CAD/CAM Technology and Zirconium Oxide with Feather-edge ...
CAD/CAM Technology and Zirconium Oxide with Feather-edge ...
CAD/CAM Technology and Zirconium Oxide with Feather-edge ...
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CLINICAL APPLICATION<br />
<strong>CAD</strong>/<strong>CAM</strong> <strong>Technology</strong> <strong>and</strong><br />
<strong>Zirconium</strong> <strong>Oxide</strong> <strong>with</strong> <strong>Feather</strong>-<strong>edge</strong><br />
Marginal Preparation<br />
Stefano Patroni, MD, DDS, LMD<br />
University of Geneva, Switzerl<strong>and</strong><br />
Visiting professor, University of Parma, Italy<br />
Private practice, Piacenza, Italy<br />
Giuseppe Chiodera, DMD<br />
Private practice, Brescia, Italy<br />
Carlotta Caliceti, DMD<br />
Private Practice, Piacenza, Italy<br />
Paolo Ferrari, DMD, LMD<br />
University of Geneva, Switzerl<strong>and</strong><br />
Private Practice, Parma, Italy<br />
Correspondence to: Dr. Stefano Patroni<br />
Via S. Giovanni, 28; 29100 Piacenza, Italy<br />
phone: +39 0523 331852; e-mail: stefano.patroni@libero.it; www.stefanopatroni.com<br />
THE EUROPEAN JOURNAL OF ESTHETIC DENTISTRY<br />
VOLUME 5 • NUMBER 1 • SPRING 2010
Abstract<br />
Clinical needs <strong>and</strong> growing patient expectations<br />
have forced modern dentistry to focus<br />
on finding ever simpler protocols <strong>and</strong><br />
to develop materials which offer high performance<br />
in their mechanical resistance<br />
<strong>and</strong> esthetics.<br />
In recent years, the scientific community<br />
has been venturing into the world of<br />
<strong>CAD</strong>/<strong>CAM</strong>, a significant technological innovation<br />
imported from the world of engineering.<br />
This innovation has made it<br />
possible to exploit a material which has<br />
long been noted for its mechanical <strong>and</strong><br />
mimetic qualities: zirconium oxide. While<br />
<strong>CAD</strong>/<strong>CAM</strong> is revolutionizing the laboratory<br />
work of dental technicians, the white color<br />
of zirconium has opened new avenues<br />
PATRONI<br />
which may lead not only to new options<br />
for proper treatment planning, but also to<br />
new opportunities in the choice of materials<br />
to be used in prosthetic rehabilitation<br />
<strong>and</strong> variation of the types of preparations<br />
possible.<br />
The present paper will analyze the advantages<br />
<strong>and</strong> limitations of these methodologies,<br />
which are capable of simplifying<br />
clinical protocols <strong>and</strong> st<strong>and</strong>ardizing results.<br />
These technologies have instilled<br />
great enthusiasm in the profession due to<br />
their innovative nature, but this approach<br />
needs to be verified by further scientific<br />
evidence.<br />
(Eur J Esthet Dent 2010;5:XXX–XXX)<br />
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Introduction<br />
<strong>CAD</strong>/<strong>CAM</strong> is a term borrowed, like the<br />
technology associated <strong>with</strong> it, from the<br />
world of engineering <strong>and</strong> has been applied<br />
to clinical dentistry. The end products<br />
in the two fields are very different as far as<br />
materials, forms, <strong>and</strong> dimensions are concerned,<br />
but the philosophy behind the<br />
process remains the same <strong>and</strong> involves<br />
the replacement of manpower <strong>with</strong> mechanized<br />
or robot-powered procedures. 1,2<br />
The procedures can be divided in two<br />
phases: <strong>CAD</strong> (computer-aided design),<br />
whereby the object is designed <strong>and</strong> created<br />
virtually, <strong>and</strong> <strong>CAM</strong> (computer-aided<br />
manufacturing) which takes the design<br />
from the virtual to the production of the designed<br />
object.<br />
The precision involved <strong>and</strong> the ability of<br />
this technology to st<strong>and</strong>ardize both the<br />
process <strong>and</strong> the finished product make its<br />
potential fascinating in itself; in theory it<br />
may even be possible to eliminate variables<br />
introduced by the operator in the laboratory<br />
phase <strong>and</strong> reduce the clinical steps<br />
needed to produce the final prosthesis. 3 Interest<br />
for <strong>CAD</strong>/<strong>CAM</strong> technology has been<br />
expressed by dentists the world over, as<br />
this technology makes it possible to work<br />
<strong>with</strong> different materials. Some of these include<br />
titanium as base metal alloys, acrylic<br />
