CPD Clinical Articles 5-8

CPD Clinical
Articles 5-8
by Dr David Penn
Article 5
Article AwAiting cPD Point APProvAl from the DentAl BoArD.
clinical
Please refer to our website for updates: www.australasiandentist.com.au
The Seven
Deadly Sins
Common errors to avoid when preparing crowns
for all ceramic restorations
By Dr David Penn
BDS MBA
2
2. Bevelled or feather margins
My ceramists will not hesitate to tell me
that porcelain is a nightmare to fabricate and
finish over any bevelled or feather margin. In
addition, if it makes it through the laboratory
procedures, it has a higher chance of fracturing
during try in or at some point after cementation. For optimal strength and fit with any
all-ceramic restoration, use only a shoulder
preparation (preferred for pressed ceramics and
indirect composites) or a definitive chamfer
preparation (ideal for all-ceramics built over
aluminium oxide or zirconium oxide copings).
1
Figure 1. Why did this fracture occur? Occlusal loading can often be the cause and
is often exacerbated by sharp internal line angles which propagate cracks in ceramic
copings.
1. Sharp internal line angles
Dr David Penn
58 Australasian Dentist
Sharp line angles can cause minor to major
problems with fitting and seating which result
in considerable time loss and frustration for
all of us. This is true for any type of restoration, not just all-ceramics. A premature stress
fracture occurring at the seating appointment,
or worse yet, soon after cementation (Figure
1), can be the costly result of leaving sharp
line angles under an all-ceramic or indirect
composite restorations. These issues can also
cause an erosion of confidence between us and
our patients. It is imperative to remember to
round all sharp line angles after the rest of the
preparation is completed. Use any tapered, fine
diamond bur to perform this important procedure or a Sof-lex disk.
Figure 2. The margins of this all ceramic crown
preparation are well defined and the line angles have been
smoothed as much as is practicable.
If an old PFM restoration is being replaced
with an existing bevelled (metal) margin, and
it cannot be modified into a definitive chamfer
or shoulder margin, then stay with a PFM as
the replacement crown. A wide selection of
chamfer and shoulder burs are available which
can be used to prepare all-ceramic margins successfully without unwanted bevels.
3
3. the “J” margin
One of the dangers in preparing a
definitive shoulder or chamfer margin is the unintended creation of a J
(or grooved) margin (Figure 3). This
occurs when the apex of the diamond
passes the edge of the margin and creates a groove inside the margin. If the
coping fabrication technique requires
scanning, it can lead to inaccuracy in
the scan with a resultant poor marginal fit. Possible physical degradation
of the die during handling and certain
scanning procedures, in tandem with
an area of thin porcelain created on
this fragile margin, makes a J margin
unacceptable.
Unfortunately, some of the manufacturers have tried to convince us
that any margin will work. This leads
to a basic misunderstanding that
somehow, the material or the way it
is fabricated will make up for a deficiency in the design of the preparation.
The fact is, no matter which system is
utilized, a certain minimal amount of
room is needed to allow for adequate
thickness of the underlying coping
and the aesthetic layering porcelain.
This is especially true in the gingival
margin area and cervical third. Many
times, our laboratory technicians have
to work with chamfers or shoulders
that are too shallow or even nonexistent: the resultant crowns can be bulky
and far less aesthetic with poor emergence profile. The solution is simply
to prepare a deeper, definitive chamfer
or shoulder (minimum width 1.2mm)
with sufficient axial wall reduction
(minimum 1.5 mm).
4
4. incomplete and/or non-uniform
shoulder
Figure 3. A “J” margin is created when the
apex of the chamfer bur passes the edge of
the margin, creating a groove.
If you use some of the commercially available chamfer diamonds to
prepare the margin, one must exercise
caution not to exceed a depth into the
tooth equal to more than one half of
the width of the bur tip. This means an
appropriate diameter bur must always
be selected to create the chamfer
depth desired. If you wish to correct
a J margin that has been accidentally
created, it can be converted into a
modified shoulder margin by reducing
the outer lip with an end-cutting bur.
Figure 4. End cutting bur (tissue protected)
provides well defined chamfers and rounded
shoulders which are ideal for all ceramic
crowns
This problem causes the porcelain in
the cervical areas to vary significantly
in thickness, with a potential for premature fracture during fabrication, in
the process of seating or after cementation. This can also create aesthetic
problems when the underlying abutment shade affects the final colour in
the thinner areas of porcelain.
When pressed ceramics such as
e.max or Empress Esthetic are selected
as the ceramic of choice, be sure to
maximize strength and aesthetics by
preparing a complete and uniform 360
degree shoulder with a width of 1.2- to
1.5- mm (butt-joint margin).
5
clinical
5. rough shoulder
Taking a little extra time to go back
and round any sharp spicules or undulations left on the margins will pay
huge dividends. A smooth shoulder
will help to ensure an excellent fit and
improved aesthetics at the margin. It
can also translate into a reduction in
the potential for stress fractures upon
seating or a delayed fracture after
bonding the crown in place. After the
shoulder is prepared with a shoulder
bur go back with an appropriately
sized end-cutting bur and refine the
shoulder. With this bur, it can be done
quickly and safely without any further undesired removal of axial tooth
structure.
Figure 6. These preparations on
endodontically treated teeth needed to be
prepared slightly subgingivally to mask the
dark root surface. The margins need to be
smoothed to remove the gouge marks from the
shoulder bur.
6
Figure 7. A smooth well defined shoulder ideal
for all-ceramic restorations has been recorded
in this PVS impression
6. inadequate reduction – labially
Figure 5. This is a common all ceramic
preparation error. The non-uniform nature
of the preparation means that the coping
and veneering ceramic will be compromised
causing a weakness and potential fracture site
when loaded.
This dreaded sin is very common
amongst new graduates or inexperienced crown and bridge operators. I was
often guilty of this in my early days: I
would spend an inordinate amount of
time preparing an exquisite finishing
margin but would forget about providing our lab with enough axial wall
reduction and hence, adequate room
for the coping and veneering ceramic.
“Buccal Belly” can easily be overcome by ensuring that a clear line of
sight exists from the occlusal aspect of
the preparation down to the gingival
margin without impingement. Simply
placing a mirror to view the axial wall
and then envisaging adequate room
for the restoration will soon remind
you if further reduction is required.
Aim for an even 1.5 mm axial wall
reduction with smooth walls.
Australasian Dentist 59
clinical
7
7. inadequate reduction
– occlusally
One of the most common dramas we
are faced with is under reduction on the
occlusal aspect of all ceramic crown
preparations which threaten the stress
bearing ability of the restoration and
bond between the veneering ceramic
and underlying restoration. Additionally, preparations do not match the
restoration prescribed: for example,
Figure 8. The margins of this all ceramic
crown preparation are indistinct, sharp line
angles exist circumferentially on the occlusal
aspect and further occlusal height reduction
also needs to be performed.
anatomic occlusal reduction should
be less emphasized and smoother (not
flat) in any CAD/CAM restorations
that are physically scanned, such as
Procera (Nobel Biocare) or Calypso
and need to be sympathetic to the
ground in functional occlusion of the
surrounding dentition.
Reduction (sometimes major) of
the incisal edges of opposing mandibular teeth is done far too frequently
to “make room” for the lack of maxillary anatomic lingual reduction. If the
proper anterior coupling cannot be
accomplished with anatomic reduction of the maxillary teeth alone, then
one should consider other alternatives,
such as an appropriate opening of the
occlusal vertical dimension This often
involves having an understanding
of comprehensive, interdisciplinary
restorative dentistry.
conclusion
The seven deadly sins wreak havoc
with the life-spans of all ceramic restorations on a daily basis. At SCDL,
we encourage dentists to improve their
clinical skills by asking our clinical
advisory panel and our laboratory technicians to give meaningful feedback
on preparations, tissue-management
effectiveness and impressions. If we
are open-minded and have a relationship with them that is based on mutual
trust, they will be willing to offer us a
unique blend of clinical and technical
expertise based on their unique experiences and position in dentistry.
