The Effect of Temporary Cement Cleaning Methods on the Retention of Crowns

PURPOSE

To evaluate the effect of temporary cement cleaning methods on the retention of cemented crowns using zinc phosphate cement and resin-modified glass ionomer cement.

MATERIALS AND METHODS

Forty titanium specimens were fabricated to simulate prepared molars with minimally retentive taper. The Ni-Cr cast crowns were fabricated, temporarily cemented, and separated. The specimens were divided into four groups according to the temporary cement cleaning method (n = 10) as follows: control group (no temporary cementation), orange solvent group, ultrasonic cleaning group, and air-abrasion group. After the cleaning procedures, the specimens were cemented with definitive cements (zinc phosphate cement and resin-modified glass ionomer, RMGI, cement) and subjected to thermocycling (5000 cycles, 5-55°C, dwell time, 10 seconds). The tensile bond strength of each specimen was measured using a universal testing machine, and the results were analyzed using the Kruskal-Wallis and Mann-Whitney U test (α = 0.05).

RESULTS

When cemented with zinc phosphate cement, the statistical analysis showed that the value of the air-abrasion group was significantly higher than those of the other groups (p < 0.01). There was no statistically significant difference among the other groups. When cemented with RMGI cement, the air-abrasion group showed the lowest value, and the control group showed the highest value (p < 0.01). The difference between the ultrasonic cleaning group and the orange solvent group was not statistically significant.

CONCLUSION

The use of temporary cement did not have a significant influence on retention of permanently cemented crowns when zinc phosphate cement was used for permanent cementation. Airborne-particle abrasion after provisional cementation improved retention of crowns cemented with zinc phosphate cement; however, the use of temporary cement significantly decreased retention of permanently cemented crowns when RMGI cement was used regardless of the temporary cement cleaning method.

Angle of convergence of posterior crown preparations made by predoctoral dental students

The aim of this study was to determine angle of convergence (AC) of posterior crown preparations made by predoctoral dental students at the University of Toronto. Ninety-one dies of students' crown preparations were digitally scanned with an in-Eos-Blue scanner (Sirona). Created images were virtually sliced at three similar locations of mesiodistal and buccolingual planes. Virtual protractor was used to determine AC of each section. Means and SDs were calculated, and data were statistically analyzed with ANOVA and student's t-test for operator's gender, experience, and tooth type. There were no significant differences among the groups except for AC of preparations grouped by tooth type (p<0.0001). The greatest mean mesiodistal AC was 26.4° found with mandibular molars, while the smallest was 16° found with maxillary premolars. ANOVA revealed significant difference in mean mesiodistal AC among groups (p<0.01). Also, greatest mean buccolingual AC was 25° found with mandibular molars, while the smallest was 20.8° found with maxillary premolars. ANOVA did not reveal significant difference in mean buccolingual AC among groups (p>0.05). Overall mean AC values were greater than ideal range of 2-5°; however, they were within ranges published for dentists/prosthodontists. Gender and experience had no significant effect on AC, but tooth type significantly affected AC.