Retention Form

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Certain forces (e.g., when the jaws are moved apart after biting on very sticky food) act on a cemented restoration in the same direction as the path of withdrawal. The quality of a preparation that prevents the restoration from becoming dislodged by such forces parallel to the path of withdrawal is known as retention. Only dental caries and porcelain failure outrank lack of retention as a cause of failure of crowns and fixed partial dentures . 48,49

The following factors must be considered when deciding whether retention is adequate for a given fixed restoration:

1. Magnitude of the dislodging forces

2. Geometry of the tooth preparation

3. Roughness of the fitting surface of the restoration

4. Materials being cemented

5. Film thickness of the luting agent

Magnitude of the Dislodging Forces. Forces that tend to remove a cemented restoration along its path of withdrawal are small compared to those that tend to seat or tilt it. A fixed partial denture or splint can be subjected to such forces by pulling with floss under the connectors; however, the greatest removal forces generally arise when exceptionally sticky food (e.g., caramel) is eaten. The magnitude of the dislodging forces depends on the stickiness of the food and the surface area and texture of the restoration being pulled.

Geometry of the Tooth Preparation. Most fixed prostheses depend on the geometric form of the preparation rather than on adhesion for retention because most of the traditional cements (e.g., zinc phosphate) are nonadhesive (i.e., they act by increasing the frictional resistance between tooth and restoration). The grains of cement prevent two surfaces from sliding, although they do not prevent one surface from being lifted from another. This is analogous to the effect of particles of sand or dust within machinery. They do not have a specific adhesion to metal, but they increase the friction between sliding metal parts. If sand or dust gets into an old-fashioned, mechanical camera or watch, the increase in friction can effectively jam the mechanism.

Cement is effective only if the restoration has a single path of withdrawal (i.e., the tooth is shaped to restrain the free movement of the restoration). The relationship between a nut and a bolt is an example of restrained movement (Fig. 7-26). The nut is not free to move in any direction but can move only along the precisely determined helical path of the threads on the bolt.

The relationship between two bodies, one (in this case a tooth preparation) restraining movement of

Retention Form

Fig. 7-26. A, The relationship of a nut and a bolt is an example of restrained movement; the nut must move along a precisely defined helical path (arrows). B, For effective retention, a tooth preparation must constrain the movement of a restoration. For this to occur, it must be cylindrical. (See Figure 7-27.)

Fig. 7-26. A, The relationship of a nut and a bolt is an example of restrained movement; the nut must move along a precisely defined helical path (arrows). B, For effective retention, a tooth preparation must constrain the movement of a restoration. For this to occur, it must be cylindrical. (See Figure 7-27.)

the other (a cemented restoration), has been studied mathematically and is known in analytical mechanics as a closed lower pair of kinematic elements. In fixed prosthodontics, a sliding pair is the only pair that has relevance. It is formed by two cylindrical* surfaces constrained to slide along one another. The elements are constrained if the curve that defines the cylinder is closed or shaped to prevent movement at right angles to the axis of the cylinder (Fig. 7-27).

A tooth preparation will be cylindrical if the axial surfaces are prepared by a cylindrical bur held at a constant angle. The gingival margin of the preparation becomes the fixed curve of the mathematical definition, and the occlusoaxial line angle of the tooth preparation should be a replica of the gingival margin geometry. The curve of a complete crown preparation is closed, whereas the grooves of a partial crown preparation prevent movement at right angles to the long axis of the cylinder. However, if one wall of the complete crown preparation is over-

*Cylinder is defined in its mathematical sense as the solid generated by a straight line parallel to another straight line and moving so that its ends describe a fixed curve.

Dental Bur Cross Section

Fig. 7-27. A preparation is cylindrical if the two horizontal cross sections of the prepared axial tooth surface (1 and 2) are coincident. A, This complete crown is cylindrical and therefore retentive. B, A partial crown will be retentive if its sections are coincident and perpendicular movement is prevented by grooves. C, This preparation is cylindrical (1 and 2 coincide) but not retentive, because it can move perpendicularly to the axis of the cylinder. (Redrawn from RosenstielE: Br Dent J 103:388, 1957.)

