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

STUDY DESIGN

Finite element analysis.

BACKGROUND DATA

Pedicle subtraction osteotomy (PSO) is associated with a high rate of mechanical complications and implant failures. The biomechanical reasons for these failures are unclear.

OBJECTIVES

Using finite element analysis (FEA): to analyze the biomechanical instability after a PSO, to compare the effect of constructs with different rod contours and analyze the mechanical forces acting on these constructs to explain the mechanisms of failure.

METHODS

A 3D validated FE model of the spine from L1 to the sacrum was used. The model was modified to simulate a PSO of L4 in different situations: healthy, high dehydrated and completely degenerated discs. Loads were applied and range of motion (ROM) was measured. Pedicle screw constructs from L2 to S1 with different rod contours were added to the most instable scenario. Bending, torsion, shear moments and stress were measured.

RESULTS

PSO alone had a moderate impact on the ROM of basic movements (flexion, extension and lateral bending). Secondary motion (torsion) in lateral bending increased 200 %. Greatest increase in ROM was observed with the PSO and degenerated discs. Secondary motion (torsion) in lateral bending increased +625 %. The instability after a PSO is rotational. Mean reduction of ROM was 95 % for all constructs tested. Rod contour affected the location of bending moments and stress. Sharp angle bend showed maximum bending moments (2,208 Nmm) and stress at the PSO level. Smooth contour of the rod showed maximum bending moments (1,940 Nmm) and stress at the sacral connection. Anterior support below the PSO reduced bending moments along the rod (-26 %).

CONCLUSION

The instability observed after a PSO is mainly rotational and increases with disc degeneration. Shape of rod contour affects the location of maximum stress in the constructs. These findings may explain different instrumentation failures.