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Esthetic Dentistry
Complete-crown and partial-coverage tooth preparation designsfor
bonded cast ceramic restorationsStephen P. Broderson"^
A new concept for tooth preparation design for complete- and
partial-coverage all-eeramierestorations is presented. Because
ofthe efficacy of third-generation dentinal bonding agents,the
preparations for complete-coverage and partial-coverage
restorations can be made withless emphasis on retentive form. For
partial-coverage restorations, cavosurface angles shotddbe large so
that the resultant configuration ofthe enamei rods is conducive to
optimal bond-ing. The new preparations are simple, with extreinely
tapered axial walls, to allow maximumthiekness ofthe ceiamic
material. These types of preparations result in fmished
restorationsthat are stronger and have better margins and less
chance of microleakage. Long-term successof these types of
restorations will be determined by the success or failure ofthe
dentinal bond-ing agent and resin cement system used. (Quintessence
Int 1994;25:535-539.)
Introduction
The concept of dentin-enamel bonding has revolution-ized
restorative dentistry. Many third-generation den-tinal bonding
agents are available to bond ceramic res-torations. These bonding
agents, when combined withresin cements, have bond strengths to
dentin that ap-proach those to enamel.
Etched porcelain and Dicor {Dentspiy Internation-al) are
available to provide conservative esthetic resto-rations.'"*
Several articles have provided recommenda-tions for tooth
preparation design for all-ceramic resto-rations. The consensus
seems to be that all-porcelainrestorations require absolutely
precise work, more sothan any other type of prosthetic restoration.
'
In this article the preparation designs currently advo-cated for
complete- and partial-coverage all-ceramicrestorations will be
reviewed. Changes in the basic de-sign concept will be recommended
to enable the dentistand laboratory to construct all-ceramic
restorationsthat will fit more precisely and be fabricated more
effi-
Privaie Practice, 1313 Gilman Street, Suite A, Berkeley.
Califor-nia, 94706.
ciently. Although special emphasis will be placed onDicor
restorations, the preparation design for any all-ceramic
restoration is the most important factor in thelong-term success of
the restoration, regardless of theceramic material utilized. It has
been my experiencethat the success rate of restorations bonded to
prepara-tions described in this paper is much higher than that
ofrestorations bonded to preparations utilizing conven-tional
retention form. Long-term success also dependson the strength of
the dentinal bonding agent.
Com pie te-coverage all-ceramic restorations
Current concepts of tooth preparation for
all-ceramicrestorations have evolved from the need for
retentionform—because of the limitations of the types of ce-ments
available (zinc phosphate, polycarboxylate, andglass-ionomer)—and
from the technical mechanicalneeds for the fabrication of the
original porcelain jack-ets.
Dodge et aP reported that the ideal convergenceangle for cast
metal complete crowns is 16 degrees.Their castings were cemented
with zinc phosphate ce-ment. Dodge et aP found that, as parallelism
decreases,castings seat moTe completely and retention
decreases.However, Dodge et aF also stated that in the mastica-tory
process and in oral habits there are compressive
Quintessence Intemational Volume 25, Number 8/1994 535
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Esthetic Dentistry
Roi.incisi
Cavos
ndcd »linll iotcluialline angle
Qrface angle ^120"-140"
—r—\
1 \1 \
Qndtd intei
RciundËdlirc
Bu ;nal Roundíd mtsrnal [_
0
t"
hL
K
•clusal leductioat least 2 mm^-—^J
Li
Fig 1 Anterior and posterior com-plete-crown preparations.
Com-piete-crown preparations shouldhave a cavosurface angle ot 120
to140 degrees and a convergenceangle of greater than 20 degrees.The
axiogingivai internai line angleshould be rounded and as smoothas
possible.
forces with oblique and apical components that tend totip or
unseat the casting. The occurrence of pure tensileforces is
infrequent. In real clinical situations, severalauthors have found
the average convergence angles tobe 20 to 25 degrees.
