Research Papers

Residual Stresses in Titanium Spinal Rods: Effects of Two Contouring Methods and Material Plastic Properties

[+] Author and Article Information
Francesca Berti, Agnese Piovesan, Tomaso Villa, Giancarlo Pennati

Laboratory of Biological Structure Mechanics,
Department of Chemistry,
Materials and Chemical Engineering
“Giulio Natta,”
Politecnico di Milano,
Piazza Leonardo da Vinci 32,
Milan 20133, Italy

Luigi La Barbera

Laboratory of Biological Structure Mechanics,
Department of Chemistry,
Materials and Chemical Engineering
“Giulio Natta,”
Politecnico di Milano,
Piazza Leonardo da Vinci 32,
Milan 20133, Italy
e-mail: luigi.labarbera@polimi.it

Dario Allegretti, Claudia Ottardi

Laboratory of Biological Structure Mechanics,
Department of Chemistry,
Materials and Chemical Engineering
“Giulio Natta,”
Politecnico di Milano,
Piazza Leonardo da Vinci 32,
Milan 20133, Italy

1Corresponding author.

Manuscript received January 10, 2018; final manuscript received May 24, 2018; published online August 20, 2018. Assoc. Editor: Giuseppe Vairo.

J Biomech Eng 140(11), 111001 (Aug 20, 2018) (8 pages) Paper No: BIO-18-1020; doi: 10.1115/1.4040451 History: Received January 10, 2018; Revised May 24, 2018

Posterior spinal fixation based on long spinal rods is the clinical gold standard for the treatment of severe deformities. Rods need to be contoured prior to implantation to fit the natural curvature of the spine. The contouring processes is known to introduce residual stresses and strains which affect the static and fatigue mechanical response of the implant, as determined through time- and cost-consuming experimental tests. Finite element (FE) models promise to provide an immediate understanding on residual stresses and strains within a contoured spinal rods and a further insight on their complex distribution. This study aims at investigating two rod contouring strategies, French bender (FB) contouring (clinical gold standard), and uniform contouring, through validated FE models. A careful characterization of the elastoplastic material response of commercial implants is led. Compared to uniform contouring, FB induces highly localized plasticizations in compression under the contouring pin with extensive lateral sections undergoing tensile residual stresses. The sensitivity analysis highlighted that the assumed postyielding properties significantly affect the numerical predictions; therefore, an accurate material characterization is recommended.

Copyright © 2018 by ASME
Your Session has timed out. Please sign back in to continue.


