Springer Nature is making SARS-CoV-2 and COVID-19 research free. View research | View latest news | Sign up for updates

Spinal rod gripping capacity: how do 5.5/6.0-mm dual-diameter screws compare?

  • 11 Accesses


Study design

Biomechanical comparative study.


To evaluate pedicle screw gripping capacity from five suppliers, comparing single-diameter (S-D) systems using 5.5-mm-diameter rods to dual-diameter (D-D) systems accepting 5.5- and 6.0-mm-diameter rods with both cobalt chromium (CoCr) and titanium alloy (Ti) rods.

Summary of background data

D-D systems have become increasingly prevalent; however, these systems theoretically may compromise spinal rod gripping, particularly when a smaller-diameter rod is used within a D-D pedicle screw.


D-D pedicle screw systems from three suppliers (accepting 5.5- and 6.0-mm-diameter, Ti and CoCr rods), and S-D systems from two suppliers (accepting 5.5-mm-diameter, Ti and CoCr rods) were tested on an MTS MiniBionix machine. Axial load was applied in line with the rod to measure axial gripping capacity (AGC), and torsional load was applied to measure torsional gripping capacity (TGC) for each rod material and diameter. AGC and TGC were compared between D-D and S-D constructs, suppliers, rod diameters, and materials with subsequent classification and regression tree (CART) analysis.


5.5-mm rods within D-D screws were no weaker than 5.5-mm rods in S-D systems for AGC (dual > single, p = 0.043) and TGC (p = 0.066). As a whole, D-D systems had greater AGC than S-D systems (p = 0.01). AGC differed between suppliers (p < 0.001). No rod diameter (p = 0.227) or material (p = 0.131) effect emerged. With CART analysis, Supplier was the most significant predictor for greater AGC. As a whole, D-D systems had greater TGC than S-D systems (p = 0.008). TGC differed between suppliers (p < 0.001). Rod diameter was a significant predictor of higher TGC (6.0 > 5.5 mm, p = 0.002). CoCr rods had greater TGC than Ti (p < 0.001). CART analysis revealed that Supplier and CoCr material were significant predictors for increased TGC.


Despite 30%–70% variability in gripping capacity due to rod supplier and material, overall D-D pedicle screw systems had similar AGC and TGC as S-D systems.

Level of evidence


This is a preview of subscription content, log in to check access.

Fig. 1
Fig. 2
Fig. 3
Fig. 4


  1. 1.

    Bartley CE, Yaszay B, Bastrom TP et al (2017) Perioperative and delayed major complications following surgical treatment of adolescent idiopathic scoliosis. J Bone Joint Surg 99:1206–1212

  2. 2.

    Kepler CK, Meredith DS, Green DW et al (2012) Long-term outcomes after posterior spine fusion for adolescent idiopathic scoliosis. Curr Opin Pediatr 24:68–75

  3. 3.

    Lykissas MG, Jain VV, Nathan ST et al (2013) Mid- to long-term outcomes in adolescent idiopathic scoliosis after instrumented posterior spinal fusion: a meta-analysis. Spine 38:E113–E119

  4. 4.

    Murphy RF, Mooney JF (2016) Complications following spine fusion for adolescent idiopathic scoliosis. Curr Rev Musculoskelet Med 9:462–469

  5. 5.

    Dobbs MB, Lenke LG, Kim YJ et al (2006) Selective posterior thoracic fusions for adolescent idiopathic scoliosis: comparison of hooks versus pedicle screws. Spine 31:2400–2404

  6. 6.

    Kim YJ, Lenke LG, Cho SK et al (2004) Comparative analysis of pedicle screw versus hook instrumentation in posterior spinal fusion of adolescent idiopathic scoliosis. Spine 29:2040–2048

  7. 7.

    Faraj AA, Webb JK (1997) Early complications of spinal pedicle screw. Eur Spine J 6:324–326

  8. 8.

    Katonis P, Christoforakis J, Kontakis G et al (2003) Complications and problems related to pedicle screw fixation of the spine. Clin Orthop Relat Res 411:86–94

  9. 9.

    Rawall S, Mohan K, Nagad P et al (2011) Role of “low cost Indian implants” in our practice: our experience with 1,572 pedicle screws. Eur Spine J 20:1607–1612

  10. 10.

    Schroerlucke SR, Steklov N, Mundis GM Jr et al (2014) How does a novel monoplanar pedicle screw perform biomechanically relative to monoaxial and polyaxial designs? Clin Orthop Relat Res 472:2826–2832

  11. 11.

    Fogel GR, Reitman CA, Liu W et al (2003) Physical characteristics of polyaxial-headed pedicle screws and biomechanical comparison of load with their failure. Spine 28:470–473

  12. 12.

    Amaritsakul Y, Chao CK, Lin J (2014) Biomechanical evaluation of bending strength of spinal pedicle screws, including cylindrical, conical, dual core and double dual core designs using numerical simulations and mechanical tests. Med Eng Phys 36:1218–1223

  13. 13.

    Christodoulou E, Chinthakunta S, Reddy D et al (2015) Axial pullout strength comparison of different screw designs: fenestrated screw, dual outer diameter screw and standard pedicle screw. Scoliosis 10:15

  14. 14.

