Axial Reduction and Groove Placement

path of withdrawal of the restoration must be accurately determined before axial reduction. Mesiodis-tally it should parallel the long axis of the tooth; buccolingually it should parallel the middle third or incisal two thirds of the facial surface. This will permit the preparation of proximal grooves of optimum length in an area of the tooth where sufficient bulk is present.

4. To enhance the retention and resistance form of the preparation, place a slightly exaggerated chamfer on the lingual aspect of the tooth (see Fig. 10-24, D) and a guiding groove in the

Tooth Structure Axial Wall
Fig. 10-26. A, A regular-grit diamond is used to complete the axial reduction. Mesiodistally the diamond is oriented parallel to the long axis of the tooth. B, When completed, a mesial and distal flange results that will serve as a guide during preparation of the proximal groove.

middle of the lingual wall. When alignment has been verified, the axial reduction can be performed in the same manner as the other preparations (Fig. 10-26). It is important to understand the difference between this phase of the preparation on a canine, with little bulk of lingual tooth structure as opposed to a premolar or molar. After completion, a proximal flange should result that will guide the rotary instrument during groove placement (see Figs. 10-24, E, and 10-26, B). The technical aspects of the preparation of proximal grooves are like those described for the other partial veneer preparations (Figs. 10-27 and 10-28). The primary difference is the direction in which the groove is prepared. Because the groove is placed perpendicular to the proximal wall, its deepest portion will be slightly labial to the proximal flange that results when proximoax-ial reduction is completed. As a result, the proximal flares will extend slightly farther onto the facial surface. This is even more accentuated by the curvature of the proximal wall (Fig. 10-29). Meticulous assessment of the needed extent of the initial axial reduction is a prerequisite for successful preparation (see Fig. 10-24, F, and 10-30). (The required interproximal clearance is illustrated in Fig. 10-31.)

Incisal Offset and Lingual Pinhole. Anterior partial veneer crowns require a means of reinforcement for preserving the casting's integrity. Posterior three-quarter crowns usually do not need as much additional reinforcement because the solid "corrugated" occlusal surface provides rigidity. For an anterior tooth, an incisal offset or groove is needed to create a band of thicker metal to provide a "staple"

Buccal Groove

Axial reduction

Groove

Flare

Fig. 10-27. A, Because the groove is prepared perpendicular to the proximal surface of the tooth, its deepest portion will be slightly buccal to where axial reduction was halted. B, The dotted line indicates the proposed flare. Note that the curvature of the tooth causes the final margin to be located a considerable distance buccal to where the initial axial reduction stopped. C, Completed flares.

Axial reduction

Groove

Flare

Fig. 10-27. A, Because the groove is prepared perpendicular to the proximal surface of the tooth, its deepest portion will be slightly buccal to where axial reduction was halted. B, The dotted line indicates the proposed flare. Note that the curvature of the tooth causes the final margin to be located a considerable distance buccal to where the initial axial reduction stopped. C, Completed flares.

Buccal Groove
Fig. 10-28. A, A tapered carbide is used to place the proximal groove. B, Initial groove preparation is completed. C, The carbide is moved parallel to itself. D, Mesial and distal grooves must be prepared in strict alignment.
Ridge Lap Pontic For Canine Teeth

Fig. 10-29. Differences between the proximal flares on premolars and canines. A designates where the initial proximal reduction is halted. Because a facial component is present in the direction of groove placement on the canine, as opposed to the premolar, the starting point (B) for the flare is located farther to the facial. In conjunction with the greater degree of proximal curvature of canines, it is critical that the initial axial reduction not be carried too far facially; otherwise, the final margin will extend too far onto the labial surface of the tooth and result in excessive display of metal.

Fig. 10-29. Differences between the proximal flares on premolars and canines. A designates where the initial proximal reduction is halted. Because a facial component is present in the direction of groove placement on the canine, as opposed to the premolar, the starting point (B) for the flare is located farther to the facial. In conjunction with the greater degree of proximal curvature of canines, it is critical that the initial axial reduction not be carried too far facially; otherwise, the final margin will extend too far onto the labial surface of the tooth and result in excessive display of metal.

Incisal Flare

Fig. 10-30. A, Unsupported enamel remaining after initial groove placement. B, A carbide bur can be used to flare the grooves. C, The flared groove. Note the irregularity of the margin near the cervical aspect of the groove. D, After the flaring. Note that a mesial box, rather than a groove, has been prepared. This restoration is designed to contain an intracoronal removable partial denture rest; hence, the box. Nevertheless, there is adequate resistance to lingual displacement. E, A special mandrel is placed in the box to ensure that it fits within its confines. It is identical in size to the male attachment of the RPD.

Fig. 10-30. A, Unsupported enamel remaining after initial groove placement. B, A carbide bur can be used to flare the grooves. C, The flared groove. Note the irregularity of the margin near the cervical aspect of the groove. D, After the flaring. Note that a mesial box, rather than a groove, has been prepared. This restoration is designed to contain an intracoronal removable partial denture rest; hence, the box. Nevertheless, there is adequate resistance to lingual displacement. E, A special mandrel is placed in the box to ensure that it fits within its confines. It is identical in size to the male attachment of the RPD.

Anterior Three Quarter Crown Extension
Fig. 10-31. Completed three-quarter crown preparation. Note the location of the facial margin relative to the adjacent teeth. Sufficient interproximal clearance has been established, but unnecessary display of metal is avoided.

ameter twist drill* (it must be parallel to the precise path of withdrawal of the restoration); fourth, the preparation is completed with a tapered carbide bur to a pinhole depth of approximately 2 mm; finally, a larger, round bur is used to countersink or bevel the junction between pinhole and ledge.

The technical aspects of pinhole preparation are described in the ensuing paragraphs. The completed preparation (Fig. 10-33) is carefully assessed for any remaining undercuts. The flares are a common area for undercuts, and all surfaces should be smoothed as previously described.

Fig. 10-32. An inverted-cone diamond or carbide can be used to prepare the incisal offset. Note the faciolingual inclination of the rotary instruments.

configuration. This provides additional rigidity and resistance against bending of the casting.

5. Connect the mesial and distal grooves with an incisal offset. It should improve the general resistance form of the preparation against lingual displacement and should have a V configuration. Sufficient dentin must be preserved facially to the offset to prevent the metal from being visible through the translucent tooth enamel. This is most effectively accomplished with an offset that is slightly narrower labiolingually than in-cisocervically. The offset should follow the normal configuration of the incisal edge, and its transition into the proximal flares should be smooth and continuous. An inverted-cone diamond or carbide (Fig. 10-32) can be used to prepare the offset.

6. Place a pinhole in the cingulum area slightly off center to improve the retention and resistance form of this preparation. The pinhole is prepared in five stages: first, a small horizontal ledge is made with a large, tapered carbide bur; second, a slight "dimple" is created with a round bur at the intended pinhole location; third, a pilot hole is prepared with a small-di-

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  • Sebastian
    What isaxial reduction?
    3 years ago

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