Instability and instrumentation failures after a PSO: a finite element analysis

Area of Science:

• Spinal biomechanics
• Orthopedic surgery
• Finite element analysis

Background:

• Pedicle subtraction osteotomy (PSO) is linked to frequent mechanical complications and implant failures.
• The biomechanical origins of these failures remain insufficiently understood.

Purpose of the Study:

• To analyze biomechanical instability following PSO using finite element analysis (FEA).
• To compare the effects of different spinal construct rod contours.
• To identify mechanical forces and failure mechanisms in PSO constructs.

Main Methods:

• A 3D finite element model of the spine (L1-sacrum) was utilized.
• The model simulated PSO at L4 across healthy, dehydrated, and degenerated disc conditions.
• Range of motion and construct forces (bending, torsion, shear) were measured with varying rod contours.

Main Results:

• PSO significantly increased rotational instability, particularly with degenerated discs (+625% torsion).
• Spinal constructs reduced ROM by 95%, but rod contour shifted maximal stress points.
• Sharp rod contours concentrated stress at the PSO site, while smooth contours shifted it to the sacral junction.

Conclusions:

• Post-PSO instability is primarily rotational and exacerbated by disc degeneration.
• Rod contour critically influences the location of maximal stress in spinal constructs.
• These biomechanical insights may elucidate the mechanisms behind instrumentation failures after PSO.