resins for provisional restorations, <strong>and</strong> either<br />
crystalline or vitreous ceramics. The<br />
material <strong>with</strong> the most applications is a ceramic<br />
oxide stabilized <strong>with</strong> yttrium oxide<br />
known as zirconium dioxide (zirconia). 4<br />
Many questions have arisen about this<br />
new method from both clinicians <strong>and</strong> dental<br />
technicians alike; in fact, the laboratory<br />
will be most affected by these innovations.<br />
Therefore, the clinician needs to be able<br />
to underst<strong>and</strong> how these new <strong>CAD</strong>/<strong>CAM</strong><br />
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VOLUME 5 • NUMBER 1 • SPRING 2010<br />
technologies <strong>and</strong> intraoral optical impressions<br />
can be integrated <strong>with</strong> everyday clinical<br />
practice, what type of preparations <strong>and</strong><br />
prosthetic margins will be compatible <strong>with</strong><br />
these new materials, <strong>and</strong> precise information<br />
about how to use them.<br />
From a chemical point of view, zirconium<br />
is a grey/white hard transition metal,<br />
which is not found in nature as a pure mineral.<br />
In dentistry, zirconium is not used as a<br />
pure mineral but as a ceramic oxide (zirconium<br />
dioxide) instead. From a structural<br />
point of view, zirconium dioxide comes in<br />
three states: cubic, monocline, <strong>and</strong> tetragonal.<br />
5 All of these states are unstable at<br />
room temperature <strong>and</strong> must be combined<br />
<strong>with</strong> an element to hold the desired form.<br />
The tetragonal is one of the most studied<br />
forms in published literature 6 <strong>and</strong> is stabilized<br />
<strong>with</strong> yttrium. 7 This material was used<br />
experimentally as early as 1969 in the field<br />
of orthopedics.<br />
It is the esthetic <strong>and</strong> mechanical characteristics<br />
of zirconium which makes it such<br />
an attractive metal to work <strong>with</strong>. Indeed, not<br />
only is it white in color, but its resistance to<br />
fractures is very similar to that of titanium<br />
(285 N vs 305 N). 8 In addition, it is not cytotoxic,<br />
9 does not produce mutations of the<br />
cellular genome, 8,10 <strong>and</strong> it induces a modest<br />
inflammatory response in tissues, which<br />
is less than that of titanium. 11<br />
Another attractive characteristic of the<br />
material is that it has little retention of bacterial<br />
plaque. Scarano reported a bacterial<br />
covering on the surface of zirconium of<br />
12%, as opposed to 19% for titanium. 5<br />
<strong>Zirconium</strong> has multiple applications in<br />
dentistry. It can be used as a core for onlays/overlays,<br />
as a core for single crowns,<br />
as a framework for bridges, as abutments<br />
for implants (Fig 1), <strong>and</strong> as implants themselves.
Despite the material having been well<br />
known, the technology was not available.<br />
Today, thanks to <strong>CAD</strong>/<strong>CAM</strong>, this technological<br />
gap has been filled.<br />
The structures are obtained by subtraction<br />
milling a partially sintered zirconium<br />
block; this procedure is guided by dedicated<br />
software. 12<br />
An overview of zirconium<br />
<strong>and</strong> <strong>CAD</strong>/<strong>CAM</strong><br />
It must be borne in mind that the zirconium<br />
structure is no longer made by a technician<br />
but by the milling center machine which<br />
grinds the zirconium block. This requires<br />
specific instructions <strong>and</strong> guidelines in digital<br />
form, which can be summarized in<br />
three steps: 3<br />
1) digitization: transforms the geometry of<br />
the scanned object into digital data<br />
2) processing: software processes the data<br />
<strong>and</strong>, if necessary, develops the project<br />
3) realization: technology (not necessarily<br />
a milling machine) transforms the data<br />
into the desired object.<br />
When using <strong>CAM</strong>/<strong>CAD</strong> technology, the<br />
classic clinical/technical itinerary (impression,<br />
model, waxup, casting) must interface<br />
<strong>with</strong> the digital pathway (Table 1). The<br />
transformation of analog information into<br />
digital information can take place at various<br />
points in the conventional process:<br />
1) The impression is developed in the laboratory.<br />
The laboratory forwards the<br />
model to the digital service who will<br />
digitize it, design the structure (<strong>CAD</strong>),<br />
