Injection Shrinkage аnd Water Sorption of Some Thermoplastic Dental Materials

Four thermoplastic injection-molded materials were analyzed (Bre.flex 2^nd edition, Vertex ThermoSens, Perflex Biosens and Polyan IC). A total of 432 test samples were made in the form of an "hourglass". All samples were divided into three groups: Group I (Control) - consisting of 36 test samples for each type of material, that was not exposed to artificial aging or a wet environment; Group II - consisting of 36 test samples for each type of material, that were artificially aged through dehydration; Group III - specimen were subjected to artificial aging without dehydration. The diameter of each specimen was measured with a digital caliper. Data were analysed using the Student's t-test.

Results:

Regarding to shrinkage, the samples from the Bf Control group have a mean value of 1.56 mm and was observed a shrinkage of the injection-molded polyamide material within 0.25%. The comparison between the samples from Group II and Group III showed statistically significant differences (p<0.001). There were no significant differences between groups for Thermosens and Biosens (p>0.05). The comparison between Group II and Group III for Polyan IC samples shows that Group III has a higher arithmetic mean value (p<0.01).

Conclusion:

Shrinkage of the polymers during the injection process is present in all materials. The thermocycling and the storage in a dry or in a wet environment of the samples results in a change of the diameter in almost every single type of material.

Keywords:
Dental Impression Materials; Dental Prosthesis; Denture Bases

	Control Group		Thermocycling with Dehydration		Thermocycling without Dehydration
	N	%	N	%	N	%
Up to 1.45 mm	-	-	17	47.2	-	-
1.46 - 1.47 mm	-	-	4	11.1	-	-
1.48 - 1.49 mm	-	-	4	11.1	4	11.1
1.50 - 1.51 mm	4	11.1	7	19.4	4	11.1
1.52 - 1.53 mm	8	22.2	-	-	8	22.2
1.54 - 1.55 mm	4	11.1	4	11.1	8	22.2
1.56 - 1.57 mm	12	33.3	-	-	12	33.3
1.58 - 1.59 mm	4	11.1	-	-	-	-
1.60 + mm	4	11.1	-	-	-	-
Total	36	100.0	36	100.0	36	100.00

Groups	N	Mean (SD)	S_x	u	p-value
I (Control)	36	1.56 ± 0.49	2.93	16.97	<0.001¹ 1 Comparison of the diameters between Control and Group II;
II	36	1.46 ± 0.44	2.61	4.38	<0.001² 2 Comparison of the diameters between Control and Group III;
III	36	1.53 ± 0.51	3.04	10.43	<0.001³ 3 Comparison of the diameters between Group II and III.

Groups	N	Mean (SD)	S_x	u	p-value
I (Control)	36	1.44 ± 0.34	2.01	0.75	>0.05¹ 1 Comparison between the Control group and Group II;
II	36	1.45 ± 0.69	2.22	0.55	>0.05² 2 Comparison between the Control group and Group III;
III	36	1.47 ± 0.47	3.10	0.33	>0.05³ 3 Comparison between Group II and Group III.

Groups	N	Mean (SD)	S_x	u	p-value
I (Control)	36	2.46 ± 1.11	2.22	0.02	>0.05¹ 1 Comparison between the Control group and Group II;
II	36	2.56 ± 0.77	3.23	0.65	>0.05² 2 Comparison between the Control group and Group III;
III	36	3.45 ± 1.54	2.24	0.55	>0.05³ 3 Comparison between Group II and Group III

Groups	N	Mean (SD)	S_x	u	p-value
I (Control)	36	1.465 ± 0.02	0.012	0	>0.05¹ 1 Comparison between the Control group and Group II;
II	36	1.465 ± 0.001	0.005	2.14	>0.05² 2 Comparison between the Control group and Group III;
III	36	1.525 ± 0.02	0.013	3.00	>0.01³ 3 Comparison between Group II and Group III

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[1] Author to whom correspondence should be addressed: Stoyan Yankov, Department of Prosthodontics, Faculty of Dental Medicine, Medical University Plovdiv, Bulgaria. Phone: +359 877717251. E-mail: yankov.dent@gmail.com.