Biological and Mechanical Principles of Tooth Preparation

Question 1

What is the purpose of tooth preparation?

Answer 1

Accomodating the final restorationand facilitating the subsequent procedures.

Question 2

What procedures can be conducted following tooth preparation?

Answer 2

Provisional restorations

Impressions

Cast pouring

Final restoration fabrication

Question 3

What are the types of tooth preparations and what are their indications?

Answer 3

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.

Question 4

What are the principles of optimal tooth preparations?

Answer 4

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)

Question 5

What is the aim of the biological principles of tooth preparation?

Answer 5

Preserving the health of the oral tissues

Question 6

What is the aim of the mechanical principles of tooth preparation?

Answer 6

To determine the integrity and durability of the restoration

Question 7

What is the aim of the aesthetic principles for tooth preparation?

Answer 7

Determine the appearance of the final restoration

Question 8

How can tooth damage occur during tooth preparation?

Answer 8

Damage to proximal teeth leading to caries and gingivitis

Soft tissues such as gingival tissues, tongue, and cheeks

Pulp

Question 9

How can pulp irritation take place during crown preparations?

Answer 9

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)

Question 10

How can tooth structure be affected by crown material?

Answer 10

Gold requires less reduction than PFM.

Question 11

How can tooth structure be preserved?

Answer 11

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

Question 12

What is the problem with over-contouring the crown?

Answer 12

Over contoured crown makes it difficult for the patient to clean effectively

Gives rise to gingivitis

Question 13

How should contouring be done during a preparation?

Answer 13

Ideal contours of the restoration should follow the contours of the tooth and surrounding tissue.

Question 14

What are the benefits of having supragingival margins?

Answer 14

Easily prepared

Easier to take impressions

Easily maintained by patient

Easily evaluated at recalls

Periodontium is preserved

Question 15

What are the indications for using subgingival margins?

Answer 15

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

Question 16

Why should margins always be smooth in a crown preparation?

Answer 16

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.

Question 17

What is biological width?

Answer 17

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

Question 18

Why should prep follow papillary and gingival contour?

Answer 18

To avoid violation of the biological width preventing gingivitis and inflammation

Question 19

What are the design requirements?

Answer 19

Simplicity of preparation

On intact tooth surface

No unsupported enamel

Ease of identification

Allows sufficient bulk of restoration (durability and aesthetics)

Conservative

Question 20

What are the possible designs of the margins/finishing lines?

Answer 20

Feather edge

Bevel

Chamfer

Shoulder

Shoulder with bevel

Question 21

What are the advantages and disadvantages of a feather edge finish line?

Answer 21

Advantages: Conservation of tooth structure

Disadvantages: Fail to provide adequate bulk at margins

Question 22

What are the advantages and disadvantages to bevel finish lines?

Answer 22

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

Question 23

What are the advantages to shoulder finish lines?

Answer 23

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

Question 24

What are the advantages to chamfer finish lines?

Answer 24

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

Question 25

What is a chamfer finish line?

Answer 25

A hook shaped finish line

Question 26

How is harmonious occlusion approached with crowns?

Answer 26

Confirmative or reorganized approach

Decide upon a satisfactory occlusal scheme prior to tooth preparation

Using diagnostic wax ups can be extremely helpful

Question 27

When is harmonious occlusion especially important?

Answer 27

Eventual occlusal plane adjustment

Possibility of elective endodontic treatment

Possibility of crown lengthening surgery

Question 28

What should be ensured prior to core placement?

Answer 28

Ensure all the weakened tooth structure is removed first before core placement

This ensures protection of remaining structures.

Question 29

Why is retention form important?

Answer 29

It prevents displacement of the crown occlusally

You cannot rely on cements alone to retain the restoration in place.

Question 30

What does retention form of a crown depend on?

Answer 30

Preparation height

Degree of tape

Freedom of displacement

Roughness of the surfaces

Physical properties of the luting cement

Question 31

How do luting cements help with retention?

Answer 31

They fill the crown-tooth space and create friction

Prevent the surface from sliding

Question 32

How does preparation geometry impact function?

Answer 32

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.

Question 33

Why must tooth prep be tapered?

Answer 33

To permit more complete seating of the restoration

Prevents undercuts

Visualize preparation walls and finish line

Compensate for inacccuracies during fabrication

Question 34

How much taper must be given to a preparation?

Answer 34

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

Question 35

How can taper be visualized?

Answer 35

One eye vision should show all the margins.

In the mouth by mirror with a birds eye view

Question 36

38/52

Answer 36

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