realize it (<strong>CAM</strong>), <strong>and</strong> then send it back<br />
to the technician for ceramic stratification<br />
(Table 2).<br />
PATRONI<br />
Fig 1 Example of a custom implant abutment completely<br />
realized in zirconia.<br />
Table 1<br />
Table 2<br />
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Table 3a<br />
Table 3b<br />
Table 4<br />
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2) The impression is developed in the laboratory.<br />
The technician digitizes the<br />
model <strong>with</strong> a scanner <strong>and</strong> <strong>with</strong> special<br />
software designs the structure (<strong>CAD</strong>)<br />
(Table 3a). Alternatively, the technician<br />
can realize a traditional waxup <strong>and</strong> scan<br />
it (Table 3b). In both cases, the laboratory<br />
will be able to issue a digital file to the<br />
milling center for manufacturing the<br />
structure (<strong>CAM</strong>). The structure is then<br />
sent back to the laboratory for ceramic<br />
stratification .<br />
3) The impression can take the digital route<br />
from its beginning (eg, using a 3M ESPE<br />
video camera or similar). The planning<br />
can take place either in the laboratory or<br />
in the digital service center (<strong>CAD</strong>). However,<br />
the manufacture of the core (<strong>CAM</strong>),<br />
must take place in the milling center <strong>and</strong><br />
is then delivered to the laboratory along<br />
<strong>with</strong> a resin model for ceramic stratification<br />
(Table 4).<br />
In the authors' clinical experience, the<br />
method that gives the best frameworks/<br />
cores for supporting ceramics is that<br />
which starts <strong>with</strong> a conventional waxup of<br />
the structure made in the laboratory which<br />
is then scanned. This technique allows the<br />
creation of an individualized structure,<br />
which means that the design can be customized<br />
<strong>with</strong> an adequate cusp <strong>and</strong> marginal<br />
crest. This creates, where necessary,<br />
a palatine framework thickening <strong>and</strong> adequate<br />
connection design (Fig 2a <strong>and</strong> b).
a b<br />
Clinical application<br />
For a fixed prosthesis, the base material of<br />
the ceramic crown which is most commonly<br />
used <strong>and</strong> which, according to scientific<br />
evidence, guarantees the highest st<strong>and</strong>ards<br />
of resistance, is a noble alloy (ceramometal<br />
technique).<br />
The greatest limitation of this technique<br />
is the fact that the metal restoration has limited<br />
translucency, which means that esthetically<br />
it cannot compete <strong>with</strong> the st<strong>and</strong>ard<br />
that can be achieved when integral ceramics<br />
are used. Integral ceramics are indicated<br />
in cases where excellent esthetic results<br />
are required as these materials best imitate<br />
the color <strong>and</strong> luminosity of the natural tooth<br />
(Fig 3). One disadvantage however, is that<br />
these restorations have low mechanical resistance.<br />
Collarless ceramics supported by a<br />
metal core represent a solution that, in<br />
theory, combines the advantages of both<br />
techniques, eliminating the metal in the<br />
marginal portions so as to reduce the umbrella<br />
effect. 13 Today there is also a third option,<br />
which cuts a middle road <strong>and</strong> which<br />
is increasingly being used: zirconium.<br />
PATRONI<br />
Fig 2a <strong>and</strong> b Examples of zirconia copings <strong>with</strong> a well-represented margin to guarantee the mechanical<br />
strength.<br />
Fig 3 The best material for prosthetic reconstructions,<br />
in terms of esthetics, is the material able to mimic<br />
the translucency of natural teeth.<br />
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a b<br />
Fig 4a Discolored natural tooth; the use of a high<br />
translucency crown would end in a unsatisfactory result.<br />
a b c<br />
a b<br />
Fig 6a <strong>and</strong> b A clinical case: the smile before <strong>and</strong> after treatment.<br />
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Fig 4b The ceramic crown supported by a zirconia<br />
core once cemented. The unfavorable tooth value is<br />
partially masked, increasing the core thickness. In spite<br />
of that, however, a discolored abutment appears<br />
through the marginal tissues anyway.<br />
Fig 5a to c Manufacturers<br />
advise a round<br />
shoulder or a chamfer<br />
preparation for a zirconia<br />
ceramic crown.
