Operative dentistry aje qualtrough, jd satterthwaite la morrow, pa brunton

Computer-aided design and computer-aided machining
techniques (CAD/CAM)
Several propriety systems exist for creation of ceramic restorations at
the chairside. A system-specific intra-oral camera captures an image
of the cavity or preparation (often termed a ‘three-dimensional optical
impression’). Most software packages then allow some modification
of the image (e.g. identification of margins). The planned shape of the
final restoration is then machined from a block of ceramic.
This method has the obvious advantage of avoiding multiple 
visits. Also the restorations produced are strong when compared with
traditional dental porcelains, as the restorations can be milled from a
block of industrially sintered ceramic, which will have fewer flaws.
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Indirect restorations – further considerations


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Although the laboratory costs are eliminated, CAD/CAM systems
require a large initial outlay. In addition the occlusal surface of
restorations is not as accurately produced compared with laboratory
restorations, and some systems require the occlusal surface to be
shaped intra-orally by the operator. A further disadvantage is that
aesthetics are limited because of the small range of available colours
and monochromacity, as the restoration is machined from a single
block.
LIMITED RESISTANCE AND RETENTION
When a preparation has limited retentive and resistance form, there
are a number of steps that may be taken to reduce the potential loss of
the restoration. These include limiting influence of lateral forces with
additional preparation features, utilising adhesive luting cements and
increasing crown height.
Additional preparation features
If a groove is placed into the axial wall(s) of a preparation, this will
reduce the moment arm of the force that acts to dislodge a restoration
about a point of rotation. Shortening this radius of rotation will
increase the resistance form of a preparation. This effect can be 
visualised as a short tapered preparation in which the placement of 
a groove results in the restoration ‘engaging’ against a wall of the
preparation at an earlier point, so that a larger portion of the prepara-
tion acts to resist the rotational force. This is shown schematically in
Fig. 6.5. Multiple parallel grooves will significantly limit the path of
withdrawal and increase both resistance and retention of the final
restoration.
Fig. 6.5
Grooves providing additional resistance form.
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