When dentists encounter problems
with their all ceramic work, our clinical team examine every stage of the
clinical and lab procedure to try and
ascertain the cause of the dilemma.
The solution is often as simple as taking the time to understand the limitations of each material and collectively
apply a greater attention to detail and
concentrating on avoiding these seven
deadly sins.
u
CPD Questionnaire
1. Bevelled margins are acceptable for pressed
ceramic restorations
True/False
2. Porcelain fused to Zirconia crowns need less
occlusal reduction as the copings are so
strong
True/False
3. “J” shaped margins are a problem for preparations
that are scanned
True/False
4. Preparations for all ceramic restorations should be
smoothed with sandpaper discs where
possible
True/False
60 Australasian Dentist
5. Modern day bonding procedures with all ceramic
crowns diminish the problems caused by sharp
internal line angles
True/False
6. End cutting burs help to define and smooth
rough margins
True/False
7. Shoulders with an internal right angle are more
suitable than chamfers for all ceramic
restorations
True/False
8. Under prepared labial axial walls should be
overcome by over contouring the crown and
changing the emergence profile
True/False
clinical
Ideal Preparation for Full Ceramic Restorations
Australasian Dentist 61
clinical
Article 6
Article AwAiting cPD Point APProvAl from the DentAl boArD.
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fully milled
Zirconia crowns
(FMZ)
by Dr David Penn
This new generation of monolithic all zirconia crowns
promise so much but impeccable case selection
is paramount due to the danger of damaging the
opposing dentition.
T
Dr David Penn
he importance of balancing the requirements for strength, aesthetics and avoidance
of further tooth wear can be extremely challenging in posterior full coverage restorations.
Fundamentally, the rationale driving this
choice is based on:
1. The demands on the strength of the restoration/tooth unit
2. Amount of sound tooth structure available
3. Occlusal clearance
4. Aesthetic assessment of the need for translucency versus opacity and
5. Parafunctional activity
A further consideration is the abrasiveness
of the particular material against natural tooth
structure. Ideally, this should match that of natural tooth enamel. It is this interplay between
wear, strength and clinical requirements and
light transmission, which must be considered
when choosing the type of material to be used
(Fons-Font et al 2006).
Where strength is the prime concern, (virtually in almost every posterior case), as the
occlusal loading is high, then clearly a highstrength material is required and the accompanying higher opacity means that aesthetics
may have to compromised.
As far as abrasiveness is concerned, low-fusing ceramic (e.max, Procera) has been shown to
cause less wear of opposing teeth than conventional porcelain (Christenson 2000).
It is also well documented that rough,
abraded porcelain is extremely damaging to
opposing unrestored teeth. One recent study
compared various ceramic materials with gold
(Elmaria 2006). While gold, unsurprisingly,
proved to be the least abrasive, polished lowfusing porcelains also resulted in minimal
tooth wear.
what about the concept of using zirconia
only to make full coverage crowns?
Unquestionably, they are very strong compared
to more traditional ceramics and in theory at
least, its opacity might be useful when trying to
mask heavy discolouration. Additionally, these
restorations can be fabricated in thin cross-sections, meaning tooth reduction is minimised
and conservative “full gold crown” type reparations can be utilised. Patients also prefer a tooth
54 Australasian Dentist
clinical
coloured restoration where possible
and these crowns are generally more
aesthetic than full cast metal or metal
occlusal surfaces on PFM crowns.
Unfortunately, as a stand- alone
material for full crowns, it has some
significant drawbacks which the
manufacturers sometimes gloss over
and as there is a paucity of long term
in-vivo studies, a cautious approach
utilising ideal case selection should be
adopted until a body of evidence based
research data is available.
what is fmZ?
FMZ is a new generation fully milled
monolithic solid zirconia crown or
bridge restoration with no veneering
ceramic.
The principal qualities of FMZ
are:
a. Strength and accuracy; designed
and milled using CAD/CAM technology, FMZ crowns and bridges
are sintered for 6.5 hours at 1,530
degrees Celsius. The final FMZ
solid zirconia crown or bridge
emerges nearly “bulletproof” with
reported compressive strengths of
over 1200 mpa.
b. No danger of delamination of
traditional porcelain veneer layer
as with some porcelain fused to
zirconia crowns
c. Acceptable
aesthetics
when
inserted in lieu of metal occlusal
PFM and full-cast metal restorations in the posterior segment
d. Virtually chip-proof, it is an ideal
restoration for bruxers and grinders, if opposed to metal or other all
zirconia restorations
e. Conservative preparations similar
to FGC are acceptable
FMZ crowns and bridges are made
from the highest quality zirconia
that is uniquely colloidal processed
without mechanical pressing or contaminate organic binders for enhanced
aesthetics. Other dental zirconia mate-
rials on the market today use organic
binders to hold the powders together
during high pressure pressing to form
the milling blanks which diminishes
aesthetic properties.
laboratory and intra-orally) which is
reported to be able to minimise this
wear. Other commercially available
all zirconia crowns appear to be glazecovered zirconia only which may exacerbate occlusal wear significantly.
wear Studies on full-contour
Zirconia
fmZ indications
Recent research from the University
of Zurich demonstrates that zirconia
which is only glazed and improperly
polished can be particularly destructive to opposing tooth structure.
Not all zirconia crowns are identical
and a number of manufacturers are
now using abrasive milled zirconia
blocks which are difficult to polish
properly and where the glaze is easily
removed.
FMZir (Southern Cross) has a
specific polishing protocol (both
FMZ Zirconia crowns or bridges are
indicated for bruxers and grinders
when PFM metal occlusal or full-cast
restorations are not desired and where
the antagonist is metallic, zirconia
and no dentine is exposed. FMZ is ideally suited for posterior molar crowns
when the patient desires a tooth
coloured restoration but lacks the
preparation space for, or has broken a
PFM crown in the past due to bruxing.
FMZ can also be utilised for inlays
and onlays.
Australasian Dentist 55
clinical
fmZ contraindications
1. The first problem is that, whilst
being very strong, it is also
extremely opaque and is therefore
only suitable in limited scenarios
(posteriors)
2. The second problem is that the
fitting surface of a zirconium restoration cannot be etched in the
way that materials like e.max or
Procera can be and so cannot rely
on micro-mechanical
3. Short clinical crowns thus need
mechanical retention and further
tooth reduction
4. Adjustment of zirconia is difficult and needs specific armamentarium. Extreme heat can be
generated and irreversible pulpal
damage can occur if not performed
correctly.
5. Incorrect and insufficient polishing creates an even more abrasive
finish which can wreak havoc
with the opposing dentition. If
dentine is exposed on the antagonist after adjustment, this wear is
exacerbated even further.
6. Removal, especially of inlays,
is extremely difficult and can
lead to large amounts of tooth
destruction
7. Cases where the dentine is exposed
on the antagonist pre-operatively
should be precluded from FMZ
and full gold or PFM options are
preferred.
Preparation requirements
u A full shoulder preparation is not
required, chamfer and feather
edges are acceptable.
u A conservative preparation similar
to that for full-cast gold, so any
preparation with at least 0.7 mm
of occlusal space (1mm is ideal)
will work.
instructions for Adjustment and
Polishing
u Adjust FMZ crowns and bridges
using water and air spray to keep
the restoration cool and to avoid
micro-fractures with a fine grit
diamond. If using air only, use
the lightest touch possible when
making adjustments. A football
shaped bur is the most effective
for occlusal and lingual surfaces; a
tapered bur is the ideal choice for
buccal and lingual surfaces.
u Polishing FMZ restorations is a
critical element of this procedure
and if not performed properly,
can wreak havoc on opposing
dentitions.
cementation recommendations
u Resin reinforced glass ionomer
cement (RelyX Luting Cement,
3M ESPE; GC Fuji Plus, GC)
u Resin cements for short or overtapered
preparations
(RelyX
Unicem, 3M ESPE; Panavia F2.0,
Kuraray)
Summary
All zirconia crowns have characteristics which are very appealing in many
respects. Low biological cost, almost
indestructible by nature and tooth
coloured characteristics are almost
idyllic but these are offset by the need
for meticulous case selection. Excessive wear on the antagonist caused by
the intrinsic physical abrasiveness of
zirconia is problematic and despite
the claims of the manufacturers, needs
continual vigilance.
