Effect of Screw Insertion Torque on Mechanical Properties of Four Locking Systems

Authors

  • Bianca Boudreau DVM,

    1. Collaborative Orthopaedic Investigations Laboratory, Department of Small Animal Clinical Sciences, College of Veterinary Medicine, Michigan State University, East Lansing, Michigan
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  • Jérôme Benamou DVM,

    1. Collaborative Orthopaedic Investigations Laboratory, Department of Small Animal Clinical Sciences, College of Veterinary Medicine, Michigan State University, East Lansing, Michigan
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  • Dirsko J. F. von Pfeil Dr.Med.Vet, DVM, Diplomate ECVS & ACVS,

    1. Collaborative Orthopaedic Investigations Laboratory, Department of Small Animal Clinical Sciences, College of Veterinary Medicine, Michigan State University, East Lansing, Michigan
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  • Reunan P. Guillou Doc Vét,

    1. Collaborative Orthopaedic Investigations Laboratory, Department of Small Animal Clinical Sciences, College of Veterinary Medicine, Michigan State University, East Lansing, Michigan
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  • Clifford Beckett,

    1. The Orthopaedic Biomechanics Laboratories, College of Osteopathic Medicine, Michigan State University, East Lansing, Michigan
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  • Loïc M. Déjardin DVM, MS, Diplomate ACVS & ECVS

    Corresponding author
    • Collaborative Orthopaedic Investigations Laboratory, Department of Small Animal Clinical Sciences, College of Veterinary Medicine, Michigan State University, East Lansing, Michigan
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Corresponding Author

Dr. Loïc M. Déjardin, DVM, MS, Diplomate ACVS & ECVS, College of Veterinary Medicine, Michigan State University, 736 Wilson Rd, East Lansing, MI 48824.

E-mail: dejardin@cvm.msu.edu

Abstract

Objective

To evaluate the effect of screw insertion torque on the mechanical properties of four 3.5 mm locking systems: New Generation Devices (NGD), Securos (PAX), Synthes (SYN), and Veterinary Orthopedic Implants (VOI).

Study Design

In vitro mechanical study.

Methods

Screws were inserted at 1.5 Nm, 2.5 Nm, and 3.5 Nm torques, using dedicated drill guides and a calibrated torque screwdriver. Locking mechanisms were tested under shear loading conditions using a custom-design probe. Tests were conducted under displacement control until failure. Load to failure and interface stiffness were compared between and within groups using a two-factor ANOVA (P < .05). Failure modes were described.

Results

The SYN group had significantly greater mechanical properties compared to all other groups at any given insertion torque. Insertion torque had a significant effect on the PAX group, increasing the load to failure by 126% when increasing the insertion torque from 1.5 Nm to 3.5 Nm. Insertion torque had no significant effect on the mechanical properties of the VOI group and limited effect on the NGD and SYN groups. Single failure mode, by screw head decoupling or screw shaft fracture, occurred in the SYN and VOI groups, respectively. In contrast, NGD and PAX systems failed through a combination of screw decoupling and/or bending.

Conclusions

Regardless of insertion torque, the SYN locking mechanism showed the highest failure loads, interfacial stiffness as well as consistent failure mode. These findings highlight the dependability of this system. When using the PAX system an insertion torque of at least 2.5 Nm should be recommended.

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