Crown and bridges lec 4

5/3/2013

Designing and Planning Crowns and

Bridges

If somebody comes to your clinic with an old bridge, and then this bridge fall, don’t

ask your patient:"shall I replace it and make a new one?" until there is a strong

evidence that it should be replaced, so you should ask yourself :"can I make the

patient's life easier? More comfortable?", because once you cut the tooth it's

irreversible, you need to plan before you start, you know that the bur turns half million

cycles per minute, so when you use it don't rush without planning.

Principles of crown preparation design:

-Materials:

a. metal (noble/base metal) : if you want to make the crown of metal minimal

reduction is required (0.5 mm).

b. Porcelain: if esthetics is needed and you will make all-porcelain crown, then you

need not less than 1mm of reduction, of course functional cusp bevel needs more, and

if you want to make ceramo-metal crown it needs not less than 1.5mm

so you must decide the type of material you want to use before you start preparation.

Function:-

a) occlusion: suppose you have a patient with class II devision 2 , and you want

to crown an upper tooth, which means there is no enough space to put ceramo-

metal crown, you will think of metal, you can make the labial surface of

ceramic and the palatal surface of metal, all of this should be planned for before

you go ahead.

b) Future wear: if your patient has bruxism , then it's better to go for metal, and

if esthetics is also required (remember it's multifactorial decision) you do

ceramo-metal, but in this case more structure should be removed to

accommodate for more ceramic and metal.

- Appearance:

a) Buccal, incisal and proximal reduction: if it's upper central incisor and you

need incisal translucency, and you didn't remove enough structure , the

technician will make the crown over-contoured to achieve translucency because

metal will show through the ceramic, so ceramic should be thick enough.

b) Occlusal reduction of posterior teeth: for mastication ,better function and

structural durability, so you need more reduction. (actually, I don't know what this

has to make with appearance!! )

c) Crown margins: if esthetics is important you go for subgingival crown,

when we say "subgingival" we mean just to cover the finish line by the gingiva,

do NOT go further.

tooth structure but remember the 1st principle in tooth preparation is

"preservation of tooth structure" in this case it is better to make resin bonded

bridge, also you can make partial denture, implant, or you can do nothing.

-Adjacent teeth:

a) Clearance to avoid damage to adjacent teeth: sometimes teeth are very tight,

and maybe with some overlap, so you should over reduce the proximal surfaces

to prevent damage to adjacent teeth.

b) Path of insertion: if you have a bridge and two abutments which are not

parallel, you have two choices, either fixed-movable bridge, or resin bonded

bridge ( because resin bonded bridge is fitted on the palatal surfaces of the teeth

only), suppose that for other reasons fixed-fixed bridge is the choice, so you

need to do over reduction on one tooth to achieve parallelism.

c) Technical consideration: this is mentioned in the book, it talks about

sectioning of the teeth in the lab, however the doctor doesn't believe in this, as it

doesn't justify the loss of tooth structure.

Look at the canine in this photo,

the incisal third is wider than the

cervica third, now suppose that the

lateral incisor is missing and you

want to replace it and prepare the

canine, so you should reduce the

size and eliminate the undercut,

and because the cervix is narrow

and you need to get rid of the

undercut you will end up with no

tooth structure,

We all know that old pulp is smaller than younger one, (actually it's not that much, but

what counts is gingival recession), so if you want to make a crown for a 13 yrs old

patient you must take an x-ray to see how close you are to the pulp, if you are worried

then you can make metal crown with minimal preparation.

Note: **

-You can make a crown for a young patient but NOT a bridge , because bone grows in

young patients, but teeth do not.

Periodontal tissue:-

Before you start the preparation you should check the abutment tooth and the

periodontium, and you should take an x-ray at least for the adjacent and opposing teeth

to check bone level.

-Path of insertion:

As mentioned earlier in this script, if you want to make a bridge and the abutment

teeth are not parallel you may need to do over reduction in one tooth,( parallelism is

important to have a path of insertion) , so you may think about endangering the pulp

and if this can be avoided by metal crown… and so on, so you should think about all

of this.

