Assessment of Retention and Resistance Form of Tooth Preparations

Assessment of Retention and Resistance Form of Tooth Preparations

Define retention and resistance form in fixed partial dentures. What are the factors affecting retention and resistance in posterior tooth preparation?

Define Retention In Fixed Partial Denture

Retention prevents removal of the restoration along the path of insertion or along the path of the tooth preparation.

It is the quality of preparation that prevents the restoration from becoming dislodged by such forces parallel to the path of withdrawal.

Read And Learn More: Fixed Partial Denture Short Essay Question And Answers

Define Resistance In Fixed Partial Denture

Prevents dislodgement of the restoration, by forces directed in an apical or oblique direction and prevents any movement of restoration under occlusal forces.

Factors Affecting Retention

• The magnitude of the dislodging forces
• Geometry of the tooth preparation
• Roughness of the fitting surface of the restoration
• Type of restorative materials being cemented
• Type of luting agent
• Film thickness of the luting agent.

The magnitude of the dislodging forces

Factors influencing dislodging forces:

• Stickiness of the food
• The surface area of the restoration
• The texture of the restoration is being pulled.

Of all the types of forces, sticky food exhibits large dislodging forces.

Geometry of the tooth preparation

Factors influencing geometry of tooth preparation:

• Taper
• Surface area
• Stress concentration
• Type of preparation.

Taper

• Maximum retention is obtained if a tooth preparation has parallel walls
• Smaller degrees of taper have more retention
• Two opposing surfaces each with a 3° taper provide a 6° taper for the preparation
• The ideal convergence between opposing walls is 6°. This amount of convergence is required to restrain the cemented restoration.

Factors decreasing retention in regard to taper:

• Less amount of taper will form an undercut causing divergence between opposing axial walls, in an occlusocervical direction
• An increased amount of taper increases free movement of the restoration and retention will be reduced (Experimented by Jorgensen in 1955).

Preparation rule:

A rotary instrument of the desired taper held at a constant angulation produces the amount of taper required.

The rotary instrument of ideal taper when moved through a cylindrical path during tooth preparation will produce the desired axial wall taper on the completed preparation.

Surface area

• The greater the length of the clinical crown, the more is the retention
• In short clinical crowns, the surface area needs to be increased with the help of grooves and box preparation
• The greater the width of the clinical crown, the better the retention
• A restoration with limited path of withdrawal is more retentive depending on the length of the surface area in sliding contact.

Stress concentration

• Round margins reduce stress concentrations, which in turn increases the retention of the restoration
• Stresses are generally concentrated around the junction of the axial and occlusal surfaces
• Changes in the geometry of the preparation (e.g. rounding the internal line angles) reduce stress concentrations and hence, increase the retention of the restoration.

Type of preparation

• Additional grooves and boxes to a preparation increase the retention as the surface area is increased
• Retention is double for complete crowns than for a partial crown
• Luting agent is only effective if the restoration has a single path of withdrawal
• The occluso-axial line angle of the tooth preparation should be a replica of the gingival margin geometry
• Fixed prostheses depend on the geometric form than on adhesion of the luting agents for retention.

Roughness of the surfaces

• Retention is increased if the restoration is roughened or grooved by air abrading the fitting surface with alumina
• A smooth internal surface of a restoration can cause retentive failure at the cement restoration interface.

Materials being cemented

• The more reactive base-metal alloys have more adhesion with certain luting agents than less reactive high-gold content metals
• Cement adheres better to amalgam than to composite resin or cast gold.

Type of luting agent

Adhesive resin cements are the most retentive.

Factors Affecting Resistance Form

Lateral forces tending to displace the restoration by causing rotation around the gingival margin is prevented by areas of the tooth preparation that are placed in compression called resistance area.

Resistance depends on:

• Magnitude and direction of the dislodging forces
• Geometry of the tooth preparation
• Physical properties of the luting agent.

Magnitude and direction of the dislodging forces

• Patients with abnormal biting forces should be given a complete metal crown. A proper design and preparation can help to resist these displacing forces
• In patients with parafunctional habits and pipe smoking to prevent large oblique forces from being applied to a restoration, additional retentive grooves and the restoration should be luted with adhesive cements.

Geometry of the tooth preparation

• Increased preparation taper and rounding of axial angles tend to reduce resistance
• Short tooth preparations with large diameters have less resistance form
• A partial-coverage restoration has less resistance when compared to complete crown.

Resistance can be increased by:

• Preparation of boxes or grooves with walls that are perpendicular to the direction of the applied force
• U-shaped grooves and flared boxes provide more resistance than V-shaped ones
• Pinholes increase resistance as they prevent rotational movement and subject additional areas of the luting agent to compression. The buccal and lingual walls must meet the axial walls at 90° to resist rotational forces.

Physical properties of the luting agent.

Factors affecting resistance to deformation of luting agent are as follows:

• Compressive strength
• Modulus of elasticity.

Compressive strength

• Zinc phosphate cement should have compressive strength above 70 MPa at 24 hours
• Silico-phosphate cements have a higher compressive strength
• Glass ionomer, resin, polycarboxylate and reinforced zinc oxide-eugenol have lower values
• Increase in temperature during manipulation, decreases the strength of the luting agent.

Modulus of elasticity

Zinc phosphate cements have a higher modulus of elasticity than polycarboxylate cements, hence, polycarboxylate cement depends more on the taper of the preparation than zinc phosphate cement.

Posterior tooth Conclusion

• A prepared teeth should have the following charecteristics as 10–20 degrees of total occlusal convergence, an occlusocervical dimension of 4 mm and occlusocervical to faciolingual ratio greater than 0.4
• Facio- and lingoproximal line angles should be preserved if not auxillary features as box or grooves need to be added
• All line angles should be rounded and supragingival finish lines are more acceptable in case of esthetic requirements finish lines at gingival level or sometimes subgingival if restoration are present.

Posterior tooth Reference

• Goodacre CJ, et al. Tooth preparations for complete crowns: An art form based on scientific principles. J Prosthet Dent. 2001;85:363-76.