a b<br />
PATRONI<br />
Fig 7a <strong>and</strong> b The initial intraoral situation (a). Final result (b): the four incisors <strong>with</strong> zirconia crowns cemented.<br />
The m<strong>and</strong>ible was rehabilitated <strong>with</strong> an implant-supported fixed prosthesis.<br />
a b<br />
c d<br />
Fig 8a to d Maxillary anterior teeth before treatment (a). Four incisors prepared <strong>with</strong> a marginal chamfer (b).<br />
Four zirconia ceramic crowns on the master model (c). The end of treatment; the four upper incisors were reconstructed<br />
<strong>with</strong> zirconia ceramic crowns, the two cusps were restored by ceramic veneering (d).<br />
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CLINICAL APPLICATION<br />
a b<br />
Fig 9a <strong>and</strong> b Some details of the crown margins <strong>and</strong> their tissue integration.<br />
a<br />
c<br />
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b<br />
Fig 10a to c In the case of a feather-<strong>edge</strong> preparation,<br />
it is necessary to finish <strong>with</strong> a metallic margin.<br />
When using a zirconia core, the margin should also be<br />
well represented, but the white color simplifies the esthetic<br />
result.
<strong>Zirconium</strong> has excellent mechanical <strong>and</strong><br />
esthetic characteristics. Mechanically, this<br />
is observed if the material is used in thickness<br />
of 0.5 mm or more. This thickness<br />
guarantees excellent mechanical resistance<br />
but partially diminishes the esthetic<br />
result. In some cases however, the thickness<br />
of the structure makes it possible to<br />
mask highly discolored prepared teeth, optimizing<br />
the esthetic results (Fig 4a <strong>and</strong> b).<br />
In order to obtain the very best esthetic<br />
results, it is necessary to use a thickness of<br />
0.3 mm, as this gives a high degree of luminosity<br />
<strong>and</strong> translucency. 14 It should be<br />
noted however, that these characteristics<br />
are inferior to those that can be obtained<br />
from integral ceramics (light transmission<br />
through 0.6 mm thick flat specimens of<br />
densely sintered alumina is 72% versus<br />
48% for densely sintered zirconia). 15<br />
For restorations <strong>with</strong> a zirconium oxide<br />
core, the producers recommend round<br />
shouldered or chamfered preparations as<br />
these are considered to be best for precision,<br />
resistance, <strong>and</strong> esthetic outcome<br />
(Figs 5 to 9).<br />
The presence of a margin means that<br />
excellent definition is required in the<br />
preparation phase <strong>and</strong> then adequate<br />
transferral of that information to the laboratory.<br />
These problems are easier to solve<br />
<strong>with</strong> a feather-<strong>edge</strong> preparation where the<br />
finishing line is no longer represented by<br />
a line but rather by an area (Table 5).<br />
The advantage of a butt preparation<br />
(shoulder or chamfer) is that it allows, from<br />
the margin, an adequate thickness of<br />
restoration materials (metal-opaque ceramics),<br />
reducing as much as possible<br />
the unsightliness of a metal border.<br />
However, it is necessary <strong>with</strong> a feather<strong>edge</strong><br />
preparation to have a well-represented<br />
<strong>and</strong> clinically evident metallic margin<br />
Table 5<br />
PATRONI<br />
(Fig 10). This may be considered an esthetic<br />
compromise <strong>and</strong> is usually indicated for<br />
complex perio-prosthetic rehabilitations.<br />
This limitation can be overcome by using<br />
a material such as zirconium, which makes<br />
it possible to create a white margin. While<br />
being clinically acceptable, this integrates<br />
well esthetically <strong>with</strong> the periodontal tissues<br />
<strong>with</strong>out needing to “invade” the dentogingival<br />
sulcus, which would complicate<br />
the prosthetic procedures <strong>and</strong> risk damage<br />
to the periodontal attachment.<br />
This led to the idea of combining zirconium<br />
oxide <strong>with</strong> the feather-<strong>edge</strong> preparation,<br />
a solution which, while not advised, is<br />
allowed by the producers on the condition<br />
that certain fundamental clinical <strong>and</strong> technical<br />
considerations are taken into account.<br />
