Lect.1. Crown

Lecture 1. Crown and Bridge د. امير حمدي العميدي

HISTORY:
The earliest record of crown and Bridge work is from the Etruscans over 2500 years ago. After that, crowns disappear until the1700S.
During that time period, the work done was a bit crude due to the absence of good materials. Typical materials used were from animal or human teeth and held in place by metal pivots.
In 1746, a gold crown and post to be retained in the root canal, also recommends white enameling for gold crowns for a more esthetic appearance.
In The 1800s, changes in crown restoration Porcelain began being used and new machine was invented, the allowed for faster and more accurate tooth preparation.
Improvement during the 1900s was largely due to the advancement of metal casting technology and the creation of an adjustable detached post crown.
In 1903, porcelain jacket crown. In 1990s, new tooth-colored restorative materials were devises such as veneers, for esthetic dentistry.
Today crown and bridge are most comfortable and the crowns can be created in shade similar to natural vital teeth.


1-CROWN:
1- Is a restoration which encompasses coronal tooth tissue.
2- Covering remaining tooth substance and restorations or dental implant.
3- Dental crowns may be used to anchor a dental bridge and is typically bonded to the tooth using dental cement.
4- Crowns can be made from many materials of metal, or porcelain to restore strength and eliminate discomfort.
5- When insufficient tooth tissue remains, the root canal can be used to aid retention - a post crown.

TYPES OF CROWNS:

Full coverage:
(Full veneer crowns, porcelain jacket crowns, metal-ceramic crowns) This provides better retention and resistances because all the axial surface of the teeth are included in the preparation.

Post crowns:
(Cast gold core and prefabricated core) It is a fixed restoration which replace the coronal part of the tooth completely and are retained by means of post extended and cemented to the root canal the post crown will reinforce the remaining tooth structure against forces by distributing the forces to the surrounding tooth structure.

Partial coverage:
(Three-quarter crowns and reverse three- quarter crowns) useful to preserve a single intact cusp.



ANTERIOR CROWNS:

Indications:
Protection of heavily restored teeth, aesthetics, bridge retainer, toothwwear,iimplantwretainedecrown.

TYPES OF ANTERIOR CROWNS:

Metal-ceramic crown:
Used when limited occlusal space and high functional loads. Relies on ability of porcelain to bond to metal oxide. Modem metal-ceramic crowns have excellent aesthetic. Can have metal (when very limited occlusal space)or porcelain palatal surface. Often have butt joint labially (1.5 mm) shoulder to allow adequate metal and porcelain for aesthetics) and chamfer margin palatally.




Porcelain jacket Crown:
Used when aesthetics of prime Problem in high-load situation as porcelain in thin section and liable to fracture Not usually suitable for posterior teeth. Usually butt joint around whole preparation (minimum 1 mm shoulder to allow adequate porcelain for aesthetics). Need 1.5 mm thickness of porcelain incisally.

Other anterior Crowns:
Porcelain crowns with superior aesthetics and with higher tensile strength than conventional porcelain jacket cores or injection moulding of ceramic. Require even reduction; preparation similar to a conventional aluminous, also nickel chrome cobalt alloy crown fused to acrylic resin to impart esthetic result, this are also generally cheaper compared to ceramic crown


POSTERIOR CROWNS:

Indication:
Aesthetics (some posterior teeth only), bridge retainer, tooth wear, protection of heavily restorediteeth,ipartialidentureiabutments.

TYPES OF POSTERIOR CROWNS;

Metal-ceramic crown:
Used when insufficient occlusal space, high functional loads, or aesthetics important. Metal (when limited occlusal space) or porcelain occlusal surface. Junction of metal and porcelain should not be in area of high occlusal stress. Can have metal or porcelain (superior aesthetics) labial margin. Often have butt joint labially (1.5 mm shoulder to allow adequate metal and porcelain for aesthetics) and chamfer margin palatally or lingually. Functional cusps (in Class I occlusion upper palatal cusps and lower buccal cusps) need further tooth reduction by means of a functional cusp bevel.




Full-veneer crown:
Used when aesthetics of minimal concern (usually second or third molars). Usually made of cast gold. Tooth preparation should be as conservative as possible with the following features: buccolingually and approximally, a 5° taper is ideal; chamfer margin removing all undercut areas. Should finish supra-gingivally not always possible as preparation should extend more gingivally than existing restorations so that preparation finishes on sound dentine, require minimum of 1 mm reduction occlusally to allow for gold to cover preparation. Functional cusp bevel is needed to allow more occlusal clearance (1.5 mm) over functional cusps.

Post retained crowns:
When there is insufficient coronal dentine to withstand occlusal forces or retain a crown. Root dentine is used and loads transmitted via a post to the root dentine. The post retains the crown. Usually root filled teeth (but not every root filled tooth requires a post crown).




