Transcript الشريحة 1 - جامعة فلسطين

Tooth preparation
Tooth preparation is defined as the mechanical
treatment of dental lesions, or injury of hard tissue of
teeth to restore a toth to orginal form
Adequte tooth preparation,can determine the success of
fixed prosthesis .
Principles of preparation
Biologic
-
Pulp
consideration
- Periodontal
consideration
Mechanical
esthetical
- Preservation of
structure tooth
- Color selection
- path of insertion
- Restoration design
- Retention and
resistance
- Adequate reduction of
tooth structures
- Marginal integrity
(finish line )
- Smoothing and
rounding the angles
-Type of material
Mechanical principles
1- preservation of tooth structures
- minimal reduction
- partial veener crown or laminate (partial coverage)
versus full veener crown (full coverage)
- Supragingival margin versus subgingival margin
Supra-ging
At crest
Sub-ging
2- path of insertion
It’s the specific direction in which a prosthesis is placed
onto the abutment or removed from the abutment . Any
diviation (undercut) from this line will result in distoration
of patterm wax and failure of restoration seating
- Axial reduction must be with Taper
- facio-lingual reduction
{ height of contour must be removed to eliminate any divergent (undercut)
with taper 5 – 10 “ }
- mesio-distal reduction (proximal)
{ removal of divergence in occlusal third to eliminate any undercuts , and
mesio-distal inclination (taper 5-10”) of the path must parallel to adjacent
proximal contact of adjacent teeth }
Taper
its defined as a convergance between two opposing axial
walls , in occluso-axial junction
Two opposing surfaces , each with a 3 degree of
inclination would give the preparation a total of 6 degree
taper
taper must be 5- 10 degree of inclination
Why tapered ?
1- To visualize preparation walls
2- To prevent the undercuts
3- To compensate for inaccuracies in
the fabrication process
4- To permit complete seating of the
restoration during cementation
Taper
More tilting toward the
tooth create excessive
convergence(Taper)
tilting bur slightly toward the
tooth create taper 5 – 10 C”
Weak
retention
Undercut
Its defined as a divergence between opposing axial walls , in a
cervical –occlusal direction . If the cervical diameter of tooth
preparation at the margin is narrow than at the occluso-axial junction
(reverse taper) , it well be impossible to seat a complete crown
Undercuts can be present wherer tw axial walls faces in opposite
directions
For example :- the mesial wall of a complete crown prparation can
be undercut relative to the distal wall , and the buccal wall can be
undercut relative to the lingual wall
Tilting the bur
away from tooth
create undercut
No setting
3- Retention and
resistance
Retention
It’s the quality of the preparation to prevent removal of
the restoration along the axis of the tooth ( its path of
insertion )
Retention depend on the factors :1 - magnitude of the dislodging forces
2 - geometry of preparation
- taper preparation
- duplicating anatomical form of occlusal surface preparation
- type of prparation (full crown more retentive than partial
crown)
3 - roughness of the internal surface of the restoration
( air-abrasion – oxide layer --
microretention
4 - type and film thickness of luting agent ( cement)
Resistance
It’s the ability of the preparation to prevent dislodgement of the
restoration by forces , directed in apical , oblique or horizontal
direction
Factors affecting resistance :1- length of preparation
The more occluso-gingival dimension ,the more resisatnce
resistance is increased due to the amount of tooth structure interfering
the arc of rotation with (lateral or oblique force)
2- Taper
The resisatnce is increased by decreasing the taper of the preparation
3 – Ratio of diameter to length of preparation
There is a balance ratio between the diameter and height ,which provide
more resisatnce
4 – occlusal irrgularity of reduction
Duplicating the anatomical form of occlusal surface during preparation ,
increase the resisatnce .(flat reduction of the occlusal surface of the tooth
may decrease the resistance )
4 –adequate Reduction of the tooth structure
Steps of tooth reduction (preparation)
Occlusal (incisal) reduction
Axial reduction
- buccal (facial)
- lingual (palatal)
- mesial
proximal
- distal
1- occlusal (incisal) reduction
Occlusal and incisal reduction is preformed toprovide
adequate clearance between he preparaed surface and
opposing teeth .
