crown and bridge

Finishing lines:
It is the final margin that separates 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 easily recognized in the impression and the cast.
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: it’s better to place the finishing line supragingivally for the following reasons:
A. Easy to be prepared and finished by dentist without trauma to the soft tissues.
B. Patient can keep it clean easily.
C. 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.

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 gingival margin are removed only. It is the least acceptable type, 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 provides the best marginal seal and it’s the most conservative type.
Chamfer finishing line:
This type is prepared with a tapered round-ended fissure diamond bur. It is 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.



Shoulder finishing line:
Itis very well defined finishing line so it’s easily detected on the cast. It is the least conservative type and regarded as 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. This finishing line is prepared with flattened end tapered diamond fissure bur.
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 which enhances the retention. This type of finishing lines is used on the labial or buccal tooth surface for porcelain fused to metal and full cast veneer with acrylic facing.







Types of crowns:
1. Full metal crown:
This provides better retention and resistance because all the axial surfaces of the teeth are included in the preparation.
Indications:
1. Posterior abutment teeth with excessive caries.
2. As a retainer on a tooth that will receive a clasp (posterior teeth).
3. High caries index.
4. Necessity of maximum retention and strength.
Contraindications:
1. Teeth located in the appearance zone.
2. Low caries index.
Advantages:
1. Strong.
2. More conservative and easy to be prepared.
3. Provide more retention and resistance compared to partial veneer crowns.
Disadvantages:
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 (DOG) must be prepared on the surface of the tooth to act as guide or reference to determine when enough tooth structure is removed, without these grooves we may remove much or less tooth structure or we loss time for repeated checking.
1. The preparation for a full veneer crown usually begin 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 tapered fissure bur or a round-end tapered diamond is used to create DOGs on the ridges and in the primary grooves of the occlusal surface.
The remaining tooth structure between the DOGs is removed to accomplish the occlusal reduction then smooth any roughness left by the grooves, keeping the 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 round-end tapered diamond bur. The functional cusp bevel should be placed on the buccal inclines of mandibular buccal cusps and the palatal inclines of maxillary palatal cusps. After completion of the 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. For the occlusal one third a (DOG) is placed in the center of this area by placing the bur at 45 degree with the long axis of the tooth and by moving the bur with the curvature of the surface to be prepared. This type of preparation is called two plane preparation or two steps preparation. The two-plane preparation is needed on the buccal surface of the lower molar and 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 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 we are going to create a rough surface in addition to removing the outer layer of enamel which is saturated with fluoride which leads to high caries susceptibility. 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 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. The advantages of these seating grooves are:
• To prevent the rotation of the restoration.
• Increase the surface area of preparation so it enhance the retention and the resistance.
• Improves the seating of crown as it enhance 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. 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
Indications:
1. Esthetics needs (carious teeth, malposed teeth, peg shaped lateral incisor, discolored teeth).
2. Fractured tooth without pulp exposure.
3. Teeth with large filling.
4. As a bridge retainer, especially for 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.
Advantages:
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 brittle especially if poorly designed.


Preparation
1. Preparation for posterior teeth
We should follow the same principles as in full metal crown with 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.
2. Preparation for anterior teeth
A. Incisal edge reduction:
We started by creating DOG of 2 mm in the center of the incisal edge and reducing the incisal edge with tapered bur. 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 and enhances the strength of brittle ceramic.
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 created parallel to the longitudinal axis of the tooth and by moving the bur mesially and distally this part will be reduced. For the incisal reduction, we place a DOG with the inclination of this area, otherwise 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 reduction is accomplished by following the inclination of the tooth in this area. The remaining axial lingual surface should he reduced using a wheel diamond bur, pear-shaped or american football-like diamond bur. We must keep in mind not to remove or over reduce the area at the junction between the cingulum and the remaining part of the lingual axial surface, otherwise, we may create a conical shape preparation which will lead to lack of retention and resistance. Finally, we should smooth and round the lie angles 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 inclinations of the tooth and not to disturb the surface geometry.
2. To increase the surface area which will increase the retention and resistance of the final restoration.
3. To avoid exposing the pulp chamber during preparation.
4. To give enough space for the restorative material, which will enhance the structural durability, otherwise, we will have bulky restoration, bulky facing or poor shade of the tooth.