crown cutting

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Lecture three------------------------------------------------------------------------------------crown and bridge

.

Finishing lines:

Is the final margin that separate between the prepared axial tooth surface and the

remaining unprepared tooth surface.

Requirements of finishing line:

1. It must be clear, well defined and smooth, so it can be reproduced on working

model.

2. It must be continuous from one surface to another.

3. Whenever possible the finishing line should be placed on sound tooth structure.

Position of finishing line:

1. With the level of free gingival margin.

2. Supra gingival finishing line,: its better to place the finishing line

supragingivally for the following reasons:

A. Easily to be prepared without trauma to the soft tissues.

B. Easy to be prepared and finished by dentist.

C. Patient can keep it clean easily.

D. Impression is easily made and can be removed without tearing or deficiency.

3. Subgingival finishing line: indicated in

A. Esthetic.

B. Caries or filling at the area of finishing line.

C. To increase retention of short teeth.

Types of finishing line:1. Feather edge (knife edge).

2. Chamfer.

3. Shoulder.

4. Bevel shoulder.

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The selection of certain type of finishing line depends on:

1. The materials used to construct the restoration.

2. The position of the tooth.

3. The tooth aspect to be prepared.

Feather edge (knife edge):

In this type all convexities coronal to the margin are removed only, its mostly

unacceptable but it was advocated already before the development of high speed cutting

instruments and improvement of impression materials and techniques, this type of

margins lack strength, difficult to locate on the cast and difficult to fabricate the wax

pattern, however it provide the best marginal seal and its the most conservative type.

Chamfer finishing line:

This type is prepared with a tapered round ended fissure diamond bur, its regarded as

the line of choice for most veneer cast metal restorations and lingual margins of

porcelain fused to metal restoration. It has been shown to exhibit the least stress.

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Shoulder finishing line:

This is the best choice for jacket crowns; the wide ledge provides resistance to occlusal

forces and minimizes stresses that might lead to fracture of the restoration, and its less

conservative. This finishing line is prepared with flattened end tapered diamond fissure

bur. Its very well defined finishing line so its easily detected on the cast.

Shoulder with bevel:

In this type we create a bevel on the end margin of unprepared tooth structure, this lies

between the prepared and unprepared tooth structure and is very critical area. This type

of finishing line is recommended for extremely short walls, since the axial walls of this

type is nearly parallel to each other so enhances retention. This type of finishing lines is

used for porcelain fused to metal and full cast veneer with acrylic facing.

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Types of crowns:

1. Full metal (veneer) crown:

This provides better retention and resistance because all the axial surfaces of the teeth

are included in the preparation.

Indication:

1. Posterior abutment teeth with excessive caries.

2. As retainer on tooth receive clasp (posterior teeth).

3. High caries index.

4. Necessity of maximum retention and strength.

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Contraindication:

1. Teeth located in the appearance zone.

2. Low caries index.

Advantage:

1. Strong.

2. More conservative and easy to prepared.

3. Provide more retention and resistance compared to partial veneer crowns.

Disadvantage:

1. Poor esthetic.

2. Tarnish and corrosion, so it needs prophylactic measures.

3. Difficulty to test the vitality of the abutment teeth.

Steps of preparation:

Depth orientation grooves must be prepared on the surface of the tooth to act as guide or

reference to determine when sufficient amount of tooth structure is removed, without

these grooves we may remove much or less tooth structure or we loss time in repeated

checking.

1. The preparation for a full veneer crown is begun with the occlusal reduction. By

accomplishing this step first, the occlusso gingival length of the preparation can be

determined. The potential retention of the preparation can then be assessed, and

auxiliary features can be added if necessary there should be 1.5 mm clearance on the

functional cusps and about 1.0 mm on the non functional cusps. A No. 170 taper fissure

bur or a round-end tapered diamond is used to place the grooves on the ridges and in the

primary grooves of the occlusal surface. If there is already sonic clearance with the

opposing tooth because of malpositioning or fracture of the tooth being prepared, do not

make the grooves as deep.

The tooth structure remaining between the orientation grooves is removed to accomplish

the occlusal reduction then smooth any roughness left by the grooves. Keeping the

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occlusal surface in the configuration of the geometric inclines that make up the occlusal

surface of any posterior tooth after that a wide bevel is placed on the functional cusp

again using the No. 170 buy or rounded tapered diamond. The functional cusp bevel

placed on the buccal inclines of mandibular buccal cusps and the lingual inclines of

maxillary lingual cusps after completion of occlusal surface preparation we should

check the occlusion of the patient in centric and eccentric positions of jaw relationship.

2. Buccal surface: because of the anatomy of the buccal surface of the lower posterior

teeth, this surface should be divided into two parts: gingival two thirds and occlusal one

third for the gingival two thirds we should place a (DOG) in the center of this surface

parallel to the long axis of the tooth and by moving the bur mesially and distally

following the inclination of the surface so this surface prepared. For the occlusal one

third a (DOG) is placed in the center of this area by placing the bur 45 degree with the

long axis of the tooth and by moving the bur with the curvature of the surface to be

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prepared. This type of preparation is called two plane preparation or two steps

preparation The two plane preparation is done on the buccal surface of the lower molarand the palatal surface of the upper molar.

