Prosthodontics MCQ Answer 49

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The correct answer is e. (4 and 5)

1. is false. ADA Specification No. 8 Type I states that film thickness be 25 um (maximum).

2. is false. Zinc phosphate is generally the thinnest of the luting agents, with a thickness of 20 um.

3. is false. Polycarboxylate cement has one of the lowest compressive strengths, but, does meet the maximum thickness guidelines with a thickness rivaling zinc phosphate (21um).

4. is true. Polycarboxylate cements appear to be much more viscous than is a comparable mix of zinc phosphate cement. As the mix is classified as pseudoplastic, it undergoes thinning at an increased shear rate. Clinically, the cement yields a film thickness of 25 um or less due to the action of spatulation and seating with a vibratory action to reduce the viscosity.

5. is true. Don’t’ confuse particle size with film thickness. GIC film thickness is more viscous than Zinc phosphate polycarboxylate cements; it has a minimum thickness of approximately 24 um.

It is important to know the thickness of various cements. Note the following:

Cement type	Setting time (min)	Film thickness (um)	24 hour compressive strength (Mpa)	24 hour tensile strength (Mpa)	Elastic modulus (Gpa)	Solubility in water (Wt%)	Pulp response
ANSI/ADA specification 8 (Type I)	5.0 min.	25 max.	69	na	na	0.2 max	*see note
Zinc phosphate	5.5	20	104	5.5	13.5	0.06	Moderate
ZOE (TYPE I)	4-10	25	6-28	__	__	0.04	Mild
ZOE-EBA (TYPE II)	9.5	25	55	4.1	5.0	0.05	Mild
ZOE PLUS POLYMER (TYPE II)	6-10	32	48	4.1	2.5	.08	Mild
SILICOPHOSPHATE	3.5-4	25	145	7.6	---	.4	Moderate
RESIN	2-4	<25	70-172	---	2.1-3.1	0-0.01	Moderate
POLYCARBOXYLATE	6	21	55	6.2	5.1	.06	Mild
GLASS IONOMER	7	24	86	6.2	7.3	1.25	Mild

*Note-based on comparison with silicate cement, a severe irritant

References: Anusavice K: Philips’ Science of Dental Materials, 10^th Ed., WB Saunders, 1996.