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Featheredge

Conservative of tooth structure

Does not provide sufficient bulk

Not recommended

Chisel edge

Conservative of tooth structure

Location of margin difficult to control

Occasionally on tilted teeth

Bevel

Removes unsupported enamel, allows finishing of metal

Extends preparation into sulcus if used on apical margin

Facial margin of maxillary partial-coverage restorations and inlay/onlay margins

Chamfer

Distinct margin, adequate bulk, easier to control

Care needed to avoid unsupported lip of enamel

Cast metal restorations, lingual margin of metal-ceramic crowns

Shoulder

Bulk of restorative material

Less conservative of tooth structure

Facial margin of metal-ceramic crowns, complete ceramic crowns

Sloped shoulder

Bulk of material, advantages of bevel

Less conservative of tooth structure

Facial margins of metal-ceramic crowns

Shoulder with bevel

Bulk of material, advantages of bevel

Less conservative, extends preparation apically

Facial margin of posterior metal-ceramic crowns with supragingi-val margins

ing its bulk beyond the original contours. A variation of the featheredge, the chisel edge margin (Fig. 7-17, B), is formed when there is a larger angle between the axial surfaces and the unprepared tooth structure. Unfortunately, this margin is frequently associated with an excessively tapered preparation or one in which the axial reduction is not correctly aligned with the long axis of the tooth.

Under most circumstances, featheredges and chisel edges are unacceptable. Historically their main advantage was that they facilitated the making of impressions with rigid modeling compound in copper bands (a technique rarely used today), because there was no ledge on which a band could catch. A chamfer margin (Fig. 7-17, C ) is particularly suitable for cast metal crowns and the

Chisel Edge Tooth Preparation
Fig. 7-17. Margin designs: A, Featheredge. B, Chisel. C, Chamfer. D, Bevel. E, Shoulder. F, Sloped shoulder. G, Beveled shoulder. Scanning electron micrographs. H, Feather-chisel edge. I, Chamfer. J, Bevel. K, Shoulder. L, Sloped shoulder. M, Beveled shoulder. (Courtesy Dr. H. Lin.)

metal-only portion of metal-ceramic crowns (Fig. 7-18). It is distinct and easily identified, provides room for adequate bulk of material, and can be placed with precision, although care is needed to avoid leaving a ledge of unsupported enamel.

Probably the most suitable instrument for making a chamfer margin is the tapered diamond with a rounded tip; the margin formed is the exact image of the instrument (Fig. 7-19). Marginal accuracy depends on having a high-quality diamond and a true-running handpiece. The gingival margin is prepared with the diamond held precisely in the in tended path of withdrawal of the restoration (Fig. 7-20). Tilting it away from the tooth will create an undercut, whereas angling it toward the tooth will lead to overreduction and loss of retention. The chamfer should never be prepared wider than half the tip of the diamond; otherwise, an unsupported lip of enamel could result (Fig. 7-21). Some authorities have recommended the use of a diamond with a noncutting guide tip to aid accurate chamfer placement.13 However, the guide has been shown to damage tooth structure beyond the intended preparation margin . 44

Chamfer Preparation Dental Demussen

Fig. 7-18. Chamfer margins are recommended for cast metal crowns (A) and the lingual margin of a metal-ceramic crown (B). Compare the scanning electron micrographs of a chamfer (C) achieved with a fine-grit diamond after initial preparation with a coarser instrument (D) and a chamfer achieved with finishing carbides (E and F). (C to F courtesy Dr. H. Lin.)

Fig. 7-18. Chamfer margins are recommended for cast metal crowns (A) and the lingual margin of a metal-ceramic crown (B). Compare the scanning electron micrographs of a chamfer (C) achieved with a fine-grit diamond after initial preparation with a coarser instrument (D) and a chamfer achieved with finishing carbides (E and F). (C to F courtesy Dr. H. Lin.)

