* The three dimensional forces acting on a single tooth restoration.
　1. Compressive force: a force pushing directly against an object. The force
　　of closure directed along the line of shed of a restoration is a compressive
　　force.
　2. Tension: an subjected to equal and opposite forces at its ends. Tensile
　　force in dentistry is the opposite force of compressive force.
　3. Shear: an force on an object that is not at a right angle to a specific area
　　and is represenred by a resultant force.

* Three considerations for the three forces.
　1. Compressive forces --> Structural support.
　2. Tensile force --> Retention.
　3. Shearing force --> Resistance.

* Convergence angles:
　Convergence angle is defined as the angle formed by extending two axial
　walls into space. Where they meet,the measurement of that angle formed, is
　called convergence angle. The convergence angles of the external axial walls
　of a preparation control both the resistance form and the retention form. In
　theory, the best resistive and retentive preparation would have parallel axial
　walls. This is not humanly possible. What is practical are near-parallel axial
　walls with a convergence angle of 10 to 15 degrees.

* Height of axial walls:
　The height of the axial walls is directly proportional to the retention form of
　the pre-paration. Possible alternative remedies for short axial walls are: 1)
　subgingival margin placement; 2) crown lengthening with perio surgery; 3)
　intentional endodontic therapy with a core build-up; and 4) removal with a
　subsequent implant.

* How can we enhance the resistance form of most posterior preparations?
　Placing axial grooves: placing the groove proximally and intersecting the
　radius of their rotation.

* The considerations of increasing preparation surface area:
　a) Extra-coronal considerations: the greater the surface area of the
　　preparation usually means better resistance and retention form. Besides,
　　the greater the surface area, the better the forces are distributed and the
　　smaller th stress.
　　S = F / A
　b) Intra-coronal components: such as boxes; grooves; countersinks; pins;
　　an
$t26.jpgd com-binations of the above.

* Other considerations:
　1. Line of shed: identical with long axis of tooth.
　2. Occlusal reduction: one and one half to two mm.
　3. Finish line: chamfer, shoulder, shoulder bevel, and knife.
　4. Margin: supragingival or equigingival.

* Common errors in tooth preparation.
　1. Intentional errors: overtapering.
　2. Unintentional errors: underreduction.

* Recent general consensus is that margins should be placed supragingivally if
　the health of the periotium is to be maintained. Except:
　1. Subgingival caries.
　2. Existing subgingival restoration.
　3. Short crown.
　4. Esthetics.　　 ==> Crown lenthening.

* If the margins must be placed subgingivally, at least the margins must never
　injure or impinge on the dental epithelial junction that functions as a biologic
　seal of the perio-dontal system.
　Biologic width: 1.5 mm from the alveolar crest.

* Common errors in anterior teeth preparations.
　1. Inadequate reduction of the cervicolabial portion of facial margins.
　2. Inadequate reduction at the incisal third or establishment of labioincisal
　　slanting.
　3. Insufficient reduction of the incisal portion.

* Shoulder or shoulder bevel design is preferred to a chamfer design as the
　facial margin of ceramometal crown preparation, because the substructure
　may receive from buttressing the metal.

* Proper tooth reduction is between 1.0 to 1.5 mm, depending on the pupal
　con-figuration and size. (0.2 to 0.5 mm metal substructure, 0.1 to 0.2 mm
　opaque, and 1.0 to 1.5 mm veneering porcelain).

* Underreduction of the cervicolabial portion will result in an overcontoured
　crown, which may be harmful to the periodontium.　　 ==> Emergence
　profile.

* Inadequate reduction at the incisal third, or failure to establish the proper
　labioinci-sal slanting, will result in disclosure of the opaque through an
　insufficient thickness of the veneering porcelain (blotching).

* If the porcelain is more than 2 mm thick, the reinforcing effect of the metal
　is lost, and porcelain fracture may occur. The optimum reduction for the
　incisal portion is 2 mm.

* Overcontuoring of the cigulum portion may cause:
　1. harmful effects on the periodontal tissue.
　2. interference during excursive mandibular movement.

* Common errors in posterior teeth preparations cause:
　1. harmful periodontal effect due to oversizing or overcontouring of the
　　artificial crown.
　2. unesthetic results in the crown caused by disclosing the opaque layer.
　3. improper occlusal relationship if proper canting is not provided.

* The most common deficiencies in the posterior preparation are: axial
　reduction ignoring the cervical circumferential morphology of the tooth;
　insufficient axial reduction; insufficient reduction at the occlusal third,
　buccally and lingually (canting); inadequate reduction at the central portion
　of the occlusal surface; occlusal reduction ignoring the general morphology
　of the occlusal pattern; and sharp edges and angles on the finished
　preparation.

* The most frequent error of axial reduction is inadequate reduction in the area
　of transitional line angles. This results in a restoration with insufficient
　gingival embra-sure space to accommodate the gingival papilla and restricts
　proper oral hygiene, which leads to poor periodontal health.

* The proximal surface of posterior teeth is always flat or slightly concave
　buccolin-gually and occlusocervically. For practical hygienic reasons, a flat
　surface is preferred for easy using of dental floss. But in an excessively
　conservative preparation, it is im-possible to restore this contour without
　jeopardizing the health of the gingival papillla.

* Proper canting promotes occlusal relationship, contours, and esthetics.

* Lack of canting results in overcontouring or inadequate ceramic shield for
　opaque layer.

* Inadequate reduction of the occlusal surface*s central portion is also one of
　the common mistakes. Sufficient occlusal reduction allows reproduction of
　proper occlusal anatomy, restores function, and distributes stress equally.

* The amount of occlusal reduction varies between 1 and 2 mm, depending on
　the following considerations:
　1. the length of the tooth;
　2. the size and the configuration of the pulp chamber;
　3. the quality of the masticatory muscles (or occlusal forces);
　4. the type of material used;
　5. functional or nonfunctional areas.

* Conclusion:
　A successful restoration is one that meets both biological and biophysical
　demands rather than the longevity of the service. It should repair, restore,
　and modify the com-ponents of a dentition to enhance function, esthetics,
　and the health of the hard and soft tissues.
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Topic Seventeen:　The relative infective route of
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