Hung et al" compared Cerestore (Coors Biomédi-cal), Dicor, and
porcelain-fuscd-to-metal (PFM)crowns. They concluded that marginal
openings in-crease in size after cementation and after
thermocy-cling, and that the PFM crowns have a significantly
bet-ter marginal fit than do the Dicor or Cerestore crowns.The PFM
crowns were prepared with a 45-degree cir-cumferentiai bevel on a
shoulder, and the Dicor andCerestore crown preparations had
90-degree shoul-ders. Convergence angles for the preparations were
notdiscussed. The finished crowns were cemented withzinc phosphate
cement. Hung et aP reported that thetwo major factors resulting in
acceptable margins arecement film thickness and margin design. When
thereis less hydrostatic cement pressure" the castings seatmore
completely.
Leempoel et al'' showed that preparation design, ce-ment
utilized, and laboratory quality are the most im-portant factors
influencing the retention of a castingand that occlusal convergence
angles of 15,5 to 30.2 de-grees provide sufficient retention for a
complete castmetal crown.
The restorative process for all-ceramic restorationsis technique
sensitive and the utmost care must be tak-en to develop a cavity
preparation that makes use ofthe strengths and minimizes the
weaknesses of the ce-
ramic material.-' Ease of laboratory fabrtcation shouldalso be a
primary consideration for the tooth prepara-tion.
Hobo and Iwata'', in introducing Cerapearl (Kyo-cera), described
the ideal preparation as having 2.0-mm occlusal reduction, 1.5-mm
axial reduction, and a1.2-mm shoulder with smooth internal hne
angles andno bevels. Malamed,'" preparing for a complete
Dicorcrown, described a marginal design with a 135-degreechamfer
with a cavosurface junction of 90 degrees. Thedepth ofthe shoulder
should be 1.2 to 1,5 mm. Henselet al" described the ideal margin
for a complete Dicorcrown as having a 1,2-mm shoulder perpendicular
tothe axial surface and occlusal reductions of 1.5 to 2,0mm. None
of these authors discussed convergenceangles.
New preparation design
It is this author's opinion that the availability of
third-generation dentinal bonding agents has had a major ef-fect on
the preparation design; that is, retention formcan be less
emphasized. Complete-crown preparationswith greater convergence
angles, which allow greaterseating of the restorations, are
recommended; thiscloser fit results in a higher compressive
strength,'̂Sharp internal or external line angles and
preparationmargins with sharp corners should be avoided. Becausethe
casting must be seated with passive pressure, thepreparation design
must have a greater convergencethan is necessary for gold
restorations. It is imperative
536 Quinlessence international Volume 25, Nutnber
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Esthetic Dentistry
Fig 2 The six maxillary anterior teeth have been restoredwith
Dicor Plus compiete-crown restorations. Note the con-servative
tooth reduction at tbe marginal area, the roundedexternal and
internal iine angles, and tbe convergenceangles.
Fig 3 Design for an onlay type of preparation as recom-mended in
tbe Dicor Ciinical Procedures Manual, This typeof preparation
creates too many stress points in the ceramicrestoration.
that the preparation be as smooth as possible, so thatoptimal
accuracy in impressioning, waxing {in the caseof Dicor), and die
duplication can be obtained.
This author's experience with Dicor over the past 8years has led
to modifications in the complete-crownpreparation for cast ceramic
restorations (Fig 1):
1. The occlusal convergence angle should be no lessthan 20
degrees. This allows for better seating of therestoration.
2. Al! line angles should have exaggerated rounding.Sharp
shoulders should be eliminated, because theycan result in stress
points in the restoration. It is alsovery important to smooth and
round all axio-occlu-sal line angles. For the fabrication of
castings onsharp shoulder preparations with near-parallel
axialwalls, it is usually necessary to relieve the inside ofthe
shoulder area of the casting to allow completeseating of the
casting on the die. This leads to weak-ness in the cemented
casting,
3. The marginal configuration should be a chamfer
ofapproximately 120 to 140 degrees with a roundedinternal line
angle to the axial wall.
