This document discusses various types of partial coverage crowns, including their indications and preparation techniques. It describes the key features of standard three-quarter crowns, including occlusal reduction with depth orientation grooves, axial reduction with chamfer finish lines, and proximal grooves or boxes for retention. The document also covers preparations for other partial crowns like seven-eighths crowns and pin-modified three-quarter crowns. Overall, it provides detailed information on conservative crown preparations that preserve tooth structure.
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Partial veneer crown preparations
1.
2. Also called partial coverage crowns
It is a conservative restoration that requires less destruction of
tooth structure than does a full coverage crown
Based on the premise that an intact surface of too5th
structure should not be covered by a crown if its inclusion is
not essential to the retention, strength, or esthetic result of
the definitive restoration.
3. Tooth structure is spared
Much of the margin is accessible for the dentist to finish and
patient for cleaning
Less restoration margin is in proximity to the gingival crevice,
reducing the possibility of gingival irritation
Open faced partial veneer crown more easily seated completely
during cementation, whereas a full coverage crown tends to act
like a hydraulic cylinder containing a highly viscous fluid
With some of the margins visible, complete seating of a partial
coverage crown is more easily verified
If an electric pulp test ever need to be conducted on the tooth, a
portion of enamel is unveneered and accessible
4. A partial coverage crown is not as retentive as a full coverage
crown, but there is adequate retention for single restorations
and retainers for short-span fixed partial dentures
Grooves compensate for this
For max effectiveness, grooves must have definite lingual
walls
Resistance to torquing achieved by lingual hook or a lock
effect by directing the bur (and groove) slightly to the
opposite corner of the tooth
5. Standard three-quarter crown :facial surface is left uncovered
Most commonly used
Occlusal finish line on a maxillary tooth terminates near the
facio-occlusal angle
Used on max posterior teeth
Satisfies esthetics in which esthetic demands are demands are
moderate and reasonable
7. Occlusal reduction is the first step in
preparing a tooth for a three quarter
crown
Depth orientation grooves are cut on to
the anatomic ridges and grooves of the
occlusal surface with a round ends
tapered diamond
Clearance should be 1.5 mm on the
functional cusp (lingual on the maxillary
teeth) and 1 mm on the non functional
cusp(facial)
8. Next the functional cusp bevel is made.
Holding the round end tapered diamond
at a 45 degree angle to the long axis of
the preparation,3-5 depth orientation
grooves are placed on the lingual or
outer incline of the lingual cusp.
The grooves are 1.5 mm deep At the
cusp tip and fade out at their apical end
9. Axial reduction is begun by reducing
the lingual surface with a torpedo
diamond, taking care not to overcline
the lingual wall
The cut is extended interproximally
As the axial reduction is done,chamfer
finish line is formed
10. Proximal access gained by using a short
needle diamond in an up and down sawing
motion
This is continued facially until contact with
the adjacent tooth is broken
Gingivofacial angle should not be
underextended; it is the most likely area of
a three quarter crown to fail
11. The axial wall and chamfer are finished
with torpedo diamond of the same size
and configuration
Proximal grooves are approximately the
size of a no. 171L bur, groove should be
placed as far facially as possible without
undermining the facial surface paralleling
the long axis of a posterior tooth
12. A flare is a flat plane that removes equal
amounts of the facial wall of the groove
and the outer surface of the tooth
It is cut from the groove outward with
the tip of a flame diamond to prevent
over extension
The flare should be smoothened with a
carbide bur matching the configuration
of flame diamond
13. The occlusal offset, a 1 mm ledge on the
lingual incline of the facial cusp. Made
with a no.171L bur
A 0.5mm bevel along the bucco occlusal
finish line, perpendicular to the path of
insertion
14.
15. A three quarter crown preparation with
proximal boxes is more retentive than than a
standard preparation with grooves.
But as the are very destructive, they can be
justified only if there has been previous caries
or restorations
16. A three quarter crown preparation on a
mandibular molar or premolar has many
features found in the preparation of the
maxillary tooth
The biggest difference is the location of the
occlusal contacts ,there is no need for an
offset on the lingual inclines of the buccal
cusps
17. The seven –eights crown in a three quarter
crown whose vertical disto occlusal margin is
positioned slightly mesial to the middle of
the buccal surface
Esthetics are good
Resistance is better than that of three quarter
crown
Easy to do
cleaning by the patient also facilitated
18. The reverse three quarter crown is used on
mandibular molars to preserve an intact lingual
surface
It is useful on fixed partial denture abutments
with severe lingual inclinations
19. The proximal half crown is a three quarter
crown that is rotated 90 degrees with the
distal rather than the buccal surface left
intact
20. Create a concave lingual reduction incisal
to the cingulum
It is necessary to create 0.7mm or more
clearance with opposing teeth
To ensure adequate reduction, depth
orientation cuts are made Iingual surface
21. Incisal reduction is done with the small
wheel diamond
It parallels the inclination of the uncut incisal
edge and breaks through the labioincisal
line angle
22. The diamond is kept parallel with the incisal
two thirds of the labial surface to initiate the
path of insertion of the preparation
The instruments comes from the lingual to
minimize display of metal layer
An up and down motion is used with care not
to nick the adjacent tooth or lean the
diamond too far into the centre of the
prepared tooth
23. The axial reduction is completed and the
finish line is accentuated with a torpedo
diamond
The groves are placed as far labially as
possible without undermining the labial
enamel plate
24. On the facial aspect of each groove , a flare is
started at the gingival end with the thin tip of
a flame diamond
The grooves are connected with an incisal
offset , staying a uniform distance from the
incisal edge
25. A 0.5 mm wide bevel is placed on the labioincisal
finish line
26.
27. Pin modified three quarter crowns
In situations where the preparations will not permit the use
of a classic preparation design
Esthetic modification
Excellent retainer for short span fixed partial dentures
28.
29. Preparations for restorations should be based on fundamental
principles i.e a preparation must satisfy all the three principles i.e
biologic, mechanical & esthetic principles which inturn are
responsible for the success of the prosthodontic treatment
Also, it should be carefully decided which restoration to choose
based on each case.
