APPLETON COMPREHENSIVE DENTISTRY
Digital Restorative Dentistry
GLENN E APPLETON DDS
CEREC Digital Impression System
allows a dental practitioner to produce an indirect ceramic restoration using
a variety of computer assisted technologies, including 3d photography and
CAD?CAM. With CEREC, teeth can be restored in a single sitting with the
patient, rather than the multiple sittings required with earlier techniques.
Additionally, with the latest software and hardware updates, crowns,
veneers, onlays and inlays can be prepared, using different types of ceramic
material.
Digital Impression
Requires strict isolation and gingival retraction
similar to conventional impression techniques
(retraction cord, electrosurgery or laser troughing)
CAD/CAM Techniques
CEREC Crown Preparations
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6o- 8o wall taper
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Rounded internal line angles
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Smooth, crisp margins; avoid bevels
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Adequate tooth reduction
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Shoulder - 90 degrees
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Chamfer - 90 - 130 degrees
CEREC Crown Preparations
similar to all ceramic restorations
Shoulder - 90 degrees
Chamfer - 90 - 130 degrees
CAD/CAM Preparations
Tooth Reduction
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Recommendations based on requirements for all-ceramic strength
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Stress-bearing (occlusal) vs. non-stress-bearing (axial) areas
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Second plane of reduction in occlusal 1/3 prevents over-thinning of the crown
>1.5 mm
>1.2 mm
Conventional 2-plane occlusal reduction for PFM crown
2-plane occlusal reduction for Cerec crown
note subtle 2nd reduction plane on occlusal and
round line angles giving the preparation a “derby
hat” like appearance. This decreases internal ceramic
stresses
Spikes: How are they handled?
Make sure the gingival margins of the
preparation are smooth. Any
irregularities (spikes) cannot be milled
Inadequate Reduction
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Adequate reduction more important in stress-bearing areas (occlusal) compared to
non-stress-bearing areas (proximal)
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Fracture is generally due to inadequate reduction
Predictable Depth Reduction
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Occlusal Reduction Burs (Meisinger)
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Patterson Item 509-9734
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Ensures uniform 2mm occlusal reduction
Onlays
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Avoid using cast restoration design concepts; avoid ferrule margins
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Ceramic retention based on surface area, not from opposing walls
Not necessary
Ferrule Effect
Onlays
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Desire 2.0mm reduction over functional cusps
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At least 1.5mm over non-functional cusps
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Desire bulk of ceramic at the margin
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Avoid sharp internal angles
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Smooth walls
2 mm
>2 mm
Axial Wall Preparation
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Axial walls for inlays/onlays may be prepared vertically,
convergent or slightly divergent to the floor:
Greater thickness of ceramic at margins.
Preservation of tooth structure.
Makes margins
thinner.
Undercuts: crowns
Possible to record undercuts with
multiple images, but the software will
not build (or mill) ceramic in the
undercut area!
Tight Corners
Milling corners and extensions:
Diameter of burs:
-Cylinder: 1.6
-Step: 1.0
We can recognize where burs cannot remove
adequate material.
Arrows indicate areas of immediate binding
Software will try and incorporate tapered bur in such areas,
but often will leave excess material that needs to be
manually adjusted
Try and ensure corners have at least 1mm diameter
Proximal Margins
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Minimal separation from adjacent
teeth = “bridging”
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Need separation to avoid connecting
the surface, prep as close to gingival
margin as possible
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Will allow easier design of contacts
Exit angle, break contact
Exit Angles
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Thin ceramic margins are prone to fracture
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Difficult to record the margin accurately
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Restoration binds – incompletely seats during try-in
Cavity Margins
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An effort is required to ensure smooth margins, especially sub gingival
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Initial prep with coarse diamonds; finish prep
with 40mm fine diamond (red stripe)
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Bur selection should fit the
specifics of the prep
– Tapered walls
– Flared boxes
– Smooth margins
– Rounded shoulders
Smooth Transitions
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Avoid sharp angles; potential to prevent seating
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Rounded internal angles ensure passive fit
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Avoid stress points under the ceramic
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Improve internal adaptation and ease of delivery
Smooth
transitions
Recurrent caries leaving unsupported enamel
under the mesio buccal cusp of a maxillary 1st
premolar
Prepared tooth has smooth margins and
rounded line angles