4. The occlusal reduction should be at least 2.0 tnm.The
finished occlusal surface of the preparation onpremolars and molars
should have a V shape mesio-distally, similar to the finished
intercuspal angle de-sired in the final restoration, to enable the
laborat-ory to develop optimum occlusal anatomy
The ultimate strength of the cast ceramic material isdependent
on its thickness. Tlie material responds to
stress in accordance with the engineering Law ofBeams (Dicor
Laboratory Techniques Course, York,PA. Dentsply International,
1984), which states thatdoubling the thickness of the material
increases itsstrength fourfold. Creating a tooth preparation
withlarge convergence angles allows greater thickness inthe axial
occlusal area of the restoration. The finishedcast ceramic
restoration fabricated in accordance withthe above-mentioned
guidelines will be stronger andhave better marginal fit (Fig
2).
Partía I-coverage all-ceramic restorations
The partial-coverage bonded cast ceramic restorationhas many
advantages over conventional types of rest-orations (cast gold and
porcelain-fused-to-metalcrowns). Maxillary and mandibular posterior
teeth cannow be restored with conservative tooth
reduction,providing the pahents with the esthetics they demand.No
longer does the dentist have to make complete-coverage ceramometal
restorations to satisfy esthetics.Preparation design is the secret
to success in utilizingpartial-coverage all-ce ramie
restorations.
Jackson and Ferguson^ described the ideal inlay-on-lay
preparation as having walls flared 11 to 15 degrees,rounded
internal line angles, rounded proximal boxeswith a chamfered margin
and no bevels, and ocelnsalreduction of at least 1,5 mm.
The Dicor Chnical Procedures Manual" shows an on-lay-type
preparation to be very similar to that for a castgold restoration,
with rounded line angles and no bev-els (Fig 3), This type of
preparation uses classic reten-
Quintessence Intemationai Volume 25, Number 8/1994 537
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Esthetic Dentistry
Buccal and linijua! cavostirface angles onmanditiular molars and
premolars 130°
Gingival eavosurfaceangle !2ü"-140°
Bueeal eavosurfaee angles onmaxillary molars and premolars
SO'-âO"
Fig 4 Complete-crown preparations for maxillary and mandiublar
premolars and molars. All line angles are rounded. The ca-vosurface
angle is 130 degrees on the buccal and lingual of surfaces
mandibular of premclars or molars, 150 degrees on theinterproximal
aspect of all posterior teeth, and 50 to 60 degrees on the buccal
surface of maxillary molars and premolars. Thisleaves an optimum
amount of enamel with properly oriented enamel rods for the best
bonding.
tion form with complex line angles and boxes, that, inmy
opinion, are unnecessary. Tliese features also pro-vide stress
points in the casi ceramic, which may lead tobreakage. It ¡s the
author's experience that Dicor resto-rations fabricated for this
type of preparation design,even when acid etched and dentin bonded,
have a highincidence of breakage.
New preparation design (Fig 4)
1. The preparation should have as much enamel tobond to as
possible.
2. The occlusal reduction should be at least 2 mm to al-low
optimal occlusal anatomy and strength.
3. The eavosurface angle should be nearly 130 degreeson the
bucea! and lingual surfaces and nearly 150 de-grees
interproximally. All internal line anglesshould be rounded, and
remnants of the box fromprevious preparation should be eliminated
as muchas possible. The preparation will have a horizontalflat
appearance with very little retention form.
4. There should be no occlusal function at the margins.5. The
preparation should have smooth, rounded cor-
ners and line angles, and the margin should be a con-tinuous,
smooth flowing line.
6. For maxillary molars and premolars, the buccaleavosurface
angle should be 50 to 60 degrees.
Rationale for preparation design changes
The continued development of third-generation denti-
nal bonding agents should enable preparations withless retention
form. Cast ceramic material has higherstrength when all line angles
are grossly rounded. Thistype of preparation leads to a casting
that is more easilyseated with a thinner cement interface;
therefore a res-toration that is superior in both fit and strength
isachieved. In a partial-coverage preparation, exposingas mueh
enamel as possible for bonding, especiallyinterproximally, results
in a finished bonded restora-tion with minimal chance for
mieroleakage. Figure 5,views of onlays placed on maxillary right
premnlarsand first molar, reveals how much enamei is availablefor
bonding, even interproximally. Note the smooth-ness of the
preparations, the lack of retention form, andthe flowing
margins.