Long term studies need to be completed before this restoration should
be considered for widespread use in
the posterior segment.
u
Dr David Penn is in private practice in Sydney,
Australia and is managing director of Southern
Cross Dental Laboratories (Australia).
Questions for CPD Points
True or False
5. FMZ can be bonded to tooth structure effectively?
1. FMZ is an ideal replacement for PFM in the posterior segment in most cases?
6. Bruxers who have metallic or all zirconia antagonists are
ideal candidates for this material?
2. All Zirconia blocks are identical in physical and chemical
characteristics?
7. Preparations need only be the same as for FG crowns?
3. Excessive wear to the antagonist is the major drawback
to this material?
4. Glazing over zirconia will protect the antagonist?
56 Australasian Dentist
8. Removal or adjustment of zirconia can be dangerous to
the pulp due to excessive heat generation?
9. Knife edge margins are acceptable?
10. Long term studies need to be completed before utilising?
Article 7
clinical
Article AwAiting cPD Point APProvAl from the DentAl boArD.
Please refer to our website for updates: www.australasiandentist.com.au
Aesthetic Failure
of Anterior Crowns
by Dr David Penn and Dr brenda baker
t
Dr David Penn
he past four decades have witnessed numerous
improvements in metal-ceramic and all-ceramic
crowns. the initial excitement of the metal-ceramic
crown was that it theoretically combined the
aesthetics of the porcelain jacket crown with the
potential for clinical longevity. shoulder-bevel margins
with metal collars were to provide optimal fit and to
obtain aesthetics, the metal margins were intended to
be hidden within the confines of the gingival sulcus.
this concept proved to be rather unpredictable and
lead to the development of numerous techniques
for fabrication of all-porcelain margins with metalceramic crowns (1).
the evolution of such simplified techniques, along
with the introduction of several innovative all-ceramic
crown modalities has eliminated the need to hide
Dr Brenda Baker
In an attempt to mask the discoloured dentine, this
preparation has a buccal margin that impinges deeply into the
sulcus and is likely to be periodontally compromised.
48 AustrAlAsiAn Dentist
metal margins deep in the gingival sulcus. However,
it is clear that it is impossible to match precisely the
shade of the restoration with the colour of the gingival
portion of the tooth with these restorations and in
most clinical situations it is still desirable to hide the
restorative margins underneath the healthy gingival
tissues. the exception to this statement is bonded
porcelain veneers, where tooth reduction is minimal
and the restoration is bonded to sound enamel. in
these situations, the contact lens effect allows margins
to be placed in a supra-gingival location.
However, the disappointment with the metalceramic restorations was the genesis for the
development of numerous all-ceramic alternatives
to the metal-ceramic restoration (2). Many different
techniques for fabricating all-porcelain labial margins
are available to improve the inherent aesthetic
performance of metal-ceramic restorations. When
there is sufficient remaining enamel, etched porcelain
laminate veneers may be considered to restore the
teeth to optimum aesthetics and function.
Failure to obtain optimum results probably has
less to do with the restorative medium chosen than
it does with failure to adequately prepare the soft and
hard tissues to receive the restorations.
in order to achieve aesthetic and functional
success with any and all of the available modalities, it
is imperative that the clinician first bring the gingival
tissues to optimal health prior to definitive tooth
preparation and maintain this stage of health through
the provisional stage. the clinician must understand
the nature of the restorative material to be used and
prepare the teeth adequately in the correct planes to
provide sufficient room for the chosen materials. the
dentist must accurately communicate the aesthetic
treatment plan to a quality laboratory technician.
in spite of these technological improvements, the
majority of aesthetic failures with such restorations
are biologic. the two primary types of aesthetic failure
have been recession of the gingival tissues resulting in
exposure of the restorative margins and the presence of
chronic marginal gingival inflammation.
there are essentials to the soft tissue management inherent in intracrevicular restorative dentistry
related to anterior teeth.
the clinician faces two basic problems with the
soft tissue management.
1. in situations in which the restorative margins are
to be placed in the gingival sulcus and are intended
clinical
to be hidden by healthy gingival tissues
the goal is to maintain tissue health
and at the same time prevent recession
and subsequent exposure of the restorative margin.
2. the restoration has to be placed deep
enough in the sulcus to avoid detection
and at the same time keep the margin
an appropriate distance from the
attachment or crest of the alveolar
bone so that the biologic width is not
violated (3).
Preventing gingivAl
receSSion
Gingival recession in adults is not just
a natural effect of aging but rather is a
result of pathology. if excellent gingival
health is attained prior to definitive margin
placement and proper clinical techniques
are utilized, the relationship between
the prepared restorative margin and the
gingival tissues can be very stable, as long
as the patient practices proper oral hygiene.
there are a number of ways to prevent
gingival recession related to anterior crown
fabrication, but most of these are under
control of the clinician.
if excellent gingival health is attained
prior to definitive margin placement and
appropriate restorative techniques are
utilised during therapy, the relationship
between the prepared cervical margin and
the gingival tissues can be very stable over
the long term. this assumes adequate professional maintenance and personal oral
hygiene over the long term (4).
there are five major strategies for preventing gingival recession when placing
anterior indirect restorations, and all are
primarily under the clinician’s control.
these strategies are:
1. Attaining optimum soft tissue health prior
to impression making;
Most often patients requiring extensive
restoration of anterior teeth do not present
with healthy gingival tissues (5). the clinical
diagnosis may range from mild gingivitis to
chronic periodontitis and it is critical that
an accurate diagnosis be made and that the
appropriate therapy be initiated.
Attainment of optimum soft tissue
health prior to determining the final cervical margin location and making the final
impression is absolutely critical.
With mild gingivitis, the teeth are often
prepared, provisional restorations are fabricated and impressions made at the same
appointment. this expedited approach is a
prescription for disaster.
With placement of the definitive
restorations a few weeks later, it is
reasonable to assume an improved effort
on behalf of the patient to comply with
oral hygiene procedures, and often in these
situations the inflammation in the gingival
tissues will resolve or at least be reduced.
in this scenario, the gingival tissues will
move in an apical direction, often exposing
the restorative margins. this can occur
during the provisional phase or shortly
after the definitive restorations are placed.
in either situation, the clinician is faced
with an aesthetic failure.
the optimum approach is to wait to
determine the final margin location when
the gingival tissues have attained a state of
optimal health.
With most anterior restorations, the
approach recommended is to prepare the
This preparation demonstrates an ideal finishing
line with optimal periodontal health. The
likelihood of bleeding is minimal and crevicular
fluids are well controlled.
This recently cemented bridge is already compromised aesthetically. Improved oral hygiene will most
likely expose the finishing line and contrast between the ceramic and root surface.
teeth after initial scaling and prophylaxis,
leaving the cervical margins in a supra or
equi-gingival position. excellent indirect
provisional restorations must be fabricated,
which restore optimum crown and gingival
tissue contours, provide access for proper
oral hygiene and serve as predictors for the
definitive restorations (6,7,8).
Gingival
enhancement
can
be
achieved by placing the patient on a weak
chlorhexidine rinse for 2 weeks (9). the
optimal location for the cervical margin in
the gingival crevice is determined, and the
margin is dropped to this predetermined
position. the provisional restorations
are relined to restore marginal integrity,
impressions are made and the patient
continues rinsing with mouth rinse until
the definitive restorations are placed.
