Marginal adaptation of Class II composite fillings: guided polymerization vs reduced light intensity

To improve the marginal adaptation of Class II composite fillings, a three-sited light-curing technique was proposed by Lutz et al in 1986 and has proven to be effective in several studies. This study investigated whether this effect, attributed to guided polymerization towards the cavity margins, might also be due to the decreased polymerization stress resulting from reduced curing light intensity caused by transdental curing.

Forty extracted human molars were mounted in casts and 40 Class II slot-type cavities were cut (cervical margins 0.5 mm above the CEJ). To measure the light being transmitted from a halogen curing light onto a composite layer within a cavity, miniature photodetectors were used and irradiance measured for a three-layer technique (1st: cervical-horizontal; 2nd and 3rd: lateral-diagonal) with occlusal or three-sited curing. To evaluate the influence of light intensity on marginal adaptation, the prepared cavities were treated as follows: after base placement and acid etching, the cavities were filled with Herculite XR (Kerr, Romulus, MI, USA) in three increments. All cavities were filled with the layering technique described above. In two groups (n = 10), curing was carried out only from occlusal. In the remaining groups, the cervical composite layer was cured through a light-transmitting, lateral reflective wedge, and the following layers transdentally with a proximal wand positioning. Using the photodetectors, the output power of an argon laser was adjusted so that the composite was cured with the equivalent irradiance calculated for occlusal (high-intensity groups) and three-sited curing occlusal (low-intensity groups). Replicas were produced after storing in water for 21 days and thermocycling (2000x, 5 degrees C to 55 degrees C). The margins of the fillings were quantitatively assessed using a SEM at 200x. The results were statistically analyzed (ANOVA, Bonferroni/Dunn; p < 0.05).

The summed values for the criteria "restoration margin fracture", "enamel margin fracture," and "marginal opening" at the cervical margins were 11.3 +/- 11.5 (three-sited curing, low intensity), 14.1 +/- 15.7 (occlusal curing, low intensity), 25.4 +/- 21.2 (three-sited curing, high intensity), and 27.8 +/- 30.6 (occlusal curing, high intensity).

No significant difference was found between curing techniques at equal energy levels; the effect of the three-sited light curing technique should therefore not be attributed to guided polymerization but to the reduction of light intensity.