Published on February 20, 2014
Principles of Tooth Preparation
I. Preservation of Tooth Structure 1- Use of partial coverage rather than full coverage restorations
2- Preparation of tooth with minimal practical convergence angle (Taper) between axial walls
3. Preparation of occlusal surface following the anatomical contour to give uniform reduction
4. Reduction of axial walls with maximal thickness of remaining dentin surrounded the pulp
5. Selection of marginal geometry which is conservative and compatible with other principles
6. Avoidance of unnecessary apical extension of the preparation
Reduce Retention Reduce Resistance Over reduction Increase Hypersensitivity Pulp inflammation and necrosis Tooth fracture
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
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
Causes of Pulp Injury; 1- Thermal 2- Chemicals 3- Bacterial
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
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.
Indications of subgingival finish line 1- 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.
Marginal Geometry Ideal 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
Types of finish line
Featheredge Chisel Chamfer Bevel Shoulder Sloped Shoulder Beveled Shoulder
Preventing of tooth fracture
Principles 1- Providing Retention form 2- Providing Resistance form 3- Preventing deformation of restoration (structure durability)
Retention & Resistance What 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.
Tensile stress Shearcompression Shear stress compression
I. Retention Form Factors affecting retention: 1- Taper. 2- Total surface area 3- Roughness of tooth structure. 4- Film thickness 5- Type of luting agent
Taper The 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 degrees The 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
Average degree of tooth preparation taper
Surface area The greatest the surface area of preparation, the greatest the retention of restoration. Grooves surface area Boxes
The longer the occluso-gingival height, the more surface area. The more diameter of prepared teeth, the more surface area.
Surface roughness The prepared teeth should not be highly polished. Microscopic roughness, increase the retention
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
II. Resistance form Mastication 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 retention Factors affecting resistance to displacement: 1. Magnitude and direction of the dislodging forces 2. Geometry of the tooth preparation 3. Physical properties of the luting agent
Magnitude and direction of the dislodging forces In a normal occlusion, biting force is distributed over all the teeth; most of it is axially directed. If a fixed prosthesis is carefully made with a properly designed occlusion, the load should be well distributed and favorably directed . However, if a patient has a biting habit such as pipe smoking or bruxing, it may be difficult to prevent fairly large oblique forces from being applied to a restoration
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
The more convergence more displacement
Short occlusogingival height more displacement
The presence of the grooves less displacement
Path of insertion Definition That direction through which the restoration could be precisely seated on the corresponding tooth or teeth Parallel to the long axis of the tooth Parallel to the incisal 2/3 of the tooth
Exception; Tipped tooth Should parallel to occlusal plane
III. Preventing deformation (Structural Durability) 1. Occlusal reduction 2. Axial reduction
Functional Cusp Bevel Palatal Buccal
Axial Reduction Proper axial reduction Thin restoration Insufficient axial reduction Over contour
1.All Ceramic Restorations 2.Metal Ceramic Restorations 3.Partial Coverage Restorations
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• Retention prevents removal of the restoration along the path of insertion or long axis of the tooth preparation. • Resistance prevents dislodgement ...
٢ Finishing line of the preparation (f.l): It’s the final margin that separates between the prepared and the unprepared tooth structure. The f.l.
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