2. Minimising iatrogenic soft tissue trauma
during margin placement and gingival
displacement procedures;
in order to avoid iatrogenic recession, it is
essential that the rotary instrumentation
used to drop the cervical finish line to
its final intracrevicular position does
not damage the soft tissue. retraction
cord soaked in water should be placed
into the sulcus for 3-5 minutes prior to
margin preparation to prevent iatrogenic
damage. On removal of the cord, a defined
space permits dropping of the margin
These failed ceramic veneers were prepared
without a finishing line and hence exhibit an
emergence profile that is impossible for the
patient to clean. The result is accumulation of
plaque, chronic gingivitis and a margin that has
moved cervically.
with minimal chance for trauma. use of
rotary instruments especially designed to
minimize trauma is recommended. (tissue
Protection end cutting burs)
it is also critical not to damage the
attachment apparatus during gingival
displacement procedures. the philosophy
of attaining optimum gingival health prior
to definitive margin location, coupled
with placement of the gingival margin a
short distance into the gingival sulcus,
permits relatively atraumatic retraction
procedures. A suitable diameter retraction
cord is placed in the gingival sulcus for
eight to ten minutes (10,11).
the cord is moistened with water prior
AustrAlAsiAn Dentist 49
clinical
to removal from the sulcus. Histological
evaluation has demonstrated that removal
of a dried cord from the sulcus tears the inner epithelial lining, initiates bleeding and
may cause irreversible recession (12).
An improved retraction system known
as traxodent (Premier usA) is now available and yields outstanding results. it
offers a convenient way to stop any crevicular seepage as it is impregnated with
Aluminium Chloride and is combined with
Defining margins with tissue protected end cutting
burs minimises iatrogenic periodontal insults.
an absorbent clay base which has an affinity towards oral fluid and blood. ideally,
it can replace the need for a second cord
in the double cord technique as it lessens
patient discomfort, displaces the tissue and
eliminates the occurrence of break-through
bleeding when removing the second cord.
3. Providing provisional restorations of
excellent quality;
the importance of fabricating quality provisional restorations cannot be overemphasized. these restorations may be made early in the restorative sequence as part of the
healing phase or after the preparations are
finalized. in either event, such provisionals
must demonstrate physiologic crown contours and excellent marginal integrity and
provide adequate aesthetics.
4. Eliminating all excess temporary cement;
if all-ceramic restorations are planned, a
non-eugenol cement should be used to prevent any inhibition of the polymerization
of the resin cement with eugenol contained
in the zinc-oxide eugenol cements (13).
if a conventional dental luting agent is
to be used such as glass ionomer or zincphosphate, a zinc oxide-eugenol temporary
cement is preferred. While it is now known
that zinc oxide-eugenol cements are not
obtundent to pulpal tissues, they do provide an excellent initial seal of the prepared
tooth. this tends to eliminate sensitivity
during the provisional stage. However, zinc
oxide-eugenol is a potent soft tissue irritant,
5. Waiting an appropriate time period to
allow the tissues to heal after periodontal
surgical therapy.
When patients require periodontal surgical
procedures such as crown lengthening, sufficient time must be allowed after the surgery to permit stabilization of the gingival
crest. it is often stated that a waiting period
of six to eight weeks is required to attain
adequate stability. However, for many patients, this time frame is far too short. in
a majority of patients, a waiting period of
five to six months is recommended (14).
it appears empirically that patients with
thin, scalloped gingival tissues are more
prone to recession that those with thick,
flat tissues. this prolonged waiting period
of 5 or 6 months would seem to be essential
with the former type of patient. this will
mean that many patients will be wearing provisional restorations for protracted
lengths of time. it is recommended that
such provisional restorations be removed
and recemented approximately every six
weeks to prevent leakage and subsequent
recurrent caries.
in summary, recession in association
with the placement of anterior restorations
is preventable. Attaining optimum soft tissue health prior to final determination of
margin location is essential. Atraumatic
tooth preparation and gingival displacement procedures are required, along with
the fabrication of excellent provisional
restorations. A meticulous technique for
provisional cementation is critical, and
provision must be made for tissue shrinkage after periodontal surgical procedures.
gingivAl inflAmmAtion
These margins are exposed but the gingival health of this patient has been exemplary. The crowns are
twenty years old and demonstrate physiological aging of the periodontium.
The two central incisors are implant retained
crowns and the periodontal health has been
stable for many years. Placement of the fixture in
the optimal labio-lingual position is critical as is
the provision for correct emergence profile.
50 AustrAlAsiAn Dentist
and care must be taken that all excess temporary luting agent is removed from the
sulcus prior to dismissing the patient.
Any residual cement left in the sulcus
will result in gingival inflammation. this
inflammatory reaction is reversible upon
removal of the irritant, but often a slight
amount of recession will occur subsequent
to the healing process. this recession, however slight, is detrimental to the long term
goal of hiding the margin beneath healthy
tissue.
Whilst recession exposing the gingival margins has been a primary cause of aesthetic
failure with metal-ceramic and all-ceramic
restorations, an equally compelling problem is the chronic marginal inflammation
in the gingival tissues associated with
restorations with subgingival margins. For
many years marginal inflammation was
attributed to poor oral hygiene, and the
patient was admonished to improve oral
physiotherapy, usually to no avail.
Highly polished metal margins or
glazed porcelain margins are smoother and
less conducive to plaque accumulation(15).
Certain cervical marginal configurations
have been demonstrated to be inherently
rough and thus to increase the potential
for plaque accumulation and retention.
therefore, they may contribute significantly
to such marginal inflammation. the use of
disappearing shoulder margins should be
discouraged for this reason.
the primary causative factor with
chronic gingival inflammation surrounding anterior restorations is violation of the
biological width (16). it is tempting for the
clinician, who is aiming to prevent margin
clinical
exposure in the event of some gingival recession to decide to place crown margins
deep into the gingival sulcus. Placing the
margin deep into the sulcus creates difficulties with gingival retraction and increase the chance that irreversible damage
leading to recession might occur.
Clinical studies have demonstrated
that the closer the restorative margin
is to the attachment, the poorer is the
periodontal response (17). the further
the margin is from the attachment, the
better is the periodontal response. specific
recommendations have been made to
place the restorative margins 0.5 mm from
the healthy free gingival margin, or more
precisely, a minimum of 3.0 mm from the
alveolar crest (18).
it is the opinion of the authors that the
aetiology of the gingival inflammation seen
in the majority of anterior crown restorations is biologic width violation because
margins are routinely placed too deep into
the sulcus.
Often, this results from the clinician
not following the anatomical sculpting of
the gingival tissues, and the interproximal
margins are placed too close to the
attachment.
While almost all authorities recommend
supragingival crown margin placement
wherever possible in order to obtain
optimum soft tissue health (19). When crown
restorations are needed, cervical margins are
usually placed in an intracrevicular location
as it is usually impossible to blend in
crowns imperceptibly with tooth structure
when left supragingivally. it is important to
consider the patient’s individual smile line
and soft tissue display prior to determining
the specific margin location when placing
anterior restorations.
An important tooth-shape criterion
for an esthetic smile is the symmetry
of the maxillary anterior teeth (20). One
excellent study demonstrated that as many
as 25 percent of patients do not display the
anterior gingival tissues with a normal or
even and exaggerated smile. this finding
has significant clinical implications in that
if patient consent is obtained, many anterior
restorations can be placed with supragingival
margins, which results in an improved
periodontal response, better evaluation
of marginal integrity, and substantially
simplified operative procedures.
in
summary,
chronic
marginal
inflammation associated with anterior
restorations can be prevented by placing
crowns with smooth precise margins in
the proper intracrevicular position. that
position is quite a short distance into the
sulcus (0.5mm) as measured from the crest
of healthy gingival tissues. Margins must
be a minimum of 3 mm from the alveolar
crest and patients must be instructed in
and encouraged to perform optimum oral
hygiene procedures. An appropriate recall
and maintenance program is vital.
