Finite element method-based study of pedicle screw-bone connection in pullout test and physiological spinal loads

Area of Science:

• Biomechanics
• Spine Surgery
• Finite Element Analysis

Background:

• Finite element (FE) methods are crucial for studying screw-bone interactions in spinal implants.
• Excluding screw threads in FE models simplifies analysis but may impact accuracy.
• The biomechanical effects of different screw-bone connection types remain largely uninvestigated.

Purpose of the Study:

• To compare the accuracy of different screw-bone connection models in FE spine simulations.
• To evaluate the influence of screw-bone connection types on spinal biomechanics and implant stress.
• To assess the predictive capability of simplified-geometry-rigid (SGR) screw models.

Main Methods:

• Three screw-bone connection types were simulated: contact, bonded, and simplified-geometry-rigid (SGR).
• FE simulations included screw pullout tests in polyurethane foam and physiological spinal loading tests (flexion, extension, lateral bending, axial rotation) on a validated lumbar spine model (L1-L5).
• The FE spine model incorporated major ligaments and resultant muscle forces.

Main Results:

• Contact connections exhibited lower holding capability and higher von Mises stress (up to 44.88%) in bone and implants compared to bonded connections.
• Flexion generated the highest von Mises stress in both bone and implants across all spinal loading conditions.
• Significant differences in von Mises stress at the screw-bone interface and screw pullout strength were observed between simplified and non-simplified screw FE models.

Conclusions:

• Spinal implants and bone experience elevated stress immediately post-surgery before full bone-screw bonding.
• The screw-bone interface is more robust and less prone to failure once fully bonded.
• The SGR screw model provides predictions of screw force and rod stress consistent with non-simplified models.