Crown Preps and Resistance Form
A preparation's Resistance Form refers to the the shape given to a prepared tooth to enable the restoration and remaining tooth structure to withstand masticatory stress. Here we will look at the effect of oblique forces acting on a crown. These might occur when biting on hard food, or during parafunction. These forces can try to cause rotation in the vertical axis.
When a conventional (non-adhesive) luting cement is used, such as Zinc Polycarboxylate, Glass Ionomer, or Zinc Phosphate, resistance form must be prepared into the tooth by the operator to prevent displacement of the crown by oblique forces.
Dentine bonding has been shown to help retention, (e.g. when a toffee is trying to pull a crown off). However, it can not be relied upon to provide resistance - the shearing forces are so great in comparison. When preparing a dentine-bonded crown (where a resin adhesive is planned), you must balance the extra tooth destruction needed against the likeleyhood of the resin bond failing.
In these animations, a small gap is shown between the prep and the crown: this is to help demonstrate the potential movements that could be caused by oblique forces on the crown. This is of course a gross exaggeration of what is happening, but will help to visualise the concepts.
||This prep has a slight taper, and a good height in relation to its width. Tilting of the crown is resisted by the wall of the preparation.
||This prep is over-tapered. Resistance is not provided by the prep's wall until there has been a lot of rotation.
||Although this prep has minimal taper, it is short. A lot of rotation can occur before it is resisted by the wall of the prep.
||This prep is both over-tapered and short. It provides no resistance to rotation.
|| Although this prep is short, the tooth is narrow. So long as the height is more than 40% of the width, good resistance form can be achieved.
Compare with the short but wide preparation above.
To summarise, resistance to rotation in the vertical axis by oblique forces is provided by a minimum taper, and a good height - to - width ratio.
A combined taper of 10° to 20° (i.e. 5°-10° each side) has been found to provide good resistance, when combined with a suitably large height of dentine prepared to this taper.
The required height is subject to many factors: see Crown Prep Principles for more information..
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