Pull-out strength comparison of a novel expanding fastener against an orthopaedic screw in an ovine vertebral body: an ex-vivo study

Abstract

The purpose of this study was to mechanically test a novel Unthreaded Expandable Fastener (UEF), manufactured using Selective Laser Melting, which was designed for fixation in the cervical lateral mass. The pull-out strength and stiffness of the prototype UEFs was evaluated in a non-osteoporotic ovine bone model against equivalent screws. The prototype UEF demonstrated a 41% increase in failure force and a 60% reduction in failure force standard deviation compared to the screws. All bone samples were micro CT-scanned and no significant differences in bone microstructural properties was found between the screw and UEF sample sets, indicating that the UEFs may be less sensitive to bone quality variation. This increased performance can potentially translate into improved surgical outcome and reduced surgical risk for lateral mass fixation. With further design optimisation, additional improvement in performance over screws may be possible in future studies.

Keywords:

• Expandable fastener
• lateral mass
• spine
• orthopaedic screws
• selective laser melting

Acknowledgements

The authors acknowledge the help of Astrid Armitrage, a Senior Technician in Charge at the Large Animal Facility, UWA, for facilitating the acquisition of the sheep spines. The authors acknowledge the facilities and the scientific and technical assistance of the National Imaging Facility at the Centre for Microscopy, Characterisation & Analysis, UWA, a facility funded by the University, State and Commonwealth Governments. The authors also acknowledge the use of the Realizer SLM100 machine at UWA and the Instron 8874 servo-hydraulic materials testing machine at RPH.

Disclosure statement

Matthew Oldakowki, Intan Oldakowska, Thomas B. Kirk and Philip Hardcastle are named inventors in the patent application (PCT/AU2015/000099) related to the design of the fastener tested in this experiment. Chris T. Ford, Tim B. Sercombe and Robert E. Day report no conflicts of interest.