It was shown by Retief et al" that severe microieak-age occurs
in sealant-lined butt-joint restorations inetched cavities. They
also found that bonding to sur-faces of longitudinally cut etched
enamel prisms ob-tained in butt-joint preparations is not as strong
asbonding to cross-cut etched enamel surfaces obtainedin beveled
preparations. Cheung,''* in a scanning elec-tron microscopic
investigation of acid-etched cervicalmargins of Class II cavities,
found that beveling resultsin an oblique orientation of the enamel
rods, resultingin a better pattern for bonding after acid
conditioning.Cheung''' concluded that beveled eavosurface
marginsresult in reduced mieroleakage.
The horizontally oriented long bevel described forthe cast
ceramic preparation results in the same orien-tation of enamel
rods. Tliis author's clinical experience
538 Quintessence International Volume 25, Number 8/199
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Esthetic Dentistry
Figs 5a and 5b Dicor onlay restorations on the maxillary right
first and second premoiars and first molar. The etched
prepara-tions show how much enamel is available for bonding. The
buccai margins are exceiient.
Fig 5c Oniay-complete-crown preparation. Note thetapered chamfer
on the mesiolingual aspect, the roundedmesio-axio-occlusal line
angle, and the smooth-fiowingcavosurface margin.
has been that the laboratory fabrication becomes much
easier and that the casting has a better clinical fit than
that obtained with a shoulder type of preparation.
Summary
Concepts for preparation of complete- and partial-
coverage cast ceramic restorations were reviewed.
Recotnmendations for changes from more conven-
tional types of retention form were presented. It has
been my clinical experience that these changes will re-
sult in faster preparation, more accurate duplication of
the preparation, easier laboratory fabrication, and a
stronger cemented finished restoration. These recom-
mendations can be utilized with any type of all-ceramic
restoration. Ultimately, the long-term success of these
restorations will be determined by the success or fail-
ure oí the dentinal bonding agent and resin cement
system utilized. To the present time, however, this
system of preparation design has been successful clin-
ically.
References
1. Shaffer H. Zobler C. Complete restoration witti resin
bondedporcelain inlays. Quintessence Int 1991;22:87-!)3.
2. HungSH.etal. Marginal fit of porcelain fused to melaJ and
twotypes of ceramic crowns. J Prosthel Dent 1990;63:26-31.
."i. Cavel W,et al. A pilot study of (lie clinical evaluation of
castableceramic inlays and a dual cure cement. Quintessence
Int1988:19:257-262.
4. Hobo S. Iwata I. Castable apatite ceramic as a new
biocompat-ible resiorative materials. Quintessence Int
1985;16:207-216.
5. Jackson KD, Ferguson RW. An esltietic, bonded
inlay-onlaytechnique for posterior te et ti. Quinlessence Int
1990;21:7-12.
6. Dicor Ctinicat Procedures Manual. York, PA' Dantsply
Inler-national, 1984.
7. Dodge WW, et al. Tbe effect of convergence angle on
retentionand resistance form. Quinlessence Int 1985;16:191-194.
8. GaviMs JR, et al. The effect of various finish line
preparationson the marginal seal and occlusal seat of full crown
prepara-tions. J Prosthet Denl 19S1;45:13K-145
9. Leempoel PJB. et al. The convergence angle of tooth
prepara-tions for complete crowns. J Prosthet Dent
1987;58:414-416.
10. Malamed KA. Considerations in posterior glass-ceramic
res-toraiions. Int J Periodont Rest Dent 1988:8(4) ;33^9.
11. Hensel W, ct al. A clinical case report utilizing a eastable
ce-ramic crown system. Quintessence Int 1987;18:323-334.
12. BrukI CE, Philip GK. The fil of molded all ceramic, twin
foil,and conventional ceramic crowns. J Prosthet Dent
1987;58:408-413.
13. Relief DH, et al. Surface topography ot the enamel margins
ofbutt and beveled class V preparations. J Prosthet
Dent1982;48:]66-17O.
14. Cheung G. A scanning electron microscopic investigation oí
theacid-etched cervical margin of Class II cavities.
Quintessence
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