The occlusal and incisal surfaces must be reduced by minimum
1,5 mm clearance for (full metal crown) and 2mm clearance for
(full ceramic and metal ceramic crown)
To allow:- adequate thickness of restorative material (metal – ceramic)
to be enough strong (to prevent fracture of crown)
- To form anatomical morphology in occlusal surface of the
restoration to be more esthetic
Adequate clearance
more strength of restoration
Adequate clearance
adequate anatomical form
and more chewing effect
more esthetic
- Occulsal depth cuts
Occlusal cut acts as a guide for reduction ,and placed using
round bur No2 (1,5mm) or by round ended-taper bur on the
grooves and cusps
Depth cuts
To complete occlusal reduction the remaining tooth structures
between the depth cuts is then uniformly removed by (round endedtaper bur ) or by (wheel-shaped bur ).
The reduction of occlusal surface should be uniformly and follows
anatomical contour ,which permit to increase the retention and resistance of
the restoration
Note : flat plane reduction of the occlusal surface may be acceptable in
older patients with attretion on the occlusal surface
- Functional cusp bevel
Its provided in the functional cusp area (deep contact of occlusion) to
increase the thickness of the restoration in occluso-axial junction
The functional cusp is reduced more than nonfunctional cusp for 0,5 mm
Note:
Functional cusp bevel in the mandibular molars
buccal cusps
Functional cusp bevel in the maxillary molars
palatal cusps
Funct. Cusp bevel
Lack of functional cusp bevel may
produce several problems :1- can cause a thin area or perforation
2- may result in over- contouring and poor occlusion
- Evaluation of the clearance
To evaluate clearance between the preparaed and opposing teeth
, the mandible and maxilla must be closed in centric occlusion ,
1,5 – 2 mm of interocclusal space must present , this space space
can be measured by using the strip of utility red wax hwn the
patient in bite or by using reduction gauge its should pass
through the occlusal surface without obstraction
Complete reduction of occlusal surface
Axial Reduction
Axial reduction commonly includes the entire circumference
of the tooth ( buccal – lingual – mesial distal surfaces )
Buccal and lingual
reduction
The buccal and lingual surfaces must be reduced so that the
restoraton can be reestablish tooth contour `
- Depth cuts
for adequate and uniform reduction is achieved , can use the depth
cuts in buccal and lingual surfaces as guide to preparation . Depth
cut of reduction in the buccal and ligual surfaces 1 – 1.5 mm . By
taper round-ended diamnod bur makiing 2 -3 depth cuts equally
placed along the mesio-distal dimension of buccal and lingual
surfaces in the posteriro teeth .
Placing depth cuts
To complete the preparation of buccal and lingaul surfaces in
posteriro teeth , use the taper round – ended diamond bur , to
remove the reaminimg structures between depth cut to obtain
uniformly reduction . The bur should be tilted toward the surface
during reduction to obtain taper .
The reduction of buccal and ligual surfaces should be taper 5 – 10
degree occluso-gingivally with finish line ( chamfer or edge – knife )
tilting bur
slightly toward
the tooth create
taper 5 – 10 C”
In the buccal and lingual surfaces the hieght of contour and any
divergence should be removed to eliminate the undercuts , the over
reduction of the axail surfaces will lead to loss of retention .
Tilting the
bur away
from tooth
create
undercut
No setting
Hieght of
contour
and
undrcut
More tilting
toward the
tooth create
excessive
convergence(
Taper)
Mesial and distal (proximal)
reduction
The reduction form of the proximal walls ( surfaces) , should be
taper occlusogingivaly 5 – 10 degree with chamfer or edge-knife
finish line
Reduction of proximal surfaces complete by using the taper
round-ended diamond bur with tip 0.8 mm , interproximal space
must be 0.8 – 1mm and not less than 0.6 mm . Insufficient
reduction of the proximal walls leads to inadequate embrasure
with predictable periodontal implications
Access to proximal reduction
Care must be taken during the proximal reduction to avoid the
damage of adjacent tooth or restoration with rotary instrument .
- break the contact with adjacent teeth by carbide bur 169L or by
needle diamond bur , work bur through the proximal area in
occluso-gingival and buccolingual “sawing’ motion until gingival
contact is broken with adjacent teeth .
break the contact with
adjacent teeth
Making or creating “flanges or enamel lip” on the buccal and ligual surfaces during their reduction , which serves as a guide to avoid
the contact with adjacent teeth and help to complete the preparation
of the proximal surfaces .
