Objectives: Physical and chemical properties of the luting cementare strictly linked to conversion degree of polymerization, since an insufficient curing determines a loss of properties and a potentially unfavourable prognosis of the indirect composite restoration. In our study the curing percentage of four different composite cements was evaluated. Materials and methods: Composite onlays of different thickness (1 mm, 2 mm, 3 mm, 4 mm) were interposed between curing lamp and luting cement, to evaluated the influence of the thickness of the indirect restoration on luting cements conversion. Onlays of different and defined thickness were made in composite material shade A 3.5 and they were cemented onto a glass plate with different luting materials: 2 dual-curing and 2 light-curing. Then, curing was started with the lamp, with a shielded-all-around tip to make the light reach the cement only through the onlay. 48 onlays were divided into 12 groups (n=3), differing in onlay thickness and luting material. Each sample was examined in three points by Micro-Raman spectrophotometer to appreciate the polymer conversion degree and the results were statistical analyzed by ANOVA and Student Newman Keuls tests. Results: The average of conversion rate obtained provided the results as shown in the table:

Average conversion rate of tested composite materials
Onlay thickness	Venus flow	Variolink II	Twinlook cement	Tetric flow
Onlay  1mm	84,21	81,78	90,56	79,32
Onlay  2mm	80,54	77,06	91,34	69,19
Onlay  3mm	61,88	41,48	47,77	48,39
Onlay 4mm	32,07	30,76	0,52	32,05

Conclusions: Restoration thickness influences highly the composite polymer conversion rate. All tested materials in case of 1 and 2 mm onlays show sufficient curing, while with onlays thicker than 3 mm a constant conversion degree could not be found.

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