Luca, A. , Ottardi, C. , Sasso, M. , Prosdocimo, L. , La Barbera, L. , Brayda-Bruno, M. , Galbusera, F. , and Villa, T. , 2016, “Instrumentation Failure Following Pedicle Subtraction Osteotomy: The Role of Rod Material, Diameter, and Multi-Rod Constructs,” Eur. Spine J., 26(3), pp. 764–770. [CrossRef] [PubMed]
Barton, C. , Noshchenko, A. , Patel, V. , Cain, C. , Kleck, C. , and Burger, E. , 2015, “Risk Factors for Rod Fracture After Posterior Correction of Adult Spinal Deformity With Osteotomy: A Retrospective Case-Series,” Scoliosis, 10(1), p. 30. [CrossRef] [PubMed]
Smith, J. S. , Shaffrey, E. , Klineberg, E. , Shaffrey, C. I. , Lafage, V. , Schwab, F. J. , Protopsaltis, T. , Scheer, J. K. , Mundis, G. M. , Fu, K. M. , Gupta, M. C. , Hostin, R. , Deviren, V. , Kebaish, K. , Hart, R. , Burton, D. C. , Line, B. , Bess, S. , and Ames, C. P. , and International Spine Study Group, 2014, “Prospective Multicenter Assessment of Risk Factors for Rod Fracture Following Surgery for Adult Spinal Deformity,” J. Neurosurg. Spine, 21(6), pp. 994–1003. [CrossRef] [PubMed]
Smith, J. S. , Shaffrey, C. I. , Ames, C. P. , Demakakos, J. , Fu, K. M. , Keshavarzi, S. , Li, C. M. , Deviren, V. , Schwab, F. J. , Lafage, V. , and Bess, S. , and International Spine Study Group, 2012, “Assessment of Symptomatic Rod Fracture After Posterior Instrumented Fusion for Adult Spinal Deformity,” Neurosurgery, 71(4), pp. 862–867. [CrossRef] [PubMed]
Berjano, P. , Bassani, R. , Casero, G. , Sinigaglia, A. , Cecchinato, R. , and Lamartina, C. , 2013, “Failures and Revisions in Surgery for Sagittal Imbalance: Analysis of Factors Influencing Failure,” Eur. Spine J., 22(S6), pp. 853–858. [CrossRef]
Charosky, S. , Guigui, P. , Blamoutier, A. , Roussouly, P. , and Chopin, P. , 2012, “Complications and Risk Factors of Primary Adult Scoliosis Surgery: A Multicenter Study of 306 Patients,” Spine, 37(8), pp. 693–700. [CrossRef] [PubMed]
Yang, J. S. , Sponseller, P. D. , Thompson, G. H. , Akbarnia, B. A. , Emans, J. B. , Yazici, M. , Skaggs, D. L. , Shah, S. A. , Salari, P. , and Poe-Kochert, C. , 2011, “Growing Rod Fractures: Risk Factors and Opportunities for Prevention,” Spine, 36(20), pp. 1639–1644. [CrossRef] [PubMed]
La Barbera, L. , Galbusera, F. , Wilke, H. J. , and Villa, T. , 2016, “Preclinical Evaluation of Posterior Spine Stabilization Devices: Can the Current Standards Represent Basic Everyday Life Activities?,” Eur. Spine J., 25(9), pp. 2909–2918. [CrossRef] [PubMed]
La Barbera, L. , Brayda-Bruno, M. , Liebsch, C. , Villa, T. , Luca, A. , Galbusera, F. , and Wilke, H. J. , 2018, “Biomechanical Advantages of Supplemental Accessory and Satellite Rods With and Without Interbody Cages Implantation for the Stabilization of Pedicle Subtraction Osteotomy,” Eur. Spine J. (accepted).
Dick, J. C. , and Bourgeault, C. , 2001, “Notch Sensitivity of Titanium Alloy, Commercially Pure Titanium, and Stainless Steel Spinal Implants,” Spine, 26(15), pp. 1668–1672. [CrossRef] [PubMed]
Demura, S. , Murakami, H. , Hayashi, H. , Kato, S. , Yoshioka, K. , Yokogawa, N. , Ishii, T. , Igarashi, T. , Fang, X. , and Tsuchiya, H. , 2015, “Influence of Rod Contouring on Rod Strength and Stiffness in Spine Surgery,” Orthopedics, 38(6), pp. e520–e523. [CrossRef] [PubMed]
Noshchenko, A. , Xianfeng, Y. , Armour, G. A. , Baldini, T. , Patel, V. V. , Ayers, R. , and Burger, E. , 2011, “Evaluation of Spinal Instrumentation Rod Bending Characteristics for in-Situ Contouring,” J. Biomed. Mater. Res. B: Appl. Biomater., 98(1), pp. 192–200. [CrossRef] [PubMed]
Tang, J. A. , Leasure, J. M. , Smith, J. S. , Buckley, J. M. , Kondrashov, D. , and Ames, C. P. , 2013, “Effect of Severity of Rod Contour on Posterior Rod Failure in the Setting of Lumbar Pedicle Subtraction Osteotomy (PSO): A Biomechanical Study,” Neurosurgery, 72(2), pp. 276–282. [CrossRef] [PubMed]
Slivka, M. A. , Fan, Y. K. , and Eck, J. C. , 2013, “The Effect of Contouring on Fatigue Strength of Spinal Rods: Is It Okay to Re-Bend and Which Materials Are Best?,” Spine Deform., 1(6), pp. 395–400. [CrossRef] [PubMed]
Lindsey, C. , Deviren, V. , Xu, Z. , Yeh, R. F. , and Puttlitz, C. M. , 2006, “The Effects of Rod Contouring on Spinal Construct Fatigue Strength,” Spine, 31(15), pp. 1680–1687. [CrossRef] [PubMed]
ASTM, 2016, “Standard Test Methods for Tension Testing of Metallic Materials,” American Society for Testing and Materials, West Conshohocken, PA, Standard No. ASTM E8/E8M-16a. http://www.astm.org/cgi-bin/resolver.cgi?E8E8M
Pérez-Pevida, E. , Brizuela-Velasco, A. , Chávarri-Prado, D. , Jiménez-Garrudo, A. , Sánchez-Lasheras, F. , Solaberrieta-Méndez, E. , Diéguez-Pereira, M. , Fernández-González, F. J. , Dehesa-Ibarra, B. , and Monticelli, F. , 2016, “Biomechanical Consequences of the Elastic Properties of Dental Implant Alloys on the Supporting Bone: Finite Element Analysis,” Biomed. Res. Int., 2016, p. 1850401.
ASTM, 2014, “Standard Specifications and Test Methods for Metallic Angled Orthopedic Fracture Fixation Devices,” American Society for Testing and Materials, West Conshohocken, PA, Standard No. ASTM F2193-14.
Eberle, S. , Gerber, C. , von Oldenburg, G. , Högel, F. , and Augat, P. , 2010, “A Biomechanical Evaluation of Orthopaedic Implants for Hip Fractures by Finite Element Analysis and In-Vitro Tests,” Proc. Inst. Mech. Eng. H, 224(10), pp. 1141–1152. [CrossRef] [PubMed]
Niinomi, M. , 1998, “Mechanical Properties of Biomedical Titanium Alloys,” Mater. Sci. Eng. A, 243(1–2), pp. 231–236. [CrossRef]
Davis, J. R. , 2003, Handbook of Materials for Medical Devices, ASM International, Materials Park, OH, p. 41.
Mirone, G. , Barbagallo, R. , Corallo, D. , and Di Bella, S. , 2016, “Static and Dynamic Response of Titanium Alloy Produced by Electron Beam Melting,” Procedia Struct. Integr., 2, pp. 2355–2366. [CrossRef]
ASTM, 2013, “Standard Specification for Wrought Titanium-6Aluminum-4Vanadium ELI (Extra Low Interstitial) Alloy for Surgical Implant Applications (UNS R56401),” American Society for Testing and Materials, West Conshohocken, PA, Standard No. ASTM F136-13. http://www.astm.org/cgi-bin/resolver.cgi?F136
Melkerson, M. N. , Griffith, S. L. , and Kirkpatrick, J. S. , 2003, “Spinal Implants: Are We Evaluating Them Appropriately?,” ASTM International, West Conshohocken, PA, Standard No. ASTM STP1431-EB. https://www.astm.org/DIGITAL_LIBRARY/STP/SOURCE_PAGES/STP1431.htm
Nguyen, T. Q. , Buckley, J. M. , Ames, C. , and Deviren, V. , 2011, “The Fatigue Life of Contoured Cobalt Chrome Posterior Spinal Fusion Rods,” Proc. Inst. Mech. Eng. H, 225(2), pp. 194–198. [CrossRef] [PubMed]
Oberwinkler, B. , 2016, “On the Anomalous Mean Stress Sensitivity of Ti-6Al-4V and Its Consideration in High Cycle Fatigue Lifetime Analysis,” Int. J. Fatigue, 92, pp. 368–381. [CrossRef]
Takakuwa, O. , Nakai, M. , Narita, K. , Niinomi, M. , Hasegawa, K. , and Soyama, H. , 2016, “Enhancing the Durability of Spinal Implant Fixture Applications Made of Ti–6Al–4V ELI by Means of Cavitation Peening,” Int. J. Fatigue, 92, pp. 360–367. [CrossRef]


