Principles of tooth preparation

By Dr. Vishaal Bhat on Friday 20 June 2008 with 0 comments



3 principles of tooth preparation:

  1. biologic
  2. mechanical
  3. esthetic

Biologic considerations:

  1. pulpal (temperature, chemical irritants, desiccation, age, and tooth morphology)

  2. adjacent teeth (you may have to enameloplasty adjacent proximal surfaces of teeth before making the final impression)

  3. periodontal considerations (gingival health, margin location, ability to see the margins, and non-violation of the biologic width)

Indications for subgingival margins:
  1. caries

  2. retention/resistance form

  3. esthetics

  4. the need to end the margin on tooth structure and not restorative material.

-a disappearing metal margin is contraindicated as it places porcelain in tension instead of compression and can lead to opaque showing through or fracturing of the porcelain.

-a featheredge margin is also contraindicated to prevent overcontouring.

-“lipping” of the prep is also contraindicated and leads to unsupported tooth structure and porcelain.

  1. conservation of tooth structure (2-plane reduction, adequate occlusal/incisal reduction, need for enough thickness for the metal, and the least amount of taper)

- for porcelain fused to metal crowns, the reduction should be 1.2-1.5mm

- for a butt and 1.5-2.0mm for incisal/occlusal porcelain. The occlusal contact should be 1.5-2.0mm from the porc/metal junction.

Mechanical considerations:

  1. structural durability (corrugation effect on occlusal surface of molars)

  2. the effect of mastication forces on the cement (tensile and shear are bad/compression is favored)

  3. retention and resistance form

-retention: path of insertion or withdrawal (should restrict the possible paths)

-6 retention variables:

  1. taper

  2. diameter

  3. height

  4. surface area

  5. surface roughness

  6. effects of boxes and grooves

-taper has a greater effect on retention than prep height.

-for preps with the same taper, the prep with the larger diameter

will have more retention.

-when taper and diameter are the same, the prep with the greater

height will be more retentive.

-when taper and height are equal, the prep with the greater

diameter will be more retentive.

-when diameter and height are equal, the prep with the least taper

will be more retentive.

-resistance: prevents dislodgment in a horizontal direction (lateral forces)

3 resistance variables:

  1. taper

  2. diameter

  3. height

-occlusally to gingivally, the restoration is under compression, shear, and

tensile stresses respectively. This is based on the tangent line---the area

above the tangent line is under compression and the area below is under

predominantly shear and tensile forces. The best resistance is when the

tangent line is at least ½ way down the prep.

-when the heights of the crowns stay the same and the heights of the preps are

different, the larger prep has more resistance.

-when the heights of the preps stay the same, but the crown heights differ, the

shorter crown is more resistant.

-when tapers and prep heights are equal, the prep with the smallest diameter

will be more resistant. This is because the tangent line is lower on the

opposing wall, creating an increased resistant area. So, for retention you

want a bigger diameter and for resistance you want a smaller one.

-the resistance for a short, wide prep can be enhanced with boxes and grooves.

-when prep height and diameter are equal, the prep with the greater taper will

have a decrease in resistance.

-***the taper that provides resistance for a prep where the height is equal to

the base is 2x that of a prep where the height equals ½ the base. So, a shorter

prep needs to be more parallel. To achieve adequate resistance, the ht to base

ratio should be at least 0.4 or greater, provided you have minimal taper. 3mm

of prep ht should be adequate for anterior teeth while 4mm should be

adequate for molars.

-10-22 degrees of tooth preparation is more realistic and achievable.

-limiting tapers for the following teeth:

-incisors: 29 (due to a ht to base ratio of 1:1)

-canines: 33

-premolars: 10

-molars: 8.4 (due to a ht to base ratio of 1:2)

-auxiliary resistance forms are boxes, grooves, pins, or a shoulder with a bevel. Boxes and grooves are the most common forms of auxiliary resistance.

-for crowns, the mesial and distal surfaces have less convergence than facial and lingual surfaces. Here, it is suggested that boxes and grooves be placed on the mesial and distal surfaces to enhance their effectiveness.

-in FPDs, mesial and distal boxes will offset buccal and lingual forces—especially if the pontics are located facial to the rotation axis. If the span length is long, facial and lingual grooves will offset mesial and distal movement.

-4 esthetic considerations:

  1. retainer selection

  2. amount of reduction

  3. contour

  4. placement of margin

-4 ways to ensure you’re taking off the required amount of tooth structure when prepping:

  1. know the diameter of your bur

  2. make depth cuts

  3. use a template to evaluate your reduction

  4. prep ½ the tooth at a time

Category: Prosthodontics Notes

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