Other posterior crowns:
Cast gold partial-veneer crowns such as three-quarter crowns are occasionally useful to preserve a single intact cusp (usually mesio-buccal cusp of upper first molar). Porcelain crowns using sintered alumina cores or injection moulding of ceramic are finding increasing use in posterior crown situations.

ASSESSMENT OF TOOTH FOR CROWNS:

Case selection is important. In order to plan treatment approprialy, when considering crowns, assess:
tooth vitality, periodontal support and gingival condition, oral hygiene, caries control, occlusion radiographic appearance, aesthetics (including patient’s expectations), adjacent teeth. In some cases study casts, clinical photographs and a diagnostic wax-up of anticipated appearance may beeuseful.

CLINICAL STAGES IN MAKING CROWNS

1-Preparation: Crown preparation involves removal of enough tooth substance allowing sufficient thickness of material (from which the crown is to be made) to provide strength and aesthetics.
2- Preparation must not damage the pulp. Preparation must provide sufficient retention for the crown. This can be achieved by taper of 5-20o (especially in cervical third of preparation), and inclusion of retention grooves or slots is useful in teeth of reduced occluso-gingival height. Preparation should involve minimal gingival trauma. Preparation should have smooth curves, not right angles or sharp edges. Finishing lines depend on the material from which the crown is to be made.

Options for finishing lines:
Butt joint: e.g. Porcelain jacket crown.
Chamfer: e.g. palatal margin metal ceramic crown.
Taper: e.g. full veneer gold crown.
Preparation is usually achieved by a selection of high-speed diamond burs.




2-BRIDGE:
A fixed bridge (fixed partial denture) is a dental restoration that spans an area that has no teeth, and is connected to natural teeth at each end, it replace one or more teeth and cannot be removed by the patient, the components of bridge are retainer, soldered joint and pontic.



Indication:
When the abutment tooth can carry the occlusal load of the artificial tooth and when the occlusion is protected against potentially damaging rotational forces.

Some types:
Spring cantilever bridge:
A prosthesis where the artificial tooth is supported by a connecting bar to the abutment tooth or teeth.






precision-attachment partial denture: is retained by proprietary attachments and is removable by the patient.


Preparation for fixed bridge abutments:
Modification of classic crown preparation designs are often required to increase retention and resistance to make provision for accessible margins adjacent to the ponitic connectors and to create space for precision attachment.
Additional grooves should be placed in bridge retainer preparation to help resist dislodgment, the most efficient location for groove depend on the direction of anticipated torque, posterior bridge is subjected to torque around a facio-ligual axis as the ponitic span flexes under a load loosening a crown by this type of torque can be prevented by addition of facial and lingual grooves.
The load on the pontic that lies facial to the inter abutment axis line produces torque primarily around that axis .the most effective location for supplemental grooves in this situation is on the mesial and distal surfaces.
Adding box in the abutment tooth to accommodate non-ridged connector .the box most be large enough to the bulk of the female portion of the connector to be contained within the normal contour of the completed crown.








Preparation for Removable Bridge Abutments:
The cingulum rest seat on the anterior crown is recommended by grooves and off seat. Occlusal rest 1.4 mm thick is occupying the middle third of posterior teeth facio-lingualy.


The occlusal countersink for a rest seat. As shown alone for a full veneer crown preparation (A). Shown tied in with the wing for porcelain fused to metal crown (B). Shown in conjunction with a proximal box on a three quarter crown preparation (C).
The most common method of crowning a tooth involves:
1-Using a dental impression of a prepared tooth by a dentist to fabricate the crown outside of the mouth, which is usually fabricated using indirect methods.
2-As new technology and materials science has evolved, computers are increasingly becoming a part of crown and bridge fabrication, such as in CAD/CAM Dentistry.

COMMON FAULTS WITH CROWNS:

Despite careful attention to detail the following faults with crowns occuricommonly:

Overhanging margin:
Usually due to poor impression or poor technical work. can in some cases be corrected by trimming with a bur, but often requires a remake. If uncorrected leads to plaque accumulation, gingival inflammation or recurrent caries.

Negative margin:
Usually due to poor finishing line delineation, over-trimming of die or over-vigorous polishing of crown margins. Often patient feels sensitivity, risks recurrent caries or poor aesthetics.

Poor gingival emergence angle:
Usually due to lack of communication between dentist and technician. Over bulking of material at the gingival margin leads to plaque accumulation.

Poor contact point:
Usually due to under-preparation of mesial and distal walls or over bulking of interdental area by technician. Hinders interdental cleaning.

Poor aesthetics:
Can be due to incorrect shade, shape or under-preparation leading to insufficient space for material.iOccasionallyipatientsihaveiunrealisticiexpectations.

Persistent debonding:
Often due to inadequate retention on preparation. May be due to occlusal interference. In post crowns may be due to poor post design or longitudinal root fracture.