Crown and bridge

Crown and Bridge

BY
Balsam M.Mirdan

Fixed Prosthodontics(Crown and Bridge Prosthesis)

Is that branch of dental science that deals with restoration of damaged teeth with artificial crown and replacing the missing natural tooth by a dental permanent prosthesis cemented in place
Crown
It is a fixed coronal artificial restoration of the coronal portion of the damaged tooth, it must restore the morphology, contour and function of the tooth and should protect the remaining tooth structure from farther damage.

Bridge

Component of several number of crowns connected together to replace one or more of missing teeth and can't be removed by the patients. Supported by number of natural teeth and roots, those tooth are called abutment tooth

Crown and Bridge is required to:

• Replace a large restoration when there isn’t enough tooth remaining.
•

2. Restore a weak tooth from fracturing

3.Cover dental implant

4- Cover discolored or poorly shaped tooth

5- Cover a root canal treated tooth

6- Restore mastication, esthetic and photonics
7- Prevent over eruption of opposing arch teeth(preserve occlusion)

Classification of Crowns

According to the covered surface to:


1- Complete crown: it covers all the coronal portion of
the tooth, such as full metal crown, porcelain fused to
metal crown and full zircon crown.

2- Partial crown: it covers part of the

coronal portion of the tooth such3/4, 7/8 crown.

3- complete replacement ( post and core): It replaces the natural crown entirely. This type of crown retains itself by means of a dowel(post) extended inside the root
canal space













According to the material used:
1. Metal material: full metal crown
2. Plastic material: such as acrylic resin or porcelain
3. A combination: metal and plastic material as
in full veneer crown.
Bridge Classification
• Fixed-fixed bridge: most common used
anteriorly and posteriorly. The pontics
are connected rigidly to the retainers at
both ends of bridge by solder joint,
so we have only one path of insertion.
2. Fixed-movable bridge: the pontic is
attached to fixed retainer on one side
while the other side is movable joint
that connected with other retainer.





3. Cantilever bridge: is used when support can be obtained only from one side of the edentulous space. These dentures have compromised support. The abutment teeth on the supporting side should be strong enough to withstand the additional torsional forces. Support can be obtained from more than one tooth on the same side of the edentulous space
a/ simple consist of one or two retainer with pontic that replace the missing tooth

Advantages

Very conservative design especially when a single abutment is involved. When secondary abutments are used, parallel preparation can be easily obtained because the abutments are adjacent to one another. Easy to fabricate.
Disadvantages
Produces torqueing forces on the abutment. Cannot be used to restore long span edentulous spaces. Minor design errors can affect the abutments in a large scale.



b. Spring cantilever: This is a special cantilever bridge exclusively designed for replacing maxillary incisors but these dentures can support only a single pontic. Support is obtained from posterior abutments (usually a single molar or a pair of splinted premolars).

Advantages.

Can be used for diastema cases. Metal crown retainers that require minimal tooth preparation, can be used in posterior teeth to replace missing incisors.

Disadvantages.

The connector bar may interfere with speech and mastication. Deformation of the connector bar may produce coronal displacement of the pontic. There may be food entrapment under the connector bar, which may lead to tissue hyperplasia.

Components of a bridge;

• Retainer: its part that seats over (on or in) the abutment tooth. It could be major or minor.
• Pontic : It is the suspended member of fixed partial denture that replaces the missing tooth or teeth. It usually occupies the position of the missing natural tooth.
• Connector: It is that part of fixed partial denture that joins the individual component of the bridge together ( the retainer and the pontic).


It could be fixed (rigid) or movable (flexible) connector. When the retainer is attached to a fixed connector, it is ”major retainer”, but when it is attached to a flexible (movable) connector it is called “minor retainer”

Diagnosis

To decide weather the case is indicated for crown and bridge or not, examination of the followings are required:
1/ Periodontal Examination: The patient should have a proper oral hygiene to ensure no plaque accumulation would occur at the crown margins which might lead to carries.

2/ Dental examination:

Visual examination: the occlusion of the patient
Crowding, Spacing, tilting, supra-eruption of the abutment, presence of carries, quality of existing old filling in the abutment.

Radiographic examination:

The radiograph reveals the shape and number of the root, the condition of the surrounding structure bone support of the tooth( crown/root ration), ideal crown root ratio of a tooth to be used as an abutment for a bridge should be 1:2.
Presence of lesion in the bone surrounding the root, fracture in the tooth or root, bone loss, unerrupted teeth, these thing affect the prognosis.

Principles of tooth preparation

Objectives of tooth preparation:
The main objective of tooth preparation
1- To eliminate the undercut from the axial surface of the tooth.





