Influence of cusp coverage on marginal integrity and enamel cracks

Objectives: This in vitro study of ceramic inlays (CI) and partial ceramic crowns (PCC) tested the effect of coverage of a thin non-functional cusp on marginal integrity and crack formation of enamel.

Methods: CI (n=48) and PCC (n=48) preparations were performed in extracted human cariesfree molars. Functional cusps were adjusted to 2.5mm thickness, for PCC the functional cusps were additionally reduced by 2.0mm. Non-functional cusps were adjusted to (group1) 1.0mm thickness and (group2) additionally with 2.0mm reduction. Ceramic restorations (Vita Mark II,Cerec3 System) were adhesively luted to the cavities with RelyX Unicem (3M Espe). The specimens were exposed to thermocycling and central mechanical loading (TCML:5000x8°C-55°C;30s/cycle;500000x72.5N,1.6Hz). Marginal adaptation was assessed by dye penetration (fuchsine) on multiple sections at ceramic- (C), enamel- (E) and dentin- (D) luting agent (LA) interfaces. Cracks of enamel were documented before and after TCML. The data were statistically analysed with the Mann Whitney U-test, the Chi-Square-test (frequency of cracks) and the Error Rates Method (ERM).

Results: Dye penetration and crack formation (%); Median (25%/75%percentiles)          

interface		Group1		Group2
C/LA	CI	44.8(23.9/69.5)		24.8(13.2/57.6)
	PCC	8.5(3.2/20.3)		26.8(14.3/50.2)
E/LA	CI	54.1(41.5/67.5)		68.9(55.3/97.9)
	PCC	58.1(41.4/96.2)		71.3(40.9/100)
D/LA	CI	42.1(24.0/90.3)		96.7(54.9/98.4)
	PCC	30.8(24.8/46.3)		95.9(37.3/100)
Crack		before TCML	after TCML	before TCML	after TCML
	CI	4.0(3.0/5.0)	6.0(4.25/7.0)	3.0(2.0/4.0)	4.0(3.0/5.0)
	PCC	4.0(3.0/5.0)	5.0(4.0/7.0)	4.0(3.0/5.0)	6.0(4.0/8.0)

Dye penetration data indicated that cavity design, cusp design and interface had a statistically significant influence upon marginal integrity, in general, irrespective of all other parameters (ERM). The results of pairwise comparisons are inconsistent: in 3 cases no differences were found, in 3 cases group2 showed more microleakage than group1. Crack formation of CI increased significantly for group1 compared with group2.

Conclusion: Thin non-functional cusp walls of CI should be covered or completely reduced to prevent crack formation and – in the long run – may avoid tooth fracture.