Why feather-<strong>edge</strong><br />
marginal preparation?<br />
In the last 10 years, the prosthetic treatment<br />
plan has changed due to the introduction<br />
of implants. Indeed, it is because of implants<br />
that it is possible to treat individual<br />
or multiple edentulous areas (Fig 11), <strong>with</strong>out<br />
resorting to the preparation of healthy<br />
11<br />
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a<br />
CLINICAL APPLICATION<br />
c d<br />
Fig 11a to d In the last decade, the vast majority of clinicians prefer, in the case of partially edentulous areas<br />
between two healthy teeth, to place an implant instead of conventional prosthesis.<br />
teeth <strong>and</strong> sacrificing sound tissue to accommodate<br />
bridges. In the same way,<br />
many seriously compromised teeth, which<br />
until recently would have been treated in a<br />
prosthetic manner <strong>with</strong> complete crowns,<br />
can today be rehabilitated <strong>with</strong> indirect adhesive<br />
restorations.<br />
The indications for a fixed prosthesis<br />
have changed, <strong>and</strong> it is much less frequent<br />
that a healthy tooth is rehabilitated<br />
<strong>with</strong> a fixed prosthesis. It is even rarer that<br />
the clinician performs endodontic treatment<br />
for prosthetic reasons as vital teeth<br />
can be treated <strong>with</strong> restorative therapy.<br />
Therefore, prosthetic dentistry almost always<br />
follows the endodontic treatment<br />
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b<br />
needed to recover teeth which have been<br />
destroyed by decay.<br />
The carious lesion, moreover, does not<br />
only extend horizontally towards the pulp<br />
chamber but follows the line of the dental<br />
tubules <strong>and</strong> often extends apically <strong>and</strong> involves<br />
the periodontal attachment. For this<br />
reason, the majority of cases require a<br />
crown-lengthening procedure before the<br />
prosthetic phase in order to make the<br />
residual healthy tissue accessible <strong>and</strong> to<br />
re-establish a correct relationship <strong>with</strong><br />
these <strong>with</strong>out damaging the periodontal<br />
attachment (Figs 12 <strong>and</strong> 13).<br />
In the same surgical time, it is possible<br />
to carry out a feather-<strong>edge</strong> preparation of
a<br />
Fig 12a to c Surgical crown lengthening is frequently<br />
necessary to properly rehabilitate a seriously<br />
decayed tooth. During the surgery, the tooth is prepared<br />
up to the bony crest, <strong>and</strong> after suturing the temporary<br />
is relined. With a feather-<strong>edge</strong> preparation it is easy to<br />
obtain, 4 months after surgery, healthy tissues around<br />
the prepared tooth.<br />
a b<br />
PATRONI<br />
Fig 13 The tissues maturing around natural abutments 4 months after periodontal surgery (a). The teeth were<br />
prepared once the flap was raised <strong>and</strong> the final impression was obtained <strong>with</strong>out any further marginal preparation.<br />
The zirconia crowns once cemented (b).<br />
b<br />
c<br />
13<br />
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a b<br />
Fig 14a <strong>and</strong> b A clinical case: the initial smile (a) <strong>and</strong> the final smile (b).<br />
a b<br />
Fig 15a <strong>and</strong> b The tooth wear, the lost anterior occlusal relationships, <strong>and</strong> unpleasant esthetic (a). The final<br />
result (b).<br />
the abutments once the flap is raised 16-18<br />
while, at the same time, optimizing all<br />
other parameters that render the longterm<br />
results more predictable.<br />
These parameters involve:<br />
■ re-establishing biological dimensions<br />
■ lengthening short abutments<br />
■ recovering the ferrule effect<br />
■ correcting root proximities<br />
■ eliminating residual concavities following<br />
furcation barreling<br />
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■ bone surgery to eliminate the shallow<br />
infra-osseous pockets<br />
■ surgery to correct esthetic problems<br />
such as a gummy smile or to harmonize<br />
unsightly gingival trajectory arcs.<br />
The present approach involves, once the<br />
flap has been rised, a feather-<strong>edge</strong> preparation<br />
of the teeth as far as the osseous<br />
crest.