Look at this picture, the first

premolar is heavily carious, and

pulp recession is expected , if you

want to prepare this tooth you will

endanger the pulp, in such case it's

better to make a post crown, also

by post crown you can restore

badly destructed teeth (like the one

in the photo) because there is a core

foundation .

Criteria for selecting a bridge design

Support:-1

Here it comes ante's law which we will talk about it soon.

If you have a patient with 4 missing upper incisors, the bridge will be from canine to

canine (canine is a strong abutment), so if the span is long you should think of a

strong bridge which is fixed-fixed, and if there is no parallelism the choice is fixed-

movable bridge.

Conservation of tooth structure:-2

The most conservative bridge is resin bonded.

3- Appearence

4- Cleansability

ANTE'S LAW:

(which we will invalidate)

"The surface area of the roots of the abutments should be equal or more than the

surface area of the roots of the missing teeth."

If 2 central incisors are missing, and lateral incisors are still available, according to

ante's law we can't make a bridge, because the combined surface area of the central

incisors is more than the combined surface area of lateral incisors, but in reality it

works, and it's successful.

So here it comes that we should oppose this law.

So what is the mechanism behind Ante's law?... look

Normally: If a force equals (X) is exerted on the tooth structure this will be resisted by

the same force (X).

-If we have 3 unit bridge: we have three Xs, so we should increase the strength of the

2 abutments to resist the force, this is the situation in bridges ( real bridges), but if this

occurs in human's mouth (if the tooth resist more than it can) this will result in trauma

and damage to the tooth.

- What occurs in fact is different; in the PDL we have proprioceptors (receptors which

are important in determining the position of the mandible and bite forces), when you

bite proprioceptors send massage to the muscles of mastication to stop, so that why

sometimes when you bite hard you suddenly open your mouth, so it is not a constant

force, there is physiological control on mastication, this is the difference between

human being and a bridge

(اللي بمشوا عليه السيارات)

Planning bridges:

1- History and examination : for example if the patient has a dental infection, or if

he is mentally compromised…. In general crowns and bridges take more time and

sessions than partial denture.

2- consideration of the whole patient: cost, attitude, occupation…

3- Assessing abutment teeth: if it is carious, vital or non vital, if there is a

periodontal disease…

4- Length of the span: If it is long then you need fixed-fixed bridge, because it is

strong.

5- Occlusion: you should see if there is a space for the pontic, sometimes the

opposing tooth is supraerupted then there will be no enough space for the pontic.

6- Shape of the ridge: if the ridge is resorbed in the area of the missing tooth and you

put a bridge, there will be a window above the pontic, in this case you should think of

partial denture.

Predicting final result

By study casts, selecting the design, and wax-up, sometimes you may decide to change

the design during the wax-up, but if you change it after tooth preparation and

fabrication of the final design this will be very late.

In early duckling stage (during mixed dentition) it's normal to see a midline

diastema, and this shouldn't be treated, so if you have a patient like this you

should tell him that this is acceptable, and it will be corrected later.

Types of burs used in preparation

1- Chamfer bur (parallel and tapered) : made of diamond, the shape of the tip is

like the chamfer, so don't exert pressure, once you select the correct bur (type and size)

with the correct direction you will have the shape of the finish line you want.

Chamfer bur is used for metal crowns.

2- Feather edge bur: not used a lot, but still some dentists use it, however we don't

believe in it, because there will be no finish line to put the crown on it, so it will be

over-hanged, we call it "positive margin".

3- Shoulder bur: used for shoulder finish line.

4- Shoulder beveled bur

5- Tapered interproximal bur : made of diamond (tungsten carbide burs are only

used for grooves), used to cut proximally, to open the proximal spaces.

6- Flame shape bur: for palatal side (cingulum preparation), also you can use the

wheel, but the doctor prefers to use the flame shape bur.

- you can start preparation anywhere, they say you should start incisally, but the

doctor doesn't favor this, he will learn us how to decide……………….

THE END

Done by: Lana Zedan