Subgingival Margin Preparation

the development of good axial contours. This will enable the junction between the restoration and the tooth to be smooth and free of any ledges or abrupt changes in direction.

Under most circumstances a crown should duplicate the contours and profile of the original tooth (unless the restoration is needed to correct a malformed or malpositioned tooth). If an error is made, a slightly undercontoured flat restoration is better because it is easier to keep free of plaque; however, increasing proximal contour on anterior crowns to maintain the interproximal papilla23 (see Chapter 5) may be beneficial. Sufficient tooth structure must be removed to allow the development of correctly formed axial contours (Fig. 7-14), particularly in the interproximal and furcation areas of posterior teeth, where periodontal disease often begins.

Margin Placement. Whenever possible, the margin of the preparation should be supragingival. Subgingival margins of cemented restorations have been identified as a major factor in periodontal disease, particularly where they encroach on the ep ithelial attachment (see Chapter 5). Supragingival margins are easier to prepare accurately without trauma to the soft tissues. They can usually also be situated on hard enamel, whereas subgingival margins are often on dentin or cementum.

Other advantages of supragingival margins include the following:

1. They can be easily finished.

2. They are more easily kept clean.

3. Impressions are more easily made, with less potential for soft tissue damage.

4. Restorations can be easily evaluated at recall appointments.

However, a subgingival margin (Fig. 7-15) is justified if any of the following pertain:

1. Dental caries, cervical erosion, or restorations extend subgingivally, and a crown-lengthening procedure (see Chapter 6) is not indicated.

2. The proximal contact area extends to the gingival crest.

3. Additional retention is needed.

4. The margin of a metal-ceramic crown is to be hidden behind the labiogingival crest.

Undercontouring Dentistry

Fig. 7-14. A and B, Tooth preparations with adequate axial reduction allow the development of properly contoured embrasures. Tissue is conserved by using partial coverage and supragingival margins where possible. C, Preparing furcation areas adequately is important; otherwise, the restoration will be excessively contoured, making plaque control difficult.

Fig. 7-14. A and B, Tooth preparations with adequate axial reduction allow the development of properly contoured embrasures. Tissue is conserved by using partial coverage and supragingival margins where possible. C, Preparing furcation areas adequately is important; otherwise, the restoration will be excessively contoured, making plaque control difficult.

Proximal Contact Areas
Fig. 7-15. Examples where subgingival margins are indicated. A, To include an existing restoration. B, To extend apical to the proximal contact (adequate proximal clearance). C and D, To hide the metal collar of metal-ceramic crowns.

5. Root sensitivity cannot be controlled by more conservative procedures, such as the application of dentin bonding agents.

6. Modification of the axial contour is indicated.

Margin Adaptation. The junction between a cemented restoration and the tooth is always a potential site for recurrent caries because of dissolution of the luting agent and inherent roughness. The more accurately the restoration is adapted to the tooth, the lesser the chance of recurrent caries or periodontal disease . 3° Although a precise figure for acceptable margin adaptation is not available, a skilled technician can make a casting that fits to within 10 u and a porcelain margin that fits to within 50 um, provided the tooth is properly prepared. A well-designed preparation has a smooth and even margin. Rough, irregular, or "stepped" j unctions greatly increase the length of the margin and substantially reduce the adaptation of the restoration (Fig. 7-16). The importance of preparing smooth margins cannot be overemphasized. Time spent obtaining a smooth margin will make the subsequent steps of tissue displacement, impression making, die formation, waxing, and finishing much easier and will ultimately provide the patient with a longer-lasting restoration.

Margin Geometry. The cross-sectional configuration of the margin has been the subject of much analysis and debate .33 Different shapes have been described and advocated -4142 For evaluation, the following guidelines for margin design should be considered:

1. Ease of preparation without overextension or unsupported enamel

2. Ease of identification in the impression and on the die

3. A distinct boundary to which the wax pattern can be finished

4. Sufficient bulk of material (to enable the wax pattern to be handled without distortion and to give the restoration strength and, when porcelain is used, esthetics)

5. Conservation of tooth structure (provided the other criteria are met)

Proposed margin designs are presented in Table 7-2.

Although they are conservative of tooth structure, featheredge or shoulderless crown preparations (Fig. 7-17, A) should be avoided because they fail to provide adequate bulk at the margins. Over-contoured restorations often result from feath-eredge margins because the technician can handle the wax pattern without distortion only by increas

Subgingival Margins
Fig. 7-16. A and B, Poor preparation design, leading to increased margin length. C, A rough, irregular margin will make the fabrication of an accurately fitted restoration almost impossible. D, An accurately fitting margin is possible only if it is prepared smoothly.
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  • sebastian
    How to make subgingival tooth preparation?
    1 year ago

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