a b<br />
c d<br />
e f<br />
In order to be able to benefit from all of the<br />
advantages offered by a feather-<strong>edge</strong><br />
preparation, it is important to not modify<br />
this preparation once the surgical phase is<br />
over, unless to carry out small adjustments<br />
which do not involve the marginal area. Indeed,<br />
once the abutment is re-prepared in<br />
the impression stage, this may create a reference<br />
margin in the marginal finish area<br />
which is binding for the final restoration<br />
(Figs 14 to 20). When the tissues have ma-<br />
PATRONI<br />
Fig 16a to f Abutments immediately before surgery (a). Once the flap was raised during periodontal surgery,<br />
the teeth were prepared until the osseous crest (b). The flap sutured at the osseous crest (c). The abutments <strong>with</strong><br />
the first retraction cord inserted 10 months after surgery (d). The abutments <strong>with</strong> the second retraction cord inserted<br />
immediately before the final impression (e). The final cemented restorations (f).<br />
tured completely it will be possible to take<br />
a definitive impression <strong>and</strong> finalize the<br />
prosthetic restoration (Fig 21).<br />
There is a published technique which<br />
makes it possible to exploit the advantages<br />
of the feather-<strong>edge</strong> preparation carried<br />
out “blind,” that is, <strong>with</strong>out surgical access.<br />
19,20 Naturally, each preparation type<br />
has advantages <strong>and</strong> disadvantages. The<br />
choice of one rather than another is often<br />
guided by the different degrees of difficul-<br />
15<br />
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a<br />
CLINICAL APPLICATION<br />
Fig 17a to c The palatal view showing the healthy tissues around the crowns.<br />
Fig 18 The buccal emergence profile respecting the<br />
tissue health.<br />
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b c<br />
a<br />
b<br />
c<br />
Fig 19a to c Six months after the surgery, when<br />
teeth were prepared up to the osseous crest, the abutments<br />
were ready for the final prosthetic phase: occlusal<br />
(a) <strong>and</strong> buccal view (b). Cemented zirconium<br />
oxide prostheses (c).
a b<br />
c d<br />
e f<br />
g h<br />
Fig 20 The old restorations (a). The abutments once<br />
the previous prosthesis was removed (b). The periodontal<br />
intervention <strong>with</strong> the open flap abutment preparation<br />
(c). The final abutments 10 months after surgery (d). The<br />
red line on the master model represents where the technician<br />
placed the apical limit of the restoration after ditching;<br />
the black line represents the gingival level (e). The<br />
zirconia copings try-in (f). The tracing of the gingival scallop<br />
on the zirconia copings (g). Zirconia copings removed<br />
after try-in <strong>and</strong> tracing (the amount of the coping<br />
margin positioned into the sulcus is evident) (h). Oneyear<br />
follow-up, the good tissue integration is evident (i).<br />
i<br />
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CLINICAL APPLICATION<br />
a b<br />
Fig 21a <strong>and</strong> b The impression is easily obtained in<br />
cases of feather-<strong>edge</strong> preparation; the intra-crevicular<br />
part of the prepared tooth appears as a rising margin.<br />
ty <strong>and</strong> operative management. The feather-<strong>edge</strong><br />
preparation may be the solution<br />
for several clinical problems <strong>and</strong> simplifies<br />
the procedure beginning <strong>with</strong> the<br />
preparation itself <strong>and</strong> including the relining<br />
of the provisional, the final impression,<br />
<strong>and</strong> all further laboratory phases.<br />
Advantages of feather<strong>edge</strong><br />
marginal preparation<br />
■ Saves dental tissue: it is therefore indicated<br />
for teeth which have suffered serious<br />
periodontal damage as a consequence<br />
of periodontitis.<br />
■ Simplifies the final impression as there<br />
is no longer a finishing line but rather a<br />
finishing area.<br />
■ Relining the provisional is easier <strong>and</strong><br />
faster. Even in cases where the border of<br />
a temporary crown is slightly shortened,<br />
no marginal precision is lost as would<br />
happen in the case of a “short” provisional<br />
margin <strong>with</strong> a shoulder or chamfer<br />
preparation. In fact, the finishing tar-<br />
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VOLUME 5 • NUMBER 1 • SPRING 2010<br />
get is represented by an area <strong>and</strong> not a<br />
line. However, the management of the<br />
esthetic zone is more delicate as a short<br />
margin would make it impossible to<br />
condition the gingival tissues <strong>and</strong> shape<br />
the prosthetic emergence profiles at an<br />
interproximal level.<br />
■ In cases of shoulder or chamfer preparations,<br />
the finish line is well defined <strong>and</strong><br />
is the line that the technician must refer<br />
to <strong>with</strong>out any doubt. The margin of the<br />
restoration must be unequivocally positioned<br />
at this level. On the other h<strong>and</strong>,<br />
<strong>with</strong> a feather-<strong>edge</strong> preparation, the<br />
prosthetic margin can be positioned at<br />
any level in an apico-coronal direction,<br />
as long as it respects the periodontal attachment.<br />
Once the structure has been<br />
realized, it could invade too deeply into<br />
the gingival sulcus. In this case, it can be<br />
shortened during the trial. In healthy<br />
conditions, the interproximal probing<br />
depth is approximately 3 mm <strong>and</strong> it is<br />
possible to receive a structure from the<br />
laboratory which finishes at this depth.<br />
From a clinical point of view, it is often<br />
unnecessary to position the prosthetic<br />
margin at this level interproximally, as in<br />
this zone there are no important esthetic<br />
requirements but only the need for tissue<br />
conditioning to recreate an adequate<br />
emergence profile. This profile is<br />
obtainable when the margin is positioned<br />
at 1 mm inside the sulcus. Therefore,<br />
any invasion by the restoration is<br />
unnecessary <strong>and</strong> could damage the<br />
periodontal attachment <strong>and</strong> make it difficult<br />
to remove any cement excess. Furthermore,<br />
adjusting the margin <strong>and</strong><br />
shortening it does not lead to any loss<br />
of precision nor will it compromise the<br />
esthetic result.
