Preparation guide

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					Preparation guide
The Material
Zirconium oxide has a proven track
record of over ten years as a high-
strength framework material for
dental restorations. With a bending
strength of > 1000 MPa, this is the
first material that can also be used
to make long-span bridgework
for posteriors. The occlusion can
be checked during try-in and
conventional permanent cements
can be used, thus simplifying
treatment considerably.
Full ceramic crowns on a ZENO®
framework offer excellent aesthe-
tics (Figs. 1, 2 and 3), since zirco-
nium oxide is a white, translucent
material which can be stained to
resemble natural dentine. Its low
plaque affinity and low coefficient
of thermal expansion make this
biocompatible material the ideal
framework material for prosthetic

                                        Fig. 1
                                        Aesthetically pleasing ZENO® anterior
                                        crowns; made by Master Dental
                                        Technician F. Wüstefeld, Hanover

As a matter of course, the ZENO®
Tec system can be used to fabricate
partial crowns, crowns and bridges
for both anterior and posterior
Contraindications include a lack
of space in the bridge connector
area, since connectors must have
a generous cross-section.
If the patient suffers from bruxism,
the practitioner must decide on
a case by case basis whether a full
ceramic restoration is suitable or
whether a metal occlusal surface is

                                        Fig. 2                                  Fig. 3
                                        Before: Inadequate restoration          After: ZENO® crowns in situ
                                        in the posterior region
1. First steps                                                                 4. Recommendations for
                                                                                  preparing a tooth for
 Before starting treatment (and prior                                             a ceramic restoration
 to administering an anaesthetic),
 use an ultra thin articulating film                                            The preferred instruments are the
 to record the bite whilst the patient                                          established abrasive burs from the
 is still seated upright. This enables                                          zirconium oxide preparation set
 the occlusion to be verified on                                                according to Dr. H. v. Blanckenburg,
 articulated models at a later stage.                                           Gebr. Brasseler (Fig. 4a). Equally
                                                                                suitable is the zirconium oxide
                                                                                preparation set according to
2. Existing root canal pins                                                     Dr. Beuer (Fig. 4b).

 Since it is a relatively opaque                                                In general, a tooth prepared for
 material, zirconium oxide can be                                               a zirconium oxide restoration must
 used to completely cover existing                                              always have a reduced anatomical
 pin restorations. This therefore also                                          shape with rounded edges. This
 enables intact pin superstructures,                                            enables the technician to design
 which are often very difficult to                                              a framework, which is supported
 remove, to remain in place when                                                by cusps and has an even layer
 the tooth is prepared.                                                         thickness. The thicknesses required
                                                                                for the subsequent restoration are
 In the case of highly aesthetic                                                similar to those specified for PFM
 anterior crowns with a slender                                                 restorations. There is no truth in
 framework having a wall thick-                                                 the rumour that full ceramic re-
 ness of 0.4 mm, it is advisable to                                             storations always require a greater
 coat any existing metal pin with                                               reduction of hard tooth substance.
 a tooth-coloured composite or
 to increase the wall thickness in                                              For anteriors, a thickness of 0.4 mm
 order to increase the opacity of the                                           is adequate for a single zirconium
 framework.                                                                     oxide coping. For posterior crowns
                                                                                and for bridge anchors a thickness
 Ideally, a new pin should be inserted                                          of 0.5 mm to 0.6 mm should be
 and bonded to the dentine. For this                                            allowed for. The veneer then
 purpose, composite pins reinforced                                             requires a further 1.0 to 2.0 mm
 with fibreglass have proven to                                                 at the occlusal / incisal surfaces
 be suitable, since they possess an                                             (the same as for PFM restorations).
 elasticity and appearance similar
 to that of natural dentine.                                                    The prepared tooth should be
                                                                                tapered with a side angle of about
                                                                                4º. This gives the best scanning
3. Reconstruction material               Fig. 4a                                results in the lab and ensures
                                         Ergonomic preparation set according
                                                                                that the crown is sufficiently an-
                                         to Dr. H. v. Blanckenburg
 For small defects, reinforced glass                                            chored to the tooth.
 ionomer cements can be used                                                    The largest circumference of the
 as a low-cost filling material.                                                prepared tooth is clearly visible
 Larger defects should be restored                                              in the area of the gingival pre-
 with composite materials bonded                                                paration margin. It does not
 to the dentine. Compomers are                                                  matter here whether the pre-
 unsuitable for this purpose, since                                             paration has a distinct chamfer or
 they expand through swelling and                                               is a shoulder with a rounded in-
 can therefore exert undue pressure                                             ner edge. At the margins the cut
 on the ceramic.                                                                should be circular with a uniform
                                         Fig. 4b
                                         ZENO® Preparation set according
                                                                                depth of 1.0 mm.
                                         to Dr. Beuer
5. Efficient preparation
   – step-by-step

Tip: An epigingival preparation
simplifies every step of the procedure,
right up to the fitting.