Using the matrix band – matrix band can be placed around the
adjacent teeth to protect it from the abrasive contact
Note :- Tapare round –ended and needle diamond burs are
used to reduction of proximal surfaces
Preparation from
buccal or lingual
to proximal
Flange
or
enamel
lip
Inter-proximal reduction
not les than 0.6 mm
Formation of finish line
Finish line
It’s the point at which the preparation terminates on the tooth
Finish line serves many functions
1- its delinate the estent of the cut in apical direction of the
restoration
2- the finish line is one of the features that indicate and evaluate
the precision (correctness) of impression
3- the correct marginal adaptation of the wax pattern depends on
an obvious finish line
4- evident finish line helps in the evaluation of quality of the die
Finish line location
Supra –gingival location – finish line located above the gingiva and its
easy to manipulation and save for periodonatal tissue (more healthy ) but (
non- esthetic)
Sub-gingival location – finish line located in the free gingiva ( sulcus)
requires special care to avoid excessive soft tissue trauma from rotary
instruments , the gingiva must be carefully held out of contact with bur
by placing a retraction cord in the gingival sulcus ( high esthetics ) .
At the gingiva located at the gingivl margin border its not helathy
because causing secondary caries ( poor oral hyeigyne)
There are four basic types of finish lines
1- edge –knife (chesil)
Its an extremely thin finish line , this line terminate on the
cementum and formed by needle diamond burs or by thin
flame . Its used in the posterior teeth for ful metal and gold
crown , used for inaccessible areas of oral cavity (limited mouth
opening ) , and used in drifting or maloaligned teeth in mesial
surface
- difficult to wax up and cast
- difficult to produce smooth margin
- susceptible to distoration
2- Chamfer
It’s a concave extra-coronal finish line that is possess a curved
slope from the axial wall till margin . The chamfer formed by
taper round ended bur , chamfer used for cast metal crown ,
metal-acrylic veneer crown , metal ceramic crown in the
posterior teeth and in the lingual margin for metal-ceramic
crown in the anterior teeth { chamfer seem as a negative image
of a tip of round-ended bur }
- less stress and good success rate
- improper (excessive deep) reduction will produce an desirable
fragile piece of enamel (lipping)
Chamfer
depth
0.5mm
- shoulder
subgingival located finish line , which extend in gingival sulcus (crevice) to
.05 –o.8mm and has 1mm depth .it should be terminated on enamel .
Shoulder formation require using the retraction cord to prevent gingival
injury during procedure , retraction cord placed into gingival sulcus , the
cord provides about 1mm gingival displacement , which allows the
shoulder to be formed at the level of cord and located cervically without
trauma from rotary instruments
.
Shoulder is formed by using flat-end bur , and used for metal ceramic and
all-ceramic
Restorations ,specially in anterior teeth to gain the restoration more
retention and very
Esthetic appearance
0.8-1mm(free gingiva)
1,5mm(attached gingiva)
Supracrestal fiber tissue
Placing retraction cord
Cord is placed with
gingiva and
retracted
forming shoulder
Cord packer
Retraction cord
(ultra-pack)
Cord is removed and
completely
formed
shoulder
Impression with
negative details of
shoulder
Beveled shoulder – its modification or simislar to shoulder with bevel about 0.2mm of
width ,and forms an angle of 45 degree relative to the floor of shoulder .
. This finish line improve marginal adaptation of the restoration .
beveled shoulder indicated for all-ceramic and metal-ceramic restorations
To form beveled shoulder , too need a retraction of gingiva to avoid gingival trauma
Beveled shoulder is formed by using bevel-end cylinder bur or by torpedo bur
1 mm
0.2mm
0.2 mm
2- 3 mm
Bevel-end
cylinder bur
Bevel with 2mm
and 45C` angle
Ging.-sulcus
smoothing the preparation and rounding line angles
All prepared surfaces should be smoothed by using fine diamond bur or
by carbide bur to remove any rough areas and undercuts.
The sharp line angles should be rounded , this procedure reduce the
stress that develops when occlusal forces are applied to the
restoration .
When the wax pattern is invested that is less possibility to air will be
trapped and cause internal nodules on the cast .
The smoothing of the prepared tooth surfaces achieved without using
water spray to maximize visual observation . Using the air as a coolant
and short intermittent contact of abrasive instrument can avoid pulpal
irritation .
Smoothing all surfaces and rounding all
sharps lines with 45C` angles
Smoothing surfaces and
rounding incisal edge and
lingual lines