Grahic Jump Location
Fig. 1

Experimental setup (top) and FE models (bottom) of uniform contouring (a) and FB contouring (b). In (a) the supporting pin is depicted as fixed while the loading pin trajectory is displayed by an arrow, as well as the fixed central and moving roller in (b). The FE models are built based on the symmetries of the experimental setup.

Grahic Jump Location
Fig. 2

Experimental tensile loading–unloading stress–strain curve obtained on a dog-bone specimen (a). Experimental average stress–strain curve obtained by tensile test; standard deviation is also reported on yield and ultimate points (b): (a) loading–unloading tensile tests and (b) experimental average.

Grahic Jump Location
Fig. 3

Schematic drawing of the procedure adopted to assess the local curvature radius on a contoured rod (the same procedure applies in case of uniform contouring). The contoured region of interest comprised by lines s (coinciding with the Y symmetry of the rod) and r is identified. Three points are selected on the rod cross section to define r, namely A, C, and B, lying, respectively, on the concave, convex, and on the midline of the rod. D is the middle point of the rod cross section, where s line passes. ϕ is the resulting intersection angle between r and s. The assessed curvature radius was defined as: R = 0.5*z/sin(ϕ/2), with ϕ = arctan(m). Points coincided, respectively, with specific nodal elements and with pixels, respectively, for numerically and experimentally bent rods. A sensitivity analysis proved that a slight variation in the identification of these points may lead to a difference in curvature radius estimation lower than 10%.

Grahic Jump Location
Fig. 6

Residual stress (σ*) trends after release along the circumferential (top) and radial (bottom) directions for the uniform bending (a) and FB (b) models. The graphs were obtained on a transversal section both in the gage length (AA) and under the loading pin (BB): these sections collapse for the FB contouring model: (a) uniform contouring and (b) FB.

Grahic Jump Location
Fig. 5

Residual stress (σ*) fields color maps at peak bending (top) and after release (middle), predicted during uniform (a) and FB (b) contouring on the transversal yz and the frontal yx planes. A qualitative comparison with an experimentally contoured rod is also provided (bottom).

Grahic Jump Location
Fig. 4

Comparison between the numerical prediction and the experimental force–deflection curve obtained in a four-point bending. The standard deviation is also reported on yield and maximum displacement points.



Some tools below are only available to our subscribers or users with an online account.

Related Content

Customize your page view by dragging and repositioning the boxes below.

Related Journal Articles

Sorry! You do not have access to this content. For assistance or to subscribe, please contact us:

  • TELEPHONE: 1-800-843-2763 (Toll-free in the USA)
  • EMAIL: asmedigitalcollection@asme.org
Sign In