Scoliosis corrective force estimation from the implanted rod deformation using 3D-FEM analysis

Area of Science:

• Spinal surgery
• Orthopedic biomechanics
• Medical device engineering

Background:

• Advancements in spinal instrumentation have improved scoliosis correction.
• Increased mechanical strength of implants leads to higher forces at the bone-implant interface.
• Actual corrective forces and screw pull-out safety margins in scoliosis surgery remain incompletely understood.

Purpose of the Study:

• To estimate corrective forces and pull-out forces during scoliosis surgery.
• To analyze the biomechanical effects of the simultaneous double rod rotation technique (SDRRT).
• To evaluate the safety margins of pedicle screws during spinal deformity correction.

Main Methods:

• Finite Element Analysis (FEA) was employed to model spinal biomechanics.
• Data from 20 adolescent idiopathic scoliosis patients undergoing SDRRT with 6mm titanium rods were analyzed.
• Pre- and post-maneuver rod geometry was obtained from intraoperative and postoperative imaging (3D-CT).

Main Results:

• SDRRT corrected major curves from an average of 63.1 to 18.1 degrees (71.4% correction rate).
• Concave side screws experienced significantly higher estimated pull-out forces (160.0N) compared to convex side screws (35.6N).
• Estimated push-in forces at the uppermost and lowest instrumented vertebrae were higher on the concave side (305.1N) than the convex side (86.4N).

Conclusions:

• Corrective forces are approximately four times greater on the concave side during scoliosis correction.
• Estimated screw pull-out and push-in forces were within previously reported safety margins for moderate curves.
• For severe curve correction, spinal segment mobilization may be crucial to prevent implant failure, rather than solely relying on stronger implants or maneuvers.