The progression of bilateral spondylolysis to spondylolisthesis occurs most commonly in skeletally immature spines. Numerous biomechanical factors are believed to influence this progression. Two patterns of L5-S1 lytic spondylolisthesis are clinically observed:
The aim of this computational study was to evaluate and compare the biomechanical instabilities associated with type-1 and type-2 lytic spondylolisthesis. A calibrated, non-linear finite element (FE) model was generated from computed tomography (CT) scans of a healthy subject. The intact scans were manipulated in the image segmentation stage to produce type-1 and type-2 spondylolisthesis models. The L5 pars defect was created after the volumetric mesh was imported into FE analysis software. Models were loaded with pure unconstrained moments in flexion, extension, lateral bending, and torsion. The type-1 model demonstrated hypermobility in all bending modes at L5-S1 accompanied by a compensatory decrease in range of motion at L4-L5. Type-2 spondylolisthesis was associated with segmental hypermobility at both L4-L5 and L5-S1. Similarly, the type-1 model exhibited increased shear stress at the L5-S1 disc only, whereas type-2 experienced increased shear stress at L4-L5 and L5-S1. Interpedicular travel in flexion and extension increased in both spondylolisthesis models, while retrolisthesis at L4 induced the highest instabilities at L5-S1 during extension. The data demonstrated two-level instability associated with the type-2 defect, whereas type-1 spondylolisthesis induced instabilities predominantly at the L5-S1 level. Type-2 spondylolisthesis is at a greater risk of progression and extension movements in particular should be avoided. The L4-S1 sagittal plane bending instabilities associated with the type-2 defect compared to single-level instabilities of type-1 spondylolisthesis should be considered during surgical planning and treatment.