Three different screw trajectories in single segment fixation: a finite element analysis and biomechanical study

Area of Science:

• Spinal Surgery
• Biomechanical Engineering
• Orthopedic Research

Background:

• Osteoporotic spines pose challenges for conventional pedicle screw (CPS) fixation.
• Cortical bone trajectory (CBT) screws improve fixation by engaging cortical bone.
• Standard CBT (S-CBT) screws face issues like stress concentration and reduced fatigue resistance.

Purpose of the Study:

• To investigate the biomechanical behaviors of a modified CBT (M-CBT) screw technique.
• The M-CBT technique was modified to accommodate longer screws for enhanced fixation.

Main Methods:

• Utilized a validated nonlinear finite element analysis (FEA) model (L1-S1).
• Created L4-5 fusion models with CPS, S-CBT, and M-CBT screw fixations.
• Performed FEA under various motion simulations and biomechanical cadaveric studies including fatigue and pull-out tests.

Main Results:

• M-CBT fixation showed reduced peak von Mises stress on cages and screws compared to S-CBT.
• Cadaveric studies indicated M-CBT had intermediate fatigue displacement between S-CBT and CPS.
• M-CBT demonstrated significantly higher pull-out force in lower vertebrae compared to S-CBT and CPS.

Conclusions:

• The M-CBT technique enhances anterior column control by increasing screw length.
• M-CBT minimizes stress on cages and screws, optimizing anti-fatigue performance.
• M-CBT screw technique offers improved biomechanical properties for osteoporotic vertebrae fixation.