The Effect of Compromised Illumination on Bulk-fill Composite Polymerisation

Objectives: As they require fewer increments, bulk-fil composites should reduce treatment times. However, imperfect illumination conditions may affect their degree of polymerisation, consequentially reducing their mechanical properties and potentially compromising their durability. In this work, we look at the effect of angle and distance of separation between a light curing unit (LCU) tip and the hardness of a typical bulk-fill composite.
Methods: To simulate the intensity of the light at the top and bottom surfaces of a composite, the intensity of an LED LCU (Eliper Deepcure, 3M) was measured (checkMARK, BlueLight Analytics) with the LCU tip at the distances and angles shown in Table 1. Measurements were taken, either without (simulating top) or with (simulating bottom) a 3mm thick composite sample placed between the tip and the sensor.
Composite (Venus, Kulzer) disc specimens (10x3mm, n=3) were polymerised using 20s exposure on the top surface using the same LCU and identical distances and angles, Table 1. Next the specimens were stored in phosphate buffered saline at 37°C for 7 days before the Martens hardness (Z2.5, Zwick Roell) was measured on the top and bottom surfaces (n=3 per specimen).
Results: The light intensity significantly reduced when a specimen was placed on the sensor (P<0.01), when the distance increased (P<0.05) and when the angle increased (P<0.01). The top hardness was unaffected by distance or angle (P>0.05). Distance and angle significantly affected the bottom hardness, most clearly demonstrated using the top/bottom hardness ratio, which fell below 80% when the distance was 2mm or the angle 20°.
Conclusions: As the top/bottom hardness ratio reduces when increased distance or angulation occur during polymerisation it is vital that dentists realise that the degree of polymerisation of their bulk-fill composites may be reduced.