Crown Preps

Full coverage restorations should fulfill the following objectives:

1. Function

2. Esthetics

3. Preservation of periodontium

4. Marginal integrity

5. Preservation of tooth structure

Preservation of Tooth Structure

full cover restorations are only done when necessary

do not over-prepare

design your restoration to protect the remaining tooth structure

Is preservation of tooth structure critical/necessary when creating crown preps?

yes

How do you control the amount of reduction?

use matrix and depth cuts

these let you know how much of the tooth will be reduced

What can be used as a matrix?

thermoplastic sheet

To be sure of your depth cut, you should ...

measure diameter of burs

How can you measure this?

using a crown gauge or dental caliper instrument

T/F: indentations (depth cuts) made into the tooth should span the entire bur length

false

closer to half of it

Depth Cuts

aiding cuts to let us know the extent that we can go to... without these, we risk cutting too deep into/down the tooth

Restoration crown needs these in order to function appropriately:

Marginal adaptation: a

seamless meeting of the

restoration & tooth

Occlusal harmony

Patient satisfaction: free

from pain & sensitivity,

contours compatible

Not providing enough reduction leads to there being ___ in the crown design, amidst additional stress

perforations

___ also need to be considered when choosing/making a crown — a match should be found with the adjacent tooth

Esthetics

Esthetics is determined by the ___ and ___ of the restoration blending in with the adjacent dentition

shade

contours

The shade is influenced by the ___ and it's ___

material used

thickness

Thickness of the material is influenced by ___

the amount of tooth reduction

You need the restoration to function for years... this means the restoration must have good ___

longevity

the materials used should

have the strength to hold up to the oral environment

without complications

What are some potential complictions?

fracture of materials, early

cement failure, shade

stability, marginal decay

T/F: even if a crown fits a tooth, if certain

principles are not followed, the crown may not be sufficiently retentive and will

eventually fail

true

you want to prevent this from happening

What are the principles of crown preparation?

Amount of tooth reduction

Preparation geometry

Finish line geometry

Principles of preparation are based

on the ___ the prosthesis

materials used to fabricate

Different materials have different ___ requirements

tooth reduction

There are two different materials that can be used to create a crown:

all ceramic

PFM

What does PFM stand for?

porcelain-fused-(to)-metal

has cement layer, followed by metal layer, followed by porcelain outside

T/F: a porcelain fused to metal crown has different requirements than an all ceramic crown

true

this is what was being said before

Which crown type requires more reduction?

PFM

PFM has a core of a ___ with a bonded layer of ___ that provides the esthetics

metal coping

procelain

The metal must have sufficient ___ to resists flexure

thickness

The porcelain requires a certain thickness to provide an esthetic result and ___

resist fracture

To achieve these respective thicknesses, the amount of ___ is governed by the thicknesses required

tooth reduction

as you prep for the placement of a crown

Does an all ceramic crown have metal to support the ceramic material?

no

"all" ceramic

Ceramics tend to be ___ so once again, the material will affect the amount of reduction and the preparation and finish line geometry

brittle

need enough thickness to 'overcome' brittle nature

There are different types of ceramics, clearly. Give an example of one:

Zirconia

Every time a patient functions (i.e. chews),

they apply 'forces' on the materials (___ and ___)

metal

porcelain

(or ceramic)

it is our job to find out the 'ideal' thickness of each material to resist breakdown (i.e. flexure of metal/fracture of porcelain)

What are some of the various types of forces that can be applied to a material?

tensile elongation

compressive shrink

shear distortion

torsional twist

flexural bend

Porcelain has a high MOE and is very brittle — this makes it more resistant to ___ than to ___

compression

tension

(can't deform, simply fractures)

Metal has a lower MOE and is very ductile — hence it will ___ prior to ___

deform

fracture

In order for a metal-ceramic crown to be strong and durable (i.e. resist porcelain fracture), the ductile metal and brittle porcelain must be of ___to withstand the applied forces

sufficient thickness

The metal coping should be at least ___ thick

0.5 mm

The porcelain layer is ideally about ___ thick

1.5 mm

There must be a balance between the amount of reduction required by the

materials with the amount of reduction ___

a tooth can tolerate

T/F: each material requires a minimal thickness

true

So 0.5 mm is needed for metal... and 1.5 mm is needed for porcelain... can we reduce by 2.0 mm and not damage the tooth?

sometimes... but it may be risky other times...