 Step 1:
 Use a coarse diamond bur to                    Step 2:
 reduce the axial tooth surface                 Create an anatomical cusp-sup-
 (Figs. 5 and 6). In doing so, a                ported shape for the occlusal reli-
 chamfer with a depth of 1 mm                   ef/ reduce the lingual concavity of
 should be formed in a single pass.             incisors (Figs. 7 and 8).
 Tip! An accentuated preparation                Tip! Ensure that there is adequate
 margin in its final position serves            space (Fig. 9). Verify this by using
 as a guide for the final stages of             bite registration wax or a silicone
 the preparation. This initial shoulder         impression.
 and chamfer will subsequently
 guide the instrument smoothly,
 even in less accessible areas.

                                                 Fig. 7
                                                 Step 2: Removal of lingual
                                                 hard substance from anteriors

   Fig. 5
   Step 1: Initial preparation using a coarse
   diamond bur for the posterior region

                                                 Fig. 8
                                                 Step 2: Removal of occlusal hard
                                                 substance from molars

   Fig. 6
   Step 1: A thinner instrument for the
   anterior region; an accentuated chamfer
   makes all subsequent steps easier

                                                 Fig. 9
                                                 Checking the space created by means
                                                 of a bit registration
                                            Step 3:
                                            Complete the preparation with
                                            a gently tapered diamond finish-
                                            ing bur (Figs. 10 and 11) or with a
                                            special tapered carbide finishing
                                            bur used at a lower speed (Fig. 12).
                                            The different toothing on the
                                                                                   Fig. 13
                                            shaft and tip of this universal        Extremely safe and atraumatic finishing
                                            instrument    means      that    the   of the accentuated chamfer
                                            shoulder can be formed smoothly
                                            and without trauma in a single
                                            pass and the tooth walls can be
                                            smoothed to a defined degree
Fig. 10
Step 3: Short finishing bur for partial     of gentle roughness. The finely
crowns and less accessible areas            toothed tip runs very gently and
                                            evenly over the shoulder shaped
                                            during the initial stage of the pre-
                                            paration. This atraumatic instru-
                                            ment does not damage the gingi-
                                            va, but the shaft creates a surface
                                            structure which is ideal for retai-
                                            ning the crown.
                                                                                   Fig. 14
                                            Tip! Tilt the tapered instrument       For special cases a ceramic abrasive is
                                            gently towards the outside. If a       customised with a diamond abrasive

                                            channel was formed during the
                                            initial preparation, it is now easy
                                            to smooth over (Fig. 13). In special
Fig. 11
                                            cases it is also possible to use a
Step 3: Tapered diamond                     ceramic abrasive specially trim-
finishing bur for molars                    med with a diamond abrasive
                                            (Fig. 14, 15).

                                            Step 4:
                                            Now round all edges with finishers
                                            or ceramic burs (Fig. 16). Since
                                            zirconium     oxide     restorations
                                            can nowadays only be made by           Fig. 15
                                            machining industrially produced        Finishing the preparation
                                                                                   and rounding all edges
                                            blanks, this must be taken into
                                            consideration during preparation.
                                            For example, no sharp edges must
Fig. 12                                     remain after preparation, especially
Step 3: Special carbide finishing bur; an   in the anterior region.
ergonomic instrument for anterior and
posterior regions
                                            Note: Do not polish the prepared

                                                                                   Fig. 16
                                                                                   The finished preparation
Impression                                                                             As an alternative or in addition to
                                                                                       the use of threads, electrotomy or

taking                                                                                 laser techniques can also be used
                                                                                       to expose the prepared margin,
                                                                                       whereby all forms of retraction
Impressions for zirconium oxide                                                        must be as gentle as possible,
restorations must be taken using                                                       especially in areas where the
a precision impression material                                                        gingiva is visible. With the elec-
(Figs. 17 and 18). One method of                                                       trotomy method it is advisable to
exposing the prepared margin                                                           use the thinnest instrument availa-
and retracting the gingiva before                                                      ble to open the sulcus. The sub-
the impression is taken is the use                                                     sequent impression should be
of non-impregnated retraction                                                          taken using customised or indivi-
                                        Fig. 17
threads. For this, the so-called “V”    Cross-section of a (hydrocolloid)              dual impression trays.
technique is a well-proven method.      impression
                                                                                       The preferred impression material
First, a No. 1 retraction thread is
                                                                                       is polyether using the one-off
placed around the entire sulcus
                                                                                       single-phase technique or hydro-
of the prepared tooth and then
                                                                                       colloid. After removing the tray
a thicker thread is placed on top
                                                                                       from the mouth, examine the im-
of this. Both threads are left in
                                                                                       pression under a stereo microscope
the sulcus for 10 minutes. Then the
                                                                                       or through magnifying glasses.
thicker thread is removed. Provided
                                                                                       Repeat the procedure if neces-
that there is no bleeding and that
                                                                                       sary. The quality of the impression
the entire margin is visible, the
                                                                                       is the dentist’s visiting card.
impression can now be taken.

