Abstract
Polymethylmethacrylate (PMMA) has been applied clinically and biomechanically repair loose pedicle screws. Controversies have arisen over data due to uncontrolled cement properties, various locations and sizes of fenestrated holes in repair screws, irregular holes and different bone densities of specimens. In this study, the pullout strength was compared for two techniques, the modified technique to use PMMA to augment a threaded hole and the traditional technique with retrograde injection of a PMMA filling, for standard loose screws in porcine vertebrae. Both techniques provided statistically significant results for sufficiently randomized specimens and experimental procedures. The difference in the pullout strength between conical and cylindrical screws for the aforementioned cement augmentation techniques was also investigated. Twenty-four single-level fresh-frozen lumbar vertebrae from L1 to L6 were harvested from four mature pigs. A total of 0.8 ml of PMMA was retrograde injected into screw holes with a 5.5 mm diameter, followed by insertion of a 5.0 mm diameter repair screw in the traditional group (n = 12). A stiff threaded PMMA hole was created with a 4.5 mm tapping screw before insertion of repair screws in the modified group (n = 12). Two screw geometries were randomly assigned as cylindrical (n = 6) and conical (n = 6) in each group. The correlations between filling techniques, screw geometries and axial pullout strength were analyzed. An appropriate screw trajectory and insertion depth were confirmed using X-ray imaging prior to pullout testing in both groups. For a given screw geometry (cylindrical or conical), the pullout force of the modified group was significantly higher than that of the traditional group. There was no significant difference in the pullout force between the screw geometries for a given filling technique. The cement augmentation technique is far more influential than the screw outer geometry. The modified PMMA technique created a greater anchor force than the traditional method and could be an alternative for revision of pedicle screw loosening.
Highlights
Polymethylmethacrylate (PMMA) has been applied clinically and biomechanically repair loose pedicle screws
A 5.5 mm diameter enlarged screw hole was prepared (Fig. 1A, B), and cylindrical (Fig. 1C) and conical (Fig. 1D) repair screws were convergently inserted into the vertebral body along the track
A 5.5 mm diameter enlarged screw hole was formed (Fig. 2A), and a threaded PMMA screw hole was created by tapping screws (Fig. 2B, C)
Summary
Polymethylmethacrylate (PMMA) has been applied clinically and biomechanically repair loose pedicle screws. The pullout strength was compared for two techniques, the modified technique to use PMMA to augment a threaded hole and the traditional technique with retrograde injection of a PMMA filling, for standard loose screws in porcine vertebrae. Both techniques provided statistically significant results for sufficiently randomized specimens and experimental procedures. A modified transpedicular screw augmentation method was compared to the traditional filling method and used to repair loose pedicle screws[11,15]; the results were unreliable because there were too few specimens, the specimens had mixed bone density, irregular loosening holes were formed and the traditional filling method was not explained
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