- Use of partial coverage rather than complete coverege restorations
- Preparation of teeth with the minimum practical convergence angle between axial walls (taper)
- Preparation of the occlusal surface so that reduction follows the anatomic planes to give uniform thickness in restortion
- marginal geometry that is conservative yet compatible with other principles
- Avoidance of unnecessary apical extension of preparation!
CONSIDERATIONS AFFECTING FUTURE DENTAL HEALTH
- A restoration must provide sufficient space for good axial contours
- It should duplicate the contours and profile of original tooth
- Adequate axial reduction allows the development of properly contoured embrassures
- Overcontoured restoration can result in perio problems
- Over reduction can lead to loss of retention!
Following guidelines for margin designs should be considered:
- Ease of preparation without overextension or unsupported enamel.
- Ease of identification in impression and on the die
- A distinct boundary to which the wax pattern can be finished
- sufficient bulk of material
- conservation of tooth structure
- Most of the forces acting on a tooth act directly on the occlusal surface of the tooth.
- when the patient's occlusion is disrupted by the supraerupted teeth or tilted teeth, the eventual occlusal plane must be carefully analysed and the teeth reduced accordingly.
- considerable reduction needed and this may shorten the tooth preparation axial wall height with associated mechanical consequences!
- diagnostic tooth preparation and waxing procedures are helpful in determining the amount of reduction needed!
PREVENTING TOOTH FRACTURE
Tooth facture can be prevented :
- By reducing the wedging effect of forces acting on tooth
- By increasing the amount of tooth material remaining.
RETENTION FORM
Following factors must be considered :
- Taper
- Surface area
- Type of preparation
- Surface texture
- Film thickness of luting agent
- Type of luting agent
Factors that influence the resistance of cemented restorations are :
- Magnitude and direction of dislodging forces
- Taper
- Diameter of the tooth
- Height of the preparation wall
- Type of preparation
- Luting agent
- Minimum recommended clearance for non functional cusp is 1mm and for functional cusp is 1.5mm
- Occlusal reduction should follow the normal anatomic contours.
- axial reduction should parallel the long axis of tooth while alllowing for the recommended 6 degree taper or convergence between opposing axial surfaces.
- The margin should have a chamfer configuration and should ideally be located supragingivally.
- Functional cusp bevel will be placed at about 45 degrees to the long axis.
- Maxillary molars usually require an additional bevel reduction at the occlusoaxial line angles of the non functional cusp.
RESISTANCE FORM
2. MARGIN PLACEMENT
SUPRAGINGIVAL MARGINS
SUBGINGIVAL MARGINS
Preparation should have supragingival margins whenever possible. Advantages of supragingival margins are :
- Easily finished
- Easily kept plaque free
- Easy impression making
- Easy to examine and evaluate during future visits
- Easily prepared without trauma to soft tissues
INDICATIONS :
- where esthetics is main concern
- when proximal contact area extends to gingival crest
- when additional resistance and retention needed
- when margins of PFM crown is to be hidden
- root sensitivity
- subgingival caries
- cervical erosion
- where crown lengthening procedures cannnot be done
- Root canal treated teeth
PREVENTING DEFORMATION
Strength of restoration depends on the following factors:
- Type of alloy
- Adequate tooth reduction
- Margin designs
- Incorporating internal features (grooves and ledges)
3. MARGIN ADAPTATION
- should have smooth and even margins.
- Acceptable margin adaptation:
> castings that fit to within 10micrometers
> porcelain that fit to within 50 micrometers
BIOLOGIC CONSIDERATIONS
Surgical procedures involving living tissues must be carefully executed to avoid unnecessary damage to adjescent teeth, soft tissues and the pulp of the tooth being prepared!
PREVENTION OF DAMAGE DURING TOOTH PREPARATION
1. ADJESCENT TEETH
Damage can be prevented by following methods :
- using metal matrix band
- leaving a thin lip of proximal enamel
2. SOFT TISSUES
Soft tissues of tongue and cheek can be prevented by retracting them using :
- Aspiratory tip
- mouth mirror
- flanged saliva ejector
Care should be taken to prevent damage to tongue while preparing lingual surfaces of mandibular molars
3. PULP
Increased temperature damages the pulp.Heat is generated by fricion between the rotatory instrument and the surface being prepared.
Following increases the heat generation:
- Excessive pressure
- Higher rotational speed
- Type , shape and condition of cutting instrument.
Water coolent must be used!
5. BACTERIAL ACTION
4. CHEMICAL ACTION
Pulp damage under restorations have been attributed to bacteria that either were left behind or gained access to dentin because of microleakage.
however, following actibacterial agents can be used :
- Zinc phosphate cement
- vital dentin
- chlorhexidine gluconate disinfecting solution
Use of folowing can cause pulp damage particularly when applied to freshly cut dentin :
- Bases
- Restorative resins
- Solvents
- Luting agents
however cavity varnishes and dentin bonding agents can be used to form effective barrier.
CONSERVATION OF TOOTH STRUCTURE
OCCLUSAL CONSIDERATIONS
GENERAL CRITERIA FOR CROWN PREPARATION
PRINCIPLES OF TOOTH PREPARATION
- Occlusal guiding grooves
- Occlusal reduction
- Axial alignment grooves
- Axial reduction
- Finishing and evaluation
STEP BY STEP PROCEDURE:
PRINCIPLES OF TOOTH PREPARATION
Teeth require preparation to recieve restorations and these preparations must be based upon fundamental principles from which basic criteria can be developed to help predict the success of prosthodontic treatment.
COMPLETE CAST CROWN PREPARATION
METAL CERAMIC CROWN PREPARATION
I . Biologic considerations
III. Esthetic considerations
II . Mechanical considerations
ALL CERAMIC CROWN PREPARATION
I. Providing Retention Form
MECHANICAL CONSIDERATIONS
II. Providing Resistance Form
III. Preventing Deformation Of Restoration
Full Coverege Restorations
4. MARGIN GEOMETRY
- All ceramic crown
- Metal ceramic crown
- Complete cast crown
OBJECTIVES
The audience should be able to :
- describe the principles of tooth preparation
- outline the three broad categories of these principles
- discuss the biologic, mechanical and esthetic considerations of tooth preparation.
- describe the individual preparations of each type of restoration.
1. AXIAL REDUCTION
PRINCIPLES
OF TOOTH
PREPARATION
Tooth structure is conserved through adherence to the following guidelines :