Seven-eighths Crown Preparation

After verifying the alignment, remove tooth structure between the guide grooves (with a smooth continuous motion) and place a cervical chamfer (Fig. 10-7). Carry the diamond into the proximal embrasure and reduce the proximal wall (Fig. 10-3, E and F). For proper reduction of the axial tooth surface, it is important to understand the factors that determine correct positioning of the proximal groove. A proximal groove is placed parallel to the path of withdrawal. Normally, unsupported tooth structure will remain on the buccal side of the groove, and this side is flared to remove it. Figure 10-8 illustrates the relationship among the initial axial reduction, groove placement, and location of the cavosurface angle where the flare meets the intact buccal wall. The cavosurface angle is especially significant when preparing a tooth for a partial veneer that should display a minimum of metal; the further to the buccal the margin is, the more gold will be visible. A subtle but

Seven Eighths Crown Dental
Fig. 10-7. Proximal and lingual axial reduction is performed with a round-tipped diamond. The proximal reduction is stopped short of the proposed location of the buccal margin.
Groove And Pinledge Retainer

Fig. 10-8. A, Upon completion of the proximal axial reduction, a groove is placed perpendicular to the prepared surface. B, Note that some unsupported tooth structure remains at the cavosurface angle. C, After the buccal wall of the proximal groove is flared, no unsupported tooth structure remains. NOTE: It is important to anticipate in ad vance the influence of the buccal extent of the proximoaxial reduction (A) on the ultimate location of the margin.

Fig. 10-8. A, Upon completion of the proximal axial reduction, a groove is placed perpendicular to the prepared surface. B, Note that some unsupported tooth structure remains at the cavosurface angle. C, After the buccal wall of the proximal groove is flared, no unsupported tooth structure remains. NOTE: It is important to anticipate in ad vance the influence of the buccal extent of the proximoaxial reduction (A) on the ultimate location of the margin.

extremely important variable that determines the final location of the cavosurface angle is the apical extension of the preparation. As the cervical chamfer extends closer to the cementoenamel junction, more axial tooth structure is removed. Consequently, the deepest portion of the groove (its pulpal wall) will be located slightly closer to the center of the tooth. This results in a flare that can extend farther onto the facial or buccal surface than desirable. Marking the location of the intended facial flare on the tooth with a pencil before initiating the proximoaxial reduction is helpful. The intersection of this mark with the reduced occlusal surface is a convenient reference point.

10. Stop the proximal reduction well short of the pencil mark and usually slightly short of breaking the proximal contact (Fig. 10-9). The resulting flange should be parallel to the linguoaxial preparation, with the chamfer placed sufficiently cervical to provide at least 0.6 mm of clearance with the adjacent tooth and the axial wall allowing for a prox-i mal groove of at least 4 mm of length oc-clusocervically (see Fig. 10-3, F).

Groove Placement. Preparation of the proximal grooves is best done with a tapered carbide bur.

11. Position the bur against the interproximal flange parallel to the path of withdrawal and make a groove perpendicular to the axial surface. The groove need not be deeper than 1 mm at its cervical end but may be deeper near its occlusal end (Fig. 10-10). During this stage, the bur must be held precisely parallel to the selected path of withdrawal. Allowing it to tip axially will result in excessive taper between opposing proximal grooves, which is a common error. The

Occlusal Buccal Distal
Fig. 10-9. The distal proximal reduction is stopped before breaking proximal contact. After groove placement and subsequent flaring, interproximal clearance will result.

criteria that need to be met consist of the following (see Figs. 10-9 and 10-11): The grooves should resist lingual displacement of a periodontal probe or explorer. The walls of the grooves should not be undercut relative to the selected path of withdrawal. The walls should be flared toward the intact buccal surface of the tooth (see Fig. 10-3, G and H).

Seven Eighths Crowns Teeth

Fig. 10-10. Because of the rotary instrument's taper (A), the proximal groove is deeper near the occlusal table. The floor of the groove should be flat and smooth. Often the proximal chamfer will extend slightly cervically to the floor of the groove. If only minimal difference exists, as in B, the cervical margin adjacent to the groove can be beveled. The recommended occlusocervical height for a proximal groove is 4 mm.