Under some circumstances a beveled margin (Fig. 7-17, D) is more suitable for cast restorations, particularly if a ledge or shoulder already exists, possibly from dental caries, cervical erosion, or a previous restoration. The objective in beveling is threefold: (1) to allow the cast metal margin to be bent or burnished against the prepared tooth structure; (2) to minimize the marginal discrepancy33 caused by a complete crown that fails to seat completely (however, Pascoe has shown that when an oversized crown is considered, the discrepancy is increased rather than decreased [Fig. 7-22]); and (3) to protect the unprepared tooth structure from chipping (e.g., by removing unsupported enamel). NOTE: When access for burnishing is limited, there is little advantage in beveling. This applies particularly to a

Crown Margin Chamfer
Fig. 7-19. A chamfer margin is formed as the negative image of a round-ended tapered diamond.
Sloped Shoulder Margin
Fig. 7-20. Precise control of the orientation of the diamond is very important. A, Tilting away from the tooth creates an undercut. B, Tilting toward the tooth results in excessive convergence.
Al! unsupported enamel must be removed.

Fig. 7-21. A chamfer should not be wider than half the bur used to form it. Otherwise, a lip of unsupported enamel will be left.

gingival margin, where beveling would lead to subgingival extension of the preparation or placement of the margin on dentin rather than on enamel. Facial margins of maxillary partial-coverage restorations should be beveled to protect the remaining tooth structure and to allow for burnishing.

Because a shoulder margin (Fig. 7-17, E) allows room for porcelain, it is recommended for the facial part of metal-ceramic crowns, especially when the porcelain margin technique is used. It should form a 90-degree angle with the unprepared tooth surface. An acute angle is likely to chip (Fig. 7-23, A). In practice, dentists tend to underprepare the facial shoulder, leading to restorations with inferior esthetics or poor axial contour.

Some authorities46 have recommended a heavy chamfer rather than a shoulder margin, and some

Chamfer Bevel Paration

Shoulder

4 S" Bevel

Shoulder

4 S" Bevel

Properly seated castings should have minimal marginal gap widths.

Fig. 7-22. Effect on marginal fit of beveling the gingival margin. A, If the internal cross section of a crown is the same as or less than that of the prepared tooth, a 45-degree bevel will decrease the marginal discrepancy by 70%. B, If the internal diameter is slightly larger than the prepared tooth, beveling will increase the marginal discrepancy. In practice, crowns are made slightly larger than the prepared tooth to allow for the luting agent.

find a chamfer easier to prepare with precision. Earlier work36 37 found less distortion of the metal framework during porcelain application, although with modern alloys, this doesn't appear to be a problem (see Chapter 19).

Heavy Chamfer

Fig. 7-23. A, A shoulder provides more bulk of metal than a heavy chamfer, which may facilitate the laboratory steps. B, A disadvantage of the shoulder bevel is that its margin must be placed deeper in the gingival sulcus so that the wider band of metal will be hidden (compare d with D). c, Scanning electron micrograph of a shoulder margin prepared with a high-speed diamond. D, This margin has been refined with a sharp chisel. E, This has been beveled with a tungsten carbide bur. F, This bevel was placed with a sharp hand instrument.

(Microscopy by Dr. J. Sandrik, teeth prepared by Dr. G. Byrne.)

Fig. 7-23. A, A shoulder provides more bulk of metal than a heavy chamfer, which may facilitate the laboratory steps. B, A disadvantage of the shoulder bevel is that its margin must be placed deeper in the gingival sulcus so that the wider band of metal will be hidden (compare d with D). c, Scanning electron micrograph of a shoulder margin prepared with a high-speed diamond. D, This margin has been refined with a sharp chisel. E, This has been beveled with a tungsten carbide bur. F, This bevel was placed with a sharp hand instrument.

(Microscopy by Dr. J. Sandrik, teeth prepared by Dr. G. Byrne.)

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  • Carita
    What is the difference Chamfer or shoulder margin?
    8 years ago

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