    Dalal A, Upasani VV, Bastrom TP et al (2011) Apical vertebral rotation in adolescent idiopathic scoliosis: comparison of uniplanar and polyaxial pedicle screws. J Spinal Disord Tech 24:251–257

  15. 15.

    Demura S, Murakami H, Hayashi H et al (2015) Influence of rod contouring on rod strength and stiffness in spine surgery. Orthopedics 38:e520–e523

  16. 16.

    Essig DA, Miller CP, Xiao M et al (2012) Biomechanical comparison of endplate forces generated by uniaxial screws and monoaxial pedicle screws. Orthopedics 35:e1528–e1532

  17. 17.

    Ha KY, Hwang SC, Whang TH (2013) Biomechanical stability according to different configurations of screws and rods. J Spinal Disord Tech 26:155–160

  18. 18.

    Lamerain M, Bachy M, Delpont M et al (2014) CoCr rods provide better frontal correction of adolescent idiopathic scoliosis treated by all-pedicle screw fixation. Eur Spine J 23:1190–1196

  19. 19.

    Serhan H, Mhatre D, Newton P et al (2013) Would CoCr rods provide better correctional forces than stainless steel or titanium for rigid scoliosis curves? J Spinal Disord Tech 26:E70–E74

  20. 20.

    Wang X, Aubin CE, Crandall D et al (2011) Biomechanical comparison of force levels in spinal instrumentation using monoaxial versus multi degree of freedom postloading pedicle screws. Spine 36:E95–E104

  21. 21.

    Wang X, Aubin CE, Crandall D et al (2012) Biomechanical analysis of 4 types of pedicle screws for scoliotic spine instrumentation. Spine 37:E823–E835

  22. 22.

    Wang X, Aubin CE, Labelle H et al (2012) Biomechanical analysis of corrective forces in spinal instrumentation for scoliosis treatment. Spine 37:E1479–E1487

  23. 23.

    Demir T, Camuşcuz N (2012) Design and performance of spinal fixation pedicle screw system. Proc Inst Mech Eng H 226:33–40

  24. 24.

    ASTM Standard F1798-97 (2008) Evaluating the Static and Fatigue Properties of Interconnection Mechanisms and Subassemblies Used in Spinal Arthrodesis Implants. ASTM International, West Conshohocken, PA. DOI: 10.1520/F1798-97R08. https://www.astm.org

Download references


The authors thank Tracey Bastrom, MS, for statistical analysis and Samantha Farnsworth and Claire Warrenfelt for testing assistance.


This study was funded by the Division of Orthopedics, Rady Children’s Hospital–San Diego. Implants (pedicle screws, set screws, rods) and use of instrumentation (torque wrench) were provided by Alphatec, DePuy Synthes, K2M, NuVasive, OrthoPediatrics.

Author information

Correspondence to Peter O. Newton.

Ethics declarations

Conflict of interest

DGK (none), CLF (none), MEJ (none), NEM (none), BY (grants and personal fees from K2M, DePuy Synthes Spine, and NuVasive; personal fees from Medtronic, Orthopediatrics, Stryker, Globus, and Biogen; grants from Setting Scoliosis Straight Foundation, outside the submitted work; in addition, BY has a patent K2M with royalties paid), VVU (personal fees from DePuy Synthes Spine and OrthoPediatrics, outside the submitted work), PON (other from DePuy Synthes, K2M, OrthoPediatrics, NuVasive, and AlphaTec, during the conduct of the study; grants and other from Setting Scoliosis Straight Foundation; other from Rady Children’s Specialists; grants, personal fees, and nonfinancial support from DePuy Synthes Spine and K2M; grants and other from SRS; grants from EOS Imaging; personal fees from Thieme Publishing; grants from NuVasive; other from Electrocore; personal fees from Cubist; other from International Pediatric Orthopedic Think Tank; grants, nonfinancial support, and other from Orthopediatrics; grants and nonfinancial support from Alphatech; grants from Mazor Robotics, outside the submitted work; in addition, PON has a patent “Anchoring Systems and Methods for Correcting Spinal Deformities” [8540754] with royalties paid to DePuy Synthes Spine; a patent “Low Profile Spinal Tethering Systems” [8123749] licensed to DePuy Spine, Inc.; a patent “Screw Placement Guide” [7981117] licensed to DePuy Spine, Inc.; a patent “Compressor for Use in Minimally Invasive Surgery” [7189244] licensed to DePuy Spine, Inc.; and a patent “Posterior Spinal Fixation” pending to K2M).

IRB statement

This project does not involve live humans or animals; so IRB and IACUC approval are not necessary.

Additional information

Publisher's Note

Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.

Rights and permissions

Reprints and Permissions

About this article

Verify currency and authenticity via CrossMark

Cite this article

Kluck, D.G., Farnsworth, C.L., Jeffords, M.E. et al. Spinal rod gripping capacity: how do 5.5/6.0-mm dual-diameter screws compare?. Spine Deform 8, 25–32 (2020). https://doi.org/10.1007/s43390-020-00028-1

Download citation


  • Torsional gripping capacity
  • Axial gripping capacity
  • Spinal rods
  • Dual-diameter pedicle screw
  • Biomechanical testing