Biomechanical comparison of sagittal-parallel versus non-parallel pedicle screw placement

Farshad, Mazda ; Farshad-Amacker, Nadja ; Bachmann, Elias ; Snedeker, Jess ; Schmid, Samuel

In: Acta Neurochirurgica, 2014, vol. 156, no. 11, p. 2147-2151

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    Summary
    Background: While convergent placement of pedicle screws in the axial plane is known to be more advantageous biomechanically, surgeons intuitively aim toward a parallel placement of screws in the sagittal plane. It is however not clear whether parallel placement of screws in the sagittal plane is biomechanically superior to a non-parallel construct. The hypothesis of this study is that sagittal non-parallel pedicle screws do not have an inferior initial pull-out strength compared to parallel placed screws. Methods: The established lumbar calf spine model was used for determination of pull-out strength in parallel and non-parallel intersegmental pedicle screw constructs. Each of six lumbar calf spines (L1-L6) was divided into three levels: L1/L2, L3/L4 and L5/L6. Each segment was randomly instrumented with pedicle screws (6/45mm) with either the standard technique of sagittal parallel or non-parallel screw placement, respectively, under fluoroscopic control. CT was used to verify the intrapedicular positioning of all screws. The maximum pull-out forces and type of failure were registered and compared between the groups. Results: The pull-out forces were 5,394N (range 4,221N to 8,342N) for the sagittal non-parallel screws and 5,263N (range 3,589N to 7,554N) for the sagittal-parallel screws (p = 0.838). Interlevel comparisons also showed no statistically significant differences between the groups with no relevant difference in failure mode. Conclusion: Non-parallel pedicle screws in the sagittal plane have at least equal initial fixation strength compared to parallel pedicle screws in the setting of the here performed cadaveric calf spine experiments.