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Pedicle Screw Fixation Under Nonaxial Loads: A Cadaveric Study
Rohan-Jean Bianco1, Carl-Eric Aubin, Jean-Marc Mac-Thiong
1*Department of Mechanical Engineering, Polytechnique Montréal, Montreal, Quebec, Canada†Sainte-Justine University Hospital Center, Montreal, Quebec, Canada‡Laboratoire de Biomécanique Appliquée, Aix-Marseille Université, IFSTTAR, LBA UMR T24, Boulevard Pierre Dramard, Marseille Cedex, France§iLab-Spine (International Laboratory - Spine Imaging and Biomechanics), Montreal, Canada and Marseille, France¶Department of Surgery, Faculty of Medicine, Université de Montréal, Montreal, Quebec, Canada||Department of Mechanical Engineering, École de technologie supérieure, Montreal, Quebec, Canada**Department of Surgery, Hôpital du Sacré-Cœur de Montréal, Montreal (Quebec), Canada.
Nonaxial loading causes pedicle screw loosening through bone deformation, with lateral loads posing a greater risk than cranial loads. Screw design and trajectory impact fixation stability.
Area of Science:
- Spine biomechanics
- Orthopedic implant technology
- Surgical fixation methods
Background:
- Traditional pedicle screw fixation testing uses axial pullout, not reflecting complex spinal loads.
- Nonaxial load effects on pedicle screw loosening mechanisms remain incompletely understood.
Purpose of the Study:
- To experimentally characterize pedicle screw fixation under nonaxial loading.
- To analyze how screw design and placement influence fixation failure risk.
Main Methods:
- A design of experiment evaluated dual-lead vs. single-lead threads and transverse vs. sagittal trajectories.
- Pedicle screws were tested in 12 human cadaveric lumbar vertebrae under cyclic nonaxial loads (0-400 N).
- Forces, displacements, and rotations were recorded to assess screw fixation under simulated spinal loads.
Main Results:
- The first loading cycle caused significant bone deformation (0.79 mm), accumulating to 1.75 mm (plowing effect).
- Lateral loading resulted in greater bone deformation than cranial loading (P < 0.05).
- Dual-lead screws exhibited higher initial stiffness than single-lead screws (P < 0.05).
Conclusions:
- Nonaxial loads induce bone deformation (plowing), leading to screw loosening and reduced fixation strength.
- Lateral loads increase bone deformation and failure risk compared to cranial loads.
- Understanding these biomechanical responses is crucial for optimizing pedicle screw fixation in spinal surgery.
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