Cement layer thickness and shear stress resistance in cylindrical dowel spaces: pull-out test

Abstract

Purpose: This study evaluated the effects of different dowel space (DS) diameters on pull-out bond strength of a cylindrical post, of threaded steel, to dentin.

Materials and methods: Forty-five extracted human teeth were divided in 3 groups with DSs, with the same depth (6 mm), differing for the diameter (i.e. 1.5 mm, Group 1; 1.75 mm, Group 2; 2.00 mm, Group 3). Both the diameter of the post (1.3 mm) and the composite resin cement (Panavia 21) were the same for all the samples. The samples were submitted to pull-out test by means an Universal Testing Machine (Mod. 1193, Instron) (1KN load cell, crosshead speed 0.5 mm/min).

Results: The mean values of the bond strength (BS) were: Group 1, 442±128.3N; Group 2, 411.3±111N; Group 3, 448.7±142.29N. While the calculated average shear bond strengths (SBSs) were: Group 1, 14.7±4.27MPa; Group 2, 11.6±3.14MPa; Group 3, 11±3.5MPa. ANOVA test showed not significative differences, among the groups, concerning the BS: Group 1 vs Group 2 (p = 0.490); Group 1 vs Group 3 (p = 0.894); Group 2 vs Group 3 (p = 0.431). Significative differences were observed, among the groups, concerning the SBS for Group 1 vs Group 2 (p = 0.032) and Group 1 vs Group 3 (p = 0.014). While a not significative difference was found, concerning this parameter, for Group 2 vs Group 3 (p = 0.641).

Conclusion: The cement thickness can influence the SBS of the adhesively luted posts, in our setting, the best values were obtained with a thickness of 100 μm.

Keywords: cement thickness; composite resin cement; different cement thicknesses; dowel space; endodontic posts; endodontically treated teeth; pull-out test.

Figure 1

a–c) Graphics describing the trend of tensile force as a function of the time (a = Group 1, b = Group 2, c = Group 3). The failure of the adhesive bond failure is indicated by circled crosses; d) Group 1, dowel space (DS) 1.5 mm, cement thickness (CT) 100 μm; e) Group 2, DS 1.75 mm, CT 225 μm; f) Group 3, DS 2 mm, CT 350 μm;

Figure 2

a) A sample of the Group 3 prior to the mechanical test; b) a sample during the pull-out test; c) representative samples, of the three groups, after the test; d) all the samples of the three groups after the test.

Figure 3

a–c) Side vision, representative samples of the three groups, the cohesive fracture of the cement joints was observed in all groups but exhibiting a typical pattern in Group 1 only (original magnification 16X); d) in the dowel spaces, pertaining to the Group 1, has been possible to highlight the presence of small fragments of cement (arrows), still adherent on the dentin surface (original magnification 32X); e) area of foamy-chalky appearance. These are more frequent in Group 1, it is likely that their genesis can be attributed to residues of adhesive not completely removed (original magnification 56X).