5.
6.
7.
8.
9.
10.
11.
Summary and conclusions
no matter how natural and lifelike anterior
restorations may be, the final aesthetic
result is particularly dependent upon the
health and level of the surrounding gingival
tissues. the key to success is effective soft
tissue management, and the goal of this
soft tissue management has been to provide
healthy gingival tissues covering sound,
smooth restorative margins. Meticulous
attention to detail will result in clinical
success regardless of the type of restoration
chosen.
u
12.
13.
14.
15.
16.
17.
bibliogrAPhY
1.
2.
3.
4.
Jones DW, Development of dental ceramics.
An historical perspective. Dental Clin north
Am 1985:29:621-644
Donovan te. Factors successful for allceramic restorations. J Am Dent Assoc 2008
Vol 139: no suppl 4, 14-18s
nevins M, skurow HM. the intracrevicular
restorative margin, the biologic width, and
the maintenance of the gingival margin. int J
Periodontics restorative Dent 1984:4:30-39
Kourkouta s. Hemmings KW, laurell l
18.
19.
20.
restoration of periodontally compromised
dentitions using cross-arch bridges. Principles
of perio-prosthetic patient management.
nemetz H. tissue management in
fixed prosthodontics. J Prosthet Dent.
1974:31:628-636
Cho GC, Chee WWl. Custom
characterization of the provisional
restoration. J Prosthet Dent 1993 69:529-532
Derbabian K, Donovan te, Marzola r,
Cho GC. Arcidiancono A. the science of
communicating the art of esthetic dentistry.
ii Diagnostic provisional restorations. J esthet
Dent 2000:12:238-247
Donovan te, Cho GC. Diagnostic provisional
restoration in restorative dentistry: the
blueprint for success. J Can Dent Assoc
1999:65:272-275
Marzola r, Derbabian K, Donovan
te, Arcidiancono A. the science of
communicating the art of esthetic dentistry
i. Patient-dentist-patient communication.
J esthet Dent 2000:12:131-138
Donovan te, Gandara BK, nemetz H. review
and survery of medicaments used with
gingival retraction cords .J Prosthet Dent
1985: 53:525-531
nemetz H, Donovan te, landesman HM.
exposing the gingival margin. A systematic
approach to the control of hemorrhage
.J Prosthet Dent 1984:51:647-651
Anneroth G. nordenram A. reaction of
the gingiva to the application of threads in
the gingival pocket for taking impressions
with elastic materials. Odontol revy
1969:20:301-310
rosenstiel s, Gegauff A. effects of provisional
luting cements on provisional resins.
J Prosthet Dent 1988:59:29-33
Wise MD. stability of gingival crest after
surgery and before anterior crown placement.
J Prosthet Dent 1985:53:20-23
Wise MD, Dykema rW. the plaque-retaining
capacity of four dental materials .J Prosthet
Dent 1975:33:178-190
ingber Js, rose lF, Coslet JG. the biologic
width: a concept in periodontics and
restorative dentistry. Alpha Omegan
1977:70:62-65
Gargiulo AW, Wentz FM, Orban B.
Dimensions and relations of the
dentogingival junction in man.
J Periodontology 1961:32:261-267
Block Pl. restorative margins and
periodontal health: a new look at an old
problem. J Prosthet Dent 1987:57:683-689
silness J. Periodontal conditions in
patients treated with dental bridges. iii the
relationship between the location of the
crown margins and the periodontal condition.
J Periodontal res 1970:5:225-229
Fradeani M. esthetic rehabilitation in Fixed
Prosthodontics. Vol 1 Quintessence
CPDQuestionnaire
Questionnaire
CPD
(True or False):
1. Gingival recession in adults is a natural effect of aging.
1. The metal-ceramic crown was an initial clinical disappointment to
6. What tissue type makes patients more susceptible to
2. What are the two essentials in soft tissue management in
7. What gingival condition is associated with restorations with
2. Most
often patients
with healthy gingival tissues.
the profession.
Why?requiring extensive restoration of anterior teeth present
recession?
3. Patients should be placed on weak chlorhexidine for 2 weeks prior to definitive impressions and continue until the definitive restoration is placed.
4. A intracrevicular
single retraction
cord placed
in the
sulcus
for 8-10minutes
combinedsubgingival
with Traxodent
can replace the double cord technique.
restorative
dentistry
related
to anterior
teeth?
margins?
5. A non-eugenol temporary cement should be used if all-ceramic restorations are planned.
3. Why is excellent gingival health achievement required prior to
8. Why should margins not be placed deep into the sulcus?
6. If definitive
patients margin
have periodontal
placement?surgery, stabilization of the gingival crest occurs within 6-8weeks.
9. How far into the sulcus should the margin be placed?
7. The primary causative factor with chronic gingival imflammation surrounding anterior restorations is violation of the biologic width.
4. How do you avoid iatrogenic recession during tooth preparation?
10.crest.
How do you work out the appropriate recall and maintenance
8. Restorative margins should be a minimum of 3mm from the alveolar
5. What is the function of the provisional restoration?
program?
9. As many as 25% of patients do not display the anterior gingival tissues with a normal or even exaggerated smile.
10. Patients object to supragingival margin placement anteriorly.
AustrAlAsiAn Dentist 51
Article 8
Category
CliniCal
ARTICLE AwAITINg CPD POINT APPROvAL FROM THE DENTAL BOARD.
Please refer to our website for updates: www.australasiandentist.com.au
Modern Day
Crown Preparations
An evidence based examination
By Dr David Penn BDs MBA, Head of Clinical and technical research, sCDl
and Dr Brenda Baker BDs(Hons) Msc, Director of Clinical education, sCDl
t
Dr David Penn
his three part series of articles identifies principles
that can assist dentists design, assess and modify
complete coverage preparations to ensure clinical
success for the treatment of various unprepared
and previously prepared teeth. the individual step
by step approach to tooth form preparation will be
subsequently reviewed. Finally, troubleshooting of
clinical cases will be analysed and causes of failure
reviewed.
the authors have conducted an extensive search
of the literature to provide a detailed “evidence based
approach”.
PART 1:
the form of prepared teeth and the amount of tooth
structure removed are important contributors to the
mechanical, biologic and aesthetic success of the
overlying crown or fixed partial denture.
Clinical established guidelines are presented to
optimise success and understand the biomechanical
implications of physical preparation.
Dr Brenda Baker
FEATURES AND IMPORTANCE OF
PHYSICAL PREPARATION
the following features will be reviewed in the
preparation characteristics of crowns:
1. total occlusal convergence
2. Occlusalcervical/incisocervical dimension
3. ratio of occlusocervical/incisocervical dimension
of buccolingual dimension
4. Circumferential form of the prepared tooth
5. reduction uniformity
6. Finish line location
7. Finish line form
8. Axial and incisal/occlusal reduction depths
9. line angle form
10. surface texture
1. Total Occlusal Convergence
total occlusal convergence (tOC) is the angle formed
between two opposing prepared axial surfaces.
shillingburg et al recently suggested that the tOC
should be between 10 and 22 degrees (1).
therefore, during clinical tooth preparation, the
use of a mirror has been recommended so that a buccal
or lingual view of the prepared teeth is established.