Disadvantages of the<br />
feather-<strong>edge</strong> marginal<br />
preparation<br />
The disadvantages of a feather-<strong>edge</strong><br />
preparation are emphasized in the case<br />
of a prosthetic rehabilitation using the<br />
ceramo-metal technique:<br />
■ The presence of a visible metal collar is<br />
necessary to guarantee precision <strong>and</strong><br />
resistance at the margin. This border,<br />
when not hidden in the gingival sulcus,<br />
is particularly unattractive <strong>and</strong> certainly<br />
unacceptable in the esthetic zone.<br />
■ If a metallic border is not properly<br />
formed there is a risk that a horizontal<br />
overhang will be created. 21<br />
This is potentially damaging for the periodontal<br />
tissues as it can retain bacteria<br />
<strong>and</strong> plaque. 22-25 The zirconia margin is<br />
m<strong>and</strong>atory in this area especially during<br />
cementing <strong>and</strong> while chewing; in fact, in<br />
these conditions there is a high concentration<br />
of stress on the margins which could<br />
cause ceramic chipping. 26,27<br />
In order to create enough space for the<br />
restoration material, there is risk of creating<br />
an over-tapered preparation. This does not<br />
guarantee a retentive <strong>and</strong> stable form for<br />
the restoration unless one resorts to additional<br />
retentions. 28<br />
All of the above is particularly disadvantageous<br />
in cases of:<br />
■ patients <strong>with</strong> a thin gingival biotype in<br />
whom the metal borders, even if intracrevicularly<br />
positioned, could show<br />
through the gingival margins, resulting<br />
in an esthetically unsatisfactory rehabilitation<br />
■ short abutments <strong>with</strong> a high risk of<br />
crown debonding. The vertical preparation<br />
is considered to be unsightly as the<br />
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elimination of less dental material gives<br />
the technician less space for ceramic<br />
stratification <strong>and</strong>, especially in the anterior<br />
teeth, in cases of even minimal gingival<br />
recession the metallic margin of<br />
the prosthesis is visible. With regard to<br />
precision, a vertical preparation (feather<br />
<strong>edge</strong> or bevel) may ensure a better seal<br />
than a horizontal one (shoulder or<br />
chamfer) before luting. In contrast, after<br />
the cementing it provides worse seating.<br />
This is due to the difficult defluxion of the<br />
cement at the prosthetic margin. 29,30 The<br />
final result therefore, in terms of precision,<br />
can be considered totally superimposable<br />
<strong>and</strong>, in any case, clinically acceptable<br />
for both types of preparation<br />
(vertical <strong>and</strong> horizontal). 29,31<br />
<strong>Zirconium</strong> oxide <strong>and</strong><br />
feather-<strong>edge</strong> preparation<br />
The marginal design recommended by<br />
the producers of zirconium oxide is a<br />
chamfer or round shouldered margin<br />
combined <strong>with</strong> a preparation that has a<br />
slightly increased taper compared to the<br />
conventional ones: 4 degrees per side<br />
compared to the vertical as opposed to 2<br />
to 3 degrees per side (Fig 22). 15,26,32 The authors'<br />
clinical experience suggests that zirconium<br />
oxide could also be used in vertical<br />
preparations, thereby combining a<br />
much simpler <strong>and</strong> much faster preparation<br />
<strong>with</strong> a material which is at least as mechanically<br />
resistant as gold, yet more esthetically<br />
pleasing.<br />
The design of the zirconium core must<br />
not differ from a core for a ceramo-metal<br />
restoration in the case of the feather-<strong>edge</strong><br />
preparation. The emergence profile must<br />
respect the periodontium <strong>and</strong> be easy to<br />
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20<br />
CLINICAL APPLICATION<br />
a b<br />
Fig 22a <strong>and</strong> b The ideal taper recommended in prosthetic dentistry is 2 degrees on each side (a). The recommended<br />
taper in the case of zirconia crowns is 4 degrees on each side (b).<br />
