A biomechanical comparison of polymethylmethacrylate-reinforced and expansive pedicle screws in pedicle-screw revisions

Objectives: Different techniques and choices exist for re¬vision of pedicle screws, two of which are pedicle screw combined with cement augmentation and expansive pedi¬cle screw fixation. This biomechanical study was designed to compare the pullout strengths of two different revision techniques. Methods: Fourteen lumbar vertebrae obtained from four calves (mean age 15 months) were divided into two groups equal in number. Monoaxial 6.0-mm pedicle screws were inserted into the right pedicles, and axial pullout test¬ing was performed at a rate of 10 mm/min and failure strengths were recorded. Revision was performed with the same-sized pedicle screws reinforced with polymethyl- methacrylate in group 1, and with 7.0-mm expansive pedi¬cle screws in group 2, and pullout testing was repeated to record maximum revision pullout strengths. Results: The mean pullout strengths recorded before and after revision were significantly different in both groups, being 2,162.9±718.5 N and 2,794.3±979.2 N in group 1 (p=0.041) and 2,605.0±487.6 N and 3,327.1±640.8 N in group 2 (p=0.012), respectively. However, the mean pullout strengths recorded before and after revision did not differ significantly between the two groups (p>0.05). Conclusion: Our results showed that expansive pedicle screws 1 mm larger in diameter provide similar pullout strengths to those of same-sized, polymethylmethacrylate- reinforced screws in revision of pedicle screw fixation, sug¬gesting that they can be preferred with the additional advan¬tages of ease of application and avoiding risks for pedicle fracture and cement leakage.