The effect of pedicle morphometry on pedicle screw loading. A synthetic model

Area of Science:

• Spinal biomechanics
• Orthopedic implant research
• Surgical instrumentation analysis

Background:

• High clinical failure rates of pedicle screws in short-segment spinal instrumentation for unstable fractures.
• Pedicle screws predominantly fail in sagittal bending within the vertebral pedicle.
• Limited understanding of external factors influencing in situ loads on pedicle screws.

Purpose of the Study:

• To investigate the impact of pedicle morphometry on sagittal bending moments experienced by pedicle screws.
• To quantify how variations in pedicle height, length, and width affect screw loading.
• To identify key anatomical factors contributing to pedicle screw mechanical failure.

Main Methods:

• Static, nondestructive bending analysis of pedicle screws in synthetic vertebral analogues.
• Fabrication of analogues with independent variations in pedicle height, length, and width.
• Utilized internally strain-gaged pedicle screws as load transducers to measure in situ bending moments.

Main Results:

• Screw bending moments significantly increased with greater pedicle length (30% increase from 8.0 to 12.0 mm).
• A 20% increase in screw moment was observed when pedicle height decreased below 15.0 mm, indicating a threshold effect.
• Pedicle width variations did not demonstrate a significant impact on screw bending loads.

Conclusions:

• In situ pedicle screw loads are significantly influenced by pedicle length and height.
• Increased pedicle length and decreased pedicle height elevate the risk of pedicle screw failure.
• Strain-gaged pedicle screws provide an effective method for measuring in situ spinal instrumentation loads.