Study design: A biomechanical study.
Objective: To study the different biomechanical property among fixed-axis, monoplanar and polyaxial screws in the static and dynamic tests.
Summary of background data: Correction loss is a common phenomenon in the patients with thoracolumbar vertebral fractures who underwent the posterior pedicle screw fixation. The incidence varies with the kinds of fixation instrumentation used. There is higher incidence in polyaxial pedicle screws group than in fixed-axis pedicle screws. Monoplanar pedicle screws, which are mobile in the axial plane but fixed in the sagittal plane, can be a better fixation instrumentation for thoracolumbar vertebral fractures in theory.
Methods: A total of 30 porcine spinal units (L2-L4) were used for the static and dynamic tests, which were randomized into six groups (A1, A2, A3, B1, B2, and B3). Static test was performed in A1, A2, and A3. In this test, fixed-axis, monoplanar, and polyaxial screws were performed in A1, A2, and A3, respectively. The ultimate load was noted after tested. In addition, dynamic test was performed in B1, B2, and B3, used fixed-axis, monoplanar, and polyaxial screws, respectively. Correction loss (head-shank angle shift and anterior vertebral body height shift) was obtained and analyzed in each mode.
Results: In static test, fixed-axis and monoplanar screws had significantly higher ultimate load than polyaxial screws (P < 0.05) and fixed-axis screws had a little higher ultimate load than monoplanar screws (P < 0.05). In dynamic test, correction loss was minimal in fixed-axis screws, medium in monoplanar screws, and maximal in polyaxial screws. However, the differences were statistically significant in all comparisons but not in the comparison of fixed-axis and monoplanar screws (P > 0.05).
Conclusion: The findings from the current study suggest that monoplanar screws can significantly increase the stiffness in axial direction compared with polyaxial screws, and reduce the risks of correction loss. For thoracolumbar vertebral fractures, monoplanar screw is a better optional instrumentation for minimally invasive surgery.
Level of evidence: N/A.