Hello!!
My studies have started and I am having anatomy this month, so you probably know how much to learn I have right now because as far as I saw on Instagram most of you are dentistry students or doctors... Anyway, I think quality beats quantity haha
This is the second part of "The Metal-Ceramix Crown Preparation". I am also planning to do the third one with short summary of part 1 and part 2 + videos.
Axial reduction of the proximal and lingual surface
Sufficient
tooth structure must be removed to provide a distinct, smooth chamfer of about
0.5 mm width.
M and N: Proximal reduction. O:Placing a 0.5-mm lingual chamfer. 
A football-shaped diamond is recommended for lingual reduction of anterior teeth.
6.
Reduce the proximoaxial and linguoaxial surfaces with the diamond held parallel
to the intended path of placment of the restoration. These walls should
converge slightly from cervical to incisal or occlusal. A taper of
approximately 6 degrees, as measured as the angle between opposing axial walls,
is recommended. On anterior teeth, a lingual concavity is prepared for adequate
clearance for the restorative materials. Typically, 1 mm is required if the
centric contacts in the completed restoration are to be located on metal. When
contact is on porcelain, additional reduction is necessary. For anterior teeth,
usually only one groove is placed, in the center of the lingual surface. For
molars, three grooves can be placed in a manner similar to that described for
the all-metal complete cast crown.
7.
Make a lingual alignment groove by positioning the diamond parallel to the
cervical plane of the facial reduction. When the round-tipped diamond of
appropriate size and shape is aligned properly, it is submerged almost halfway
into the tooth structure. Verify the alignment of the groove, and carry the
axial reduction from the groove along the lingual surface into the proximal; maintain
the originally selected alignment of the diamond at all times.
8.
As the lingual chamfer is developed, extend it buccally into the proximal to
blend with the interproximal shoulder placed earlier.
A lingual chamfer is prepared to allow adequate space for metal. A smooth transition from interproximal shoulder to chamfer is essential.
Alternatively, a facial approach may be used. Although this is slightly more
difficult initially, after some practice it should be easy to eliminate the
lingual guiding groove and to perform the proximal and lingual axial reduction
in one step; however, this requires that the diamond be held freehand parallel
to the path of placement. The proximal flange that resulted from the shoulder
preparation can be used as a reference for judging alignment of the rotary instrument.
A: Proximal reduction of the flange with a facial approach. B: One sufficient tooth structure has been removed, the cervical chamfer is prepared simultaneously with the lingual axial surface. After distolingual preparation had been completed, the mesial chamfer is blended into a smooth transition with the shoulder.
The interproximal margin should not be inadvertently placed too
far gingivally and thereby infringe on the attachment apparatus. It must follow
the soft tissue contour. On posterior teeth, the lingual wall
reduction blends into the functional cusp bevel placed during the occlusal
reduction. Anterior teeth require an additional step: After preparation of the
cingulum wall, one or more depth grooves are placed in the lingual surface. These
are approximately 1 mm deep.
9.
Use a football-shaped diamond to reduce the lingual surface of anterior teeth. It is helpful to stop when half this reduction has been
completed, to evaluate clearance in maximum intercuspation and all excursions.
The remaining intact tooth structure can serve as a reference.
Finishing
The
margin must provide distinct resistance to vertical displacement of an explorer
tip, and it must be
smooth
and continuous circumferentially. (A properly finished margin should feel like
smooth glass slab.) All other line angles should be rounded, and the completed
preparation should have a satin finish free from obvious diamond scratch marks.
Tissue displacement is particularly helpful when subgingival margins are being
finished. Sometimes this step is postponed until just before impression
making after tissue displacement.
Controlled tissue displacement can be helpful when finishing the margin with a fine-grit diamond or another rotary instrument.
10.
Finish the margins with diamonds, hand instruments, such as the off-angle
hatchets or carbides.
All internal line
angles should be rounded to facilitate the impression-making and die –pouring steps.