Fig. 10-10. Because of the rotary instrument's taper (A), the proximal groove is deeper near the occlusal table. The floor of the groove should be flat and smooth. Often the proximal chamfer will extend slightly cervically to the floor of the groove. If only minimal difference exists, as in B, the cervical margin adjacent to the groove can be beveled. The recommended occlusocervical height for a proximal groove is 4 mm.

Fig. 10-1 1. The 90-degree angle between the lingual walls of the proximal grooves will resist lingual displacement. Because the buccal aspect of the grooves has been adequately flared, no unsupported tooth structure remains.

Fig. 10-1 1. The 90-degree angle between the lingual walls of the proximal grooves will resist lingual displacement. Because the buccal aspect of the grooves has been adequately flared, no unsupported tooth structure remains.

Depending on available access, it may be feasible to complete the flaring with the same rotary instrument that was used to place the groove (Fig. 10-12). However, removing the last lip of unsupported tooth structure with a chisel is often a better option, because this minimizes the risk of damage to the adjacent tooth.

Buccocclusal Contrabevel

12. Connect the mesial and distal flares with a narrow contrabevel that follows the buccal cusp ridges. This can be placed with a dia mond, a carbide, or even a hand instrument. Its primary purpose is to remove any unsupported enamel and thereby protect the buccal cusp tip from chipping during function. If group function is planned (as opposed to a mutually protected occlusion), a heavier bevel, chamfer, or occlusal offset will be needed, because tooth contact occurs in this area during excursive movement. The bevel should remain within the curvature of the cusp tip rather than extend onto the buccal wall (Fig. 10-13). This will result in a convex shape of the restoration, and light will be prevented from reflecting back to a casual observer (see Fig. 10-6). Thus the restoration will be less obvious, and the outline form of remaining buccal enamel will be perceived as the shape of the tooth.

Occlusal Offset. If additional bulk is needed to ensure rigidity of the restoration, it can be provided with an occlusal offset. This V-shaped groove extends from the proximal grooves along the buccal cusp. It is not usually necessary for posterior partial veneer crowns but is essential for the structural durability of anterior partial veneer crowns. This is described in detail on p. 243.

Tooth Preparation Instruments

Fig. 10-12. A, Initial preparation of the mesial proximal groove. Note that the carbide is oriented parallel to the path of withdrawal as dictated by the lingual surface of the tooth. B, Initial flaring has resulted in elimination of most unsupported tooth structure. C, Hand or rotary instruments will be used to refine these proximal flares and remove all unsupported enamel.

Fig. 10-12. A, Initial preparation of the mesial proximal groove. Note that the carbide is oriented parallel to the path of withdrawal as dictated by the lingual surface of the tooth. B, Initial flaring has resulted in elimination of most unsupported tooth structure. C, Hand or rotary instruments will be used to refine these proximal flares and remove all unsupported enamel.

Contrabevel
Fig. 10-13. The buccocclusal contrabevel remains within the curvature of the cusp tip rather than extending onto the buccal surface.
Seven Eighths Crown
Fig. 10-14. A fine-grit diamond in a low-speed contraangle is used to place the buccocclusal contrabevel connecting the mesioproximal and distoproximal flares.

Finishing

13. Round all sharp internal line angles to facilitate subsequent procedures. A fine-grit diamond or carbide can be used to blend the surfaces (Fig. 10-14).

14. Reevaluate the flares, paying particular attention to any remaining undercuts, which must be removed. The flares should be straight and smooth, with sufficient clear-

Groove And Pinledge Retainer
Fig. 10-15. Three-quarter crown preparation on a maxillary molar. Note that the occlusal reduction follows normal anatomic form.
Three Quarter Crown Preparation Dent
Fig. 10-16. The three-quarter crown preparation on a maxillary first molar.

ance between them and the adjacent tooth. A minimum clearance of 0.6 mm is recommended. The mesial flare cannot extend beyond the transitional line angle. However, because the distal margin is less visible, it may extend slightly farther to the buccal, allowing better access for oral hygiene.

