Objectives: determine in vitro the minimal degree of cure required by the base of a restoration to maintain marginal seal under thermal-mechanical stress conditions. Methods: Composite (Z250/Single Bond, 3M ESPE) was placed in bulk or incrementally (control) into Class 2 slot preparations with dentinal gingival margins in human molars and light cured with various energy densities and subjected to 1000 thermal-cycles & 500,000 fatigue cycles from 18 - 85 N with a stainless-steel sphere (n=8). Marginal integrity was evaluated using ridit analysis. Degree of conversion (DC) and Knoop hardness (KHN) were determined at the occlusal & gingival surfaces using a reusable tooth template with identical preparation dimensions (n=3). Percentage of maximum DC and KHN were determined. Results: Thermal-mechanical stressing did not significantly affect gingival marginal defects (p=0.17). Water had a significant dissolving effect on marginal integrity at low degrees of conversion (paired t-test; p<0.05). Energy density had a significant effect on gingival marginal defects (ANOVA/Tukey’s; p<.001). Heavy marginal degradation was seen in the 4000 mJ/cm2 group. Although there were no significant differences in ridit scores between the control, 24000 and 6000 mJ/cm2 groups there was evidence of other marginal deterioration (discoloration, opacity) with 6000 mJ/cm2 group. Conclusion: lower limit of gingival margin acceptability in bulk-filled Z250 composite restoration was created by energy densities producing a recommended 80% of maximum DC & 73% of maximum KHN as seen with 24000 mJ/cm2. Funding by 3M and NIH/NIDCR 09431. Table: a-c denote significant differences by energy density.
|
Energy Density mJ/cm2 |
4000(water only) |
4000 |
6000 |
8000 |
12000 |
24000 |
Control (24000x3) |
|
Ging DC (% max) |
24.5(5.1) |
24.5(5.1) |
45.0(4.4) |
57.3(0.2) |
67.6(2.3) |
80.0(0.9) |
97.0(1.6) |
|
Ging KHN (% max) |
2.8(3.6) |
2.8(3.6) |
22.6(0.8) |
39.2(4.5) |
54.1(0.3) |
72.8(0.3) |
96.5(1.2) |
|
Ridit (pre-stress) |
4.0(1.9) |
4.7(2.7) |
2.5(1.2) |
3.3(1.4) |
3.9(0.9) |
3.0(0.6) |
2.4(1.2) |
|
Ridit (500K cycles) |
6.3(2.4)c |
6.0(3.8)c |
2.8(1.3)a |
4.2(1.0)b,c |
4.1(0.8)b,c |
3.2(0.6)a,b |
2.3(0.8)a |