Aquira Ishikiriama

Fracture strength of human teeth with cavity preparations

F ailure of dental restorations is a major concern. The most common causes are faulty cavity preparation and improper manipulation of restorative materials. Another cause of failure is the geometric form of the cavity preparation, which may lead to fracture of the tooth. The anatomic forms of posterior teeth with cusps and fossae present a design possessing a tendency to deflect the cusps under stress (Fig. 1, A).’ While sound teeth rarely fracture from the stresses of mastication, fracture of a cusp may occur in teeth which have been weakened by caries and the cavity preparation required for restoration. The typical intracoronal cavity preparation, especially in the maxillary premolars, exaggerates the height of the cusps (Fig. 1, B). The weakened or unsupported cusps are subjected to stresses which tend to deflect or fracture them at the buccopulpal or linguopulpal line angles (Fig., 1, C). While a fracture may not occur, deflection of a weakened cusp may open the tooth-restoration interface and lead to subsequent marginal leakage (Fig. 1, C). It is evident that the buccolingual width of the occlusal preparation may contribute toward weakening of the cusps. Cavity forms have been based on Black’s principles.’ For Class I and II cavity preparations, Black advocated an occlusal width of one-third the buccolingual intercuspal distance. Present-day equipment and restorative materials permit more conservative geometric forms. Modern cavity preparation permits narrow occlusal designs, one-fourth the buccolingual

Influence of some factors on the fit of cemented crowns

In the study, cast gold crowns were cemented similar to methods used for patients. Conclusions were that: 1. Fresh cement painted with a camel brush in the part of the crown to be cemented promotes a better fit than when the crown is completely filled with cement. 2. Mechanical vibration of the crown at cementation promotes a better fit. 3. Venting the crown, an internal relief by acid etching, or a combination of both these procedures improve the seating of the crown during cementation. 4. The association of one or numerous variables used in this study considerably improves the fit of the cemented crown.

The Influence of ultrasound in removing intraradicular posts

Thirty single-rooted teeth were prepared to receive cast posts. After cementation of the post the teeth were divided into three groups. Group I did not receive any treatment, whereas the posts in groups II and III were treated with an ultrasonic device, for 2 min and 5 min, respectively. The force required for post removal was determined using a universal test machine. No statistical differences (P > 0.05) were found between the groups where ultrasound had been used. Significantly reduced forces were necessary to remove the posts from group II and III, compared with group I which did not receive ultrasonic treatment (P < 0.05).

Marginal microleakage in cemented complete crowns

An in vivo study was developed to observe the penetration of 131INa around the margins of complete crown castings cemented with zinc phosphate cement with and without the application of different protective agents, with the following results. 1. Complete crowns cemented with zinc phosphate cement showed 131INa marginal leakage differences depending on the protective material used prior to cementation. 2. The best protective agent against marginal leakage according to this study was two layers of cavity varnish applied to the margins. 3. The amount of marginal leakage can be influenced by crown adaptation and the film thickness of the cement. 4. There were no differences between test periods of 1 hour or 21 days after cementation.

Influence of pressure and vibration during cementation

Vibration, applied at the beginning of cementation, improves the fit of MOD inlays and cast complete crowns when they are compared to the same castings cemented without vibration. The Medart pressure applicator produced better adaptation. Orange wood blocks and Burlew disks produced similar results. They were less efficient than the Medart pressure applicator. Cotton rolls resulted in the highest fit discrepancy.

Cross-splinting a weakened tooth with a horizontal pin: A new method

This study determined that the attachment of composite to unetched enamel had no significant effect on the bond strength of composite to the same enamel surface after etching. Half of the teeth were prepared (0.5 mm enamel reduction) to simulate the clinical indirect veneer technique, and the others were unprepared. Composite interim restorations were attached (without etching) to half of both groups of teeth. When composite restorations were subsequently attached by acid etching and debonded, there were no significant differences in bond strengths among the four groups. Consequently, the technique of composite attachment to enamel before etching may be used by dentists without fear of affecting the bond strength of composite to that same enamel after etching.

Tensile strength of five types of retention for resin-bonded prostheses

The tensile bond strengths of five retention techniques for bonded cast restorations were tested. The retentive methods were the electrolytic etch, Rochettes perforations, retention beads, the recessed surface, and soldered orthodontic mesh. The electrolytic etch and recessed surface methods gave the highest values of bond strength, followed by retention beads. Rochettes perforations and the soldered orthodontic mesh failed to surpass the bond strength of the control group (no additional retention) after 1 hour and provided the lowest bond strength after 24 hours.

Influence of proximal retention on the fracture strength of Class II amalgam restorations

T he conservative concept of Class II amalgam restorations suggests that the resistance and retention forms of the cavity preparation should be independent of each other. The convergence of the buccal and lingual walls toward the occlusal surface provides sufficient retention against displacement of the occlusal and proximal portions of the restoration. However, to ensure retention in the proximal portion, regardless of the occlusal dovetail, additional retention must be provided to resist proximal displacement and increase the strength of the Class II amalgam restoration.‘~s Repeated compressive stress applied to the proximal portion may cause its displacement or fracture because of the narrow isthmus of the dovetail69 7 The concept suggested by Markley” recommends eliminating the occlusal portion in the absence of occlusal caries. This view was recently supported by Almquist et al.9 Many authors agree that additional proximal retention is a basic principle toward a more conservative Class II amalgam cavity preparation.10-‘4 They also agree that the dovetail is of paramount importance. There are three designs for retention at the axiobuccal and axiolingual line angles: (1) an angular area from the axiogingivobuccal and axiogingivolingual point angles to a narrow vertex just apical to the axiopulpal line angle,“‘, 12S2o (2) a cone-shaped vertical groove which diverges toward the occlusal surface; the greater diameter is located at the level of the occlusal dentinoenamel junction, while the minor diameter is located close to the axiogingivobuccal and axiogingivolingual point angles,6’ ‘3 21-24 and (3) a cylindrical groove made with a straight fissure bur for deciduous teeth.25-28

Physical properties of dental amalgam containing metal pins

No type of pin inclusion investigated in this study produced consistent increases or decreases in the strength of amalgam specimens under a variety of loading conditions and specimen ages. However, comparison of relative values for specimens tested under the same conditions showed that the highest fracture loads were found more often for specimens containing threaded gold pins than for any other type of specimen.