Finite Element Analysis of Biomechanical Differences in Posterior Instrumentation for Congenital Kyphosis Correction

This study compared dual-rod and two four-rod (In Line vs Out Line satellite) posterior constructs after corrective osteotomy for congenital kyphoscoliosis using a patient-specific finite element model. A T۷–L۴ thoracolumbar model reconstructed from CT of a ۱۷-year-old male with type III congenital kyphoscoliosis underwent virtual eggshell (Schwab grade ۴) osteotomy at T۱۱–T۱۲. Four constructs were simulated: (۱) corrected fusion without instrumentation (control), (۲) dual-rod (۲R۸L), (۳) four-rod In Line (۴R۸L-In Line), and (۴) four-rod Out Line (۴R۸L-Out Line). A ۴۰۰ N compressive load, along with ۸ Nm of pure moments (flexion, extension, lateral bending, and axial rotation), was applied. Global (T۷–L۴) and osteotomy-site (T۱۰–T۱۲) ranges of motion (ROM) and maximum von Mises stresses in rods and pedicle screws were recorded. All instrumented constructs reduced global ROM versus control (up to ۹۶% in flexion/lateral bending). Axial rotation increased modestly (≤۲۱%) in instrumented models, attributed to the absence of ligament modeling. Four-rod constructs further reduced osteotomy-site ROM versus dual-rod, with In Line more restrictive in lateral bending and Out Line slightly superior in extension and axial rotation. Maximum rod stress decreased up to ۴۷% (extension) and ۳۵% (flexion) with four-rod configurations relative to dual-rod. Satellite rods redistributed load and lowered peak rod stress. The in-line configuration optimized lateral bending stiffness, while the out-line improved extension and rotational control. Model limitations (no ligaments, single patient, simplified contacts) warrant cautious clinical extrapolation.

Keywords:

Finite Element Analysis , Biomechanical , Posterior Instrumentation , Congenital Kyphosis , Correction