Buccal/lingual clinical views are the most effective
means of assessing tOC because convergence of
mesial and distal surfaces is readily visible (2).
the dental literature has also presented data
on several factors likely to create greater tOC
and perhaps even necessitate the formation of
auxiliary characteristics that enhance resistance to
dislodgement.
a. Posterior teeth were prepared with greater tOC
than anterior teeth (3,4).
b. Mandibular teeth were prepared with greater
convergence than maxillary teeth (5,6).
c. Mandibular molars were prepared with the greatest
tOC (7).
d. Buccolingual surfaces had greater convergence
than mesiodistal surfaces (3). However, another
study (6) determined that mesiodistal convergence
was greater than buccolingual convergence.
e. Fixed partial denture (FPD) abutments were
prepared with greater tOC than individual crowns
(8)
.
f. Monocular vision (one eye) created greater
tOC than binocular vision (both eyes) (8). in
the presence of the factors that increase tOC
beyond the recommended 11 degree to 20 degree
range, it is recommended that auxiliary tooth
preparation features such as grooves or boxes, be
added to enhance the resistance of restorations to
dislodgment.
Attributes of Single Tooth Permanent Restorations
Size of
lesion
Longevity
rating
FPD
abutment
RPD
abutment
Aesthetics
Retention
Restoration
Protects
tooth
Replaces
Cusp
Full-metal crown
Large
1
Yes
Yes
Poor
Good
Yes
Yes
Good
Poor
All
Yes
Metal-ceramic crown
Large
2
Yes
Yes
Good
Good
Yes
Yes
Good
Good
All
Yes
All-ceramic crown (veneered)
Large
3
No
No
Excellent
Good
Yes
Yes
Adequate
Good
All
Some
Full Monolithic Zirconia
Large
1
Yes
Yes
Fair
Good
Yes
Yes
Good
Poor
All
Yes
Full Monolithic Lithium Disilicate
Large
3
Some
No
Excellent
Good
Yes
Yes
Adequate
Good
All
Some
42 AustrAlAsiAn Dentist
Occlusal
Incisal
Buccal
Endodontic
restoration restoration restoration restoration
CliniCal
2. Occlusocervical/Incisocervical
Dimension
it is proposed that anterior teeth and
premolars have a minimal occlusocervical
(OC) dimension of 3mm and that molars
have a minimal dimension of 4mm.
Critical convergence angles have been
mathematically calculated and used to
identify angles beyond which a crown
would theoretically not possess adequate
resistance to dislodgement (9,10). An assess
ment of the resistance form of dies from
clinically failed restorations supported a
relationship between convergence angles
and clinical failure (11).
the resistance of crowns made for
dies the size of prepared incisors and
premolars has been tested, and it was
concluded that 3mm provides adequate
resistance(12), supporting the recommended
OC dimension for premolars and anterior
teeth.
the tipping resistance of molarsized
crowns has also been measured (13). three
millimetres of OC dimension provided
adequate resistance but only at 10 degree
tOC. three millimetres was inadequate
at 20 degree of tOC, an angle frequently
found on molars.
teeth lacking these minimal dimen
sions should be modified to enhance their
resistance form through formation of
proximal grooves/boxes.
rectangular form and most premolars and
anterior teeth have an oval form. these
shapes produce circumferential irregularity.
the value of these irregularities has been
evaluated by comparing the resistance areas
of conical and pyramidal tooth preparation.
the pyramidal preparation provided
increased resistance (15). therefore, it is
important to preserve the “corners” of a
tooth preparation whenever possible.
When prepared teeth have no corners
due to their round morphologic form or
existing condition, they should be modified
by forming axial grooves or boxes that
provide resistance to dislodging forces. As
molars are frequently prepared with greater
convergence angles than other teeth and
because they usually have a smaller OC
dimension and less favourable OC/Bl
dimension ratio, they often benefit from
axial grooves or boxes. it is suggested that
axial grooves/boxes be routinely used
when mandibular molars are prepared for
bridgework. Chewing and parafunctional
habits place dislodging forces on single
crowns and fixed partial dentures that are
largely Bl in direction, auxiliary resistance
form features should be located in the tooth
where they provide optimal resistance to
these forces.
Proximal grooves provided complete
resistance to Bl crown dislodgment,
whereas buccal or lingual grooves provide
only partial resistance (13). therefore,
auxiliary resistance form features such as
grooves and boxes should be located on the
proximal surfaces of fixed partial denture
abutments.
3. Ratio of Occlusocervical/
Incisocervical Dimension to
Buccolingual Dimension
the ratio of the OC dimension to the
buccolingual (Bl) dimension should be
0.4 or higher for all teeth. 96% percent of
incisor crowns, 92 % of canine crowns,
and 81% of premolar crowns possess
adequate resistance despite variations in
their preparation form and dimensions,
However, only 46% of molars possess
appropriate resistance (14).
When anterior teeth are prepared for
complete coverage crowns, they usually
possess a favourable ratio between the
incisocervical crown dimension and the
mesiodistal/buccolingual dimension. Molars
have less favorable ratios between the OC
dimension and the Bl dimension than
anterior teeth. When mandibular molars
are prepared they have a rectangular form
with rounded corners that enhance
resistance form.
4. Circumferential form of the
prepared tooth
teeth should be prepared so they possess
circumferential irregularity whenever pos
sible. When teeth are anatomically reduced,
they possess characteristic geometric forms.
For instance, when prepared maxillary
molars are viewed occlusally, they have a
rhomboidal form. Mandibular molars have a
AustrAlAsiAn Dentist 43
CliniCal
5. Reduction Uniformity
teeth should be uniformly reduced, thereby
enhancing the potential for normal crown
form and an improved aesthetic result.
reduction uniformity is best achieved by
placing depth grooves into the surface to
be reduced and then reducing the tooth in
accordance with the grooves.
6. Finish Line Location
Finish lines should be positioned equi or
supragingivally(16) whenever the aesthetic
and resistance form requirements permit
such a location. However, subgingival
finish lines often are used for appropriate
reasons that include the need to achieve
adequate OC dimension for retention
and resistance form, to extend beyond
caries, fractures, and erosion/abrasion;or
to encompass a variety of tooth structure
defects. subgingival finish lines are also
used to produce a cervical crown ferrule
on an endodontically treated teeth and to
improve the aesthetic result achieved on
discoloured teeth. Periodontal health can
be retained when subgingival margins are
used if other factors must be present. the
restorations must be properly contoured
and exhibit good marginal fit (17). When a
subgingival finish line is required, multiple
studies indicate that extension to the level
of the epithelial attachment should be
avoided. Pocket deepening does not occur
when the margin is at least 0.4mm occlusal
to the depth of the gingival crevice,(18)
whereas more severe gingivitis occurs when
subgingival margins approximate the depth
of the crevice (19). When teeth were prepared
so provisional crown margins were located
farther apically than recommended , about
a millimetre of gingival recession was
noted within 2 weeks and little over 1mm
of recession was recorded within 8 weeks
(20)
. Histologic evaluation indicated the
recession mechanism was activated during
the first 7 days (20) when crown margins
were extended to the bone crest, 1 mm of
crestal bone loss was observed (21).
7. Finish Line Form
A. All-metal crowns
Chamfer finish lines frequently have been
used for allmetal crowns. no scientific
studies have stated that chamfers are
superior to other finish lines. However,
they are used with allmetal crowns because
they are easy to form with a tapered, round
end diamond instrument and because they
are distinct, being readily visible on the
prepared tooth, impression and die.
Chamfers also possess adequate bulk
for restorative rigidity and their depth is
sufficient to permit the development of
normal axial contours. recommended
chamfer depth is determined by the
minimal metal thickness for strength
and minimal space required to develop a
physiologic emergence profile. Authors
have recommended chamfer finish line
reduction depths of 0.3 to 0.5mm (22,23).
it is recommended that chamfer
finish lines for all metal crowns possess a
minimum depth of approximately 0.3mm(2).
