Biomechanical Principles of Tooth Preparation (Esthetic)

7/22/2019 Biomechanical Principles of Tooth Preparation (Esthetic)

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Dr. Mohamed Abd-el Aziz



Presented by

Dr. Mohamed Abd-el AzizLecturer of Fixed Prosthodontics



• At the initial examination, it is important tomake a full assessment of the appearance of each patient, noting which areas of which teethshow during smiling, talking and laughing.

• The patient’s esthetic requirements must be

discussed and related to oral hygiene needsand the potential for disease.

• The final decision regarding an appropriate

restoration can then be made with the fullcooperation and informed consent of thepatient.



The poor appearance of some metal-ceramicrestorations is often due to :◦ Insufficient porcelain thickness.

◦ The labial margin of a metal-ceramic crown is not

accurately placed.

To correct these deficiencies, certain principlesare recommended during tooth preparation that

will ensure sufficient room for porcelain andaccurate placement of the margins.



(a) Facial tooth reduction :

• Two principles should be fulfilled during facialtooth reduction to obtain excellent esthetics:

◦ Adequate thickness of porcelain is needed tocreate a sense of color depth and translucency.



◦ The labial surfaces of anterior teeth should beprepared for metal-ceramic restoration in twodistinct planes.

◦ If they are prepared in a single plane,insufficient reduction in either the cervical orthe incisal area of the preparation will result.



(b) Incisal reduction: ◦ • An incisal reduction of 2 mm is

recommended for good esthetics.◦ • The incisal edge of a metal-ceramic

restoration is constructed with no metalbacking and can be made with a translucency

similar to that of natural tooth structure.◦ • Excessive incisal reduction must be avoided

because it will reduce the resistance andretention form of the preparation.



(c) Proximal Reduction :

◦ The proximal surfaces of an anterior teeth willlook most natural if they are restored as theincisal edges, without metal backing. This willallow some light to pass through the restoration

in a manner similar to what occurs on a naturalboth.



(d) Labial margin placement : ◦ • Although supragingival margin plcement

has many biologic advantages, subgingivalmargins may be indicated for esthetic reasons.

◦ • The patient’s smile is observed as part of

the initial examination. It is important torecord which teeth and which parts of eachtooth are exposed.



◦ • Patients with high lip line,which exposes considerablegingival tissue, present thegreatest problem if completecrowns are needed. The toothcan be restored with a metal-ceramic restoration having a

supragingival porcelain-labialmargin.

◦ • If the patient has a low lip

line, a metal supragingival

collar may be placed becausethe metal is not seen duringnormal function.



◦ Whenever possible, partial-coveragerestorations are preferred, not only becausetooth structure in conserved but also because no

restorative material can approach theappearance of intact tooth enamel.

◦ Esthetic partial-coverage restorations dependon accurate placement of proximal and facial

margins.



◦ • The proximal margin should be placed justbuccal to the proximal contact area, wheremetal will be hidden by the distal line angle of the neighbouring tooth.



◦ • Tooth preparation angulation is critical andshould normally follow the long axes of

posterior teeth and incisal two thirds of thefacial surface of anterior. If a buccal or lingualtilt is given to the tooth preparation, metalmay be visible.



(2) Facial margin: ◦ • The facial margin of a maxillary partial-

coverage restoration should be extended justbeyond the occlusofacial line angle.If the buccal margin is skilfully placedfollowing the original cuspal contour, the finalrestoration will have an acceptableappearance.



◦ • Anterior partial-coverage restorations can

be fabricated with no metal display, but theirpreparation requires considerable care. Thefacial margin is extended just beyond thehighest contour of the incisal edge but not

quite to the incisolabial line angle. Here themetal will protect the tooth from chipping butwill not be visible.

Biomechanical Principles of Tooth Preparation (Esthetic)

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