The finishing steps for the facial margin depend on the design
of margin chosen
A: completed preparation. Note that the transition from incisal to axial walls is rounded, and a distinct 90-degree or slightly sloping shoulder had been established. B: An even chamfer width and a smooth transition between lingual and axial surfaces. The chamfer is distinct and blends smoothly into the facial shoulder.
A porcelain
labial margin requires proper support for the porcelain. A shoulder with a
90-degree cavosurface angle is recommended. This type of shoulder can also be
used for a crown with a conventional metal collar and offers the advantage of
allowing the collar to be kept narrow. However, there is then the risk of
leaving unsupported enamel. For this reason, the margin is often beveled or
sloped to create a more obtuse cavosurface angle. A flat-ended diamond
in a low-speed handpiece creates the 90-degree shoulder. Any unsupported enamel
must be removed subsequently by careful planing with a sharp chisel. Care must
also be taken to orient the rotary instrument as it moves around the tooth if
inadvertent undercuts are to be avoided. When a metal-collar design of ceramic
restoration is planned, the need for a 90-degree shoulder is less crucial. A
sloping shoulder has been advocated to ensure the elimination of unsupported
enamel and to minimize marginal gap width. Such a shoulder
(cavosurface angle of about 120 degrees) can be accomplished with a flat-ended
diamond
by changing its alignment, with particular attention to the configuration of
the tooth structure cervical to the margin. Alternatively, a hatchet can be
used to plane the margin to the correct angulation. Again, be careful to avoid undercutting
the axial wall of the preparation where it meets the shoulder during finishing.
A shoulder-bevel margin is most effectively achieved with a flame-shaped carbide
bur or hand instrument, depending on the length of bevel required.
The shoulder bevel.
In general, a short bevel with a cavosurface angle of 135 degrees is advocated,
although longer bevels have been recommended
for improved marginal fit. Special care must be exerted where the bevel meets
the interproximal chamfer. The chamfer and bevel should be continuous with each
other. Care must be taken not to damage the epithelial attachment during
beveling; tissue displace ment before preparation of subgingival bevels is recommended.
11.
After a satisfactory facial margin has been obtained, round all sharp line
angles within the preparation.
S: Finishing the preparation with a fine-grit diamond. T: The completed preparation.
This facilitates surface wetting
and expedites subsequent procedures (impression making, pouring of casts, waxing,
and investing). A fine-grit diamond operating at low speed is particularly
useful. However, where access allows, a slightly larger tapered diamond may be preferred
because the greater diameter of its tip prevents "ditching" of the
chamfer. Blend all surfaces together, and remove any sharp transitions.
Facial (A) and lingual (B) views of metal-ceramic preparations.
Evaluation
Areas
often missed during finishing are the incisal edges of anterior preparations
and the transition from occlusal to axial wall of posterior preparations. The completed
chamfer should provide 0.5 mm of space for the restoration at the margin. The
chamfer must be smooth and continuous, and when it is evaluated, a distinct
resistance to vertical displacement of the tip of an explorer or periodontal
probe should be felt. The chamfer should be continuous with the interproximal shoulder
or beveled shoulder. The cavosurface angle of the chamfer should be slightly
obtuse or 90 degrees. Under no circumstances should any unsupported tooth
structure remain, especially at the facial margin. Care is also needed to avoid
creating an undercut between the facial and lingual walls. This aspect of the
preparation should be thoroughly evaluated. Excessive convergence should also
be avoided, because this may lead to pulpal exposure. All residual debris is
removed with thorough irrigation.
Metal-ceramic crowns used to restore maxillary incisor teeth.
A: Metal-ceramic preparations on the maxillary premolars in conjunction with more conservative preparations on the molars. B: Buccal view of the preparations. Note that, by comparison, considerable tooth reduction was needed on the premolars to accomodate metal-ceramic restorations. C: Except for the molars, all remaining teeth in this patient have been prepared for metal-ceramic restorations. Note that subtle variations and modifications of the same underlying theme: wing - type preparations on the anterior teeth, wingless on the premolars. D: Mandibular arch of the same patient. Many of the smaller mandibular teeth were prepared with wingless restorations. Because of previously existing restorations, excessively heavy shoulder - like chamfers resulted on some of the posterior teeth.