Fig 23 Chipping of the marginal crests: the two zirconia cores were designed in a st<strong>and</strong>ard way <strong>with</strong>out considering<br />
the necessity of ceramic support in relation to the occlusal contact points.<br />
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VOLUME 5 • NUMBER 1 • SPRING 2010
a b<br />
c d<br />
clean, simulating the horizontal natural<br />
contour of the cementoenamel junction.<br />
It is possible to realize the <strong>edge</strong> of the<br />
restoration in zirconium <strong>with</strong> a form <strong>and</strong><br />
thickness similar to that of gold. The zirconium<br />
<strong>edge</strong> however, integrates better<br />
than a metal one. This is more noticeable<br />
in the molar region as it does not necessarily<br />
require intra-crevicular positioning<br />
to achieve satisfying esthetic results. 33 The<br />
zirconium oxide <strong>edge</strong> must be 0.5 to 0.7<br />
mm wide in order to guarantee resistance<br />
to chipping during insertion <strong>and</strong> luting<br />
phases.<br />
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Fig 24a to d False root realized, creating a forced pathway for the interproximal brush to clean the distal tooth<br />
surface.<br />
The main condition to be respected in order<br />
to minimize the chipping risk <strong>and</strong> obtain<br />
a successful long-term result, is that<br />
the ceramic has to be adequately supported<br />
by the underlying structure so that<br />
it will work during compression (Figs 23<br />
<strong>and</strong> 24). 34<br />
In extended structures, planning the<br />
connections is of the utmost importance<br />
<strong>and</strong> should be orientated appropriately: in<br />
the molar region the corono-apical sections<br />
should be most represented, while in<br />
the esthetic zone the vestibular-palatal<br />
thickness should be greater.<br />
21<br />
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VOLUME 5 • NUMBER 1 • SPRING 2010
22<br />
a<br />
CLINICAL APPLICATION<br />
c d e<br />
f g<br />
Fig 25a to g A clinical case: frontal view of the abutments (a) <strong>and</strong> occlusal view (b). Details of healthy marginal<br />
tissues once the temporaries were removed: in the case of feather-<strong>edge</strong> preparation it is easier to guarantee<br />
the periodontal health around the prosthesis (c to e). Zirconia crowns on the master model (f). Frontal view<br />
of the final result (g).<br />
It is important to plan for a ceramic thickness<br />
that guarantees a good esthetic result<br />
<strong>with</strong>out the connections needing to be adjusted<br />
after the structure is finished, as any<br />
adjustments at that point cause the structure<br />
to be seriously weakened (Fig 25).<br />
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VOLUME 5 • NUMBER 1 • SPRING 2010<br />
b<br />
Conclusions<br />
Although the combination of zirconium<br />
<strong>and</strong> feather-<strong>edge</strong> preparation requires further<br />
r<strong>and</strong>omized clinical studies in the<br />
medium <strong>and</strong> long term, it would seem to
offer an extremely interesting solution from<br />
a clinical point of view.<br />
Indeed, the zirconium confers to the<br />
restoration excellent esthetic <strong>and</strong> mechanical<br />
characteristics, as long as the plan<br />
allows for a rigorously personalized support<br />
structure for the overlaying ceramic.<br />
<strong>Feather</strong>-<strong>edge</strong> marginal preparation adapts<br />
well to the increasingly frequent requirement<br />
of pre-prosthetic periodontal surgical<br />
interventions. Moreover, the possibility of<br />
performing the final tooth preparation once<br />
the flap is raised reduces the number of interventions<br />
<strong>and</strong> the risk of invading the sulcus<br />
<strong>and</strong> damaging the supra-crestal fibers.<br />
This type of preparation, <strong>with</strong> its white<br />
zirconium borders, does not require that<br />
the margins be positioned deeply into the<br />
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