3. Lingual surface :the (DOG) is placed in the middle parallel to the long axis of the

tooth and by moving the bur mesially and distally so we complete the reduction, this

type of preparation is done in one plane as it is indicated for the lingual lower and

buccal upper molar and premolar teeth.

4. Proximal surfaces: we start with a fine tapered diamond fissure bur (needle type) to

open and remove the contact area carefully without touching the adjacent tooth because

caries will be developed in the damaged surface later on, because we are going to create

a rough surface in addition to removing the outer layer of enamel which is saturated

with fluoride. The bur should be rested on the prepared tooth itself and by moving the

bar up and down the contact will be removed, finally any sharp angle should be

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removed to prevent fracture due to stress concentration, sometimes seating groove is

placed in the buccal surface of the lower and the palatal surface of the upper molar teeth

which act as a guide during placement of the crown, to prevent the rotation of the

restoration, increase the surface area of preparation so it enhance the retention and the

resistance, finally it improves the seating of crown as it let the escape of the excess

cement during cementation.

2-Full metal crown with facing

It is a full metal crown having the labial or buccal surface covered by a tooth colored

materials (acrylic, Porcelain), it combines the strength of full metal crown and the

cosmetic effect of the tooth colored material, and it is not a conservative type of crown

since it includes excessive tooth preparation to provide enough space for the metal and

the facing material in addition to that there is excessive contact with the gingival tissue

when the margin of the crown is placed close or below the gingival margin ,it can be

used on anterior and posterior teeth

Indication:

1. Improvement of esthetics (carious teeth, malposed teeth, peg shaped lateral

incisor, discolored teeth).

2. Fracture of tooth without pulp exposure.

3. Teeth with large filling.

4. As a bridge retainer especially in long span bridge.

5. Endodontically treated teeth with sufficient remaining tooth structure.

Contraindications:

1. Teeth with large pulp.

2. Teeth with short crown.

3. Patient with poor oral hygiene.

Advantage:

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1. It combines the strength of full metal crown and the cosmetic effect of the tooth

colored material.

2. Natural appearance can be closely matched by good technique and if desired

through characterization of the restoration with internally or externally applied

stains.

Disadvantages:

1. Requires significant tooth reduction to provide sufficient space for the restorative

materials.

2. Increases the potential for periodontal disease.

3. Because of the glasslike nature of the veneering material, a metal-ceramic crown is

subject to brittle fracture (although such failure can usually be attributed to poor design

or fabrication of the restoration).

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Preparation

1. Preparation for posterior teeth

We should follow the same principles as in full metal crown as in the full metal crownwith one exception only, by doing a deep reduction on the buccal surface to provide

enough space for the metal and the facing material and also to gain bulk for proper

shade of the final crown The finishing line is shoulder on the buccal surface and

chamfer all around the remaining tooth aspects.

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2. Preparation for anterior teeth

A. Incisal edge reduction:

We started by basing a depth orientation groove of 2 mm in the center of the of the

incisal edge and by using a fissure bur placed with the palatal inclination of the incisal

edge, this edge will be reduced (in the lower anterior teeth the bur should be placed with

the labial inclination to follow the anatomy of the tooth) the 2mm reduction of the

incisal edge is to provide a space for the facing material and metal to get a better

translucency of the incisal edge.

B. Preparation of the labial surface:

This surface should be divided into two parts, gingival and incisal, for the gingival part

a DOG of 1.5 mm is placed in the gingival part parallel to the longitudinal axis of tile

tooth and by moving the bur mesially and distally this part will be reduced, while for the

incisal reduction we place a DOG with the inclination of this area since the preparation

if done without inclination we may have pulp exposure.

C. Lingual surface preparation:

For the cingulum area a DOG of 0.5 mm depth is placed in the center of the cingulum

area parallel to the longitudinal axis of the tooth and by following the inclination of thetooth this area will be reduced The remaining axial lingual surface should he reduced

using a wheel diamond bur, we must keep in mind not to remove or over reduce the

junction between the cingulum and the axial and the remaining part of the lingual axial

surface if not we may create a conical shape preparation which will lead to lack of

retention and resistance, finally we should smooth and round the lie angle to facilitate

the next steps of crown construction.

Why we do the two steps preparation on the lower buccal, upper palatal surfaces of the

posterior and labial surfaces of anterior teeth:

1. To follow the anatomy and the inclination of the tooth and not to disturb the

surface geometry.

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2. To increase the surface area this will give increase in retention and resistance of

the final restoration.

3. To avoid hitting of the pulp chamber during preparation.

4. To give enough space for the restorative material so this will enhance the

structural durability other vise we will have bulky restoration, bulky facing or

poor shade of the tooth.

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