2- To provide enough space for the crown restoration to withstand the force of mastication depends on material used, so the metal material needs little space, while other material need more space .

3- Not to enlarge the size of the tooth.

4- To provide good esthetic.

PRINCIPLES OF TOOTH PREPARATION

Marginal location


1/ supra gingival
2/Equiginigival
3/ subgingival

Margin should be place supra gingival when it is possible.

Sub gingival margin preferable for increase retention
esthetic reason
presence of carries

Factors Influencing Fixed Bridge Design

1. Crown Length
-teeth must have adequate occlusocervical crown length to achieve sufficient retention

2. Crown Form

- some teeth have tapered crown form which interferes with parallelism
- incisors possessing very thin highly translucent incisal edges


3. Degree of Mutilation
- size, number and location of carious lesions or restorations affect whether full or partial coverage retainers are indicated
- fractured or carious teeth not restorable should be removed thereby altering design and creating the need for a prosthesis

4. Root Length and Form

- roots with parallel sides and developmental depressions are better able to resist additional occlusal forces than are smooth-sided conical roots
- multirooted teeth generally provide greater stability than single-rooted teeth
- longer root has better retention than short root

5. Crown-Root Ratio

- 1:1.5 ratio has been generally acceptable whereas 1:1 ratio is considered minimal and requires consideration of other factors (ex. # of tooth being replaced, tooth mobility, periodontal health) before it can be used as an abutment







6. Ante’s Law
-periodontal ligament area/pericemental area of the abutment teeth should be equal or greater than the periodontal ligament area/pericemental area of the missing tooth/teeth


1

7. Periodontal Health

- absence of any form of periodontal disease such as bone resorption and gingival recession

8. Mobility

– MILLER MOBILITY VALUE
1o mobility – normal
2o mobility – still acceptable provided that you must know the factor that cause the mobility (px age, presence of calculas deposit) and consider the # of tooth being replaced
3o mobility – can not be used as an abutment/for extraction

9. Span Length

-distance between abutments affects the feasibility of placing fixed prosthesis
- ideal for 1-2 missing tooth
- loss of 3 adjacent tooth requires careful evaluation of other factors (crown-root ratio, root length and form, periodontal health, mobility)

Primary abutment

Secondary
abutment

10. Axial Alignment

- crowns of proposed abutments must be well aligned
- minor alterations in axial alignment (tipped/rotated) often necessitate the use of full coverage crowns to achieve retention or acceptable esthetics

11. Arch Form

fulcrum line
fulcrum line
lever
lever


counter-balancing

12. Occlusion

- occlusal forces brought to bear on a prostheses are related to the :
a. degree of muscular activity
b. patients habit
c. # of tooth being replaced
d. leverage on the bridge
e. adequacy of bone support

13. Pulpal Health

- abutment/s should not be sensitive to percussion or vitality testing
- abutments with poor pulpal health should undergo endodontic tx prior to tooth preparation

14. Alveolar Ridge Form

- not indicated for FPD if there is considerable bone loss

Vertical bone loss

Horizontal bone loss


15. Age of Patient
- not indicated in older patient as well as adolescents when teeth are not fully erupted or with large pulps

16. Phonetics

- patients prefer FPD for good phonation (provides sufficient resistance to the flow of air to allow normal speech sounds to be produced) rather than RPD

17. Long-Term Abutment Prognosis

- take note of the oral hygiene
-if there is question on the ability of the remaining supporting structure to accept additional occlusal forces, RPD is indicated
- tooth with sufficient loss of periodontal support and questionable prognosis may be best treated with an RPD rather than an FPD

18. Esthetics

-prefer FPD because it resembles natural tooth
-but RPD may be indicated when the use of a pontic produces large and unsightly proximal embrasures in a fixed prostheses.

19. Psychological Factors

- to most pxs an FPD feels more normal than an RPD and more quickly becomes an accepted part of the oral environment
- px feels more confident and looks good wearing FPD than RPD

Tooth Preparations

• The tooth is prepared so the cast restoration can slide into place and be able to withstand the forces of occlusion.
• Rotary instruments are used to reduce the height and contour of the tooth.
• Hand cutting and rotary instruments prepare the gingival margins.
Tooth Preparation

Preparation of full metal crown

Depth orientation grooves (d.o.g)
These are grooves placed on the surfaces of the tooth to act as a reference to determine when a sufficient amount of the tooth structure is removed.


Steps of preparation
1. Occlusal surface preparation

1.5 mm reduction on functional (centric) cusps (buccal for lower and palatal for upper).

