2. Flashcards
The shape of the tooth restoration provides: (3)
Retention
stability
mechanical strength
Purpose of tooth preparation? (2)
- esthetics
- function
What do we protect during tooth prep? (4)
- adjacent teeth
- periodontum
- pulp
- surrounding soft tissues
What do we need after tooth preparation? (3)
- Biological seal
- Adequate emergence profile
- Respect for the biological width
Where do we start doing the preparations on the tooth?
supragingivally, and only at the end take it to the desired
height
How do you prevent the pulp from thermal harm? (4)
- water cooling
- Sharp burs
- Gentle and intermittent techniques
- Use of provisionals
How do you protect the pulp from mechanical harm? (3)
- Getting into the pulp with the bur
- Knowledge of the anatomy
- Diameter of the burs
What is the biological seal?
The fit between the margins of the restoration and preparation
What is the purpose of the biological seal? (2)
- prevents cement from
dissolving - prevents penetration of
bacteria
What is an adequate emergence profile?
Restoration reproduces the tooth’s contour
What is the advantage of an adequate emergence profile?
- Prevents dental plaque gathering preventing caries/periodontal disease
How do we respect the biological width with preparation margins?
Should be at least 2mm away from the crest bone
How do we respect the biological width with the finish line?
Should never be more than 0.5-1mm under the sulcus
What is the consequence of not respecting the biological width? (3)
- Chronic gingival inflammation
- Periodontal disease
- Bone resorption
What components are in the biological width and their size? (3)
Biological width (2mm):
- Sulcus (0.5-0.6mm)
- Epithelial attachment (1mm)
- Connective tissue attachment (1mm)
What are the 5 mechanical principles?
- Retention
- Stability
- Path of insertion
- Structural strength
- Finish line
Define retention
resist the forces of dislodgment along the path of placement
What does NOT provide retention except in porcelain veneers?
Cement
The essential element of retention is…
two opposing vertical surfaces in the same preparation
What does a cement do?
Only SEALS the interface between tooth and restoration
What does cement provide in porcelain veneers?
Retention
What type of retention do extracoronal restorations have?
External retention:
- internal surface of restoration
- external surface of preparation
What type of retention do intracoronal restorations have?
Internal retention:
- External surface of restoration
- Internal surface of preparation
What factors influence retention?
- Magnitude of dislodging forces
- Marginal adaptation
- Cements
- Morphology of the prep
What factors minimize crown retentiveness? (4)
- Short teeth
- No marginal adaptation
- Cementation technique
- Sticky food
What dislodging forces can influence retention?
Magnitude of vertical forces in occlusal directions
How can marginal adaption influence retention? (2)
- proper adaptation gives friction and retention
- impression quality and lab procedure affects marginal adaptation
How do cements influence retention? (3)
- increase retention by increasing friction
- micromechanical bonding
- there is NO chemical bonding
Whats the relation between friction and retention?
Directly proportional. Increasing one increases the other
What are the most important factors that influence retention? (2)
- Marginal adaptation
- Morphology of the preparation
What factors of tooth preparation influence retention? (4)
- Slight conicity
- Volume
- Type of restoration
- Unique path of insertion
How is conicity related to retention?
Inversely proportional
What is the ideal conicity situation?
Most retentive preparation is no conicity with parallel walls
What is the purpose of giving conicity to the preparation?
- Allows insertion and complete sealing of restoration
- Provides aqeduqate retention/friction
Why should tooth preparation in conicity be kept minimal? (3)
- Adverse effect on retention
- Preserves as much tooth as possible
- Limits the number of paths that a restoration can be removed
How many degrees do we prepare the tooth’s conicity?
3 degrees on each side
How do we prepare the tooth’s conicity? (3)
- Use tapered diamond bur
- Hold it parallel to the path of insertion
- Converge in occlusal direction
How does volume relate to retention? (2)
- Directly proportional
- More surface = more friction= more retention
How does height relate to retention?
More height = more retention
If a tooth is short, how can we maximize height? (3)
- Move the finish line apically
- Preparing the occlusal surface as little as possible
- Build boxes and guiding grooves
How does diameter relate to retention? (2)
- More diameter=more friction=more retention
- If a tooth is too narrow, can build boxes and guiding grooves in the proximal surfaces
External or internal retention: PFM crown?
External
External or internal retention: Inlay?
Internal
Define stability (2)
- to be firm/steady
- resist displacement by horizontal or rotational stresses
What factors influence stability? (3)
- Magnitude and direction of the force
- Geometry of dental preparation
- Physical properties of the cement
What forces interfere with stability? (2)
- Tangenital forces
- Forces due to interferences /pre maturities
What factors of tooth preparation affect stability? (4)
- Adequate conicity
- lateral boxes and grooves
- length : mesiodistal width
- length : faciolingual width
The MORE conical is the preparation, there
will be a higher probability for the restoration to …
Rotate
How does conicity relate to stability?
