• Peter B Derman, Erik Waldorff, Nianli Zhang, and Ram Haddas.
• Texas Back Institute, 6020 West Parker Road, Plano, TX, 75093, USA.
• Eur Spine J. 2021 Feb 1; 30 (2): 554-559.

PurposeThe traditional teaching has been that proper function of a cervical disc replacement is dependent upon appropriate placement, which includes centering the device in the coronal plane. The purpose of this study was to identify the most reliable anatomical landmark for determining midline placement of an implant within the cervical disc space under fluoroscopy.MethodsDigital fluoroscopy images were taken for each cervical level at 0 °, 2.5 °, 5 °, 7.5 °, 10 °, and 15 ° from the mid-axis by rotating the C-arm beam of six cadavers. Thin-slice CT scanning of the same levels was subsequently performed. Three independent reviewers measured the distance between anatomic structures: (a) tip of the right uncinate; (b) medial border of the right pedicle; and (c) center of the spinous processes for different x-ray angles across cervical levels C3-7.ResultsBoth the uncinate and pedicle demonstrated superior overall accuracy to that of the spinous process (p ≤ 0.02) at all angles except at 0 ° for the pedicle where the difference was not statistically significant. Overall (pooled C3-7), the accuracy of the uncinate did not differ significantly from that of the pedicle at any fluoroscopic angle. The center of the spinous process measurement was particularly sensitive to deviations from the perfect anteroposterior fluoroscopy image.ConclusionsThe results of this investigation suggest that the tip of the uncinate and the medial border of the pedicle are more accurate measures of midline in the cervical spine than the center of the spinous process and are less susceptible to inadvertent off-axis imaging.

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