B. Metal-ceramic crowns
the following types of finish line
historically have been used with metal
ceramic crowns: chamfer, bevelled
chamfer, shoulder and bevelled shoulder.
recommended
metalceramic
finish
line depths are based on the minimal
material thickness required for strength
and aesthetics as well as the minimal
space required to develop a physiologic
emergence profile. A thickness between
1.0 and 1.5mm for the porcelainveneered
marginal area of a metalceramic crown has
been suggested (1,22,23,24,25). Multiple studies
have indicated that at least 1.0mm of
translucent porcelain (not including metal
and opaque) is required to reproduce the
colour of a shade guide (26) which indicates
Table 1 Seven Key Principles of Preparation (derived from Shillingberg)
Seven space key principles
Function
Conservation of tooth tissue
To avoid weakening the tooth unnecessarily
To avoid compromising the pulp
Resistance form
To prevent dislodgement of a cemented restoration by apical or
obliquely directed forces
Retention form
To prevent displacement of cemented restoration along any of its
paths of insertion, including the long axis of the preparation
Structural durability
To provide enough space for a crown which is sufficiently thick to
prevent fracture, distortion or perforation
Marginal integrity
To prepare a finish line to accommodate a robust margin with close
adaptation to minimise micro-leakage
Preservation of the periodontium
To shape the preparation such that the crown is not over contoured
and its margin is accessible for optimal oral hygiene
Aesthetic considerations
To create sufficient space for aesthetic veneers where indicated
44 AustrAlAsiAn Dentist
that tooth reductions in excess of 1.0mm
are needed.
C. All-ceramic crowns (veneered and
monolithic lithium disilicate)
Chamfer finish lines produced lower
strength with nonbonded crowns in
laboratory tests (27). However, the negative
effect was not replicated when the crowns
were bonded (internally etched crowns
cemented to etched prepared teeth with
resin) to the teeth (28). it therefore seems
appropriate to recommended shoulder
finish lines for allceramic crowns that
are not to be bonded to underlying tooth
structure, whereas chamfer or shoulder
finish lines can be used when the crowns
are to be bonded.
D. All-ceramic crowns (monolithic zirconia)
this new generation restoration can
be finished with any of the traditional
methods: chamfer, bevelled chamfer, shoul
der and bevelled shoulder. recommended
finish line depths are based on the
minimal material thickness required
for strength and aesthetics as well as
the minimal space required to develop a
physiologic emergence profile. Authors
and manufacturers have recommended
thicknesses between a minimal 0.4 mm
and 1.0mm for the marginal area of this
monolithic allzirconia crown.
8. Axial and incisal/occlusal
reduction depths
the required depth of reduction varies
with different types of crowns and various
surfaces of a tooth. reduction also is
affected by the position and alignment of
teeth in the arch, occlusal relationship,
aesthetics, periodontal considerations
and tooth morphology. Deep occlusal
interdigitation of posterior teeth or
appreciable vertical overlap of the anterior
teeth often necessities greater overall
reduction of occluding surfaces. Mal
aligned teeth commonly have required
greater reduction of protruding surfaces
to permit restoration alignment and or
satisfactory retention and resistance form.
Periodontal health is enhanced through
the development of normal cervical crown
contours, but overcontoured restorations
promote plaque accumulation.
Allmetal crowns and monolithic
zirconia allceramic crowns should have
chamfer depths of at least 0.3mm, axial
surface reductions of at least 0.5 to 0.8mm
and occlusal reduction depths of 1 to 1.5mm
(2)
. For metalceramic crowns, depths of
1.0mm or more have been proposed and are
aesthetically desirable for the finish line
and buccal reductions. Finish line (> 1.0
mm) and buccal reductions depths of 1.5
2.0 mm are recommended for veneered and
monolithic zirconia allceramic crowns and
monolithic lithium disilicate crowns. An
CliniCal
incisal/occlusal reduction of 2mm for all
ceramic crowns permit the development of
appropriate colour, strength, translucency
and morphology.
A. All-metal crowns and monolithic zirconia
crowns
For allmetal crowns and monolithic
zirconia crowns, finish line depths of 0.3 to
0.5 mm have been recommended (22,24). For
allmetal crowns and bridges, there are no
data that identify the ideal axial reduction
depths. the experience of clinicians
and laboratory technicians recommends
0.5 at least to 0.8 mm of reduction to be
developed near the occlusal aspect of the
buccal and lingual surfaces. this depth of
reduction provided adequate space for the
development of normal axial contours and
material thickness for strength. Proximal
reduction should include the formation of
a distinct finish line and provide access for
impression making.
A minimum of 1mm of occlusal
reduction provides space for the fabrication
of these crowns, but reduction depths of
1.5mm provide the space whereby well
defined occlusal grooves and convex ridges
can be developed.
B. Metal-ceramic crowns
Finish lines for metalceramic crowns
should be 1.0 to 1.5 mm deep and the
buccal surface be reduced between 1.0 and
1.7 mm (1,22,24,25). these recommendations
are supported by research that determined
1.0mm or more of translucent porcelain
is required to reproduce shade guide
specimens (26).
When aesthetic materials are to be
placed over incisal/occlusal surfaces,
reduction depths of 2.0 to 2.5 mm have
been recommended for metalceramic
restorations to provide space for the
development of appropriate colour,
anatomic form, and occlusion (1,22,23,24).
C. All-ceramic veneered crowns and
monolithic lithium disilicate crowns
Allceramic finish line depth recommenda
tions range from 0.5 to 1.0mm (1,22,23,24,25).
From a buccal surface reduction perspec
tive, there is little improvement in shade
matching when the thickness of veneered
allceramic crowns or monolithic lithium
dislicate allceramic crowns is increased
beyond 1 mm with semitranslucent, all
ceramic systems (eg. e.Max or empress
esthetic) and high value, low chroma
shades (eg. A1)(26). However, thicknesses in
excess of 1mm are beneficial when using
more opaceous allceramic systems (lava
3M esPe or Calypso sCDl) or when using
lower value, more chromatic shades such
as C2 and A3 (26). the inherent colour of the
abutment tooth can influence the colour of
the overlying allceramic crown, requiring
greater ceramic thickness when the den
tine is discoloured.
Table 2 Suggested Preparation Features for Posterior Crowns
Crown type Posterior Crowns – Preparation Features
Occlusal Reduction* Finish Line Depth and Configuration
FCC – 1mm non-functional cusps
-1.5mm functional cusp
0.3-0.5 mm
Chamfer, knife –edge,
Shoulder or shoulder with bevel
HSAC – 2mm non-functional cusps
- 2.5mm functional cusps
1.0 mm shoulder or heavy chamfer
PFM – As for FCC if metal occlusal
2mm non-functional clasps
2.5mm functional cusps
1.2mm labial shoulder + or chamfer
0.5mm lingual chamfer
(metal collar)
1.2mm circumferentially of 360 degree ceramic margin
* Where tooth is tilted or where vertical dimension it to be increased is to be increased, the amount of occlusal
reduction required will vary.
+ Too deep a reduction for diminutive teeth or for long clinical crowns where a metal collar is preferable
FCC = Full Contour Crowns (Metal or Zirconia)
HSAC = High strength all-ceramic (veneered) or monolithic lithium disilicate
PFM = Porcelain Fused to Metal
Table 3 Suggested Preparation Features for Anterior Crowns
Crown type
Anterior crowns – Preparation features
Occlusal reduction * Finish line depth and configuration
AC
2mm incisally
1mm lingual aspect
0.8-1.0mm shoulder
RBPC
2mm incisally
0.5-1.0mm lingual aspect
>0.4mm chamfer
PFM
2mm incisally
1.2mm labial shoulder + or heavy chamfer
0.5-1.0mm lingual aspect
0.5mm lingual chamfer
(Porcelain guidance requires greater clearance) 1.2 mm circumferentially for 360 degree
ceramic margin
*Where the vertical dimension is to be increased, the amount of occlusal reduction required will be less or non existent.
