TOOTH PREPARATION

Summarizing the fundamental rules to remember when preparing a crown: Preservation of Tooth Structure: We should not weaken the tooth structure by over-preparing and must avoid damaging the pulp. Resistance Form: We need to prevent the restoration from being dislodged by apical or oblique forces. Retention Form: We need to...
Tooth model and dental instruments in dental surgery
TOOTH PREPARATION
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Summarizing the fundamental rules to remember when preparing a crown:

Preservation of Tooth Structure:

  • We should not weaken the tooth structure by over-preparing and must avoid damaging the pulp.

Resistance Form:

  • We need to prevent the restoration from being dislodged by apical or oblique forces.

Retention Form:

  • We need to prevent the restoration from being dislodged along the path of insertion and the length of the tooth.

Structural Durability:

  • We must create enough space to avoid breaking, distorting, or perforating the crown.

Marginal Integrity:

  • We should prepare a step or finish line that ensures the adaptation of crown margins, thus preventing microleakage.

Preservation of the Periodontium:

  • We should prepare the preparation in such a way that the crown is not over-contoured and margins facilitate optimum oral hygiene.

Aesthetic Factors:

  • We should prepare in a manner that enables the creation of an aesthetically successful restoration.

Stages of Tooth Preparation:

  • Preparing the occlusal surface or incisal edge.
  • Axial surface preparations (mesial-distal-labial-lingual).
  • Making preparations that increase retention.
  • Smoothing surfaces, rounding off sharp corners and edges.
  • Preparing the margin line.
  1. Step (Occlusal and Incisal Surface):

Occlusal Reduction:

  • The purpose of the occlusal reduction is to disconnect the tooth from its antagonist and create necessary space for the prosthesis material.
  • Typically, approximately 1.5 mm clearance is achieved in functional cusps and about 1.0 mm in non-functional cusps during occlusal reduction.
  • Deeper cuts may be required for certain restorations like porcelain crowns.
  • Grooves can be created to determine the depth of the occlusal cut.
  • In younger patients, the pulp chamber of the tooth is larger, so deep cutting can open the pulp chamber. Therefore, the condition of the pulp should be checked in radiographs.
  • During occlusal reduction, cusps and fossae are cut to an even thickness, maintaining the normal anatomical form of the surface.
  • This type of cutting is compatible with the intricate morphological character of the occlusal surface, making fixed prostheses more resistant to functional forces.
  • Barrel, pear-shaped, or small cylindrical burs are used for these cuts.

Determining Depths with a Diamond Round Bur:

  • Prepare grooves to determine the depths.

Holding the Bur Parallel to the Cusp Slopes:

  • Keep the bur parallel to the cusp slopes.

Removing Tooth Structure Between Grooves:

  • Remove tooth structure from the occlusal surface at the determined depth.

Forming the Contours of the Cusp Slopes:

  • Form the upward and downward contours of the cusp slopes.

Refining Occlusal Surface Morphology:

  • Do not alter the occlusal surface morphology; stay true to the natural contours.

Lowering and Checking the Occlusal Plane:

  • Lower the occlusal plane and check the depth.

Preparation of Incisal Edges for Maxillary and Mandibular Anterior Teeth:

  • Cylindrical and round burs are used for incisal edge preparations.
  • The cutting depth is usually 1-2 mm, and preparing a guide groove helps control the cutting depth.
  • Incisal edge preparations are crucial for the aesthetic quality of anterior teeth. Along with the incisal edge depth, cutting 1/3 of the labial axial surface at the incisal area is also important.
  • Giving the incisal edge a 45° angle in the labio-lingual direction provides resistance against forces during biting.
  • The incisal edges of canines are cut in two planes, staying true to their normal anatomical form.

Step 2 (Buccal and Lingual Surfaces):

  • The purpose of preparing the buccal and lingual surfaces:
  • Removing enough tooth structure for the planned restoration.
  • Creating a taper of 2-6° from parallelism in the gingivo-occlusal direction, so the tooth preparation narrows towards the occlusal.
  • Use a round-ended diamond fissure bur to open your guide grooves on the buccal surface, preparing them to be about 1 mm above the gingival margin.
  • Hold the bur at an approximate 3-degree angle to the long axis of the tooth.
  • Remove the tooth structures between the guide grooves to join them, ensuring natural undercuts are removed.
  • Adjust the angle of your bur to create a second plane for the buccal cusp slopes.
  • Follow the natural buccal surface contours while preparing and ensure that the cut is sufficient and uniform.
  • Apply the same process to the lingual surface, ensuring that the cut widens towards the gingival margin while maintaining a 3-degree angle.
  • Do not create a second plane for the occlusal surface on the lingual side and ensure that undercuts are removed.
  • Ensure that the margins are 0.5 to 1 mm above the gingiva and prepare a chamfer (groove) style step. You can finalize the shape of the steps after completing the axial (side) wall cuts.
  1. Step (Proximal Surfaces):

Purpose of Proximal Surface Preparation:

  • To eliminate contacts with adjacent teeth.
  • To provide sufficient axial taper for the occlusal surface.

