Biological and Mechanical Principles of Tooth Preparation Flashcards
What is the purpose of tooth preparation?
Accomodating the final restorationand facilitating the subsequent procedures.
What procedures can be conducted following tooth preparation?
Provisional restorations
Impressions
Cast pouring
Final restoration fabrication
What are the types of tooth preparations and what are their indications?
Therapeutic preparations: Recover dental structure destroyed by caries, abfraction, abrasion, erosion or fractures.
Preparation of healthy teeth for aesthetic, or orthodontic purposes as well as PRR.
What are the principles of optimal tooth preparations?
Biological principles:
Conservation of tooth structure
Avoidance of overcontouring
Supragingival margins
Harmonious occlusion
Protection against tooth fracture
Mechanical principles:
Retention and resistance form
Structural durability
Aesthetic principles:
Minimum display of metal
Maximum thickness of porcelain
Porcelain occlusal surfaces
“subgingival margins” (balance should be sought for the perfect resto)
What is the aim of the biological principles of tooth preparation?
Preserving the health of the oral tissues
What is the aim of the mechanical principles of tooth preparation?
To determine the integrity and durability of the restoration
What is the aim of the aesthetic principles for tooth preparation?
Determine the appearance of the final restoration
How can tooth damage occur during tooth preparation?
Damage to proximal teeth leading to caries and gingivitis
Soft tissues such as gingival tissues, tongue, and cheeks
Pulp
How can pulp irritation take place during crown preparations?
Thermal irritation (Fixed via lots of irrigation)
Chemical irritation caused by bases, resins, etch, solvents, and luting agents (4 - 8% of pulps die in the 10 years following crown preparations)
Bacterial irritation (either left behind or gained access due to microleakage)
How can tooth structure be affected by crown material?
Gold requires less reduction than PFM.
How can tooth structure be preserved?
Choosing material to need less reduction if possible.
Prepartion with minimal taper and allow occlusal reduction to follow the anatomical contour.
Uniform axial reduction
Conservative margins
Avoid unnecessary apical extension of the preparation
Over contouring the restoration
Supragingival margins
What is the problem with over-contouring the crown?
Over contoured crown makes it difficult for the patient to clean effectively
Gives rise to gingivitis
How should contouring be done during a preparation?
Ideal contours of the restoration should follow the contours of the tooth and surrounding tissue.
What are the benefits of having supragingival margins?
Easily prepared
Easier to take impressions
Easily maintained by patient
Easily evaluated at recalls
Periodontium is preserved
What are the indications for using subgingival margins?
Subgingival dental caries, erosion, fracture line, or restoration
Apical proximal contact area
Additional retention is needed
Aesthetic margins
Coverage of root surface
Modification of the axial contour
Why should margins always be smooth in a crown preparation?
It is a potential site for caries (dissolution of cement or inherent roughness leading to plaque accumulation)
Should be even and smooth to facilitate subsequent steps until delivery of the restoration.
What is biological width?
Mean sulcular depth of 0.69mm
Mean JE length of 0.97mm
Mean connective tissue attachment length 1.07mm
JE + CT attachment is known as biological width
Why should prep follow papillary and gingival contour?
To avoid violation of the biological width preventing gingivitis and inflammation
What are the design requirements?
Simplicity of preparation
On intact tooth surface
No unsupported enamel
Ease of identification
Allows sufficient bulk of restoration (durability and aesthetics)
Conservative
What are the possible designs of the margins/finishing lines?
Feather edge
Bevel
Chamfer
Shoulder
Shoulder with bevel
What are the advantages and disadvantages of a feather edge finish line?
Advantages: Conservation of tooth structure
Disadvantages: Fail to provide adequate bulk at margins
What are the advantages and disadvantages to bevel finish lines?
Advantages:
Allow margin of crown to be burnishing against the tooth structure
Protect unprepared tooth structure from chipping by removing unsupported enamel
Disadvantages: Leads to subgingival extension
What are the advantages to shoulder finish lines?
Margin for all materials
Easy to prepare and finish well
Easy to judge impressions
Frequently used for metal crowns,
Metal ceramic crowns and all
ceramic crown
What are the advantages to chamfer finish lines?
Most commonly used margin for all metal and new all ceramic restorations
Conserves tooth tissue whilst providing distinct finish line
Easy to read in impressions
Axio-pulpal line angle coronal to external margin
What is a chamfer finish line?
A hook shaped finish line
How is harmonious occlusion approached with crowns?
Confirmative or reorganized approach
Decide upon a satisfactory occlusal scheme prior to tooth preparation
Using diagnostic wax ups can be extremely helpful
When is harmonious occlusion especially important?
Eventual occlusal plane adjustment
Possibility of elective endodontic treatment
Possibility of crown lengthening surgery
What should be ensured prior to core placement?
Ensure all the weakened tooth structure is removed first before core placement
This ensures protection of remaining structures.
Why is retention form important?
It prevents displacement of the crown occlusally
You cannot rely on cements alone to retain the restoration in place.
What does retention form of a crown depend on?
Preparation height
Degree of tape
Freedom of displacement
Roughness of the surfaces
Physical properties of the luting cement
How do luting cements help with retention?
They fill the crown-tooth space and create friction
Prevent the surface from sliding
How does preparation geometry impact function?
It enhances the cement function
Having the most parallel preparation is ideal but very difficult to fit. This means ideally a 6 degree tiny taper is the best for this function.
Why must tooth prep be tapered?
To permit more complete seating of the restoration
Prevents undercuts
Visualize preparation walls and finish line
Compensate for inacccuracies during fabrication
How much taper must be given to a preparation?
Theoretical ideal is 6 degrees.
Practically 6 - 20 is acceptable
Tapered burs are designed to cut at 3 degree taper on each wall and should be held parallel to the long axis of the tooth
How can taper be visualized?
One eye vision should show all the margins.
In the mouth by mirror with a birds eye view
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