Micro-CT and micro-FE analysis of pedicle screw fixation under different loading conditions

Area of Science:

• Spine Biomechanics
• Orthopedic Surgery
• Biomaterials Engineering

Background:

• Pedicle screw fixation in elderly patients with osteoporotic bone presents significant challenges due to compromised bone quality.
• Accurate prediction of screw fixation strength is crucial for successful spinal instrumentation outcomes in this vulnerable population.

Purpose of the Study:

• To investigate the influence of pedicle screw design and local bone density on fixation mechanics using micro finite element (µFE) models.
• To correlate micro-computed tomography (µCT) derived bone parameters with experimentally measured screw fixation properties.

Main Methods:

• Creation of µFE models from µCT scans of vertebrae implanted with two pedicle screw types.
• Analysis of local bone volume fraction (BV/TV) in the pedicle and vertebral body regions.
• Correlation of predicted stiffness from pull-out simulations with experimental pull-out and torsional fixation data.

Main Results:

• Predicted stiffness showed excellent correlation with experimental pull-out strength (R² > 0.92), outperforming regional BV/TV alone (R² = 0.79).
• Correlations between fixation properties and BV/TV improved when focusing on the pedicle zone (R² = 0.66-0.94) for pull-out loads.
• Local bone density showed weaker correlations with torsional resistance (R² < 0.10), indicating limitations in predicting torsion performance.

Conclusions:

• Local bone density in the pedicle zone significantly influences pedicle screw fixation stiffness and pull-out strength.
• Micro finite element analysis provides a valuable tool for assessing screw fixation mechanics in compromised bone.
• Apparent bone density alone may not fully explain screw resistance under torsional loading conditions.