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

  • Dylan G. Kluck
  • Christine L. Farnsworth
  • Megan E. Jeffords
  • Nikolas E. Marino
  • Burt Yaszay
  • Vidyadhar V. Upasani
  • Peter O. NewtonEmail author


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



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



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.

Compliance with ethical standards

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.


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Copyright information

© Scoliosis Research Society 2020

Authors and Affiliations

  • Dylan G. Kluck
    • 1
  • Christine L. Farnsworth
    • 2
  • Megan E. Jeffords
    • 2
  • Nikolas E. Marino
    • 2
  • Burt Yaszay
    • 1
    • 2
  • Vidyadhar V. Upasani
    • 1
    • 2
  • Peter O. Newton
    • 1
    • 2
    Email author
  1. 1.University of California San DiegoLa JollaUSA
  2. 2.Rady Children’s Hospital-San DiegoSan DiegoUSA

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