Article
Analysis of different C1 screws in C1/2 fusion surgery – a biomechanical study
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Published: | October 25, 2022 |
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Outline
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Objectives: For surgical fusion of C1/2 in unstable fractures, the Harms technique provides sufficient stabilization with good clinical results, and screw placement in the atlas can be either in the lateral mass or in the pedicle via the posterior arch of C1. Yet little is known about the overall biomechanical stability of the construct depending on screw localization, which has been the aim of the present study.
Methods: In this presented study, we performed tests of C1/2 Harms constructs using five cadaveric specimen with both, lateral mass screws and pedicle screws as fixation in C1. After fixation of the base plate of C3 in the mold plastic block, measurements were performed in a testing machine (Zwick/Roell Z010, Zwick Roell, Ulm, Germany). The tests were performed with axial compression only, the force being introduced through the atlanto-occipital joint. It was chosen according to the typical weight of a human head. Strain gauges were attached to the midline of C1 and C2 and to the bilateral rods. Bending forces were measured as µm/m over time. The position of the single strain gauge omega was anchored in the midline in a radiographically controlled manner. Measurements were performed for each specimen with 20 cycles of force application in 50-, 75-, and 100-newton increments repeatedly. Differences and means of maximum and minimum were calculated, and the means of the 20 cycles were pooled.
Results and conclusion: Measurement of axial compression force along the centerline from C1 to C2 showed lower strain values for lateral mass screws at 50N compared to pedicle screws (142.04 µm/m versus 165.97 µm/m). This difference remained at compression forces of 75N (166.55 µm/m versus 190.57 µm/m) and 100N (188.53 µm/m versus 195.94 µm/m). The construct with lateral mass screws thus showed less ventral deflection of the omega under the respective loads than with pedicle screws, suggesting a more stable fixation in the given experimental setting.
The present study was unable to demonstrate any advantage in the stability of pedicle screws compared with massa lateral screws in C1, so that the benefit as well as the use of pedicle screws, which are supposedly associated with higher risk, must be critically questioned.