Preservation of tooth structure

The need to preserve tooth structure wherever possible is axiomatic.

With regard to the reduction and loss of tooth structure required to

place an indirect restoration, it is important to note that although

destructive, the provision of an extra-coronal restoration may actually

result in preservation of tooth substance in the long term – for

example, posterior root-filled teeth have a much higher fracture rate

compared with similar teeth that have been crowned. In other cases

in which indirect restorations are planned for an improvement

in aesthetics only, for example aesthetic veneers, then the ‘cost’ to

the tooth must be weighed carefully against the perceived benefit.

In determining the amount of removal of tooth structure for provision

of an indirect restoration, there are three main considerations:

the requirement for protection from fracture and wear, the pattern of

tooth substance removal and the type of restoration to be provided.

Protection from fracture and wear

Restorations providing cuspal coverage provide preservation of

tooth structure by protecting the remaining axial walls from stresses

that may subsequently lead to fracture of weakened, susceptible

cusps. Also, a well-made indirect restoration that can control loads on

the tooth by maintaining stable occlusal contacts may minimise

load and wear on the tooth and restoration itself. Intra-coronal

restorations, such as inlays, may also provide a similar degree of

protection if they are adhesively bonded to the tooth, though an inlay

that spans from one proximal surface to the other (i.e. mesio-occlusodistal)

will create a wedging effect that inherently predisposes the

tooth to fracture.

Pattern of tooth substance removal

Reduction of tooth structure should be appropriate and yet not

excessive. There needs to be sufficient reduction to provide enough

space to accommodate the required thickness of restorative material

without necessitating overcontouring of the restoration. If too much

tooth structure is removed, the health of the tooth will be compromised.

Reduction must be anatomical, that is the reduction planes

should broadly follow the contours of the tooth (or planned final

shape) – a flat over-reduced occlusal surface will shorten the preparation,

reducing retention as well as reducing the resistance form

(Fig. 5.4). Conversely, inadequate reduction in the occlusal grooves

will not provide adequate space for good functional morphology.

Also, a flat single plane of reduction on the axial portions of the tooth

will remove more tooth substance than is necessary, and will result in

an uneven space for the restorative material

There should be greater tooth substance removal over the functional

cusp (i.e. palatal maxillary cusps or buccal mandibular cusps).

This is usually achieved by means of a wide bevel. If this is not done

and the crown is constructed to a normal contour then the resulting

restoration will be too thin in this area. If adequate bulk is provided

by over-contouring the final restoration, then the occlusal contacts

will be incorrect, either too high or result in interferences during

excursive movements. In addition, a lack of a functional bevel leads to

uneven and uncontrolled distribution of stress and tensile forces on

the cement lute.

11.