Maxillary Molar Three-quarter Crown (Fig. 10-15 and Fig. 10-16). The principles used in a premolar preparation also apply for a maxillary molar. However, some additional leeway may exist for groove placement because more tooth structure is present on molars than on premolars. Also, because of their less prominent position in the dental arch, molars are less visible. As a result, the mesioproxi-mal flare can sometimes be extended onto the buccal surface without incurring esthetic liability.

Maxillary Molar Seven-eighths Crown (Fig. 10-17). The seven-eighths crown preparation includes, in addition to the surfaces covered by the three-quarter crown, the distal half of the buccal surface. Therefore the mesial aspect of this preparation resembles that for a three-quarter crown; the

Seven Eighths Crown

Fig. 10-17. The maxillary molar seven-eighths crown preparation. A, Occlusal depth grooves. On the lingual of the mesiobuccal cusp, they are identical to grooves for any centric cusp. On the buccal, note their difference from grooves placed on the triangular ridges. The mesial groove becomes shallower as it approaches the cuspal ridge; the distal extends through the cuspal ridge. B, Mesial half of the occlusal reduction is completed. Normal occlusal form can be recognized in the reduced area. C, Occlusal reduction completed. D, Distal half of the axial reduction completed. This is comparable to the preparation for a complete cast crown. The rotary instrument is moved parallel to the guiding grooves placed in the lingual tooth surface. E, Mesial half of the axial reduction completed and a proximal groove placed. F, The buccal groove, with flaring of the mesial groove. Note the monoplane of the flare, extending from the deepest portion of the groove to the cavosurface angle. G, A contrabevel connects the mesial flare with the buccal groove. The mesial wall of the buccal groove is smooth and has a 90-degree cavosurface angle, leaving no unsupported enamel.

Fig. 10-17. The maxillary molar seven-eighths crown preparation. A, Occlusal depth grooves. On the lingual of the mesiobuccal cusp, they are identical to grooves for any centric cusp. On the buccal, note their difference from grooves placed on the triangular ridges. The mesial groove becomes shallower as it approaches the cuspal ridge; the distal extends through the cuspal ridge. B, Mesial half of the occlusal reduction is completed. Normal occlusal form can be recognized in the reduced area. C, Occlusal reduction completed. D, Distal half of the axial reduction completed. This is comparable to the preparation for a complete cast crown. The rotary instrument is moved parallel to the guiding grooves placed in the lingual tooth surface. E, Mesial half of the axial reduction completed and a proximal groove placed. F, The buccal groove, with flaring of the mesial groove. Note the monoplane of the flare, extending from the deepest portion of the groove to the cavosurface angle. G, A contrabevel connects the mesial flare with the buccal groove. The mesial wall of the buccal groove is smooth and has a 90-degree cavosurface angle, leaving no unsupported enamel.

distal aspect resembles that for a complete crown. The mesial half of the buccal tooth surface remains intact and is protected by a narrow contrabevel or chamfer similar to the one used in the three-quarter crown preparation. A distal groove may be placed, although generally this is not necessary. A groove in the middle of the buccal surface is placed parallel to the path of withdrawal. Distal to this groove the buccal surface is reduced in two planes, cervical and occlusal, with the cervical paralleling the path of withdrawal and the occlusal following the normal anatomic contour. The lingual surface of the tooth also is reduced in two planes, and centric cusp bevels are incorporated.

Occlusal Reduction. Upon completion of the occlusal reduction, adequate clearance should exist in all excursive movements of the mandible. Minimum measurements are the same as for the three-quarter crown preparation.

1. Place depth grooves in the central and developmental grooves as well as on the crests of the triangular ridges. To delineate the extent of the lingual centric cusp bevel, they should extend onto the lingual surface of the tooth. On the lingual incline of the mesiobuccal cusp they will resemble depth cuts for the three-quarter crown preparation. On the distobuccal cusp they should be approximately 0.8 mm deep to provide sufficient occlusal clearance for this noncentric cusp (see Fig. 10-17, A).