Fig. 7-27. A preparation is cylindrical if the two horizontal cross sections of the prepared axial tooth surface (1 and 2) are coincident. A, This complete crown is cylindrical and therefore retentive. B, A partial crown will be retentive if its sections are coincident and perpendicular movement is prevented by grooves. C, This preparation is cylindrical (1 and 2 coincide) but not retentive, because it can move perpendicularly to the axis of the cylinder. (Redrawn from RosenstielE: Br Dent J 103:388, 1957.)

tapered, it will no longer be cylindrical, and the cemented restoration will not be constrained by the preparation because the restoration then has multiple paths of withdrawal. Under these circumstances, the cement particles will tend to lift away from rather than slide along the preparation, and the only retention will be a result of the cement's limited adhesion (Fig. 7-28).

Taper. Theoretically, maximum retention is obtained if a tooth preparation has parallel walls. However, it is impossible to prepare a tooth this way using current techniques and instrumentation; slight undercuts are created that prevent the restoration from seating.

An undercut is defined as a divergence between opposing axial walls, or wall segments, in a cervi-cal-occlusal direction (Fig. 7-29, A). For instance, if the cervical diameter of a tooth preparation at the margin is narrower than at the occlusoaxial junction (reverse taper), it will be impossible to seat a complete cast crown of similar geometry (Fig. 7-29, B). Undercuts can be present whenever two axial walls face in opposite directions (Fig. 7-29, C. Thus the mesial wall of a complete cast crown preparation can be undercut relative to the distal wall; in addi-

Film Thickness Luting Cement

Fig. 7-28. A, Cross sections 1 and 2 do not coincide, and the preparation thus has little retention. B, Under these circumstances, very little friction develops between the cement and the axial walls, and the cement is subjected to tensile stress. C, A retentive near-parallel preparation with frictional resistance. The cement is placed under shear stress.

(A redrawn from Rosmstiel E: Br Dent J 103:388, 1957.)

Fig. 7-28. A, Cross sections 1 and 2 do not coincide, and the preparation thus has little retention. B, Under these circumstances, very little friction develops between the cement and the axial walls, and the cement is subjected to tensile stress. C, A retentive near-parallel preparation with frictional resistance. The cement is placed under shear stress.

(A redrawn from Rosmstiel E: Br Dent J 103:388, 1957.)

tion, the buccal wall can be undercut relative to the lingual wall; finally, in a partial veneer preparation, the lingual wall of a proximal groove can be undercut relative to the lingual wall of the preparation.

A slight convergence, or taper, is necessary in the completed preparation. As long as this taper is small, the movement of the cemented restoration will be effectively restrained by the preparation and will have what is known as a limited path of withdrawal. As the taper increases, however, so does the free movement of the restoration, and retention will be reduced.

The relationship between the degree of axial wall taper and the magnitude of retention was first demonstrated experimentally by Jorgensen in 1955. He cemented brass caps on Galalith cones of different tapers and measured retention with a tensile-testing machine. The relationship was found to be hyperbolic, with retention rapidly becoming less as taper increased (Fig. 7-30), although the relation

Tooth Prep Taper Undercuts

Fig. 7-29. A, An undercut is formed if opposing walls diverge. B, A crown is prepared, because an undercut preparation cannot "seat," since it cannot pass over the divergent walls. C, Undercuts are possible in other locations when fixed partial dentures or restorations with preparation features such as grooves or boxes are prepared. Here one buccal facing wall (B) can be undercut relative to (four) lingual facing walls (L).

Fig. 7-29. A, An undercut is formed if opposing walls diverge. B, A crown is prepared, because an undercut preparation cannot "seat," since it cannot pass over the divergent walls. C, Undercuts are possible in other locations when fixed partial dentures or restorations with preparation features such as grooves or boxes are prepared. Here one buccal facing wall (B) can be undercut relative to (four) lingual facing walls (L).

ship was no longer hyperbolic when the internal surfaces of the caps were roughened. The retention of a cap with 10 degrees of taper* was approximately half that of a cap with 5 degrees. Similar results have been reported by other workers.

Selection of the appropriate degree of taper for tooth preparation involves compromise. Too small a taper may lead to unwanted undercuts; too large will no longer be retentive. The recommended convergence between opposing walls is 6 degrees, which has been shown to optimize retention for zinc phosphate cement.55 Recognizing this angle is important (Fig. 7-31), although there is no need to deliberately tilt a rotary cutting instrument to create a taper, since this will invariably lead to overprepa-ration. Rather, teeth are readily prepared with a rotary instrument of the desired taper held at a constant angulation. The rotary instrument should be moved through a cylindrical path as the tooth is prepared, and the taper of the instrument should produce the desired axial wall taper on the completed preparation. In practice, many dentists expe-

*In this discussion, as is generally the case in the dental literature, taper and convergence are used interchangeably and refer to the angle between diametrically opposed axial walls.

