cons sheet # 5 - RUAA KHAIRI

بسم الله الرحمن الرحيم

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Class І cavity preparation

In the following cases the amalgam restoration is used in:-
1) Moderate to large cavities
2) Restoration is not in highly esthetic area [as we know the amalgam has silver color that with time tirn to black so is not prefer to use it in esthetic zone]
3) Heavy occlusal contact area since the amalgam can tolerate in contrast to other restorative materials
4) The tooth can not isolated properly, although amalgam is sensitive to moisture but not as sensitive as other restorative materials
• NOTE: difficult to isolate doesn't mean that the cavity full with fluid if it is so we shouldn't restore and we must wait
5) In the case of making RPD { removable partial denture }, abutment teeth are those who hold the denture in place we can use amalgam to restore them

In the following cases we can't use amalgam:- { from the
Handout}


Outline: is the external outline of the cavity on the occlusal surface, basically it is the enamel margins of the restoration.


● One of the guidelines that is followed in class І cavity preparation:- we are follow the pits and fissures at same time the cavity must centered over the fissure, placed equidistance from buccal and lingual sides.

About the pulpal floor

◘ Pulpal floor of class І cavity preparation should be placed on dentine, the depth equal 1.5 mm from the central fissure to the pulpal floor [we can say also 2 mm from the cusps inclines]

◘ The pulpal floor should be flat, smooth and parallel.
Why flat to the occlusal plain of the tooth? → This is related to the resistance form, when there is any sharp angle will result in stress concentration on the amalgam and will break either the amalgam or the tooth

Why parallel to the occlusal plain of the tooth? → To maintain uniform depth of 1.5 mm.
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The walls should be parallel to each other [buccal and lingual walls mainly] or slight undercut and perpendicular to the pulpal floor, there is two exceptions we will discussed them later in the lecture.

■First exception:- the mesail and distal walls should be flare (tapered) {not undercut}

Buccolingual width = 1.5 mm equidistance from the fissures when we are in the center as we move towards the buccal and lingual fissures 1 mm is enough for the amalgam.

■Second exception:- the extremities of the facial and lingual grooves and that portion of the walls adjacent to the distal and mesial marginal ridges are prepared at 95 degrees to the pulpal floor[that is as same as flare]

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We use bur no.245 since this make slight undercut by itself (as mentioned before we should make undercut on the buccal and lingual) the shape of this bur is inverted-cone bur.
Bur no.245 dimension:-cutting edge = 3mm(length of plate)
Width = 1.5 mm

Bur no.330:- bear shape bur similar to the bur no.245 but smaller so it is used in highly conservative restoration.
STEPS

The initial penetration will be with bur no.245 , put in perpendicular to the fissure on move it along all the fissure

{Commonly we start the penetration on mesial fissure and finishing with distal fissure} the arrangement will be
D→B & L →M

What we are doing is restorative amalgam restoration so the
Width of the cavity should not exceed the bur width (equal to 1.5 mm).

Finished the cavity preparation with low speed bur.
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What is the thickness of marginal ridges at the end of the cavity?
Answer: 1.6 mm in premolars
2 mm in molars

Let suppose we exceed this dimension the restoration will break because there will be no dentine support for the restoration, when this happen we change the cavity from class І to class ІІ cavity

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Isthmus: the areas connecting between cusps inclines
{Like the narrow part area between buccal and distal extremities}.
The isthmuses are very important to keep in minimal width of 1 mm which is the size of smallest condenser we have.

Remember: whenever facio-lingual extension increases the resistance form decreases.

NOTE: amalgam margin is 90 degrees but around (85-95) degrees is acceptable less than o more than this either amalgam restoration or the tooth break.

How can we get the 90 degrees requirement?

→ The enamel rods on the occlusal surface has anatomical convergence if we followed the enamel rods we will ensure to get 90 degrees requirements and in same time we get undercut.

Slight variation of class І cavity
Preparation


Upper six: the variations result from the anatomy of this tooth, since there is oblique ridge and almost the caries start mesial and distal to the oblique ridge, the oblique ridge very important to maintain tooth structure so we try to preserve it.

● There is pit on the lingual surface of the upper six, the caries start from this pit and extend to the fissures (in this case we open the lingual surface in addition to the occlusal surface)
{This is a variation}

● The distolingual cusp in the upper six is the smallest cusp so if we make the margins of the cavity equidistant from the fissure this will result in weakness of the cusp , in order to avoid that and since we have strong oblique ridges we can shift the outline form towards the oblique ridge { another variation }.

● We make the flare since we are close to marginal ridges because of the nature of the enamel rods { see pic no.38 in handout there is flare on the distal surface but there is no flare in the lingual fissure because the last it isn't on the occlusal only it is continue to the lingual surface }.

Remember: That class ІІ cavity on the proximal surfaces not the lingual.

■ pic no.41 in handouts show the tapering (flared) only on the distal & distal walls only. {The Doc. Says it is seems that the other surfaces are tapered but it is wrong only the distal and mesial}

■ Pic no.42 in handout the angle is obtuse why?
Answer: we want uniform depth (thickness) and the lingual surface anatomically has an obtuse angle so when we penetrate we follow the anatomy in order to have uniform depth of 1 mm and must be parallel to the external tooth surface.

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The thickness of the groove on the lingual 1 mm is enough???
→ the amalgam in areas that have occlusal contact must be thick (1.5 mm), in the lingual we don't have occlusal stress so 1 mm enough.

Axial wall: any surface parallel to the long axis of the tooth & vertical called axial wall.


When we stop the cavity in anther words when we know there are no more caries?
Answer: → the difference between the dentine & enamel
Color.
→ the softness of dentine indicate there is caries
If it is hard no more caries
There are other indications we took it before.



◘ Page 21 handouts pic A is lower five (2nd premolar)

◘ lower six: there is pit on the buccal surface so if it is caries we open the buccal surface in addition to the caries fissure {upper six the pit on the lingual}

◘ lower four: the lingual cusp is shallow (مضمور)
So the caries will be only in the mesial and distal pits so it is easy to restore.


THE END



DONE BY: RUAA RAWA KHAIRI
LEC NO. 5
DATE: 18/10/2010