replace lost tooth tissue while also preserving the remaining tooth structure.
  • Over-preparation should be avoided.

Consequences of Over-Preparation:

  • Excessive thermal sensitivity in the tooth.
  • Inflammation and necrosis in the pulp.
  • Loss of retention and resistance.

Preserving Contact Tooth Structure:

  • Damage to adjacent teeth is a common error in operative dentistry.
  • A damaged proximal contact area becomes prone to decay.
  • Adhering to normal tooth anatomy during tooth preparation protects the pulp from irreversible damage.

Retention and Resistance

Definition and Importance of Retention:

  • A characteristic that prevents the dislodgement of the restoration along the path of insertion or the long axis of the tooth preparation.
  • Adequate retention depends on the following factors:
  • The intensity of the dislodging forces.
  • Geometry of the tooth preparation.
  • Surface roughness.
  • Properties and film thickness of the cement.
  • The intensity of dislodging forces is related to the stickiness of food, surface area, and the structure of the restoration being dislodged.

Geometry of Tooth Preparation

Cylindrical Structure and Taper:

  • Tooth preparation should be cylindrical, but an excessively tapered wall will disrupt the cylindrical structure and not restrict the movement of the restoration.
  • The more parallel the opposing walls of the tooth preparation are, the greater the retention.
  • An optimum taper angle of 2-6˚ minimizes stress concentration.
  • As the taper angle increases, retention decreases.

Cutting Surface Area and Retention:

  • The larger the cutting surface area, the greater the retention.
  • The preparation of additional boxes and grooves that will distribute stress evenly and create sufficient resistance against displacement.
  • Preparations on larger teeth are more retentive compared to smaller teeth.
  • Full crown preparations on molar teeth are more retentive than on premolars due to the larger surface area.

Surface Roughness

Inner Surface of the Restoration and Retention:

  • A very smooth inner surface of the restoration leads to retentive failure due to the interface created between the cement and the restoration.
  • Roughening the tooth preparation affects retention but makes impression taking and wax modeling more challenging.
  • Coating the prepared surface of the tooth with cavity varnish reduces retention but can be overlooked when considering the benefits to pulp health when using irritating bonding agents.
  • Cement Properties: Coating the prepared tooth surface with cavity varnish reduces retention. However, this disadvantage can be overlooked when considering the benefits to pulp health, especially when using irritating bonding agents like zinc phosphate.

How Should Preparation Be Done for Retention and Resistance?

  • There should be an appropriate angle between axial walls.
  • The crown (tooth) length should be as high as possible.
  • Parallelism should be achieved when necessary.
  • Retentive features should be used appropriately.
  • Undercuts should be avoided.
  • A path of insertion should be created.
  • The path of insertion is an imaginary line along which the restoration can be placed onto or removed from the prepared tooth. It is designed by the dentist before starting the preparation, and all details of the preparation are planned to achieve this design. The importance of parallelism in the path of insertion is especially crucial during the preparation of fixed partial prosthesis (bridge) abutments.

Structural Durability: The restoration must have enough mass to resist occlusal forces. This volume is confined by the space created by the tooth preparation. Only in this way can the restoration have compatible occlusion and normal axial contours.

To Ensure Structural Durability:

  • Adequate occlusal reduction must be performed.
  • Beveling of the functional cusp.
  • Adequate axial reduction must be performed.

Occlusal Reduction:

  • One of the most critical conditions for achieving sufficient mass of metal and durability for the restoration is the occlusal clearance.
  • For full metal crowns, 1.5 mm clearance is needed in functional cusps (palatal cusps of upper molars and premolars, buccal cusps of lower molars and premolars).
  • In non-functional cusps, 1 mm is sufficient.

Occlusal Reduction:

  • The purpose of creating sloping planes on the occlusal surface is to provide adequate clearance with the opposing arch.
  • A flat occlusal surface will lead to excessive shortening of the preparation.

Insufficient Occlusal Reduction:

  • Weakens the restoration.
  • Failure to adequately reduce the anatomical grooves on the occlusal surface will prevent achieving sufficient space for good functional morphology.
  • The restoration may become more prone to perforation due to finishing procedures or wear in the mouth.

Beveling of the Functional Cusp:

  • Beveling of the functional cusp is part of the occlusal reduction.
  • Wide beveling in functional cusps creates space for sufficient metal mass in areas with intense occlusal contact. Functional cusp slopes are given to the buccal cusps of mandibular teeth and the palatal cusps of maxillary teeth.

Lack of Adequate Beveling of Functional Cusp Can Cause Problems:

  • May result in thin areas and perforation in the restoration, i.e., crown surface.
  • Can lead to excessive contour formation and poor occlusion.
  • Over beveling of the buccal surface reduces retention and can also cause excessive loss of tooth structure.

Axial Reduction:

  • Axial preparation plays a significant role in creating space for restorative material to achieve adequate thickness.
  • Insufficient axial reduction can result in thin walls prone to deformation or excessive contour leading to periodontal problems.
  1. MARGINAL INTEGRITY: The edge of the restoration should perfectly match the inner surface of the finishing line of the preparation.

Margins (Finishing Lines):

  • Should be distinct.
  • Must show continuity.
  • Should be smooth and even.
  • Must terminate at an appropriate level.
  • Should follow the contour of the gingiva.
  • Contacts with adjacent teeth should be open.

Types of Finishing Lines:

Chamfer (groove)

  • Deep chamfer
  • Shoulder
  • Rounded shoulder
  • Beveled shoulder
  • Knife edge (feather edge)

1)KNIFE EDGE:

  • The thinnest and least substantial type of finish line among fixed prosthesis gingival edge types, resembling a knife edge.
  • Easiest to apply in the posterior molar region. Due to difficulty in intraoral preparation in this area, this type of finish line requires careful cutting. Otherwise, axial reduction may fail to produce a distinct finish line and become blurred.
  • Used on lingual surfaces of mandibular anterior and posterior teeth, excessively contoured teeth, and partial veneer crowns.
  • Challenging to accurately create this thin finish line during wax modeling and casting. Metal restorations with a knife-edge gingival finish line may deform under occlusal forces due to insufficient rigidity.

2)CHAMFER:

  • Groove-like preparations terminate with a broad angle at the gingival margin.
  • Allows for restorations with adequate edge thickness and strength.
  • Preferred for full metal restorations, veneer restorations, and all metal-ceramic restorations.
  • This finish line creates the least stress on the cement.
  • Prepared with a round-ended tapered diamond bur.
  • More conservative than the shoulder finish line.

3)SHOULDER (BLUNT STEP):

  • Preferred for porcelain jacket crowns, the wide step provides resistance against occlusal forces and reduces stress that can cause porcelain fracture. This finish line can also be prepared on the labial surface of full crowns.
  • Although distinct, it requires difficult cutting. There is a 90° angle between the axial surface and the step in the shoulder finish line. The shoulder can be modified by adding a bevel to the existing step.

4)PRESERVATION OF THE PERIODONTIUM:

  • The position of finish lines is critical for the long-term success and ease of fabrication of the restoration.
  • Best results are achieved with edges that are as smooth and easily cleanable as possible.

Marginal Classifications in Fixed Prosthetic Restorations Based on Relationship with Gingiva:

  • Sub-gingival (Below the gingival margin)
  • Supra-gingival (Above the gingival margin)
  • At the level of the gingival margin

Common Mistakes in Tooth Preparation:

  • Insufficient preparation
  • Overcutting
  • Lack of uniform preparation
  • Creation of undercuts

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