Biomechanical evaluation of expansive pedicle screws and injectable absorbable bone cement augmentation in the osteoporotic vertebral environment

Abstract

Objective To evaluate the fixation strengths of expansive pedicle screws in human cadaveric vertebrae with different bone mineral densities and with the augmentation by newly-developed injectable absorbable bone cement(IABC).Methods Fresh human cadaveric spines used for this study were grouped into 4 levels according to the measurements of bone mineral density:normal,osteopenia,osteoporosis and severe osteoporosis.The vertebrae were bilaterally instrumented with 4 pedicle screw protocols:conventional pedicle screw without augmentation,expansive pedicle screw without augmentation,conventional pedicle screw with augmentation and expansive pedicle screw with augmentation.The statistical method of balanced incomplete randomized blocks was used to assign the pedicles for the 4 protocols,resulting in 9 screws per group per level.Screw pullout tests were conducted.On the load-stroke curve,the maximum pullout strength and stiffness were determined.The differences between groups at the same bone mineral density level but with different screw protocols,or between groups at different bone mineral density levels but with the same screw protocol,were compared.Results Given the same bone mineral density level,the maximum pullout strength and stiffness of expansive screws were significantly higher than those of conventional screws (P ＜ 0.05).When the same type of screw was used,the maximum pullout strength in the augmented group was significantly higher than in the non-augmented group (P ＜ 0.05),but the cement augmentation had insignificant impact on the stiffness of either conventional or expansive pedicle screws (P ＞ 0.05).The pullout strength and stiffness of expansive screws,augmented conventional screws and augmented expansive screws at the osteoporotic level were comparable to those of conventional screws at the osteopenic lcvel (P ＞ 0.05).However,the pullout strength of any of the 4 protocols in the severe osteoporosis group was significantly lower than that of conventional screws in the osteopenia group (P ＜ 0.05).Conclusions Expansive pedicle screws appear feasible and safe in either osteopenic or osteoporotic spines.Augmentation by the newly-developed IABC can further improve the initial fixation strength of either conventional or expansive pedicle screws. Key words: Osteoporosis; Bone screw; Biomechanics; Cement; Pedicle screw augmentation