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Limiting interpedicular screw displacement increases shear forces in screws: A finite element study
L V P C Lima1, Y P Charles2, P Rouch3
1Institut de biomécanique humaine Georges-Charpak/LBM, arts et métiers ParisTech, 151, boulevard de l'Hôpital, 75013 Paris, France; Universidade Estadual do Rio de Janeiro, Instituto Politécnico Rua Bonfim, 25, Vila Amélia, 28.625-570 Nova Friburgo, RJ, Brazil.
Posterior spinal implants allowing interpedicular displacement (ID) showed reduced screw loosening. Future designs should balance stiffness for load-bearing with controlled ID to enhance spinal fusion device longevity.
Area of Science:
- Spinal biomechanics
- Finite element analysis
- Orthopedic implant design
Background:
- Screw loosening is a common complication in non-fusion spinal devices.
- Pedicle screw forces may correlate with kinematic parameters like interpedicular displacement (ID).
Purpose of the Study:
- To investigate the relationship between interpedicular displacement (ID) and screw loosening.
- To compare different posterior implant designs using a finite element model.
Main Methods:
- A validated L3-sacrum finite element (FE) model was utilized.
- Three posterior implant designs (rigid, flexible, sliding rod) were simulated.
- Simulated spinal rotations (flexion-extension, lateral bending, axial rotation) and varied friction coefficients.
Main Results:
- The sliding implant (S3) and intact spine exhibited greater ID (5.5mm) than rigid (R) and flexible (F) constructs (<2mm).
- Screw shear forces were significantly lower for the sliding implant (43N) compared to flexible (153N) and rigid (272N) constructs.
Conclusions:
- Spinal implants that permit interpedicular displacement (ID) experience reduced screw loads.
- Optimizing implant design requires balancing longitudinal stiffness with controlled ID to mitigate screw loosening.
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