2. Remove the tooth structure between the depth grooves. Concave shaping of the resulting mesiobuccal incline may again prove useful because it will permit the occlusocervical height of the cusp to be maintained. When completed, this bevel should provide 1.5 mm of clearance in the intercuspal position as well as throughout all excursive movements of the mandible (see Fig. 10-17, B and C).

Axial Reduction. In principle, the steps for axial reduction follow those for occlusal reduction.

3. Place three alignment grooves in the lingual wall and transfer the selected path of withdrawal to the distobuccal transitional line angle area, where a fourth alignment groove can be placed.

4. Start the reduction in the middle of the lingual surface. The mesial half is prepared like a three-quarter crown and the distal half like a complete crown (see Fig. 10-17, D).

5. Carry the facial reduction sufficiently mesial to include the buccal groove. Although the occlusal half of the buccal surface of maxil lary molars is rather flat, some additional reduction may be necessary in the occlusal third. This follows the normal anatomic configuration of the tooth and often resembles a small version of the centric cusp bevel. If correctly performed, the reduction will allow for contouring of the restoration so that when viewed from the mesial, the distal half of the restoration is hidden behind the mesiobuccal cusp. A frequent error is to overtaper the

Seven Eighths Crown
Fig. 10-18. The seven-eighths crown preparation. Note that adequate clearance has been established. From this perspective it is evident why little or no flaring is necessary for the buccal groove as opposed to the considerable flaring needed for the mesial groove.

buccal wall segment, with resulting loss of retention.

Groove Placement, Flaring, and Contrabevel

6. Prepare the mesial groove like the three-quarter crown (see Fig. 10-17, E and F).

7. Place the buccal groove parallel to the mesial groove and perpendicular to the buccoaxial wall. Often it is not necessary to flare the buc cal groove because the flat configuration of this area of the tooth precludes any unsupported enamel after the groove is placed. The buccal groove should resist mesiodistal displacement of a probe.

8. Connect the two grooves with a smooth con-trabevel that follows the ridge of the mesiobuccal cusp (see Fig. 10-17, G). This bevel should meet the same criteria as described in the three-quarter crown preparation. Adequate clearance must be established interproximally upon completion (Fig. 10-18). All surfaces are finished to the same specifications as the preceding preparations (Fig. 10-19).

Mandibular Premolar Modified Three-quarter Crown (Fig. 10-20). Mandibular partial veneer preparations are made more often on premolars than on molars. They differ from maxillary molar three-quarter crown preparations in two respects:

Additional retention is required because of the shorter crown lengths of mandibular teeth. This can be obtained by extending the preparation buccally, although because of their rather prominent position in the dental arch, these teeth should be modified only distal to their height of contour (Fig. 10-21).

The axial surface that is not prepared (the buccal) includes the functional cusp. This means that additional tooth structure must be removed to provide sufficient bulk of metal for strength.

Three Quarter Tooth Restoration

Fig. 10-2°. The mandibular premolar modified three-quarter crown preparation. A, Depth holes placed in the mesial and distal fossae approximately 0.8 mm deep. B, The holes are connected by a guiding groove that extends through the central groove and the mesial and distal marginal ridges. Guiding grooves are also placed in the buccal and lingual triangular ridges, extending through the cuspal ridges on both sides. C, Half the occlusal reduction is completed. D, Occlusal reduction and mesial half of the axial reduction are completed. E, Axial reduction is completed. The proximal grooves have been placed. Note that the distal groove is located close to the buccolingual center of the tooth. This permits retention of considerable tooth structure in the area of the distobuccal line angle, enhancing the resistance form of the preparation. F, The mesial groove has been flared and the centric cusp chamfer placed. G, Facial view. There is considerable width of the chamfer on the centric cusp. Note that the distobuccal cervical margin angles occlusally as it progresses mesially. This permits a more conservative tooth preparation in the area of the distobuccal modification that is placed to improve resistance form.