                                        Fig. 18
                                        Impression: all contours are clearly visible

Temporary restorations
The best way to make a matrix          helps you to determine when the
for a temporary restoration in the     acrylic material passes from the soft
dental laboratory is by vacuum         plastic stage to the heat-emitting
forming a polyethylene laminate        polymerization phase. Shortly after
                                                                                        Fig. 19
over an independently made model       this transformation stage, remove                Temporary restoration
of the original dentition. The         the matrix from the mouth and
thickness of the layers allowed        place it in a water bath heated to
                                                                                       Tip! For anteriors, shorten the tem-
for by the preparation can be          50° C in order to accelerate the
                                                                                       porary restoration at the labial sur-
checked before the impression is       curing process. Now remove the
                                                                                       face by approx. 0.5 mm to avoid ir-
taken by placing the clear matrix      temporary restoration from the
                                                                                       ritation of the gingiva and prevent
in the mouth and observing the         polyethylene matrix and trim.
amount of tooth substance that has     If possible, the final trimming
been removed. With bridgework,         should be carried out on a po-                  Finally the temporary restoration is
an artificial tooth can be inserted    lishing lathe in the dental labora-             lathe polished to a high lustre and
into the model and then integrated     tory. After buffing with pumice,                fixed onto the prepared teeth with
into the temporary restoration         insert the temporary restoration                eugenol free cement (Fig. 20)
to achieve perfect results. The        in the patient’s mouth once more,
temporary bridge also acts as a        check the static and dynamic
splint and prevents the abutment       occlusions, correct as necessary
teeth from straying.                   and double-check the edge mar-
                                       gins and the approximal contact
To check the hardness, first
dispense a pea-sized reference
sample onto the treatment tray,        With the ZENO® Tec System it is
then fill the matrix with auto         also possible for the technician to
polymer and position it in the         make a long-lasting temporary
mouth over the abutment teeth          crown from PMMA on the CAD /
and pontics and ask the patient to     CAM system within a few hours
close their mouth. The reference       (Fig. 19).                                       Fig. 20
                                                                                        Temporary restoration made
sample on the treatment tray                                                            from ZENO® Tec PMMA
Framework try-in                                                                     Fitting

 In special cases, it may be advisable                                                When the occlusion, shade and
 to try in the zirconium oxide                                                        approximal contacts have been
 framework before completing the                                                      checked, the restoration can be
 restoration. When doing so, check                                                    fitted into the mouth.
 the marginal fit by applying a
                                                                                      The question of whether to use
 thin-flowing impression material
                                                                                      adhesive or cement continues
 to the gap between the prepared
                                                                                      to be a controversial issue. From
 tooth and the restoration. Then
                                                                                      a clinical point of view it does
 clean the framework with alcohol
                                                                                      not matter whether the cement
 to ensure that no traces of silicone
                                                                                      used is phosphate cement mixed
 are left inside the restoration.
                                                                                      by hand or a glass ionomer ce-
 After the impression material has
                                                                                      ment supplied in a pre-mixed cap-
 set, remove the restoration from         Fig. 21                                     sule. The restoration can also be
 the mouth. If the marginal fit is a      Cementing with dual cement;
                                          the excess is easy to remove                fixed by means of autopolymers or
 good one, the material will tear
                                                                                      dual cements (e.g. RelyX® Unicem,
 off cleanly at the crown margin.
                                                                                      3M ESPE; Panavia F, KURARAY
 At this stage, a refit can be carried
                                                                                      DENTAL). The translucent material
 out using the using the zirconium
                                                                                      allows light to pass from the re-
 oxide framework as a basis.
                                                                                      storation to the tooth preparation
                                                                                      and through the preparation
                                                                                      margin into the root dentine.
                                                                                      A paramarginal preparation does
                                                                                      not reveal the cement gap (Figs.
                                                                                      21 and 22).

                                          Fig. 22
                                          High translucence and light transmission
                                          into the sulcus and the gingiva

Trephination and removal
 Although the low thermal con-           in other words at a tangent to               A similar procedure should be
 ductivity of a zirconium oxide          the tooth. This ensures that the             followed when removing a rest-
 restoration provides excellent          diamond bur is always adequately-            oration. It may be necessary to
 insulation for the dental pulp,         cooled and prevents it from over-            remove the ceramic veneer from
 it may be necessary to carry            heating (Fig. 23).                           the approximal regions in order
 out endodontic treatment on                                                          to break up the framework.
 the restored teeth. In this case,                                                    On the whole it can be said that
 the following procedure must be                                                      the trephination or removal of
 observed: First use a coarse                                                         a zirconium oxide restoration is
 diamond bur to completely                                                            just as straightforward as that
 remove the ceramic veneer from                                                       of a framework made from a
 the point at which entry is to                                                       non-precious metal.
 be made. Only then can the
 framework of the restoration
 be penetrated, again using a
 coarse diamond bur. Hold the
 bur at an angle of approx. 45º to
 the zirconium oxide framework,

                                           Fig. 23
                                           Trepanation of a ZENO® crown;
                                           use generous water cooling
                                                                                    Clinical work:
                                                                                    Dr. Hartmut von Blanckenburg

                                                                                    Lab work:
                                                                                    Master Dental Technician
                                                                                    Frank Wüstefeld

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