Retention and resistance

The ability of an indirect restoration to resist dislodging forces relies

primarily on the retentive and resistance form of the preparation.

•

of the restoration in its long axis.

•

dislodgement due to forces outside the path of withdrawal of the

restoration, i.e. lateral or rotational forces.

Resistance is probably the more important of the two. There exists a

relationship between the two but this is not direct. The role of the

cement lute should also be considered; traditional cements are strong

in compression but weak in shear loads, therefore good resistance

form (and retention) is necessary to minimise the shear loading on

the luting cement. Adhesive cement lutes offer large increases in

resistance and retention, but they should not be used simply to compensate

for poor preparation. Both traditional and adhesive lutes

may undergo fatigue failure, and uncertainty exists regarding the

longevity of adhesive bonds. Therefore whichever type of lute is

used, attention should still be given to achieving good retentive and

resistance form.

Retentive form – those features of the preparation that resist removalResistance form – those features of the preparation that resist

Retention

Retention is primarily a surface area effect, depending on:

• Height of preparation

• Diameter

• Surface texture

• Taper

Of these, taper is the most critical factor. Theoretically, the more

nearly parallel the opposing walls of a preparation, the greater the

retention

seating of the crown, a slight taper is cut. Various suggestions have

been made regarding optimal taper, commonly 6° (5–10°) is quoted,

though higher figures are often given for molars. The rationale for

this is that full seating of a restoration is more important than a tight

casting for good retention

long teeth may need a greater taper in order to allow seating of the

final crown. Resistance and retention will be excellent with a long

crown. Less taper should be produced on short teeth when retention

and resistance will be poor. Despite these arguments, the use of

die-spacer (to provide space for the cement lute) will largely reduce

problems with seating of restorations. Also, in order to maximise

retention, it would be wise to aim for near parallelism in all cases,

especially as most clinicians underestimate the amount of taper that

has been produced

should be remembered that burs commonly used for tooth preparation

are tapered and simply need to be held in the long axis of the

preparation in order to produce a taper.

13. In order to avoid production of undercuts and to allow14 (due to the role of the cement lute) and15,16. When aiming to achieve near-parallelism, it

Resistance

In order to increase resistance to displacement due to lateral or

rotational forces, the preparation requires minimal taper and also

increased height. The increased height of preparation must have a

constant diameter as parallel walls that are not on the same base do

not provide stability (Fig. 5.7).

When the clinical crown is short, or it is not possible to obtain near

parallelism, additional features such as grooves, slots or boxes can

enhance the resistance form significantly by reducing the radius of

rotation of the final crown

long axis of the preparation and not just placed in line with the axial

wall. Axial grooves should, if possible, be placed into sound tissue of a

cusp and not into core material, which may be inherently weak. There

should be a definite wall perpendicular to the direction of the force in

order to limit the freedom of displacement and provide adequate

resistance. Axial grooves effectively improve the height:diameter

ratio, and enhance retention as well as resistance.

In some situations even the use of additional features will not provide

enough resistance and in such cases surgical crown lengthening

may be utilised to increase the available clinical crown height. This

scenario, where the need to create interocclusal space by preparation

would result in short crowns with limited resistance, is discussed in17. These features should be prepared in the