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

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. 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. 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. 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.