Effect of Contact Area Size on Enamel and Composite Wear

The effect of contact area dimensions on the wear of composite specimens and their opposing enamel cusps was evaluated in vitro. Thirty-six standardized cylindrical composite specimens were placed into metal cavities (8 mm x 2 mm) and divided randomly into five groups. The composite used was a fine-particle hybrid and was stressed as follows: storage in 75 % aqueous ethanol solution for 24 h, toothbrush/toothpaste-abrasion for 30 min, followed by 300 thermal cycles in water ranging from 50 to 550C and simultaneous 120,000 occlusal chewing loads at a frequency of 1.7 Hz at 53 N maximum force. In group 1 (n = 12), the occlusal chewing loads were applied by palatal cusps ofextracted human maxillary molars with natural morphology. In groups 2 to 5 (n = 6), the cusp tips had standardized contact area dimensions of 0.26, 0.38, 1.18, and 4.10 mm2, respectively. Wear of composite specimens and antagonistic enamel cusps (means ± SD) was assessed in jim by means of a 3-D scanner. Additionally, the contact surfaces of the restorations and ofthe antagonistic enamel cusps were evaluated bySEM. Increases in enamel contact areas after being loaded were measured by means ofa digitizer and expressed in percent ofthe initial size before stress exposure. The wear ofthe composite specimens varied from 69.8 ± 19.9 to 9.5 ± 3.6 pm, and that ofantagonistic enamel cusps from 31.3 ± 3.4 to 8.8 ± 1.5 pm. The increase in contact area varied between 27.8 and 0.1%. Composite and antagonistic enamel wore signifi-cantly (p < 0.001, ANOVA, Scheff6) less with increased contact area dimensions. The most severe disintegration of the composite was found in specimens occluding with the smallest enamel contact area. The extent ofwear and the wear mechanisms were related to the chewing pressure, which in turn was governed by the contact area dimension