Posterior_Direct_Restorations Salvatore_compressed
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FIG 5-17 Defining axial walls using a flame bur (a), files (b), a disk (c), a scalpel/manual enamel cutter (d), or dedicated sonic inserts (e).
Sometimes it may be necessary to modify the emergence of the cervical step. This maneuver should be performed only when strictly
necessary. This can happen when:
Cervical anatomy is not linear, and gaps arise between the matrix and the cervical step when the sectional matrix is fitted (imperfectly fitting matrix).
The cervical step is too close to the adjacent tooth, with consequent problems fitting the sectional matrix and accurately managing the emergence profile.
Changes can be made to the cervical step anatomy, provided these are contained within the enamel structure and are of minimal size. The ideal tool for this modification is a reciprocating file (see Figs 5-15y and 5-15z).
Defining the angle between the axial walls and the external surface is important for determining the strength of the restoration and the tooth tissue (Fig 5-18). The angle between the outer surface of the tooth and the box wall should be approximately 90 degrees (green angle, type 1 in Fig 5-18b). More acute angles (red angle, type 3 in Fig 5-18b) must be avoided because they indicate that the dental tissue is very thin and more likely to fracture during function. More obtuse angles (orange angle, type 2 in Fig 5-18b) are acceptable, but they result in areas that are more difficult to fill during reconstruction, making it necessary to apply only very thin layers of restorative material.
FIG 5-18 (a and b) Defining the angle between the axial walls and the external surface of the tooth.
Sonic instruments and associated inserts are recommended as a user-friendly method. For the same tip size, rotary instruments need much more working space than a sonic instrument. If preparation is carried out using sonic instruments, cavity preparation on the most distal teeth is a simpler process for the clinician and less stressful for patients, who will not have to put so much effort into keeping their mouths open. Because sonic inserts come in many types with many angles, they can easily reach the most inaccessible areas of the tooth (Fig 5-19). If the most appropriate angle is selected, they comply with the principle of preserving healthy dental tissue where possible, which is not always feasible with the most common rotary instruments.