+Too deep a reduction for diminutive teeth eg. Lower incisors or for long clinical crowns where metal collar is preferable)
AC : All-ceramic (veneered or monolithic)
RBPC: Resin Bonded Porcelain Crowns
PFM : Porcelain fused to metal
9. Line Angle Form
10. Surface texture
line angles are formed when prepared
tooth surfaces meet each other. sharp
line angles create stress concentration(27).
line angles should be rounded during
tooth preparation to enhance strength and
minimise crack propagation, especially
in allceramic veneered restorations and
monolithic lithium disilicate crowns. the
purpose of rounding line angles with all
metal and metalceramic crowns is related
more to facilitating laboratory procedures
and optimizing fit than to enhancing
restoration strength. round line angles
facilitate the fabrication of gypsum casts
from impressions without trapping air
bubbles as well as the investment of wax
patterns without air inclusions. trapped air
bubbles can lead to nodules in castings that
impede complete seating of a restoration.
Casting nodules also are easier to remove
when the line angles are rounded during
tooth preparation.
tooth preparation should be reasonably
smooth to enhance restoration fit.
tooth preparation smoothness (28) has
been found to improve the marginal fit
to restorations in two studies whereas
another study found no difference in the
marginal seating of complete crowns when
the axial surfaces were prepared with
coarse diamond instruments (120 um grit
size) and when they were prepared with
fine diamond (50 um grit size) instruments.
11. SUMMARY
the following guidelines are proposed
when preparing teeth for compete crowns
and fixed partial dentures:
1. the tOC (angle of convergence between
opposing prepared axial surfaces)
should range between 10 degrees and
20 degrees. However, posterior teeth
are frequently prepared with greater
convergence angles as are fixed denture
partial denture abutments. When the
tOC angles exceed the recommended
AustrAlAsiAn Dentist 45
CliniCal
levels, the tooth preparation should be
modified to include auxiliary features
such as grooves or boxes.
tOC and frequently have limited OC
dimension and large Bl dimensions
that lead to unfavourable ratios.
occlusal reduction depths of 2mm
are achievable due to the available
thickness of tooth structure.
2. three millimetres should be the mini
mal
occlusocervical/incisocervical
(OC/iC) dimension of incisors and pre
molars when they are prepared within
the recommended tOC range of 10
degree to 20 degree. the minimal OC
dimension of molars should be 4 mm
when prepared with 10 degree to 20 de
gree tOC. When the OC dimension is
less than the recommended dimension,
the tooth preparation should be modi
fied to include auxiliary features such
as grooves or boxes.
6. For the purpose of optimizing
periodontal health, finish lines should
be located equi or supragingivally when
the condition of the tooth and aesthetic
requirements permits such a location.
When subgingival finish lines are
required, they should not be extended
to the epithelial attachment.
10. rounded line angles on tooth prepara
tions for allceramic crowns decrease
the stress placed on the crowns and
thereby increase crown longevity. With
crowns that contain metal (allmetal
and metalceramic crowns), line an
gles are rounded to facilitate pouring
impression and investing wax patterns
without trapping air bubbles and to fa
cilitate the removal of casting nodules.
3. the ratio of the OC/iC dimension
to the Bl dimension should be 0.4 or
higher for all teeth. When this ratio
is not present, as on large diameter
molars, the tooth preparation should be
modified to include auxiliary features
such as grooves or boxes.
4. teeth should be prepared in a manner
that preserves the buccoproximal and
linguoproximal corners whenever pos
sible because circumferential irregu
larities enhance resistance form. When
prepared teeth lack “corners” and are
round after tooth preparation, they
should be modified to include auxiliary
features such as grooves or boxes.
5. When auxiliary features are placed into
teeth, the preferred locations are the
proximal surfaces. the buccal/lingual
surfaces are secondary locations to be
used when the addition of proximal
features leaves the tooth in a state of
questionable resistance form. Proximal
grooves/boxes should routinely be used
when mandibular molars are prepared
for bridges because mandibular molars
often are prepared with the greatest
7. For allmetal or fully monolithic zir
conia allceramic crowns, a minimum
0.3 mm deep chamfer finish lines
should be used. the axial and occlusal
reduction depths for allmetal crowns
should be at least 0.5 mm and 1.0mm,
respectively.
8. For metalceramic crowns, finish
line selection should be based on
formation ease, personal preference,
aesthetic requirements and the type of
crown being fabricated rather than on
expectations of enhanced marginal fit
with one type of finish line compared
with the others. Many teeth, because
of available tooth structure thickness
external to the pulp, cannot be reduced
buccally to depths that exceed 1mm.
11. tooth preparation smoothness seems
to enhance restoration fit, but its effect
on retention appears to be related to the
type of cement used. surface roughness
generally increased retention with zinc
phosphate cement, but no definitive
relationship has been established when
crowns are cemented with adhesive
cements (e.g. polycarboxylate, glass
ionomer, resin).
u
For a full list of references, please email
[email protected]
9. shoulder finish lines are recommended
for allceramic crowns when they are
not bonded to the underlying tooth.
However, shoulder and chamfer
finish lines can be used with all
ceramic crowns that are bonded
to the prepared tooth using a resin
cement and acid etching. Finish line
and buccal reduction depths > 1mm
are not required when using a semi
translucent type of allceramic crown
but are beneficial when more opaceous
porcelain systems are used or when the
tooth structure is discoloured. incisal/
CPDQuestionnaire
Questions (True or False):
CPD
1. What is the ideal total occlusal convergence(TOC) angle and
7. Circumferential irregularity with tooth preparation enhances
Total
should with
be between
10 and 22 degrees. what important anatomical features?
whichocclusal
teeth areconvergence
frequently prepared
greater convergence
angles?
If this feature has been lost, how is it possible to compensate
2. Buccal lingual clinical views are not the most effective means of assessing total occlusal convergence.
for it in the preparation?
How isabutments
the TOC best
assessed
clinically?
3.2. Bridge
are
prepared
with greater total occlusal convergence than individual crowns.
8. Where should auxilary resistance form features such as
If TOC isteeth
exceeded,
what is thehave
best a
way
to modify
the
4.3. Anterior
and premolars
minimal
occlusocervical
dimension
of 3mm
and be
molars
a minimal dimension of 4mm.
grooves
or boxes
placed?
preparation?
5. Circumferential irregularity is not a beneficial preparation feature.9. What is the recommended finish line and depth for all-metal
4. What are the minimal occlusocervical dimensions for anterior
crowns
and monolithic
disilicate
6. Auxiliary resistance form features such as grooves and boxes should crowns/all-ceramic
be placed on buccal
and (veneered
lingual surfaces
of bridge
abutments.
teeth and premolars and molars?
and monolithic zirconia)?
1.
7.
Posterior full contour crown either metal or zirconia should have a finish line depth of .3-.5mm
5. What is the ideal ratio of the occlusocervical/incisocervical
10. What finish lines are recommended for all-ceramic crowns
8. The
finish line
depth of high
strength all-ceramic(veneered) or monolithic
is .5-1mm.
dimension
to buccolingual
dimension?
whenlithium
they aredisilicate
not bonded
to the underlying tooth?
What
finish
lines
are
recommended
for all-ceramic crowns
9.6. The
inherent
colour
ofdo
theprepared
abutment
tooth
does not influence the colour of the overlying all-ceramic crown.
What
geometric
forms
teeth
exhibit?
when they are bonded to the prepared tooth using a resin
cement and acid etching?
10. Anterior all-ceramic (veneered or monolithic) should have .8-1mm shoulder.
46 AustrAlAsiAn Dentist
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Article 6
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sOUTHERN CROSS DENTAL LABORATORIES
DR BRENDA BAKER
PO Box 607 Double Bay NSW 1360
or FAX: (02) 9362 -1199 EMAIL: [email protected]
COURIER ADDRESS:
19 Gum Tree Lane, Double Bay NSW 2028
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PO Box 607 Double Bay NSW 1360
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