Revision of the failed pedicle screw in osteoporotic lumbar spine: Biomechanical comparison of kyphoplasty versus pedicle hole polymethylmethacrylate augmentation techniques

Over the years, spine surgeons have been obliged to revise pedicle screws due to the increasing number of the pedicle screw instrumentation in elderly osteoporotic population. There are many ways to salvage pedicle screws such as using larger and/or longer size pedicle screws, augmentation or inserting screws in a different trajectory. Lumbar vertebrae were harvested from 4 calves and the bone mineral density (BMD) value was measured. Each vertebra was then placed into a beher-glass filled with hydrochloric acid decalcifier solution for 24 h to obtain osteoporotic specimens. Stage 1 : Primary polyaxial pedicle screws were randomly inserted to left or right pedicle. The screws were pulled out in a random order at 10 mm/min with Instron Material Testing Machine. The pull-out strength (POS) results of all specimens were recorded. Stage 2 : Revision pedicle screws were randomly assigned to be replaced in the same pedicles by either pedicle hole PMMA augmentation (Group 1) or kyphoplasty (Xvoid™) augmented pedicle screws (Group 2). The pull-out tests of the specimens were repeated. The mean BMD result of the specimens before the decalcification was 1.686±227.9 and 1.432±157.1 g/cm 2 after decalcification. There was a statistically significant difference between BMD results before and after the decalcification ( p Group 1 : The mean POS of primary screws was 3443±1086 N/m 2 . After pedicle hole augmentation, the mean POS was 2088±924 N/m 2 . The statistical difference was found between the primary and revision screws POS in group 1 ( p =0.006). Group 2 : The mean POS of primary screws was 3702±1063 N/m 2 and after kyphoplasty augmentation, the mean POS was decreased to 3664±1057 N/m 2 . There was no statistical significant difference between the primary and kyphoplasty augmented screws in group 2 ( p =0.934). There was also significant difference between the pedicle hole versus kyphoplasty augmented revision screw’s POS ( p =0.002). There was a strong correlation between POS and BMD for the primary screw ( p p =0.808). Even thought pedicle hole PMMA augmentation has been known to be the gold standard for the failed screw in an osteoporotic bone, kyhoplasty augmented pedicle screw seems to be the most powerful salvage technique for a failed pedicle screw.