• Permsak Paholpak, Blake Formanek, Andrew Vega, Koji Tamai, Zorica Buser, and Jeffrey C Wang.
• Department of Orthopaedic Surgery, Keck School of Medicine, University of Southern California, 1450 Biggy Street, NRT-2509N, Mail Code: 9601, Los Angeles, CA, 90033, USA.
• Eur Spine J. 2019 Oct 1; 28 (10): 2325-2332.

PurposeTo evaluate which cervical level is the most appropriate level to measure occipitocervical inclination (OCI).MethodsSixty-two patients with multi-positional MRI: 24 males and 38 females, who had cervical lordosis and had a disk degeneration grade of 3 or less were included. We measured patient's OCI at C3, C4, and C5, occipitocervical angle (OCA), occipitocervical distance (OCD), C2-7 angle, and cervical sagittal vertical axis (cSVA) in neutral, flexion, and extension position. The correlation between OCI and OCA, OCD, C2-7 angle, and cSVA on each cervical level in all three positions was tested using Pearson's correlation coefficient test. The difference between OCIs at each cervical level was tested by Wilcoxon signed-rank test. p value of less than 0.05 was set as a statistically significant level.ResultsC5 OCI showed statistically significant correlation with OCA, OCD, C2-7 angle, and cSVA in all three positions (p < 0.05, r = 0.214-0.525). C3 OCI in flexion (p = 0.393, r = 0.081) and C4 OCI in neutral and flexion (neutral p = 0.275, r 0.104; flexion p = 0.987, r = 0.002) did not show significant correlation with C2-7 angle. There was a statistically significant difference between C3, C4, and C5 OCIs in neutral and extension position (p < 0.05). At the same time, OCI showed statistically strong correlation between adjacent cervical levels (p < 0.001, r = 0.627-0.822).ConclusionC5 cervical level is the most appropriate level for OCI measurement. OCI should be measured at the same cervical level at all time. C4 OCI can reliably substitute C5 OCI in case C5 which is invisible on radiographic image. These slides can be retrieved under Electronic Supplementary Material.

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