What factors do we need to consider in terms of risk?

Preservation of pulp vitality

Optimum thickness of materials for function and longevity

Optimum thickness of materials for esthetics

Tooth fracture

___ are variable sizes in different teeth

Pulp chambers

___ of tooth is also significant in determining pulp chamber size

Age

___ pulp chambers are larger than ___ pulp chambers

Young

more advanced

So, reducing by 2 mm on a child's tooth — or a worn down tooth — may cause for you to hit the pulp chambers?

yes... this is bad news

There is a significant chance of restoration fracture, as well, if the remaining tooth substance is not ___ enough

robust (thick/defined)

What is the average thickness of tooth structure overlaying the pulp in the occlusal direction?

5-7 mM

What is the average thickness of tooth structure overlaying the pulp in an axial direction?

2.5 - 3 mm

How much occlusal reduction is required for a metal + porcelain (PFM) crown?

2.0 mm

Why 2 mm?

you need to have enough thickness for porcelain to be placed in an esthetic manner

2 mm is biologically safe

Cements are stronger in ___ than they are in tension and shear

compression

Any forces that act to dislodge the crown create ___ in the cement layer

tension

The shape or 'geometry' of our preparation is designed to maximize the longevity of the dental cement

by placing as much of the cement layer in '___' as possible

compression

(instead of/over tension)

What are some of the geometric features of the preparation>

1) Total occlusal convergence

2) Preparation height and width

3) Shape/circumferential outline form

4) Auxiliary retentive features

T/F: no single feature of the preparation geometry will

act alone to maximize the properties of the

cement layer

true

Maximum retention is achieved when there is ___

only one path of insertion

(imaginary line along which the restoration will be placed onto or removed from the preparation)

Taper

gradual diminution of width or thickness in an elongated object

gives us maximum retention if placed properly

A taper occurs when the circumference of the preparation ___ in the apical direction

increases

What is the maximum degree taper that we want for a crown prep?

20 max

with ideal range being 6-12 degrees

The total occlusal convergence is the sum of the taper of the ___

two opposing axial walls

(mesial and distal)

It becomes harder to ___ a crown when taper is applied to the prep underneath/supporting it

dislodge

Retention

the ability of the preparation to resist the crown restoration from removal along its path of insertion

Resistance

the ability of the preparation to resist the dislodgment of the restoration by forces directed obliquely or horizontally to the

restoration

A parallel-walled preparation without any taper does not facilitate ___ flow

cement

Cement is a fluid, and considerable ___ can build, preventing complete seating of a crown on a preparation

hydraulic forces

Another problem of parallel-walled preps without taper is that there will always be ___ with these designs

undercuts

these affect seating of the crown

An undercut occurs when the diameter of the preparation ___ in an apical direction

decreases

When evaluating for presence of undercuts, you should ...

look at prep with one eye closed, then examine prep with both eyes open at the same time

In an actual crown prep, an undercut appears as a more ___

gradual slope

It is possible to ___ an undercut to prevent seating issues of a crown

blockout

How can you ensure that your prep has no undercuts?

need to be able to see the whole shoulder, wrapped all the way around the tooth, from a bird's eye POV

The height of the prep can influence the amount of cement under compression... the (taller/shorter) prep has better resistance, all else held the same

taller

The width of the prep also have an influence... the (thicker/narrower) prep has better resistance, all else held the same

narrower

The minimum prep height for a molar should be ___

4 mm

Premolars and anterior teeth, being narrower than the wide molars, should have a minimum prep height of ___

3 mm

When viewed from the occlusal, teeth resemble shapes with their crowns... which shape is good for resistance and which shape is bad?

square/rectangle

triangle

Line angles should be maintained... explain how?

as a rounded, definitive LA

not a sharp corner

When the preparation taper becomes excessive, or the crown prep is not short, auxiliary retention is

generally recommended to improve resistance form:

1. Cervical less-tapered wall

2. Box/groove placed at 1

When preparation taper becomes excessive,

auxiliary retention can be achieved by reducing the

taper in the ___

cervical third