Retention Taper Crowns

Fig. 7-30. Relationship between retention and convergence angle. *, Experimental values; x, calculated values outside the experimental range.

(Redrawn from Jorgensen KD: Acta Odontol Scand 13:35, 1955.)

rience difficulty consistently avoiding excessively tapered preparations, particularly when preparing posterior teeth with limited access . 56 Some authorities recommend the routine use of grooves to reduce the incidence of restoration displacement. It is unclear, however, whether accurate groove alignment is more easily achieved than axial wall convergence, and skillfully prepared axial walls at a minimal convergence are very conservative of tooth structure.

Surface Area. Provided the restoration has a limited path of withdrawal, its retention depends on the length of this path or, more precisely, on the surface area in sliding contact. Therefore, crowns with long axial walls are more retentive than those with short axial walls,-'' and molar crowns are more retentive than premolar crowns of similar taper. Surfaces where the crown is essentially being pulled away from rather than sliding along the tooth, such as the occlusal surface, do not add much to total retention.

Stress Concentration. When a retentive failure occurs, cement often adheres to both the tooth preparation and the fitting surface of the restoration. In these cases, cohesive failure occurs through the cement layer because the strength of the cement is less than the induced stresses. A computerized analysis of these stresses-', reveals that they are not uniform throughout the cement but are concentrated around the junction of the axial and occlusal surfaces. Changes in the geometry of the preparation (e.g., rounding the internal line angles) may reduce stress concentrations and thus increase the retention of the restoration.

Type of Preparation. Different types of preparation have different retentive values that correspond fairly closely to the surface area of the axial

Fig. 7-30. Relationship between retention and convergence angle. *, Experimental values; x, calculated values outside the experimental range.

(Redrawn from Jorgensen KD: Acta Odontol Scand 13:35, 1955.)

Convergence Angle Tooth
Fig. 7-31. The recommended convergence angle is 6 degrees. This is a very slight taper. (The angle between the hands of a clock showing 12:01 is 5 1/2, degrees.)

walls, as long as other factors (e.g., taper) are kept constant. Thus the retention of a complete crown is about double that of partial-coverage restorations 59 (Fig. 7-32).

Adding grooves or boxes (Fig. 7-33) to a preparation with a limited path of withdrawal does not markedly affect its retention because the surface area is not increased significantly. However, where the addition of a groove limits the paths of withdrawal, retention is increased 60,61

Roughness of the Surfaces Being Cemented.

When the internal surface of a restoration is very smooth, retentive failure occurs not through the cement but at the cement-restoration interface. Under these circumstances, retention will be increased if the

Tooth Restoration Interface Pictures

restoration is roughened or grooved . The casting is most effectively prepared by air-abrading the fitting surface with 50 um of alumina. This should be done carefully to avoid abrading the polished surfaces or margins. Airborne particle abrasion has been shown65 to increase in vitro retention by 64%.

Failure rarely occurs at the cement-tooth interface. Therefore, deliberately roughening the tooth preparation hardly influences retention and is not recommended, because roughness adds to the difficulty of impression making and waxing.

Materials Being Cemented. Retention is affected by both the casting alloy and the core or buildup material. Laboratory testing results have yet to be confirmed by longer-term clinical studies, but it appears that the more reactive the alloy is, the more adhesion there will be with certain luting agents. Therefore, base metal alloys are better retained than less reactive high-gold content metals . The effect of adhesion to different core materials also has been tested, with conflicting results. One laboratory study67 examining adhesion between cements and core materials found that the cement adhered better to amalgam than to composite resin or cast gold. However, when crowns were tested for retention, higher values were found with the composite resin than with amalgam cores . The differences may have been due to dimensional changes of the core materials, although the clinical implications of this finding are not clear.

Type of Luting Agent. The type of luting agent chosen affects the retention of a cemented restoration .69 However, the decision regarding which agent to use is also based on other factors. In general, the data suggest that adhesive resin cements are the most retentive (Fig 7-34), although long-term clinical evidence about the durability of the bond is not available.

Film Thickness of the Luting Agent. There is conflicting evidence about the effect of increased thickness of the cement film on retention of a restoration. This may be important if a slightly oversized casting is made (as when the die-spacer technique is used).

The factors that influence the retention of a cemented restoration are summarized in Table 7-3.

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