Removing Proximal Contacts:

  • Initially use a smaller diameter round-ended fissure bur to remove proximal contacts.
  • Move your small, thin bur in the proximal area bucco-lingually and occluso-gingivally, making cuts on both mesial and distal surfaces.
  • Avoid contact with adjacent teeth with your bur.
  • Once you have created a space where you can comfortably move your bur without touching the adjacent tooth, prepare your chamfer step with the thicker diameter bur used previously, ensuring it is about 0.5 mm above the gingival margin.
  • Ensure that you maintain a 3-degree angle on each proximal surface.
  • Recheck whether you have made a uniform cut on the proximal surfaces, whether the chamfer margin is 0.5 mm, and whether contact with adjacent teeth has been removed.

Considerations in Preparing Axial Surfaces:

  • The occluso-gingival dimension obtained at the end of the tooth preparation must provide adequate retention for the crown prosthesis.
  • Axial walls should be aligned as much as possible with the long axis, with exceptions being the different crown and root axes in lower molars. The structural feature of the mandibular molar should be remembered, and the crown preparation can be inclined lingually.
  • Buccal and lingual walls, along with the occlusal surface cuts, should allow for the final bucco-lingual width of the crown prostheses.
  • In cases of short axial walls resulting from tooth preparation, additional retentive preparations can be utilized.

Preparation of Lingual Axial Walls of Anterior Teeth:

  • The “cingulum” area is very important in cutting the lingual axial walls of anterior teeth. Therefore, the lingual axial cut should be made in a way that preserves the “cingulum.” This is crucial for the retention of the crown to be made.
  1. Step (Functional Cusp Slopes):

  • The purpose of forming functional cusp slopes is to reduce the height of these cusps by an additional 0.5 mm, providing sufficient thickness for the restorative material in these areas that bear significant functional loads.
  • In functional cusps, prepare a wide taper by following the cusp slopes and contours of the tooth.
  1. Step (Preparation of Steps):

  • The purpose of preparing margins or steps is to achieve a distinct finishing line at this terminal area of the preparation.
  • A chamfer step is preferred for full metal crowns. Its width is 0.5 mm and is prepared supragingivally, encircling the tooth at the cervical area and being 0.5 mm above the gingiva.
  • Place a round-ended fissure bur at a 0.5 mm depth and above the gingival margin by 0.5 mm to define the surfaces you roughly removed along the cervical margin of the tooth earlier.
  • Follow the contours of the gingival margin.
  • Smooth any rough and irregular surfaces along the margin.
  • Ensure you have prepared a smooth, even, and distinct chamfer margin.
  1. Step (Completion of Preparation):

  • The purpose of finishing the preparation:
  • To achieve a smooth preparation, avoiding irregular structures.
  • To create a clear and smooth margin.
  • Use a round-ended fissure bur to smooth all axial surfaces of the preparation and remove any surface irregularities.
  • Round off all sharp edges and corners (beveling).
  • For margins, use a finishing bur similar to a round-ended fissure bur to remove surface irregularities and achieve a smooth and distinct margin.

Procedures to Increase Retention:

  • In teeth with extremely short clinical crown lengths, further shortening of the occluso-gingival length due to occlusal cutting can lead to a loss of retention.
  • In such cases where axial surfaces cannot provide retention, adding special retentive features to the teeth can be beneficial.
  • These retentive preparations made after occlusal and axial cuts are referred to as “intracoronal retainers.”
  • Apart from teeth with short and insufficient occluso-gingival dimensions, such preparations are routinely made in 3/4, 4/5 crowns, proximal boxes, and incisal edge grooves.
  • Additionally, steps prepared in the areas of pin holes in pinlays and in pin regions in pinledge can be considered under the same classification.

Smoothing Surfaces and Rounding Edges:

  • Disadvantages of leaving sharp corners and edges in tooth preparation:
  • Sharp corners and edges often form inaccurately in plaster models. The failure of the plaster to reach these areas reduces the accuracy of the model.
  • Sharp corners and edges make it difficult for the crown prosthesis to adapt to the supporting tooth.
  • In wax pattern work, sharp corners and edges are prone to wearing or breaking.
  • Another critical aspect of tooth preparation requiring thorough control is whether any rough surfaces, undercuts, or other retentive areas remain after preparation. Such areas hinder the precise adaptation of crown prostheses to the prepared surfaces. These surfaces should be smoothed before taking impressions.

Importance of the Margin Line:

  • The adaptation of crown prostheses in the gingival area and their relationship with the periodontium are aspects that dentists should focus on critically. The placement of crown prostheses within the gingival sulcus and their morphological characteristics in this area directly affect the success of the outcome.

Final Stage: Self-Assessment

Occlusal Reduction:

  • Did you make a 1 mm reduction from the occlusal surface? Less or more?
  • Did you apply the shape – contours of the occlusal surface to your cut? Is the occlusal surface flat?

Buccal and Lingual Reductions:

  • Did you remove any undercuts? Did you create new ones?
  • Do you have 3-degree tapers on each surface? Less or more?
  • Did you form a second occlusal plane on the buccal surface?

Proximal Reduction:

  • Did you eliminate contact with adjacent teeth?
  • Do you have sufficient occlusal convergence (taper)?

Functional Cusp Slopes:

  • Did you make an additional 0.5 mm reduction from the functional cusps?
  • Did you cut from the cusps where functional loads are applied?

Preparation of Margins:

  • Are your margins prepared at 0.5 mm?
  • Are they positioned 0.5 mm above the gingiva and follow the contour of the gingival margin?

Completion of Preparation:

  • Are the preparation surfaces smooth, and irregularities removed?
  • Is the margin distinct and well-followed, prepared as a smooth surface?

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