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bruxing, it may be difficult to prevent fairly large oblique forces from being applied to a restoration. Consequently the completed tooth preparation and restoration must be able to withstand considerable oblique forces as well as the normal axial ones.

Geometry of the Tooth Preparation. As with retention, preparation geometry plays a key role in attaining desirable resistance form. The tooth preparation must be shaped so that particular areas of the axial wall will prevent rotation of the crown.

Fig. 7-36. Mr. H. sitting beside 443 kg of gymnasium weights to illustrate the magnitude of his biting strength. (Reproduced from Gibbs CHet al: J Prosthet Dent 56:226,1986.)

Hegdahl and Silness79 analyzed how these resisting areas alter as changes are made in the geometry of the tooth preparation. They demonstrated that increased preparation taper and rounding of axial angles tend to reduce resistance. Short tooth preparations with large diameters were found to have very little resistance form. In general, molar teeth require more parallel preparation than premolar or anterior teeth to achieve adequate resistance form."" The relationship between preparation height, or diameter, and resistance to displacement is approximately linear. ,'

A partial-coverage restoration may have less resistance (Fig. 7-38) than a complete crown because it

Fig. 7-37. Resistance of different preparation designs. The line connects preparations with statistically similar displacement forces (p > 0.05).

Modified from Kishimoto M et al: J Prosthet Dent 49:188, 1983.

Fig. 7-37. Resistance of different preparation designs. The line connects preparations with statistically similar displacement forces (p > 0.05).

Modified from Kishimoto M et al: J Prosthet Dent 49:188, 1983.

Mesiodistal Diameter Crown

Fig. 7-38. Resistance form of partial and complete crowns. A, The buccoaxial wall (RA) of a complete crown should provide good resistance to rotation around a lingual axis. B, In a partial crown, resistance must be furnished by mesial and distal grooves. C, In a short or excessively tapered complete crown, resistance form is minimal because most of the buccal wall is missing. A mesiodistal groove should be placed to increase resistance form. D, Poor resistance form is less a problem in a short partial crown, provided the grooves have sufficient definition. However, lack of retention form may indicate the need for complete coverage.

Fig. 7-38. Resistance form of partial and complete crowns. A, The buccoaxial wall (RA) of a complete crown should provide good resistance to rotation around a lingual axis. B, In a partial crown, resistance must be furnished by mesial and distal grooves. C, In a short or excessively tapered complete crown, resistance form is minimal because most of the buccal wall is missing. A mesiodistal groove should be placed to increase resistance form. D, Poor resistance form is less a problem in a short partial crown, provided the grooves have sufficient definition. However, lack of retention form may indicate the need for complete coverage.

has no buccal resistance areas. Resistance must be provided by boxes or grooves (Fig. 7-39) and will be greatest if they have walls that are perpendicular to the direction of the applied force. Thus U-shaped grooves or flared boxes provide more resistance than V-shaped ones. -yThe resistance form of an excessively tapered preparation can be improved by adding grooves or pinholes, because these interfere with rotational movement and in so doing subject additional areas of the luting agent to compression.

Physical Properties of the Luting Agent. Resistance to deformation is affected by physical properties of the luting agent, such as compressive strength and modulus of elasticity. To satisfy ADA/ANSI specification no. 96 (ISO 9917), the compressive strength of zinc phosphate cement must exceed 70 MPa* at 24 hours (Fig. 7-40). Glass

*One megapascal (MPa) equals 1 million newtons per square meter.

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