principles of tooth preparation fixed orthodontic
TRANSCRIPT
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Principles of Tooth Preparation
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Biological Considerations
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I. Preservation of Tooth Structure1- Use of partial coverage rather than full coverage restorations
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2- Preparation of tooth with minimal practical convergence angle (Taper) between axial walls
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3. Preparation of occlusal surface following the anatomical contour to give uniform reduction
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4. Reduction of axial walls with maximal thickness of remaining dentin surrounded the pulp
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5. Selection of marginal geometry which is conservative and compatible with other principles
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6. Avoidance of unnecessary apical extension of the preparation
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Reduce Retention
Reduce Resistance
Increase Hypersensitivity
Pulp inflammation and
necrosis
Tooth fracture
Over reduction
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On of the most common violation of this principle is seen in discriminate use of full porcelain coverage in a situation when partial veneer coverage can be used
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Also you should be careful about;1- Adjacent tooth - by using matrix band of the adjacent tooth - cutting in the enamel of prepared tooth with fine tapered stone 2- Soft tissues - by using mirror or the flange of saliva ejector
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3- Pulp
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Causes of Pulp Injury;
1- Thermal
2- Chemicals
3- Bacterial
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Avoidance of over contour• Gingival inflammation is commonly associated with crowns and FPD
abutments having excessive axial contours, probably because it is more difficult for the patient to maintain plaque control around the gingival margin. A tooth preparation must provide sufficient space for the development of good axial contours.
• Under most circumstances a crown should duplicate the contours and profile of the original tooth (unless the restoration is needed to correct a malformed or malpositioned tooth). If an error is made, a slightly under contoured flat restoration is better because it is easier to keep free of plaque; however, increasing proximal contour on anterior crowns to maintain the interproximal papilla may be beneficial.
• Sufficient tooth structure must be removed to allow the development of correctly formed axial contours, particularly in the interproximal and furcation areas of posterior teeth, where periodontal disease often begins
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Margin placement
Whenever possible, the margin of preparation should be supragingivally, where the supragingival margin is prepared on sound enamel, while the subgingival margin is often prepared on cementum advantages:1- Easier to prepare accurately without trauma to soft tissue.2- Prepared on hard enamel.3- Easy to finished.4- More easily to kept clean.5- Impression are more easily made.6- Restoration can be easily to evaluated at recall appointment.
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Indications of subgingival finish line1- Dental caries, cervical erosion, restoration extend subgingivally.2- Proximal contact area extend near gingival crest.3- Short tooth and additional retention is required.4- The margin of metal-ceramic restoration is to be hidden subgingivally.5- Root sensitivity cannot be controlled by
conservative procedures.
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Marginal GeometryIdeal requirements of finish lines:1. They must fit closely to the finish line of the
preparation.
2. They must have sufficient strength to withstand the force of mastication.
3. Whenever possible, finish line should be located in areas where the dentist can finish and inspect them, and the patient can clean them
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Types of finish line
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Featheredge Chisel Chamfer Bevel Shoulder Sloped Shoulder
Beveled Shoulder
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√
X
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Occlusal Considerations
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Preventing of tooth fracture
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Mechanical Considerations
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Principles
1- Providing Retention form 2- Providing Resistance form 3- Preventing deformation of restoration (structure durability)
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Retention & ResistanceWhat is the Retention?The ability of the preparation to prevent removal of the restoration along the path of insertion.What is the Resistance?The ability of the preparation to prevent dislodgment of restoration by force directed in an along apical oblique and horizontal.
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Tensile stress Shear stress
Shear-compression
compression
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I. Retention Form
Factors affecting retention:1- Taper.2- Total surface area3- Roughness of tooth structure.4- Film thickness 5- Type of luting agent
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TaperThe more nearly parallel the opposing walls of preparation the greatest will be the retention.
The optimum degree of taper is 2.5 to 6.5 degreesThe recommended degrees of taper are: 3 : 5, 6, 10 : 14 the degree of taper stress in the cement interface between the preparation and restoration the degree of taper retention
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Average degree of tooth preparation taper
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Surface areaThe greatest the surface area of preparation, the greatest the retention of restoration.
Grooves surface areaBoxes
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The longer the occluso-gingival height, the more surface area.
The more diameter of prepared teeth, the more surface area.
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Surface roughnessThe prepared teeth should not be highly polished.
Microscopic roughness, increase the retention
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Film Thickness there is a doubt (no certain data)
Type of luting agent 1- Resin cement 2- Glass ionomer cement 3- Polycarboxylate cement 4- Zinc phosphate cement 5- Zinc oxide eugenol cement
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II. Resistance formMastication and parafunctional activity may subject a prosthesis to substantial horizontal , oblique or apical forces. These forces are normally much greater than the ones overcome by retentionFactors affecting resistance to displacement:1. Magnitude and direction of the dislodging forces2. Geometry of the tooth preparation3. Physical properties of the luting agent
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Magnitude and direction of the dislodging forcesIn a normal occlusion, biting force is distributedover all the teeth; most of it is axially directed. If afixed prosthesis is carefully made with a properly designed occlusion, the load should be well distributed and favorably directed . However, if apatient has a biting habit such as pipe smoking or bruxing, it may be difficult to prevent fairly largeoblique forces from being applied to a restoration
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Geometry of the tooth preparation
What is the definition of Tipping Path?It is the path along which the restoration, could be displaced under the displacing occlusal force
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The more convergence more displacement
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Short occlusogingival height more displacement
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The presence of the grooves less displacement
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Path of insertion
DefinitionThat direction through which the restoration could be precisely seated on the corresponding tooth or teeth
Parallel to the long axis of the toothParallel to the incisal 2/3 of the tooth
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Exception;Tipped tooth
Should parallel to occlusal plane
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III. Preventing deformation (Structural Durability)
1. Occlusal reduction2. Axial reduction
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Functional Cusp Bevel
Buccal
Palatal
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Axial Reduction
Insufficient axial reduction
Thin restoration Over contourProper axial reduction
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Esthetic Considerations
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1.All Ceramic Restorations2.Metal Ceramic Restorations3.Partial Coverage Restorations