1 mm reduction of non-function cusps.
By using tapered fissure bur, placing one end of which in the central groove and the other end of the bur on the tip of the cusp, then the remaining occlusal surface should be reduced to the depth of the groove following the anatomy of the tooth.

2. Buccal surface preparation

Due to the anatomy of the lower posterior teeth, the buccal surface should be reduced into two planes (gingival 2/3 and occlusal 1/3).
For gingival two thirds, the fissure bur should be placed parallel to the long axis of the tooth.
For occlusal third, we should place the fissure bur in remaining occlusal surface at 45º to the long axis of the tooth.
This type of preparation is called two planes or two steps preparation.
The finishing line is chamfer

3. Lingual surface preparation

We should placed (d.o.g) on the center of the lingual surface, and then the remaining part of the tooth is reduced in one step according to anatomy of the tooth. The finishing line is chamfer.

4. Proximal surfaces preparation

The contact area should be removed carefully by using narrow fissure bur to avoid hitting the adjacent teeth, because caries may develop later on this surface.
Finally, all sharp angles should be removed. Complete the preparation with chamfer bur.
The preparation should not be started from the proximal surfaces because this may damage the adjacent teeth and cause loss of correct taper and loss of final occlusogingival crown length.
Taper of the axial walls (buccal, lingual, mesial and distal) = 6º


5. Smoothing and finishing
All line angles should be rounded.
Finishing line should be smooth and continuous.
All undercuts should be eliminated.
Indications for full metal crown
a) Teeth with extensive caries or large amalgam in order to protect the remaining tooth structure from fracture.
b) As retainer for FPD in case of long span for maximum retention.
c) To protect endodontically treated teeth.
d) Recontouring of posterior over erupted teeth for better occlusal relation.
e) For patients with high caries index.

Contraindications

a) Areas where esthetic is important.
b) Less than maximum retention is required.

Disadvantages

a) Removal of large amount of tooth structure.
b) Not esthetic.
c) Vitality test could not be applied.

Full veneer with facing

Indications

1. When a complete crown is to be constructed for smiling areas.
a) Anterior diastema.
b) Peg- shaped laterals.
c) Discolored endodontic treated teeth that have sound tooth structure.
d) Fractured teeth without pulp exposure
2. To correct (restore) the lost occlusal vertical dimension (in case of sever attrition).
3. As a retainer for FPD.

Contraindications

1. Teeth with large pulp chambers.
2. Teeth with short clinical crown.
3. Patients with poor oral hygiene.

Disadvantages

1. More tooth structure is removed (than full metal crown).
2. Fracture of facing material (mechanical retention).
3. Discoloration and wear of facing material (acrylic resin).
4. Shade selection sometimes doesn’t match teeth.
5. Subgingival margin (gingival involvement).

Laws of preparation

Preservation of tooth structure.
Retention and resistance
1. Occlusogingival height.
2. Taper of axial wall.
3. Surface area of preparation.

Margin integrity of preparation: cervical finishing line is

smooth and continuous and readily seen.
Steps of preparation
a) Labial surface

Two planes (steps) preparation for central and lateral incisor.

1st step (gingival 2/3) with axial wall taper of 6º.
Cervical finishing line shoulder (1-2 mm) for metal rein crown.
2nd step (incisal 1/3) 45º in order to:

1. Prevent protrusion of incisal edge that could result in bad esthetics.

2. One plane reduction can cause over tapering of incisal edge and could cause preparation to be too close to pulp.

b) Lingual surface

Two planes preparation
1st plane (lingual axial wall) taper of 6º.
2nd plane (cingulum and lingual concavity) lingual concavity for
canines is done in two concavities one on each side of cervical
lingual height contour (0.7-1 mm only metal).
Cervical finishing line: knife edge or chamfer.
Junction between cingulum and lingual axial wall should not be
over reduced to prevent over shortening the axial wall
(decrease of retention).
c) Proximal surface

Start with flame shaped diamond bur to remove the enamel mesial

and distal without damaging adjacent tooth.
Complete preparation with chamfer bur.
Taper of proximal wall = 6º.
The finishing line should meet


d) Incisal surface

Two mm reduction with palatal inclination 45º for upper central and lateral.

Labial inclination 45º for lower anterior teeth.
Straight reduction for incisors.
For canines (2 planes reduction mesial and distal slops).
Inadequate preparation leads to poor incisal translucency of restoration.
e) Finishing and smoothing

All line angles should be rounded.

All undercuts removed.
Cervical finishing line well smoothed.

GOOD LUCK