1379 Stiffness and Stress with Nickel-Titanium Rotary File Pitch and Cross-section

Saturday, March 24, 2012: 9:45 a.m. - 11 a.m.
Presentation Type: Poster Session
A. VERSLUIS, Bioscience Research, University of Tennessee Health Science Center, Memphis, TN, C. LEE, Division of Precision Manufacturing Systems, Pusan National University, Busan, South Korea, D. KO, Industrial Liaison Innovation, Pusan National University, Busan, South Korea, and H. KIM, Department of Conservative Dentistry, Pusan National University, Yangsan, South Korea

Objectives: Shape is the main determinant of mechanical performance for Nickel-Titanium (NiTi) rotary instruments. This study evaluated how pitch and cross-sectional geometry affected flexural stiffness and stresses.

Methods: Finite-element models of rotary instruments with four cross-sectional geometries (triangle, slender-rectangle, rectangle, square) and three pitches (5-, 10-, 15-threads) were created, featuring NiTi shape-memory properties. All models had the same length, taper and external peripheral radius; cross-sectional area (CSA) and/or centre-core area (CCA) varied. The clamped shaft was rotated axially, while the tip was deflected 5mm. Flexural stiffness (bending force/deflection) and von Mises equivalent stresses in were calculated. Maximum stress was the mean of the top-5% values.

Results: Stiffness and maximum stress decreased with decreasing pitch (increasing threads). Doubling or tripling the threads for the triangular or rectangular cross-sections decreased the stiffness and stress about 6 and 12%, respectively; square cross-sections were less affected (1 and 3% decrease, respectively). Square cross-sections (higher cross-sectional and center-core areas) had higher stiffness and stresses than other models under the same deflection. Rectangular and triangular models with the same center-core areas had similar stresses, but the rectangular model was 30-40% stiffer. The slender-rectangle had the smallest center-core area and the lowest stiffness and stresses. Both rectangular cross-sections caused stiffness and stress variations with rotation angle (up to 13% for the slender-rectangle); larger pitch caused more variation.

 

 

CSA (mm3)

CCA (mm3)

stiffness (N/m)

maximum stress (MPa)

threads

 

 

5

10

15

5

10

15

triangle

0.33

0.20

990

940

861

4041

3780

3510

slender-rectangle

0.33

0.10

604

552

511

31717

3018

2855

rectangle

0.43

0.20

1335

1243

1152

40114

3847

3724

square

0.50

0.39

2040

2010

1970

4481

4431

4361

Conclusions: Maximum stress, and thus fracture risk, in NiTi rotary files may be reduced by decreasing pitch. Reducing cross-sectional and center-core surface-area also reduces flexural stiffness and stress.


Keywords: Biomechanics, Dental materials, Endodontics, Finite analysis and Stress