Fig. 10-2°. The mandibular premolar modified three-quarter crown preparation. A, Depth holes placed in the mesial and distal fossae approximately 0.8 mm deep. B, The holes are connected by a guiding groove that extends through the central groove and the mesial and distal marginal ridges. Guiding grooves are also placed in the buccal and lingual triangular ridges, extending through the cuspal ridges on both sides. C, Half the occlusal reduction is completed. D, Occlusal reduction and mesial half of the axial reduction are completed. E, Axial reduction is completed. The proximal grooves have been placed. Note that the distal groove is located close to the buccolingual center of the tooth. This permits retention of considerable tooth structure in the area of the distobuccal line angle, enhancing the resistance form of the preparation. F, The mesial groove has been flared and the centric cusp chamfer placed. G, Facial view. There is considerable width of the chamfer on the centric cusp. Note that the distobuccal cervical margin angles occlusally as it progresses mesially. This permits a more conservative tooth preparation in the area of the distobuccal modification that is placed to improve resistance form.

Occlusal Buccal Distal

Fig. 10-21. Modified three-quarter crown restoring a mandibular second premolar. It is serving as the anterior retainer for a three-unit FPD. Because the distobuccal modification remains in the distal fourth of the buccal preparation, it is hidden behind the normal height of contour of the buccal tooth surface. Note the considerable thickness of gold that protects the buccal cusp.

Fig. 10-21. Modified three-quarter crown restoring a mandibular second premolar. It is serving as the anterior retainer for a three-unit FPD. Because the distobuccal modification remains in the distal fourth of the buccal preparation, it is hidden behind the normal height of contour of the buccal tooth surface. Note the considerable thickness of gold that protects the buccal cusp.

Occlusal Reduction

1. Place 0.8-mm depth grooves on the buccal inclines of the lingual cusp and 1.3-mm grooves on the lingual inclines of the buccal cusp (see Fig. 10-20, A and B). These guiding grooves are once again placed to follow the basic groove and fissure pattern of the oc-clusal surface. Only one depth cut needs to be placed to accommodate the functional cusp bevel on the distal aspect of the distal ridge.

2. Reduce the occlusal surface by removing the tooth structure between the grooves (see Fig. 10-20, C).

aAxial Reduction

3. Place guiding grooves on the lingual surface to parallel the proposed path of withdrawal and the long axis of the tooth.

4. Prepare the mesial as already described for the three-quarter and seven-eighths crown (see Fig. 10-20, D).

5. Reduce the distal surface as for a complete crown, extending the preparation to the transitional line angle and onto the buccal sur face. However, it should not extend mesially beyond the middle of the distal half of the buccal surface, and the chamfer should not extend too far cervically; otherwise, the dis-tobuccal line angle will be unnecessarily reduced, which would decrease the resistance form (see Fig. 10-20, E).

Finishing. The modified three-quarter crown preparation can include two or three grooves.

6. Place the mesial and buccal grooves as described for the seven-eighths crown (see Fig. 10-20, F). Another distal groove may be placed. In general, to gain as much length as possible, the grooves of the three-quarter crown should be slightly buccal. Care must be taken so that the distal groove is slightly closer to the center of the distal wall (so the distobuccal line angle will not be undermined).

7. Connect the mesial and buccal grooves with a centric cusp chamfer after the grooves and mesial flare have been placed and evaluated. The chamfer must be heavy enough to allow 1.5 mm of clearance in the area of occlusal contact (see Fig. 10-20, G). A regular or thick diamond is used to place the chamfer, which should connect the grooves and provide a protective "staple" linkage of alloy in the completed restoration. Insufficient tooth reduction where this chamfer meets the mesial flare is a common error. Finally, all prepared surfaces are smoothed and the internal line angles rounded.

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  • falco
    Where to place proximal grooves on crown?
    8 years ago
  • Veera
    Where is the buccal wall located?
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  • Fre-Swera
    What is a tooth preparation flare?
    7 years ago
  • damien hughes
    How to cut a seven eights crown?
    4 years ago
  • Vernice
    What is meaning of flaring during tooth prep?
    3 years ago
  • amethyst
    Where to place the grooves for occlusal reduction?
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