Cusp coverage for ceramic restorations: 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 Excite/Variolink II (Vivadent). 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 and the Error Rates Method (ERM), and frequency of cracks with the c²-test.

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

interface		Group1		Group2
C/LA	CI	30.4(19.2/70.4)		25.3(17.4/35.4)
	PCC	34.5(16.5/75.4)		10.3(4.9/21.3)
E/LA	CI	28.9(23.4/42.9)		37.4(26.9/63.8)
	PCC	24.3(13.0/40.8)		35.1(12.1/95.6)
D/LA	CI	96.8(74.0/100)		65.2(44.7/89.7)
	PCC	100(97.8/100)		82.3(33.4/99.9)
Crack		before TCML	after TCML	before TCML	after TCML
	CI	10(8/12)	10(10/16)	8(6/12)	10(8/13.5)
	PCC	12(10/16)	14(10.5/18)	10(6.5/12)	12(8/16)

Dye penetration data indicated that cusp design and interface had a statistically significant influence upon marginal integrity, in general, irrespective of all other parameters (ERM): Proximal dye penetration showed significant higher microleakage along the C/LA- and D/LA-interface for group1 compared with group2. Crack formation increased significantly after TCML with a statistically not significant trend of fewer cracks in the coverage group2.

Conclusions: Thin non-functional cusp walls of adhesively bonded CI and PCC should be reduced and covered to avoid marginal deficiency at the tooth/luting agent interface and possibly increased crack formation.