A novel pedicle-PMMA-augmented screw resists reverse windshield-wiper failure in osteoporotic spine fixation: parameter optimization by finite element analysis.

BACKGROUND

Conventional PMMA-augmented pedicle screws (CPPS) remain susceptible to "reverse windshield-wiper" failure in severe osteoporosis. This study aimed to elucidate this failure mechanism and develop a novel pedicle-PMMA-augmented screw (PPAS) with enhanced biomechanical stability under FE analysis.

METHODS

An L4 vertebral finite element (FE) model incorporating threaded hollow-core CPPS with anterior injection holes was established, followed by simulation of PMMA diffusion characteristics. Under 100 N craniocaudal cyclic loading, screw stress distribution and displacement were recorded, with the stress concentration point (designated as Point P) identified. Additional injection holes were defined at Point P to determine the maximum PMMA volume (0.3 mL). Subsequently, 30 PMMA-augmented FE models with additional variable injection locations (P-2 to P + 3 screw pitches) and volumes (0.06-0.30 mL in 0.06 mL increments) were established to analyze screw stress distribution and displacement under craniocaudal loads for optimization of parameters and screw design.

RESULTS

Under 100 N craniocaudal cyclic loading, FE analysis demonstrated that threaded CPPS with 1.5 mL PMMA reduced screw-tip stress, shifted the primary stress concentration from the screw tip to the narrowest pedicle notch region (Point P), and decreased screw-tail displacement. Augmentation with 0.3 mL PMMA at Point P achieved PMMA-cortex contact. FE analysis indicated that additional augmentation at this point substantially reduced screw displacement and bone stress under craniocaudal loading. Parametric analysis revealed minimal screw-tail displacement with 0.24-0.30 mL PMMA at points P or P + 1, and minimal bone stress with 0.24-0.30 mL PMMA at point P.

CONCLUSIONS

The failure mechanism of CPPS in severe osteoporosis involves a "reverse windshield-wiper" effect, driven by stress concentration in the pedicle region resulting from screw anterior PMMA-augmentation. Our novel PPAS with dedicated PMMA injection holes at the narrowest pedicle notch regions offers a computational proof-of-concept for an alternative fixation strategy to prevent this failure mode.