More conicity = less stability
- more removal paths
How does no conicity relate to insertion?
No insertion
When do we use lateral boxes and grooves?
When the conicity, height of diameter is not the recommended value
What are lateral boxes and grooves used for?
To reduce the freedom of displacement of the restoration
When do we prepare a box?
when there is not enough height. On the M and D surfaces
When / where do we prepare a groove? (4)
- When there is a lack of height and a wide M-D distance.
- Parallel to the insertion path.
- One groove
- In Buccal if lower molars, In Palatal if Upper molars
The length of a tooth must be great enough to…
Interfere with the arc of displacement
Stability: A higher preparation will prevent…
The rotation of the restoration
Stability: The length of the axial wall must be high enough to … (2)
Counter the length of the rotational radius
- depends on mesio-distal diameter
What kind of stability will a narrower preparation have?
More stability due to a shorter rotational radius
What Length to Facio-Lingual Width Ratio is needed? (2)
- 0.4
- 4mm height for 10mm wide
What is the importance of a length:facio-lingual width ratio on stability? (2)
- movements in a buccolingual sense
- (ex: lateral movements of
the jaw in a group function situation)
Define path of insertion (2)
- SPECIFIC DIRECTION the PROSTHESIS IS PLACED ON
THE ABUTMENT TEETH OR IMPLANT - Unique imaginary line determined before starting the preparation
What is the ideal path of insertion?
Parallel to the long axis of the tooth
The path of insertion is given by…
the preparation of the axial walls
The PATH OF INSERTION must be considered in two dimensions: (2)
FACIO-LINGUALLY
MESIO-DISTALLY
Path of insertion: facio-lingually? (2)
- Important in the anterior sector.
- path of insertion parallel to the long axis of the tooth.
What happens if the tooth preparation is too buccally inclined? (2)
- overcontoured restoration OR
- Unesthetic due to a lack of volume of the ceramics
What happens if the tooth preparation is too palatally inclined? (2)
- mechanical issues due
to a very short preparation OR - biological issues for invading the pulp chamber
How should the path of insertion be mesio-distally?
Parallel to the adjacent teeth so they don’t interfere with the insertion
Abutments in bridges should all be…
parallel to each other
What do we do if the abutment tooth of a bridge is inclined? (3)
- Follow the inclination of the abutments that are not inclined.
- May need endodontic treatment
- Orthodontic treatment
If you do not have a correct visual of the path of insertion is means..
There are retentive areas to correct
Structural strength depends on…?
Repescting and removing the correct volumes
What are the parameters to achieve structural strength? (3)
- Occlusal reduction
- Functional cusp bevel
- Axial reduction
What is important to remember with occlusal reduction in regards of structural strength? (2)
- Have correct volume, respecting the anatomy of the tooth.
- Must provide enough space for the material.
What happens when the occlusal surface is too flat?
Areas too close to the pulp and other areas with lack of space for the material
What is the functional cusp bevel thickness for PFM?
2mm
What is the functional cusp bevel thickness for gold?
1.5mm
What happens if the restoration material is not thick enough? (2)
- Perforation in the metallic crown
- fracture of ceramic in PFM crowns
What happens if the functional cusp bevel is thick enough but there is not enough prosthetic space? (2)
Pre-maturities and interferences
If you overcontour the axial surface, what will occur? (2)
- strengthens the restoration
- negative effect on the periodontum
Occlusal reduction of metallic crowns? (2)
Functional cusp: 1,5mm
Non-Functional cusp: 1mm
Occlusal reduction of metal-porcelain crowns? (2)
Functional cusp: 2mm
Non-Functional cusp: 1,5mm
Occlusal reduction of all ceramic crowns? (2)
Functional cusp: 2mm
Non-Functional cusp: 2mm
Axial reduction of gold crowns?
1mm (0.5 chamfer)
Axial reduction in anterior PFM crowns? (4)
Labial reduction: 1,2-1,5mm
Lingual Concavity: 1mm
Lingual Reduction: 1mm (o,5 chamfer)
Proximal 1mm
Axial reduction in posterior PFM crowns? (3)
Buccal reduction: 1,2-1,5mm
Lingual reduction: 1-1,5mm (0,5 chamfer if metallic finishing)
Proximal: 1-1,5mm (0,5 chamfer)
Axial reduction in all ceramic crowns? (3)
Buccal reduction: 1,2-1,4mm (1mm shoulder)
Lingual: 1mm
Proximal 1,2-1,4mm
