Manual Flashcards
preparation
the final shape (form of the tooth), produced by instrumentation, to receive a restoration
complete veneer crown preparation
extracoronal preparation involving the entire clinical crown
complete veneer crown (CVC)
a cast-gold extracoronal restoration which covers the clinical crown
path of withdrawal (POW)
the imaginary line along which a casting is moved when it is separated from its prepared tooth. For single crowns the path of withdrawal should be nearly coincident with the tooth’s long axis, which can be determined clinically by bisecting the angle formed by opposing axial surfaces in the gingival third of the tooth
axial wall
part of a crown prepared in the long axis of the tooth; can be designated as buccal, lingual, mesial, or distal
convergence angle
the angle or taper formed by diametrically opposed axial walls
chamfer
the cervical junction of the prepared axial wall and the unprepared portion of the tooth structure; the chamfer extends around the most cervical portion of the tooth preparation
line angles
the lines formed by the junction of any two tooth surfaces; should be rounded in complete crown preparations: axial wall line angle occlusoaxial line angle cavo-surface line angle
axial wall line angle
the junction between two axial walls
occlusoaxial line angle
the junction between the occlusal surface and an axial wall
cavo-surface line angle
the junction of prepared (cut) to unprepared (uncut) tooth structure; this is an external line angle
depth orientation grooves
grooves placed ont he surface of the tooth to provide a reference to determine when sufficient tooth structure has been reduced
functional cusp bevel
a wide bevel placed on the functional cusps; the thickness of the tooth structure removed when preparing this bevel ensures that correct contours of the final restoration can be re-established while providing correct convergence and adequate thickness of restorative materials
undercut
a crown preparation is undercut if a wax pattern cannot be withdrawn from its die without distortion. This may be caused by:
a) a depression in an axial wall caused, for example, by a carious lesion
b) diametrically opposed axial walls that do not converge occlusally caused by improper angulation of the cutting instrument
Undercuts of type (a) can often be corrected with a base material but type (b) undercuts require further tooth reduction.
occlusal clearance
the distance between the occlusal surface of the prepared tooth and the occlusal surface of the opposing tooth
this clearance must be evaluated during excursive jaw movements, as well as in intercuspal position
retention form
the characterisitics of the tooth preparation which tends to resist the removal of a restoration along its path of withdrawl
resistance form
the characteristics of a tooth preparation which tends to prevent dislodgement of a seated restoration by forces directed in an apical or oblique direction
resistance form prevents dislodgement of the restoration by occlusal forces
advantages of complete veneer crowns
- high strength
- longevity
- high resistance to displacement
- ability to modify axial contours and occlusion
disadvantages of complete veneer crowns
- display of metal
- removal of tooth substance
- vitality testing
- margin close to gingival tissue
indications for complete veneer crowns
- extensive destruction by caries or trauma
- endodontically treated teeth
- large existing restorations
- maximum retention needed (eg long span fixed partial denture abutment)
- recontouring of axial surfaces
- correction of malinclination
- correction of occlusal discrepancies
- to provide contours suitable for a removable partial denture
contraindications for complete veneer crowns
wherever a tooth can be adequately restored with a more conservative restoration
principles of tooth preparation
- Conservation of tooth structure
- marginal integrity of the restoration
- retention and resistance
- structural durability
conservation of tooth structure
Minimum tooth reduction compatible with adequate strength and correct contour of the restoration. Excess reduction will occur if:
- the tooth is prepared with excessive convergence angle (recommended range 2-10˚)
- the preparation does not follow the anatomical features of the tooth
- the path of withdrawal diverges from the long axis of the tooth
- an excessively wide margin (ideal width is 0.5mm)
- excessively gingival extention of the preparation (ideally no further cervically than retention form and existing restorations or defects dictate)
marginal integrity of the restoration
the tooth-restoration interface at the gingival margin is a potential site of failure due to cement dissolution or roughness. The preparation should be designed to enable a smooth, strong, well-adapted casting to be provided. Inadequate marginal integrity will occur if:
- the chamfer is too narrow to provide sufficient bulk of restorative material without over-contouring (ideal recommended is 0.5mm)
- the cavo-surface line angle cannot easily be determined during laboratory procedures
- the chamfer is rough or uneven
- the preparation is undercut causing distortion of the wax pattern.
retention and resistance
Inadequate retention and resistance form may lead to displacement of the restoration during function. Causes of inadequate retention and resistance are:
- excessive convergence angle (greater than 10˚)
- inadequate height and surface area of the axial walls
- insufficient surface area to resist tipping of the restoration
structural durability
The restoration must have sufficient bulk to resist distoration and allow for wear during service. However, an unnecessarily thick casting is indictative of excessive tooth preparation.
burs used to remove tooth structure
Tapered burs (242.6M and 243.6M diamonds and the 170L carbides
When moved parallel to the long axis of the tooth, will create a tapered surface converging approximatley 3-4˚ from the cervical toward the occlusal.
The tips of the 242.6M and 243.6M diamonds are hemispherical to produce a chamfer as the bur cuts into tooth structure.
sequence of steps of tooth reduction for complete veneer crown preparation
- occlusal guiding grooves
- occlusal reduction
- axial guiding grooves
- axial reduction
- finishing and evaluation
functional cusp reduction
1.5mm
central groove reduction
1.0mm
non-functional cusp reduction
1.0mm
path of withdrawal
+/- 5˚
convergence (included angle)
6˚
chamfer width
0.5mm
interproximal chamfer placement
(measured as margin clearance interproximally)
0.5mm
linguo-occlusal line angle reduction
0.8mm
functional cusps for mandibular and maxillary teeth
mandibular: buccal cusp
maxillary: lingual cusp
buccal second plane reduction
1.0mm
maxillary posterior teeth only
to provide space for restoration metal, and to restore original contours of tooth
biologic requirements
- protect pulp
- maintain periodontal health
- provide occlusal compatability
- maintain tooth position
- protect against fracture
mechanical requirements
- resist functional loads
- resist removal forces
- maintain interabutment aligment
esthetic requirements
- easily contourable
- color compatibility
- translucence
- color stability
sequence of steps for mandibular metal-ceramic crown preparation
- occlusal reduction
- buccal reduction (first and second planes)
- lingual reduction
- proximal reduction
- shoulder finish line - 1.2mm wide
- finishing and rounding of line angles
