Effect of Tightening Torque on Abutment-Fixture Joint Stability using 3-Dimensional Finite Element Analysis

Eom Tae-Gwan; Suh Seung-woo; Jeon Gyeo-Rok; Shin Jung-Wook; Chang-Mo Jeong

doi:10.4047/jkap.2009.47.2.125

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Tae-Gwan, Seung-woo, Gyeo-Rok, Jung-Wook, and Jeong: Effect of Tightening Torque on Abutment-Fixture Joint Stability using 3-Dimensional Finite Element Analysis

ORIGINAL ARTICLE

J Korean Acad Prosthodont 2009;47(2):125-135.

Published online: 18 December 2009

DOI: https://doi.org/10.4047/jkap.2009.47.2.125

Effect of Tightening Torque on Abutment-Fixture Joint Stability using 3-Dimensional Finite Element Analysis

Eom Tae-Gwan, MS^1,², Suh Seung-woo, MS², Jeon Gyeo-Rok, PhD³, Shin Jung-Wook, PhD⁴, Chang-Mo Jeong, DDS, PhD^5,^∗

^¹Graduate Student, Interdisciplinary Program in Biomedical, Pusan National University, Busan, South Korea

^²Implant R&D Center, OSSTEM Inc, Busan, South Korea

^³Professor, The School of Medicine, Pusan National University, Busan, South Korea

^⁴Professor, The Department of Biomedical Engineering. Inje University, Gimhae, South Korea

^⁵Professor, The School of Dentistry, Pusan National University, Busan, South Korea

Corresponding Author: Chang-Mo Jeong The School of Dentistry, Pusan National University, 10 Ami-dong 1ga, Seo gu, Busan, 602-739, Korea +82 51 240 7438: e-mail, cmjeong@pusan.ac.kr

Received 24 July 200810 December 2008 Accepted 24 December 2008

This is an Open Access article distributed under the terms of the Creative Commons Attribution Non-Commercial License (http://creativecommons.org/licenses/by-nc/3.0/) which permits unrestricted non-commercial use, distribution, and reproduction in any medium, provided the original work is properly cited.

Abstract

Statement of problem

Loosening or fracture of the abutment screw is one of the common problems related to the dental implant. Generally, in order to make the screw joint stable, the preload generated by tightening torque needs to be increased within the elastic limit of the screw. However, additional tensile forces can produce the plastic deformation of abutment screw when functional loads are superimposed on preload stresses, and they can elicit loosening or fracture of the abutment screw. Therefore, it is necessary to find the optimum tightening torque that maximizes a fatigue life and simultaneously offer a reasonable degree of protection against loosening.

Purpose

The purpose of this study was to present the influence of tightening torque on the implant-abutment screw joint stability with the 3 dimensional finite element analysis.

Material and methods

In this study, the finite element model of the implant system with external butt joint connection was designed and verified by comparison with additional theoretical and experimental results. Four different amount of tightening torques (10, 20, 30 and 40 Ncm) and the external loading (250 N, 30°) were applied to the model, and the equivalent stress distributions and the gap distances were calculated according to each tightening torque and the result was analyzed.

Results

Within the limitation of this study, the following results were drawn; 1) There was the proportional relation between the tightening torque and the preload. 2) In case of applying only the tightening torque, the maximum stress was found at the screw neck. 3) The maximum stress was also shown at the screw neck under the external loading condition. However in case of applying 10 Ncm tightening torque, it was found at the undersurface of the screw head. 4) The joint opening was observed under the external loading in case of applying 10 Ncm and 20 Ncm of tightening torque. 5) When the tightening torque was applied at 40 Ncm, under the external loading the maximum stress exceeded the allowable stress value of the titanium alloy.

Conclusion

Implant abutment screw must have a proper tightening torque that will be able to maintain joint stability of fixture and abutment.

Keywords: joint stability, screw loosening, tightening torque, preload, FEA

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Fig.1.

Schematic diagram of experimental materials.

Fig. 2.

Schematic diagram of this study.

Fig. 3.

Preload measuring device.

Fig. 4 .

3-dimensional FE-model and mesh shape.

Fig. 5.

Contact and boundary conditions of FE-model.

Fig. 6.

Application method of Loading Condition on the abutment screw.

Fig. 7.

Principle of preload calculation.

Fig. 8.

Preload according to holding time of tightening torque 30 Ncm.

Fig . 9.

Axial normal stress distribution for each tightening torque.

Fig. 10.

Experimental value, theoretic value or analytic value about preload.

Fig. 11.

Equivalent stress distribution for each tightening torque.

Fig. 12.

Equivalent stress distribution for each tightening torque under external loading.

Table I.

Specification of experimental materials

Components	Sizes	Material
US II Fixture	∅ 4.1 mm × L 11mm	Titanium Gr. 4
Cement Abutment	∅ 5.0 mm × H 5.5 mm	Titanium Gr. 3
Abutment Screw	M 2.0 × 0.4 P	Ti-6Al-4V

(Diameter 2.0 × Pitch 0.4 mm)

Table II.

Material properties ^29-33

	Material	Young's modulus	Poisson's ratio
Fixture	Ti Gr4	105 GPa	0.34
Abutment	Ti Gr3	104 GPa	0.34
Screw	Ti-6Al-4V	113 GPa	0.342
Cortical Bone	-	13.7 GPa	0.3
Cancellous Bone	e -	1.37 GPa	0.3

Table III.

Number of nodes and elements at FE-model

Geometry	Nodes	Elements
Cortical bone	94,345	63,531
Cancellous bone	82,314	55,685
External type fixture	130,502	87,728
Cemented abutment	15,784	9,770
Abutment screw	26,436	16,925
Total	349,381	233,639

Table IV.

Mean values ± SDs of measured preload [N]

Tightening torque	n	Mean ± SD∗ [N]
10 Ncm	5	47.7 ± 2.1 a
20 Ncm	5	126.0 ± 14.2^b
30 Ncm	5	253.0 ± 9.2 c
40 Ncm	5	396.7 ± 29.1^d

^∗ Same letters indicate values that were not statistically different.

Table V.

Preload [N] calculated by theoretical formula

	Tightening torque
	10 Ncm	20 Ncm	30 Ncm	40 Ncm
Preload [N]	90.6	181.2	271.8	362.4

Table VI.

Measured Preload [N] from FEA

	Tightening torque
	10 Ncm	20 Ncm	30 Ncm	40 Ncm
Preload [N] at Abutment and Screw interface	81.3	161.6	238.2	342
Preload [N] at Abutment and Fixture interface	81.3	161.6	238.2	342

Table VII.

Maximum equivalent stress [MPa] on the abutment screw

	Tightening torque
	10 Ncm	20 Ncm	30 Ncm	40 Ncm
Tightenign torque condition [MPa]	193	387.4	594.1	798
External loading condition [MPa]	411.1	406.4	601.6	800.7

Table VIII.

Gap distances [mm] different tightening torque

	Tightening torque
	10 Ncm	20 Ncm	30 Ncm	40 Ncm
Gap distance [mm]	0.005	0.002	0.000	0.000

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