FINISH LINES IN FIXED DENTAL PROSTHESIS

By Dr. Amit Sadhwani

Contents

Introduction Definitions Guidelines for margin preparation Types of finish lines for full veneer

crowns Summary of types of margins Criteria for selection Conclusion References

Introduction Charles Henry Land- Porcelain Jacket

crowns, as early as 1880s. Biologic, Mechanical, Esthetic, and Psychological advantages

of preserving coronal tooth structure were first presented.

Specific details later presented by Spalding and Land in 1904. The shoulder finish line.

Definitions: GPT -5Margin: The outer edge of a crown,

inlay, onlay, or other restoration. The boundary surface of a tooth

preparation and/or restoration is termed the finish line or finish curve.

The terminal portion of the prepared tooth.

Peripheral extension of a tooth preparation.

BIOLOGICAL Considerations

MECHANICAL Considerations

AESTHETIC Considerations

1

• Conservation of tooth structure

2

•Prevention against damage

3

•Harmonious Occlusion

4

• Protection against Tooth fracture

5

•Considerations affecting future dental health

i. Axial Reduction

ii. Margin Placement

iii. Margin Adaptation

iv. Margin Geometry

v. Margin Designs

Principles of tooth preparation

Shillingberg

Marginal Integrity: To prepare a finish line to accommodate a robust margin with close adaptation to minimize micro leakage

Preservation of periodontium: To shape the preparation such that the crown is not over contoured and its margin is accessible for optimal oral hygiene.

Supragingival margin Easier to prepare accurately

without trauma to the soft tissues.

Usually situated on hard enamel.

They can be easily finished. They are more easily kept

clean. Impressions are more easily

made with soft tissue damage. Restorations can be easily

evaluated on recall appointment.

Subgingival margin Dental carries, cervical erosion or restorations

extend subgingivally and the crown lengthening procedure is not included.

The proximal contact area extends to the gingival crest.

Additional retention is needed. The margin of metal ceramic crown is to be

hidden behind the labiogingival crest. It is also used to produce a cervical crown

ferrule on endodontically treated teeth. Root sensitivity cannot be controlled by more

conservative procedures, such as the application of dentin bonding agents.

Modification of the axial contour is indicated.

Guidelines for margin design Ease of preparation without

overextension or unsupported enamel Ease of identification in the impression

and on the die A distinct boundary to which the wax

pattern can be finished Sufficient bulk of material Conservation of tooth structure

Types of finish lines for full veneer crowns

FEATHER-EDGE/ SHOULDERLESS/ KNIFE EDGE

ADVANTAGES DISADVANTAGES

Conservation of tooth structure. Fail to provide adequate bulk at margins (to enable the wax pattern to be handled without distortion and to give the restoration strength and, when porcelain is used, esthetics).

This margin is used for full veneer metal crowns , small crowns and already designed margins by previous dentist.

Over contoured restorations.

BEVEL ADVANTAGES DISADVANTAGES

A beveled margin, under certain circumstances, is more suitable for cast restoration particularly if a ledge or shoulder already exists.

When access for burnishing is limited, there is little advantage in beveling.

The objective in beveling is three fold:•To allow the cast metal margin to be bent or burnished against the prepared tooth structure.•To minimize the marginal discrepancy caused by a complete crown that fails to seat completely.•To protect the unprepared tooth structure from chipping (e.g. by removing unsupported tooth enamel).

In cases of gingival margins beveling would lead to sub-gingival extension of the preparation or placement of the margin on dentin rather than on enamel.

CHAMFER

An obtuse angle between the axial wall of the tooth surface and the prepared margin.

ADVANTAGES DISADVANTAGES

Particularly suitable for cast metal crowns and the metal-only portion of metal-ceramic crowns.

Tilting it away from the tooth will create an undercut; angling it towards the tooth will lead to over-reduction and loss of retention.

Distinct and easily identified, provides room for adequate bulk of material.

Marginal accuracy depends upon having high quality diamond and a true running hand-piece.

Can be placed with precision, although care is needed to avoid leaving a ledge of unsupported enamel.

Chamfer should never be prepared wider than half the tip of the diamond an unsupported lip of enamel can result.

SHOULDER / BUTT JOINT

ADVANTAGES DISADVANTAGES

Allows room for porcelain recommended for facial part of the metal ceramic crowns.

An acute angle is likely to chip. Placement of margin deep into the gingival sulcus.

It is less conservative of tooth structure.

A 90-degree angle with the unprepared tooth surface.

SHOULDER WITH BEVEL

ADVANTAGES DISADVANTAGES

Recommended for facial surface of a metal-ceramic restoration where a metal collar (as opposed to a porcelain labial margin) is used.

Less preferred to shoulder or sloped- shoulder for biologic and esthetic reasons.

Removes unsupported enamel and allows some finishing of the metal.

Allows improved esthetics because metal margin can be trimmed down a knife-edge and hidden in the sulcus without the need for positioning the margin closer to the epithelial attachment.

SLOPED SHOULDER

A 120-degree sloped shoulder margin is used as an alternative to the 90-degree shoulderfor the facial margin of a metal-ceramic crown.

The sloped shoulder reduces the possibility of leaving unsupported enamel and yet leaves sufficient bulk to allow thinning of the metal framework to a knife-edge for acceptable esthetics

Placement of margins with respect to gingival crevice

Summary of margins

The selected margin should provide a predictable level of marginal integrity.

To minimize plaque accumulation, the selected margin should present smooth materials to the gingival sulcus.

Margin should also must provide acceptable esthetics.

CRITERIA FOR MARGIN SELECTION

ShoulderShoulder bevelSlope shoulder

ShoulderShoulder bevel

Shoulder design with a porcelain labial margin.